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I was wondering if large animals could possibly dig extensive burrows for themselves to live in. The polar bear burrows, but snow is not dirt. The aardvark goes about 4' long, ~150 lbs: similar to a sheep. I think the aardvark might be the biggest dirt-burrowing animal, the only sheep-sized burrower.
I wonder why.
Is there anything special about the aardvark that at its size it burrows, while other burrowers are smaller?
The following paper examines the link between burrowing ability and the stress and strain resistance provided by increased endosteal tissue formation (specifically compacted coarse cancellous bone (CCCB)) in aardvarks:
Physiological constraints associated with digging, however, are known to be strongly influenced by body size, and larger burrowers are likely to exhibit a histological profile more conspicuously influenced by fossorial activity. Here, we describe for the first time the limb bone histology of the aardvark (Orycteropus afer), the largest extant burrowing mammal…
We hypothesize that the unusual histological profile of the aardvark is likely the outcome of physiological constraints due to both extensive digging behavior and strong metabolic restrictions. Adaptations to fossoriality are thus the result of a physiological compromise between limited food availability, an environment with high temperature variability, and the need for biomechanical resistance during digging. These results highlight the difficulties of deciphering all factors potentially involved in bone formation in fossorial mammals. Even though the formation and maintaining of CCCB through ontogeny in the aardvark cannot be unambiguously linked with its fossorial habits, a high amount of CCCB has been observed in the limb bones of other large burrowing mammals.
- Digging the compromise: investigating the link between limb bone histology and fossoriality in the aardvark (Orycteropus afer), PeerJ Life & Environment (2018)
- Ecosystem engineering through aardvark (Orycteropus afer) burrowing: Mechanisms and effects
Aardvarks are small pig-like mammals that are found inhabiting a wide range of different habitats throughout Africa, south of the Sahara. They are mostly solitary and spend their days sleeping in underground burrows to protect them from the heat of the African sun, emerging in the cooler evening to search for food. Their name originates from the Afrikaans language in South Africa and means Earth Pig, due to their long snout and pig-like body. Aardvarks are unique among animals as they are the only surviving species in their animal family. Until recently it was widely believed that they were most closely related to other insectivores such as armadillos and pangolins but this is not the case with their closest living relatives actually thought to be elephants.
Aardvarks have a unique appearance amongst mammals (and indeed all animals) as they display physical characteristics of a number of different animal species. They have medium-sized, almost hairless bodies and long snouts that make them look distinctly pig-like at first, with thick skin that both protects them from the hot sun and also from being harmed by insect bites. They are able to close their nostrils to stop dust and insects from entering their nose. They have tubular, rabbit-like ears that can stand on end but can also be folded flat to prevent dirt from entering them when they are underground. Aardvarks have strong, claws on each of their spade-like feet that along with the fact that their hind legs are longer than their front legs, makes them strong and capable diggers able to excavate vast amounts of earth at an alarming rate. Due to the fact that they spend most of their lives underground or out hunting in the dark at night, they have poor eyesight but are able to easily navigate their surrounding using their excellent sense of smell to both find prey and to sense potential danger.
The echidnas are named after Echidna, a creature from Greek mythology who was half-woman, half-snake, as the animal was perceived to have qualities of both mammals and reptiles. An alternate explanation is a confusion with Ancient Greek: ἐχῖνος , romanized: ekhînos, lit. 'hedgehog, sea urchin' 
Echidnas are medium-sized, solitary mammals covered with coarse hair and spines. 
Superficially, they resemble the anteaters of South America and other spiny mammals such as hedgehogs and porcupines. They are usually black or brown in colour. There have been several reports of albino echidnas, their eyes pink and their spines white.  They have elongated and slender snouts that function as both mouth and nose. Like the platypus, they are equipped with electrosensors, but while the platypus has 40,000 electroreceptors on its bill, the long-beaked echidna has only 2,000. The short-beaked echidna, which lives in a drier environment, has no more than 400 at the tip of its snout.  Echidnas use their electroreceptive beaks to sense earthworms, termites, ants, and other burrowing prey. 
Echidnas have short, strong limbs with large claws, and are powerful diggers. Their claws on their hind limbs are elongated and curved backwards to aid in digging. Echidnas have tiny mouths and toothless jaws. The echidna feeds by tearing open soft logs, anthills and the like, and using its long, sticky tongue, which protrudes from its snout, to collect prey. The ears are slits on the sides of their heads that are usually unseen, as they are blanketed by their spines. The external ear is created by a large cartilaginous funnel, deep in the muscle.  At 33 °C, the echidna also possesses the second-lowest active body temperature of all mammals, behind the platypus.
The first European drawing of an echidna was made in Adventure Bay, Tasmania by HMS Providence's third lieutenant George Tobin during William Bligh's second breadfruit voyage. 
The short-beaked echidna's diet consists largely of ants and termites, while the Zaglossus (long-beaked) species typically eat worms and insect larvae.  The tongues of long-beaked echidnas have sharp, tiny spines that help them capture their prey.  They have no teeth, so they break down their food by grinding it between the bottoms of their mouths and their tongues.  Echidnas' faeces are 7 cm (3 in) long and are cylindrical in shape they are usually broken and unrounded, and composed largely of dirt and ant-hill material. 
Echidnas do not tolerate extreme temperatures they use caves and rock crevices to shelter from harsh weather conditions. Echidnas are found in forests and woodlands, hiding under vegetation, roots or piles of debris. They sometimes use the burrows of animals such as rabbits and wombats. Individual echidnas have large, mutually overlapping territories. 
Despite their appearance, echidnas are capable swimmers. When swimming, they expose their snout and some of their spines, and are known to journey to water in order to groom and bathe themselves. 
Echidnas and the platypus are the only egg-laying mammals, known as monotremes. The average lifespan of an echidna in the wild is estimated to be around 14–16 years. When fully grown, a female can weigh up to 4.5 kilograms (9.9 lb), and a male can weigh up to 6 kilograms (13 lb).  The echidnas' sex can be inferred from their size, as males are 25% larger than females on average. The reproductive organs also differ, but both sexes have a single opening called a cloaca, which they use to urinate, release their faeces and to mate. 
Male echidnas have non-venomous spurs on the hind feet. 
The neocortex makes up half of the echidna's brain,  compared to 80% of a human brain.   Due to their low metabolism and accompanying stress resistance, echidnas are long-lived for their size the longest recorded lifespan for a captive echidna is 50 years, with anecdotal accounts of wild individuals reaching 45 years.  Contrary to previous research, the echidna does enter REM sleep, but only when the ambient temperature is around 25 °C (77 °F). At temperatures of 15 °C (59 °F) and 28 °C (82 °F), REM sleep is suppressed. 
The female lays a single soft-shelled, leathery egg 22 days after mating, and deposits it directly into her pouch. An egg weighs 1.5 to 2 grams (0.05 to 0.07 oz)  and is about 1.4 centimetres (0.55 in) long. While hatching, the baby echidna opens the leather shell with a reptile-like egg tooth.  Hatching takes place after 10 days of gestation the young echidna, called a puggle,   born larval and fetus-like, then sucks milk from the pores of the two milk patches (monotremes have no nipples) and remains in the pouch for 45 to 55 days,  at which time it starts to develop spines. The mother digs a nursery burrow and deposits the young, returning every five days to suckle it until it is weaned at seven months. Puggles will stay within their mother's den for up to a year before leaving. 
Male echidnas have a four-headed penis.  During mating, the heads on one side "shut down" and do not grow in size the other two are used to release semen into the female's two-branched reproductive tract. Each time it copulates, it alternates heads in sets of two.   When not in use, the penis is retracted inside a preputial sac in the cloaca. The male echidna's penis is 7 centimetres (2.8 in) long when erect, and its shaft is covered with penile spines.  These may be used to induce ovulation in the female. 
It is a challenge to study the echidna in its natural habitat and they show no interest in mating while in captivity. Prior to 2007, no one had ever seen an echidna ejaculate. There have been previous attempts, trying to force the echidna to ejaculate through the use of electrically stimulated ejaculation in order to obtain semen samples but this has only resulted in the penis swelling. 
Breeding season begins in late June and extends through September. Males will form lines up to ten individuals long, the youngest echidna trailing last, that follow the female and attempt to mate. During a mating season an echidna may switch between lines. This is known as the "train" system. 
Echidnas are very timid animals. When they feel endangered they attempt to bury themselves or if exposed they will curl into a ball similar to that of a hedgehog, both methods using their spines to shield them. Strong front arms allow echidnas to continue to dig themselves in whilst holding fast against a predator attempting to remove them from the hole.
Although they have a way to protect themselves, the echidnas still face many dangers. Some predators include feral cats, foxes, domestic dogs and goannas. Snakes pose a large threat to the echidna species because they slither into their burrows and prey on the young spineless puggles.
Some precautions that can be taken include keeping the environment clean by picking up litter and causing less pollution, planting vegetation for echidnas to use as shelter, supervising pets, reporting hurt echidnas or just leaving them undisturbed. Merely grabbing them may cause stress, and picking them up improperly may even result in injury. 
The first divergence between oviparous (egg-laying) and viviparous (offspring develop internally) mammals is believed to have occurred during the Triassic period.  However, there is still some disagreement on this estimated time of divergence. Though most findings from genetics studies (especially those concerning nuclear genes) are in agreement with the paleontological findings, some results from other techniques and sources, like mitochondrial DNA, are in slight disagreement with findings from fossils. 
Molecular clock data suggest echidnas split from platypuses between 19 and 48 million years ago, and that platypus-like fossils dating back to over 112.5 million years ago, therefore, represent basal forms, rather than close relatives of the modern platypus.  [ further explanation needed ] This would imply that echidnas evolved from water-foraging ancestors that returned to living completely on the land, even though this put them in competition with marsupials. [ further explanation needed ] Though both existing monotremes such as the platypus and echidna have no teeth, the ancestor of monotremes once had adult teeth. Therefore, four out of the eight genes for tooth development were lost from those common ancestors. 
Further evidence of possible water-foraging ancestors can be found in some of the echidna's phenotypic traits as well. These traits include hydrodynamic streamlining, dorsally projecting hind limbs acting as rudders, and locomotion founded on hypertrophied humeral long-axis rotation, which provides a very efficient swimming stroke.  Consequently, oviparous reproduction in monotremes may have given them an advantage over marsupials, a view consistent with present ecological partitioning between the two groups.  This advantage could as well be in part responsible for the observed associated adaptive radiation of echidnas and expansion of the niche space, which together contradict the fairly common assumption of halted morphological and molecular evolution that continues to be associated with monotremes.
Furthermore, studies of mitochondrial DNA in platypuses have also found that monotremes and marsupials are most likely sister taxa. It also implies that any shared derived morphological traits between marsupials and placental mammals either occurred independently from one another or were lost in the lineage to monotremes. 
Echidnas are classified into three genera.  The genus Zaglossus includes three extant species and two species known only from fossils, while only one extant species from the genus Tachyglossus is known. The third genus, Megalibgwilia, is known only from fossils.
The three living Zaglossus species are endemic to New Guinea.  They are rare and are hunted for food. They forage in leaf litter on the forest floor, eating earthworms and insects. The species are
- (Z. bruijni), of the highland forests (Z. attenboroughi), discovered by Western science in 1961 (described in 1998) and preferring a still higher habitat (Z. bartoni), of which four distinct subspecies have been identified.
The two fossil species are
The short-beaked echidna (Tachyglossus aculeatus) is found in southern, southeast and northeast New Guinea, and also occurs in almost all Australian environments, from the snow-clad Australian Alps to the deep deserts of the Outback, essentially anywhere ants and termites are available. It is smaller than the Zaglossus species, and it has longer hair.
Despite the similar dietary habits and methods of consumption to those of an anteater, there is no evidence supporting the idea that echidna-like monotremes have been myrmecophagic (ant or termite-eating) since the Cretaceous. The fossil evidence of invertebrate-feeding bandicoots and rat-kangaroos, from around the time of the platypus–echidna divergence and pre-dating Tachyglossus, show evidence that echidnas expanded into new ecospace despite competition from marsupials. 
The genus Megalibgwilia is known only from fossils:
The Kunwinjku people of Western Arnhem Land call the echidna ngarrbek,  and regard it as a prized food and 'good medicine' (Reverend Peterson Nganjmirra, personal comment  ). Echidna is hunted at night. After being gutted it is filled with hot stones and bush herbs namely the leaves of mandak (Persoonia falcata).  According to Larrakia elders Una Thompson and Stephanie Thompson Nganjmirra, when captured echidna is carried attached to the wrist like a thick bangle.
The aardvark is vaguely pig-like in appearance. Its body is stout with a prominently arched back  and is sparsely covered with coarse hairs. The limbs are of moderate length, with the rear legs being longer than the forelegs.  The front feet have lost the pollex (or 'thumb'), resulting in four toes, while the rear feet have all five toes. Each toe bears a large, robust nail which is somewhat flattened and shovel-like, and appears to be intermediate between a claw and a hoof. Whereas the aardvark is considered digitigrade, it appears at time to be plantigrade. This confusion happens because when it squats it stands on its soles. 
An aardvark's weight is typically between 60 and 80 kilograms (130–180 lb) .  An aardvark's length is usually between 105 and 130 centimetres (3.44–4.27 ft) ,  and can reach lengths of 2.2 metres (7 ft 3 in) when its tail (which can be up to 70 centimetres (28 in) ) is taken into account. It is 60 centimetres (24 in) tall at the shoulder, and has a girth of about 100 centimetres (3.3 ft) .  It is the largest member of the proposed clade Afroinsectiphilia. The aardvark is pale yellowish-gray in color and often stained reddish-brown by soil. The aardvark's coat is thin, and the animal's primary protection is its tough skin. Its hair is short on its head and tail however its legs tend to have longer hair.  The hair on the majority of its body is grouped in clusters of 3-4 hairs.  The hair surrounding its nostrils is dense to help filter particulate matter out as it digs. Its tail is very thick at the base and gradually tapers.
The greatly elongated head is set on a short, thick neck, and the end of the snout bears a disc, which houses the nostrils. It contains a thin but complete zygomatic arch.  The head of the aardvark contains many unique and different features. One of the most distinctive characteristics of the Tubulidentata is their teeth. Instead of having a pulp cavity, each tooth has a cluster of thin, hexagonal, upright, parallel tubes of vasodentin (a modified form of dentine), with individual pulp canals, held together by cementum.  The number of columns is dependent on the size of the tooth, with the largest having about 1,500.  The teeth have no enamel coating and are worn away and regrow continuously.  The aardvark is born with conventional incisors and canines at the front of the jaw, which fall out and are not replaced. Adult aardvarks have only cheek teeth at the back of the jaw, and have a dental formula of: 0.0.2-3.3 0.0.2.3 These remaining teeth are peg-like and rootless and are of unique composition.  The teeth consist of 14 upper and 12 lower jaw molars.  The nasal area of the aardvark is another unique area, as it contains ten nasal conchae, more than any other placental mammal. 
The sides of the nostrils are thick with hair.  The tip of the snout is highly mobile and is moved by modified mimetic muscles.  The fleshy dividing tissue between its nostrils probably has sensory functions,  but it is uncertain whether they are olfactory or vibratory in nature.  Its nose is made up of more turbinate bones than any other mammal, with between 9 and 11, compared to dogs with 4 to 5.  With a large quantity of turbinate bones, the aardvark has more space for the moist epithelium, which is the location of the olfactory bulb.  The nose contains nine olfactory bulbs, more than any other mammal.  Its keen sense of smell is not just from the quantity of bulbs in the nose but also in the development of the brain, as its olfactory lobe is very developed.  The snout resembles an elongated pig snout. The mouth is small and tubular, typical of species that feed on ants and termites. The aardvark has a long, thin, snakelike, protruding tongue (as much as 30 centimetres (12 in) long)  and elaborate structures supporting a keen sense of smell.  The ears, which are very effective,  are disproportionately long, about 20–25 centimetres (7.9–9.8 in) long.  The eyes are small for its head, and consist only of rods. 
The aardvark's stomach has a muscular pyloric area that acts as a gizzard to grind swallowed food up, thereby rendering chewing unnecessary.  Its cecum is large.  Both sexes emit a strong smelling secretion from an anal gland.  Its salivary glands are highly developed and almost completely ring the neck  their output is what causes the tongue to maintain its tackiness.  The female has two pairs of teats in the inguinal region. 
Genetically speaking, the aardvark is a living fossil, as its chromosomes are highly conserved, reflecting much of the early eutherian arrangement before the divergence of the major modern taxa. 
The term echolocation was coined by the American zoologist Donald Griffin, who worked with Robert Galambos was the first to convincingly demonstrate its existence in bats in 1938.   As Griffin described in his book,  the 18th century Italian scientist Lazzaro Spallanzani had, by means of a series of elaborate experiments, concluded that when bats fly at night, they rely on some sense besides vision, but he did not discover that the other sense was hearing.   The Swiss physician and naturalist Louis Jurine repeated Spallanzani's experiments (using different species of bat), and concluded that when bats hunt at night, they rely on hearing.    In 1908, Walter Louis Hahn confirmed Spallanzani's and Jurine's findings. 
In 1912, the inventor Hiram Maxim independently proposed that bats used sound below the human auditory range to avoid obstacles.  In 1920, the English physiologist Hamilton Hartridge correctly proposed instead that bats used frequencies above the range of human hearing.  
Echolocation in odontocetes (toothed whales) was not properly described until two decades after Griffin and Galambos' work, by Schevill and McBride in 1956.  However, in 1953, Jacques Yves Cousteau suggested in his first book, The Silent World (pp. 206–207) that porpoises had something like sonar, judging by their navigational abilities.
Echolocation is the same as active sonar, using sounds made by the animal itself. Ranging is done by measuring the time delay between the animal's own sound emission and any echoes that return from the environment. The relative intensity of sound received at each ear as well as the time delay between arrival at the two ears provide information about the horizontal angle (azimuth) from which the reflected sound waves arrive. 
Unlike some human-made sonars that rely on many extremely narrow beams and many receivers to localize a target (multibeam sonar), animal echolocation has only one transmitter and two receivers (the ears) positioned slightly apart. The echoes returning to the ears arrive at different times and at different intensities, depending on the position of the object generating the echoes. The time and loudness differences are used by the animals to perceive distance and direction. With echolocation, the bat or other animal can see not only where it is going but also how big another animal is, what kind of animal it is, and other features.  
At the most basic level, echolocation is based on the neural anatomy of auditory brain circuitry. In essence, ascending brain pathways in the brain stem allow the brain to calculate the difference between the two ears to very small fractions of a second. 
Echolocating bats use echolocation to navigate and forage, often in total darkness. They generally emerge from their roosts in caves, attics, or trees at dusk and hunt for insects into the night. Using echolocation, bats can determine how far away an object is, the object's size, shape and density, and the direction (if any) that an object is moving. Their use of echolocation allows them to occupy a niche where there are often many insects (that come out at night since there are fewer predators then), less competition for food, and fewer species that may prey on the bats themselves. 
Echolocating bats generate ultrasound via the larynx and emit the sound through the open mouth or, much more rarely, the nose.  The latter is most pronounced in the horseshoe bats (Rhinolophus spp.). Bat echolocation calls range in frequency from 14,000 to well over 100,000 Hz, mostly beyond the range of the human ear (typical human hearing range is considered to be from 20 Hz to 20,000 Hz). Bats may estimate the elevation of targets by interpreting the interference patterns caused by the echoes reflecting from the tragus, a flap of skin in the external ear. 
There are two hypotheses about the evolution of echolocation in bats. The first suggests that laryngeal echolocation evolved twice in Chiroptera, once in the Yangochiroptera and once in the horseshoe bats (Rhinolophidae).   The second proposes that laryngeal echolocation had a single origin in Chiroptera, was subsequently lost in the family Pteropodidae, and later evolved as a system of tongue-clicking in the genus Rousettus. 
Individual bat species echolocate within specific frequency ranges that suit their environment and prey types. This has sometimes been used by researchers to identify bats flying in an area simply by recording their calls with ultrasonic recorders known as "bat detectors". However echolocation calls are not always species specific and some bats overlap in the type of calls they use so recordings of echolocation calls cannot be used to identify all bats. In recent years researchers in several countries have developed "bat call libraries" that contain recordings of local bat species that have been identified known as "reference calls" to assist with identification.   
Since the 1970s there has been an ongoing controversy among researchers as to whether bats use a form of processing known from radar termed coherent cross-correlation. Coherence means that the phase of the echolocation signals is used by the bats, while cross-correlation just implies that the outgoing signal is compared with the returning echoes in a running process. Today most – but not all – researchers believe that they use cross-correlation, but in an incoherent form, termed a filter bank receiver. [ citation needed ]
When searching for prey they produce sounds at a low rate (10–20 clicks/second). During the search phase the sound emission is coupled to respiration, which is again coupled to the wingbeat. This coupling appears to dramatically conserve energy as there is little to no additional energetic cost of echolocation to flying bats.  After detecting a potential prey item, echolocating bats increase the rate of pulses, ending with the terminal buzz, at rates as high as 200 clicks/second. During approach to a detected target, the duration of the sounds is gradually decreased, as is the energy of the sound. 
Calls and ecology Edit
Echolocating bats occupy a diverse set of ecological conditions – they can be found living in environments as different as Europe and Madagascar, and hunting for food sources as different as insects, frogs, nectar, fruit, and blood. Additionally, the characteristics of an echolocation call are adapted to the particular environment, hunting behavior, and food source of the particular bat. However, this adaptation of echolocation calls to ecological factors is constrained by the phylogenetic relationship of the bats, leading to a process known as descent with modification, and resulting in the diversity of the Chiroptera today.      
Flying insects are a common source of food for echolocating bats and some insects (moths in particular) can hear the calls of predatory bats. There is evidence that moth hearing has evolved in response to bat echolocation to avoid capture.  Furthermore, these moth adaptations provide selective pressure for bats to improve their insect-hunting systems and this cycle culminates in a moth-bat "evolutionary arms race."  
Acoustic features Edit
Describing the diversity of bat echolocation calls requires examination of the frequency and temporal features of the calls. It is the variations in these aspects that produce echolocation calls suited for different acoustic environments and hunting behaviors.     
Bat call frequencies range from as low as 11 kHz to as high as 212 kHz.  Insectivorous aerial-hawking bats have a call frequency between 20 kHz and 60 kHz because it is the frequency that gives the best range and image acuity and makes them less conspicuous to insects.  However, low frequencies are adaptive for some species with different prey and environments. Euderma maculatum, a species that feeds on moths, uses a particularly low frequency of 12.7 kHz that cannot be heard by moths. 
Frequency modulation and constant frequency Edit
Echolocation calls can be composed of two different types of frequency structure: frequency modulated (FM) sweeps, and constant frequency (CF) tones. A particular call can consist of one, the other, or both structures. An FM sweep is a broadband signal – that is, it contains a downward sweep through a range of frequencies. A CF tone is a narrowband signal: the sound stays constant at one frequency throughout its duration. [ citation needed ]
Echolocation calls have been measured at intensities anywhere between 60 and 140 decibels.  Certain bat species can modify their call intensity mid-call, lowering the intensity as they approach objects that reflect sound strongly. This prevents the returning echo from deafening the bat.  High-intensity calls such as those from aerial-hawking bats (133 dB) are adaptive to hunting in open skies. Their high intensity calls are necessary to even have moderate detection of surroundings because air has a high absorption of ultrasound and because insects' size only provide a small target for sound reflection.  Additionally, the so-called "whispering bats" have adapted low-amplitude echolocation so that their prey, moths, which are able to hear echolocation calls, are less able to detect and avoid an oncoming bat. 
Harmonic composition Edit
Calls can be composed of one frequency or multiple frequencies comprising a harmonic series. In the latter case, the call is usually dominated by a certain harmonic ("dominant" frequencies are those present at higher intensities than other harmonics present in the call). [ citation needed ]
Call duration Edit
A single echolocation call (a call being a single continuous trace on a sound spectrogram, and a series of calls comprising a sequence or pass) can last anywhere from 0.2 to 100 milliseconds in duration, depending on the stage of prey-catching behavior that the bat is engaged in. For example, the duration of a call usually decreases when the bat is in the final stages of prey capture – this enables the bat to call more rapidly without overlap of call and echo. Reducing duration comes at the cost of having less total sound available for reflecting off objects and being heard by the bat. 
Pulse interval Edit
The time interval between subsequent echolocation calls (or pulses) determines two aspects of a bat's perception. First, it establishes how quickly the bat's auditory scene information is updated. For example, bats increase the repetition rate of their calls (that is, decrease the pulse interval) as they home in on a target. This allows the bat to get new information regarding the target's location at a faster rate when it needs it most. Secondly, the pulse interval determines the maximum range that bats can detect objects. This is because bats can only keep track of the echoes from one call at a time as soon as they make another call they stop listening for echoes from the previously made call. For example, a pulse interval of 100 ms (typical of a bat searching for insects) allows sound to travel in air roughly 34 meters so a bat can only detect objects as far away as 17 meters (the sound has to travel out and back). With a pulse interval of 5 ms (typical of a bat in the final moments of a capture attempt), the bat can only detect objects up to 85 cm away. Therefore, the bat constantly has to make a choice between getting new information updated quickly and detecting objects far away. 
FM signal advantages Edit
The major advantage conferred by an FM signal is extremely precise range discrimination, or localization, of the target. J.A. Simmons demonstrated this effect with a series of elegant experiments that showed how bats using FM signals could distinguish between two separate targets even when the targets were less than half a millimeter apart. This ability is due to the broadband sweep of the signal, which allows for better resolution of the time delay between the call and the returning echo, thereby improving the cross correlation of the two. Additionally, if harmonic frequencies are added to the FM signal, then this localization becomes even more precise.    
One possible disadvantage of the FM signal is a decreased operational range of the call. Because the energy of the call is spread out among many frequencies, the distance at which the FM-bat can detect targets is limited.  This is in part because any echo returning at a particular frequency can only be evaluated for a brief fraction of a millisecond, as the fast downward sweep of the call does not remain at any one frequency for long. 
CF signal advantages Edit
The structure of a CF signal is adaptive in that it allows the CF-bat to detect both the velocity of a target, and the fluttering of a target's wings as Doppler shifted frequencies. A Doppler shift is an alteration in sound wave frequency, and is produced in two relevant situations: when the bat and its target are moving relative to each other, and when the target's wings are oscillating back and forth. CF-bats must compensate for Doppler shifts, lowering the frequency of their call in response to echoes of elevated frequency – this ensures that the returning echo remains at the frequency to which the ears of the bat are most finely tuned. The oscillation of a target's wings also produces amplitude shifts, which gives a CF-bat additional help in distinguishing a flying target from a stationary one.      
Additionally, because the signal energy of a CF call is concentrated into a narrow frequency band, the operational range of the call is much greater than that of an FM signal. This relies on the fact that echoes returning within the narrow frequency band can be summed over the entire length of the call, which maintains a constant frequency for up to 100 milliseconds.  
Acoustic environments of FM and CF signals Edit
A frequency modulated (FM) component is excellent for hunting prey while flying in close, cluttered environments. Two aspects of the FM signal account for this fact: the precise target localization conferred by the broadband signal, and the short duration of the call. The first of these is essential because in a cluttered environment, the bats must be able to resolve their prey from large amounts of background noise. The 3D localization abilities of the broadband signal enable the bat to do exactly that, providing it with what Simmons and Stein (1980) call a "clutter rejection strategy." This strategy is further improved by the use of harmonics, which, as previously stated, enhance the localization properties of the call. The short duration of the FM call is also best in close, cluttered environments because it enables the bat to emit many calls extremely rapidly without overlap. This means that the bat can get an almost continuous stream of information – essential when objects are close, because they will pass by quickly – without confusing which echo corresponds to which call.    
A constant frequency (CF) component is often used by bats hunting for prey while flying in open, clutter-free environments, or by bats that wait on perches for their prey to appear. The success of the former strategy is due to two aspects of the CF call, both of which confer excellent prey-detection abilities. First, the greater working range of the call allows bats to detect targets present at great distances – a common situation in open environments. Second, the length of the call is also suited for targets at great distances: in this case, there is a decreased chance that the long call will overlap with the returning echo. The latter strategy is made possible by the fact that the long, narrowband call allows the bat to detect Doppler shifts, which would be produced by an insect moving either towards or away from a perched bat.    
Neural mechanisms Edit
Because bats use echolocation to orient themselves and to locate objects, their auditory systems are adapted for this purpose, highly specialized for sensing and interpreting the stereotyped echolocation calls characteristic of their own species. This specialization is evident from the inner ear up to the highest levels of information processing in the auditory cortex. 
Inner ear and primary sensory neurons Edit
Both CF and FM bats have specialized inner ears which allow them to hear sounds in the ultrasonic range, far outside the range of human hearing. Although in most other aspects, the bat's auditory organs are similar to those of most other mammals, certain bats (horseshoe bats, Rhinolophus spp. and the moustached bat, Pteronotus parnelii) with a constant frequency (CF) component to their call (known as high duty cycle bats) do have a few additional adaptations for detecting the predominant frequency (and harmonics) of the CF vocalization. These include a narrow frequency "tuning" of the inner ear organs, with an especially large area responding to the frequency of the bat's returning echoes. 
The basilar membrane within the cochlea contains the first of these specializations for echo information processing. In bats that use CF signals, the section of the membrane that responds to the frequency of returning echoes is much larger than the region of response for any other frequency. For example, in the greater horseshoe bat, Rhinolophus ferrumequinum, there is a disproportionately lengthened and thickened section of the membrane that responds to sounds around 83 kHz, the constant frequency of the echo produced by the bat's call. This area of high sensitivity to a specific, narrow range of frequency is known as an "acoustic fovea". 
Odontocetes (toothed whales and dolphins) have similar cochlear specializations to those found in bats. Odontocetes also have the highest neural investment of any cochleae reported to date with ratios of greater than 1500 ganglion cells/mm of basilar membrane. [ citation needed ]
Further along the auditory pathway, the movement of the basilar membrane results in the stimulation of primary auditory neurons. Many of these neurons are specifically "tuned" (respond most strongly) to the narrow frequency range of returning echoes of CF calls. Because of the large size of the acoustic fovea, the number of neurons responding to this region, and thus to the echo frequency, is especially high. 
Inferior colliculus Edit
In the Inferior colliculus, a structure in the bat's midbrain, information from lower in the auditory processing pathway is integrated and sent on to the auditory cortex. As George Pollak and others showed in a series of papers in 1977, the interneurons in this region have a very high level of sensitivity to time differences, since the time delay between a call and the returning echo tells the bat its distance from the target object. While most neurons respond more quickly to stronger stimuli, collicular neurons maintain their timing accuracy even as signal intensity changes. [ citation needed ]
These interneurons are specialized for time sensitivity in several ways. First, when activated, they generally respond with only one or two action potentials. This short duration of response allows their action potentials to give a very specific indication of the exact moment of the time when the stimulus arrived, and to respond accurately to stimuli that occur close in time to one another. In addition, the neurons have a very low threshold of activation – they respond quickly even to weak stimuli. Finally, for FM signals, each interneuron is tuned to a specific frequency within the sweep, as well as to that same frequency in the following echo. There is specialization for the CF component of the call at this level as well. The high proportion of neurons responding to the frequency of the acoustic fovea actually increases at this level.   
Auditory cortex Edit
The auditory cortex in bats is quite large in comparison with other mammals.  Various characteristics of sound are processed by different regions of the cortex, each providing different information about the location or movement of a target object. Most of the existing studies on information processing in the auditory cortex of the bat have been done by Nobuo Suga on the mustached bat, Pteronotus parnellii. This bat's call has both CF tone and FM sweep components.
Suga and his colleagues have shown that the cortex contains a series of "maps" of auditory information, each of which is organized systematically based on characteristics of sound such as frequency and amplitude. The neurons in these areas respond only to a specific combination of frequency and timing (sound-echo delay), and are known as combination-sensitive neurons.
The systematically organized maps in the auditory cortex respond to various aspects of the echo signal, such as its delay and its velocity. These regions are composed of "combination sensitive" neurons that require at least two specific stimuli to elicit a response. The neurons vary systematically across the maps, which are organized by acoustic features of the sound and can be two dimensional. The different features of the call and its echo are used by the bat to determine important characteristics of their prey. The maps include:
- FM-FM area: This region of the cortex contains FM-FM combination-sensitive neurons. These cells respond only to the combination of two FM sweeps: a call and its echo. The neurons in the FM-FM region are often referred to as "delay-tuned," since each responds to a specific time delay between the original call and the echo, in order to find the distance from the target object (the range). Each neuron also shows specificity for one harmonic in the original call and a different harmonic in the echo. The neurons within the FM-FM area of the cortex of Pteronotus are organized into columns, in which the delay time is constant vertically but increases across the horizontal plane. The result is that range is encoded by location on the cortex, and increases systematically across the FM-FM area. 
- CF-CF area: Another kind of combination-sensitive neuron is the CF-CF neuron. These respond best to the combination of a CF call containing two given frequencies – a call at 30 kHz (CF1) and one of its additional harmonics around 60 or 90 kHz (CF2 or CF3) – and the corresponding echoes. Thus, within the CF-CF region, the changes in echo frequency caused by the Doppler shift can be compared to the frequency of the original call to calculate the bat's velocity relative to its target object. As in the FM-FM area, information is encoded by its location within the map-like organization of the region. The CF-CF area is first split into the distinct CF1-CF2 and CF1-CF3 areas. Within each area, the CF1 frequency is organized on an axis, perpendicular to the CF2 or CF3 frequency axis. In the resulting grid, each neuron codes for a certain combination of frequencies that is indicative of a specific velocity 
- Doppler shifted constant frequency (DSCF) area: This large section of the cortex is a map of the acoustic fovea, organized by frequency and by amplitude. Neurons in this region respond to CF signals that have been Doppler shifted (in other words, echoes only) and are within the same narrow frequency range to which the acoustic fovea responds. For Pteronotus, this is around 61 kHz. This area is organized into columns, which are arranged radially based on frequency. Within a column, each neuron responds to a specific combination of frequency and amplitude. Suga's studies have indicated that this brain region is necessary for frequency discrimination. 
Biosonar is valuable to toothed whales (suborder Odontoceti), including dolphins, porpoises, river dolphins, killer whales and sperm whales, because they live in an underwater habitat that has favourable acoustic characteristics and where vision is extremely limited in range due to absorption or turbidity. 
Cetacean evolution consisted of three main radiations. Throughout the middle and late Eocene periods (49-31.5 million years ago), archaeocetes, primitive toothed Cetacea that arose from terrestrial mammals with the creation of aquatic adaptations, were the only known archaic Cetacea.  These primitive aquatic mammals did not possess the ability to echolocate, although they did have slightly adapted underwater hearing.  The morphology of acoustically isolated ear bones in basilosaurid archaeocetes indicates that this order had directional hearing underwater at low to mid frequencies by the late middle Eocene.  However, with the extinction of archaeocete at the onset of the Oligocene, two new lineages in the early Oligocene period (31.5-28 million years ago) comprised a second radiation. These early mysticetes (baleen whales) and odontocetes can be dated back to the middle Oligocene in New Zealand.  Based on past phylogenies, it has been found that the evolution of extant odontocetes is monophyletic however, echolocation evolved twice, convergently, along the odontocete lineage: once in Xenorophus, and oligocene stem odontocete, and once in the crown odontecete  Dispersal rates routes of early odontocetes included transoceanic travel to new adaptive zones. The third radiation occurred later in the Neogene, when present dolphins and their relatives evolved to be the most common species in the modern sea.  The evolution of echolocation could be attributed to several theories. There are two proposed drives for the hypotheses of cetacean radiation, one biotic and the other abiotic in nature. The first, adaptive radiation, is the result of a rapid divergence into new adaptive zones. This results in diverse, ecologically different clades that are incomparable.  Clade Neocete (crown cetacean) has been characterized by an evolution from archaeocetes and a dispersion across the world's oceans, and even estuaries and rivers. These ecological opportunities were the result of abundant dietary resources with low competition for hunting.  This hypothesis of lineage diversification, however, can be unconvincing due to a lack of support for rapid speciation early in cetacean history. A second, more abiotic drive is better supported. Physical restructuring of the oceans has played a role in echolocation radiation. This was a result of global climate change at the Eocene-Oligocene boundary from a greenhouse to an icehouse world. Tectonic openings created the emergence of the Southern ocean with a free flowing Antarctic Circumpolar current.     These events allowed for a selection regime characterized by the ability to locate and capture prey in turbid river waters, or allow odontocetes to invade and feed at depths below the photic zone. Further studies have found that echolocation below the photic zone could have been a predation adaptation to diel migrating cephalopods.   Since its advent, there has been adaptive radiation especially in the family Delphinidae (dolphins) in which echolocation has become extremely derived. 
Two proteins have been found to play a major role in toothed whale echolocation. Prestin, a motor protein of the outer hair cells of the inner ear of the mammalian cochlea, has an association between the number of nonsynonymous substitutions and hearing sensitivity.  It has undergone two clear episodes of accelerated protein evolution in cetaceans: on the ancestral branch of odontocetes and on the branch leading to delphinioidae.  The first episode of acceleration is connected to odontocete divergence, when echolocation first developed, and the second occurs with the increase in echolocation frequency seen in the family Delphinioidae. Cldn14, a member of the tight junction proteins which form barriers between inner ear cells, shows exactly the same evolutionary pattern as Prestin.  The two events of protein evolution, for Prestin and Cldn14, occurred at the same times as the tectonic opening of the Drake Passage (34-31 Ma) and the Antarctic ice growth at the middle Miocene climate transition (14 Ma), with the divergence of odontocetes and mysticetes occurring with the former, and the speciation of delphinioidae with the latter.  There is a strong connection between these proteins, the ocean restructuring events, and the echolocation evolution.
Thirteen species of extant odontocete evolved narrow-band high-frequency (NBHF) echolocation in four separate, convergent events. These species include the families Kogiidae (pygmy sperm whales) and Phocoenidae (porpoises), as well as some species of the genus Lagenorynchus, all of Cephalorynchus, and the La Plata Dolphin. NBHF is thought to have evolved as a means of predator evasion NBHF-producing species are small relative to other odontocetes, making them viable prey to large species such as the killer whale(Orcinus orca). However, because three of the groups developed NBHF prior to the emergence of the orca, predation by other, ancient, raptorial odontocetes must have been the driving force for the development of NBHF, not predation by the orca. Orcas, and, presumably, ancient, raptorial odontocetes such as Acrophyseter are unable to hear frequencies below 100 kHz. 
Another reason for variation in echolocation is habitat. For all sonar systems the limiting factor deciding whether a returning echo is detected is the echo-to-noise ratio (ENR). The ENR is given by the emitted source level (SL) plus the target strength, minus the two-way transmission loss (absorption and spreading) and the received noise.  Animals will adapt either to maximize range under noise-limited conditions (increase source level) or to reduce noise clutter in a shallow and/or littered habitat (decrease source level). In cluttered habitats, such as coastal areas, prey ranges are smaller, and species like Commerson's dolphin (Cephalorhynchus commersonii) have lowered source levels to better suit their environment. 
Toothed whales emit a focused beam of high-frequency clicks in the direction that their head is pointing. Sounds are generated by passing air from the bony nares through the phonic lips. These sounds are reflected by the dense concave bone of the cranium and an air sac at its base. The focused beam is modulated by a large fatty organ known as the 'melon'. This acts like an acoustic lens because it is composed of lipids of differing densities. Most toothed whales use clicks in a series, or click train, for echolocation, while the sperm whale may produce clicks individually. Toothed whale whistles do not appear to be used in echolocation. Different rates of click production in a click train give rise to the familiar barks, squeals and growls of the bottlenose dolphin. A click train with a repetition rate over 600 per second is called a burst pulse. In bottlenose dolphins, the auditory brain response resolves individual clicks up to 600 per second, but yields a graded response for higher repetition rates. 
It has been suggested that some smaller toothed whales may have their tooth arrangement suited to aid in echolocation. The placement of teeth in the jaw of a bottlenose dolphin, for example, are not symmetrical when seen from a vertical plane, and this asymmetry could possibly be an aid in the dolphin sensing if echoes from its biosonar are coming from one side or the other.   However, this idea lacks experimental support.
Echoes are received using complex fatty structures around the lower jaw as the primary reception path, from where they are transmitted to the middle ear via a continuous fat body. Lateral sound may be received through fatty lobes surrounding the ears with a similar density to water. Some researchers believe that when they approach the object of interest, they protect themselves against the louder echo by quietening the emitted sound. In bats this is known to happen, but here the hearing sensitivity is also reduced close to a target.  
The Short Answer: Debbie, that sounds like one of the many species of burrowing crayfish (also called crawfish or crawdads). They dig tunnels down to dampness or even to the water table. And they push up muddy soil out of their burrow into a mini volcano shape, with a neat hole at the top. They’re generally nocturnal, so during the day, all you’ll see are the volcanoes, which can be quite numerous. I have spent most of my life in New England, where I don’t believe any of our native crayfish are burrowers. But when I lived in Wisconsin and Kentucky, they were very common. This site has a checklist of the native species of crayfish in Michigan: http://iz.carnegiemnh.org/crayfish/country_pages/state_pages/michigan.htm. There are two burrowers on the Michigan list, the digger crayfish (Fallicambarus fodiens), and the devil crayfish (Cambarus diogenes). The digger crayfish is primarily aquatic, but sometimes digs burrows out of the water. The devil crayfish, however, is a primary burrower, meaning that it lives most of its life in its burrow. So I’m going to guess that’s what you have. For a picture, go to: http://iz.carnegiemnh.org/crayfish/NewAstacidea/species.asp?g=Cambarus&s=diogenes&ssp
More Info: Burrowing seems to be a good strategy for crayfish, as crayfish all over the world have developed a very similar lifestyle of digging complex burrows down to damp or wet soil. Like all crayfish, burrowing crayfish have gills in their abdomen under their shell. The gills are capable of gaining oxygen from air instead of water as long as they are wet.
Crayfish, as you might expect, are classified with the clawed lobsters (Nephropidae. There are over 600 species of crayfish, in three main groupings. The Astacidae and Cambaridae are restricted to the northern hemisphere and centered on Asia and North America, respectively. The Parastacidae are distributed throughout South America and Australia. There are no crayfish native to Africa. The Cambaridae are centered on the Southeastern United States, which has the most diverse crayfish assemblage in the world – more than 300 species, a remarkable number when you realize that they seem to occupy very similar ecological niches. There is also an only slightly less impressive collection of crayfish species in Australia. But that part of the world includes some interesting oddballs, including burrowing crayfish that can live far from any surface water. Then there’s the Tasmanian Giant Crayfish, which can reach 4.5 kilograms (10 pounds) – big even by the standards of an ocean-dwelling lobster. To see a picture, go to: http://yhsbiology.wikispaces.com/Crustacea
Trivia #1: The Tasmanian Giant Crayfish (Astacopsis gouldi) is the largest freshwater invertebrate in the world.
Trivia #2: There are two continents with no native crayfish. One is Antarctica. The other is Africa.
Life in Triassic Oceans: Links Between Planktonic and Benthic Recovery and Radiation
JONATHAN L. PAYNE , BAS VAN DE SCHOOTBRUGGE , in Evolution of Primary Producers in the Sea , 2007
C Feedback from the Benthos
Burrowing by macrobenthic invertebrates increases the supply of oxygen and other oxidants to sediments, thereby increasing the efficiency of organic remineralization as well as the return of buried nutrients to the water column ( Aller 1982 Thayer 1983 ). Reduced bioturbation during the Early Triassic ( Twitchett 1999 Pruss and Bottjer 2004 ), therefore, could have decreased the nutrient load in the water column by limiting the exposure of organic matter to oxygen and other oxidants, thereby allowing more efficient burial of organic matter and limiting rates of nutrient recycling. Increased depth and intensity of bioturbation during the Middle Triassic would be expected to decrease the burial efficiency of organic matter and thereby increase the quantity of essential nutrients available to phytoplankton. The magnitude of this effect is poorly documented. Based on modeling and observations, however, Thayer (1983) suggested that rates of bioturbation may have increased by as much as an order of magnitude through the Phanerozoic and thereby increased nutrient availability by a comparable amount. Observations of modern sediments suggest the first 100 to 1000 years of oxygen exposure strongly reduce burial efficiency ( Hartnett et al. 1998 ), causing burial efficiency to vary by as much as a factor of five. On the other hand, anoxia is generally argued to enhance the efficiency of phosphorus recycling from organic matter (e.g., Van Cappellen and Ingall 1994 ). Increased consumption of organic carbon by epifaunal or pelagic organisms within the water column as the food web increased in efficiency through the Triassic would also be expected to increase the recycling of nutrients prior to burial. Environmental stabilization during the Middle Triassic may have presented more favorable conditions to both the benthos and plankton, allowing this positive feedback to operate. Further quantification is required in several areas, including the relationship between nutrient availability and export production, the competing effects of bioturbation and oxygen exposure on organic carbon burial efficiency versus the recycling of phosphorus under anoxic conditions and the effect of food-web structure on the fraction of primary production that reaches the sediment surface. The ichnofossil record, itself, also remains poorly quantified in terms of the size and abundance of burrows through the Triassic and the diversity of behaviors represented.
Gibbons are not as strong as their great ape cousins, but their powerful arms, along with hook-like hands and specialized shoulder joints, help them swing from branch to branch through the jungle. This type of movement is called brachiation. It allows gibbons to reach average speeds of about 35 miles per hour and propel themselves from one tree to another over distances of up to 50 feet.
Gibbons don’t live in large family groups or spend much time on the ground, so physical strength is not their first line of defense. However, they do have large, dagger-like canine teeth that can cause serious damage.
6 Answers 6
Two things affect the possibilities:
What is is the air density. Thicker air means less wing needed, or slower speeds needed for a given amount of lift. You can do this with either higher pressure, colder temperatures, or by mixing a heavier gas as one component. E.g. Sulfur hexafluoride.
What is the local gravity? Lighter gravity means less muscle needed.
To get both lighter gravity and higher pressure, make your planet larger, but lower density.
Edit for earth conditions:
The largest birds I know of are condors and albatrosses. I've found one mention of 15 kg -- 33 pounds. Weight of a small border collie. These birds do not actually fly much, so much as soar. They are very good at finding and using updrafts. And for these critters we're talking wingspans of 10 feet. This is not a critter that maneuvers well through the treetops.
While lighter bones will help, especially if they are built like trusses, and not like columns, most of the weight is muscle. So things you can do to make a larger flying critter possible:
- A type of muscle that can produce more power per pound. Faster individual fibre recovery time. Lots more mitochondria per cell. A better reaction to create ATP from ADP
- A mechanism so that muscles can use short chain fatty acids for energy, instead of exclusively on glucose.
- A replacement for hemoglobin that can carry more oxygen per volume of blood.
- Viscosity agents to make blood slide through capillaries more easily.
- Valving in the veins in flight muscles to help the heart circulate.
- Some form of flow through lung, so lungs don't spend time exhaling. If you set it up so that air flows in through the mouth, and out through an opening further back, and set up the blood flow in the opposite direction, you have all the benefits of a counter flow exchanger. This should double to triple the effectiveness of the lungs. The animal has to have some kind of diaphram to breath when stationary, but once flying, can use flow through.
- A mechanism to store energy that can be metabolized FAST.
- A mechanism that can store oxygen for later use in burst mode. See Niven's "Legacy of Hereot" series on Grendel metabolism.
A 'tree top' ecology would also have room for really good jumpers. Think in terms of self contained catapult, where the muscles can ratchet back tendons to store energy, and then the critter leaps. Grasping limbs would be optimized for shock absorption to catch branches on the other side.
All descriptions were obtained from histological thin sections prepared and hosted at the Karoo Palaeontology Department in the National Museum, Bloemfontein, as depicted in Figs. 2–7. We sampled homologous bones from both the left and right side for one of our specimens (MVD-M 1) since left and right elements present similar histological characteristics, we do not consider sidedness to have a significant influence on the histological profile of the bones in our sample (see Woodward, Horner & Farlow, 2014). Similarly, the histological profile of one particular bone did not show much variation between sections taken from different parts of its midshaft. Hence the general description for each of the six limb bones included in this study applies to all corresponding specimens in the sample, unless mentioned otherwise.
Figure 2: Histological cross-sections of humeri, under OL (A–C, E, G–I, M), PL (D), and CPL (F, J–L, N–P).
Figure 3: Histological cross-sections of radii, under OL (A–B, K) and CPL (C–J, L–O).
Figure 4: Histological cross-sections of ulnae, under OL (A–C) and CPL (D–K).
Figure 5: Histological cross-sections of femora, under OL (A–B) and CPL (C–M).
Figure 6: Histological cross-sections of tibiae, under OL (A–B) and CPL (C–N).
Figure 7: Histological cross-sections of fibulae, under OL (A–C) and CPL (D–L).
Cross-sections present an ovoid shape, with two well-developed ridges on the anterior side, due to the strong protrusion of the deltoid and pectoral ridges at the diaphyseal level (Le Gros Clark & Sonntag, 1926 see also Lehmann et al., 2006). In the proximalmost sections, those ridges are distinct from each other (Fig. 2A), whereas in the more distal part of the midshaft, they are fused in a large deltopectoral tuberosity, resulting in an overall ‘bottleneck-like’ section shape (Fig. 2B). The cortex is relatively thin, and of even thickness throughout the section (Figs. 2A and 2B). The medullary cavity is large and well-defined, with a dense network of bony trabeculae packing its anterior side in the distal midshaft, underlying the protruding deltopectoral tuberosity (Figs. 2B and 2C). Some trabeculae are also present on its medial side (Figs. 2A and 2B). The innermost part of the cortex presents endosteal lamellar bone, mostly visible on the lateral side (Fig. 2D). The inner and mid-cortex consists of CCCB, which is formed through the compaction of trabeculae in the metaphysis, subsequently incorporated into the diaphyseal cortex during longitudinal growth (Fig. 2E see Discussion). CCCB presents a characteristic structure of compacted lamellar trabeculae, oriented either in parallel or perpendicularly to the periosteum, resulting in a mesh-like structure that is most conspicuous when observed under CPL (Fig. 2F). Most vascular canals in the inner cortex are circumferentially oriented, with a few radial anastomoses (Fig. 2G), and can locally form a more reticular pattern (Fig. 2H).
Vascularization becomes mostly longitudinal in the mid-cortex, where Haversian remodeling becomes more prevalent (Fig. 2I). The pattern of longitudinal secondary osteons becomes denser closer to the periosteum, and the outer cortex is heavily remodeled (Fig. 2J). The underlying pattern of CCCB, however, is still visible in-between secondary osteons (Fig. 2J), and thus cannot be described as dense Haversian bone (HB). Secondary osteons are more numerous on the anterior and posterior side in the deltopectoral tuberosity, some of them form a dense, obliquely-oriented pattern (Fig. 2K). The outmost cortex presents a thin layer of poorly vascularized parallel-fibered bone (PFB) on its lateral side (Fig. 2L), which contains at least one line of arrested growth (LAG), indicating a temporary cessation in growth (Fig. 2M). The periosteal edge of the PFB presents a clear resorption front, as does the outer edge of CCCB underneath this PFB layer (Fig. 2N). This contrasts with the outermost cortex on the medial side, where the PFB layer is almost completely resorbed, and the periosteal edge is more scalloped (Fig. 2O). On the anterior and posterior sides, the PFB layer, when present, is also very thin (Fig. 2P).
The shape of the cross-sections is uneven, with the small radial tuberosity protruding on the anterior side (Fig. 3A), and a concave periosteal edge on the anterolateral side, where the cortex is thinner than in the rest of the section (Fig. 3A). The radial tuberosity protrudes more toward the distal end of the midshaft, otherwise, the general shape and histology of the sections do not show any conspicuous variation on the proximodistal axis of the bone. The medullary cavity is relatively small, of elliptic shape, and exhibits a loose trabecular network in the mid-diaphysis, mostly concentrated on the posterior side of the section (Fig. 3B). Endosteal lamellae are present in the innermost part of the cortex, and are thicker on the anterolateral and anteromedial sides (Fig. 3C). Most of the cortex is comprised of CCCB (Fig. 3D) vascular canals are mostly circular in the inner cortex (Fig. 3E), and from the mid-cortex form a much more reticular pattern, with many oblique anastomoses, as the compacted trabeculae become more densely packed (Figs. 3E and 3F). The outer edge of the CCCB layer is highly remodeled (Fig. 3G). The outer cortex presents a layer of poorly vascularized PFB, of uneven thickness through the section: absent on the anterior side, where the periosteal edge of the anterior tuberosity is highly scalloped (Fig. 3H), it thickens gradually toward the medial and lateral edges (Fig. 3I), to reach its maximum thickness on the posterior side (Fig. 3J). On the posteromedial side, the PFB layer presents Sharpey’s fibers (indicating areas of muscle insertion) in almost all sections, ranging from just a few in NMBF 12311 (Fig. 3K) to large bundles of them in MVD-M 1 (Fig. 3L), covering most of the outer cortex on this side. On the posterolateral side, the PFB layer is very thick, and there is a small patch of fibrolamellar bone (FLB) in-between the CCCB and PFB, containing many small longitudinal primary osteons (Fig. 3M). Three to four conspicuous LAGs can be observed in the PFB outer layer (Fig. 3N), on at least one side. In MVD-M 1, a small bulge of PFB lies on top of this PFB outer layer on the posterolateral side, separated from it by an additional resorption line (Fig. 3O), indicating strong periosteal resorption.
The sections are elongated on their anteroposterior axis, with a flattening of the medial and lateral sides of the cortex. The general shape is highly asymetrical, with a strong outward curvature on the lateral side, where the cortex is thinner than in the rest of the section (Fig. 4A). This curvature becomes more pronounced from the proximal to the distal part of the midshaft, along with a thickening of the cortex, especially in its anterior part (Fig. 4B). The spongiosa is well developed, especially in the proximalmost part of the midshaft (Figs. 4A and 4B). The medullary cavity is small, with its endosteal edges showing signs of previous resorption (Fig. 4C). The cortex consists almost entirely of CCCB with small longitudinal canals (Fig. 4D). Much of it is highly remodeled with longitudinal secondary osteons, the distribution of which becomes denser when closer to the periosteal surface (Fig. 4E) this pattern is especially present on the posterior and anterior sides, where the periosteal edges of CCCB present patches of dense HB (Fig. 4F). A thin layer of PFB tops the outermost cortex in the proximal midshaft, it is only present in the posterolateral and anteromedial sides (Fig. 4G), whereas the rest of the outer cortex presents a scalloped outer edge, indicating periosteal resorption (Fig. 4H). Conversely, in the distal midshaft, the PFB layer becomes thicker on the anterior side, and is slightly more vascularized with small longitudinal canals (Fig. 4I). A small patch of FLB can be seen between the CCCB and PFB on the anterior side (Fig. 4I), whereas the PFB on the posterior side has been completely eroded through periosteal resorption (Fig. 4H). The anteromedial part of the PFB layer presents many radially-oriented Sharpey’s fibers (Figs. 4I, 4J and 4K). Growth marks are absent.
The sections are irregular in shape, with a predominant circular outline and a flattened ridge on the posterior side (Fig. 5A). In the proximal part of the midshaft, this ridge forms two acute angles with the adjacent medial and lateral sides (Fig. 5A). The medial angle corresponds to the pectineal tuberosity, that is, the attachment site for the pectineus muscle (Sonntag, 1925) toward the mid-diaphysis, this tuberosity progressively disappears, whereas the lateral angle protrudes even more as the third trochanter becomes more prominent (Fig. 5B Le Gros Clark & Sonntag, 1926). The medullary cavity is large, and cortical thickness remains relatively constant through the whole section (Figs. 5A and 5B). A dense network of trabeculae can be observed under each angle formed by the posterior ridge (Fig. 5A), as well as consolidating the extension of the third trochanter in more distal sections (Figs. 5B, 5C and 5M). The inner and mid-cortex consists of CCCB, with circumferentially-oriented lamellae and vascular canals in the inner cortex (Fig. 5D), and a denser, more irregular vascularization in the mid-cortex, where secondary remodeling becomes more prominent (Fig. 5E). Locally, this vascularization may form a reticular pattern (Fig. 5F).
In the outer cortex, the CCCB becomes almost completely remodeled, and its periosteal edge shows resorption on the lateral and medial sides (Fig. 5G), particularly on the third trochanter where it is highly scalloped (Fig. 5H), whereas it is topped by a layer of PFB in the posterior and anterior sides. In all sections, there is a thin, highly remodeled FLB layer sandwiched inside the PFB one, on the posterior side (Fig. 5I). The very thin PFB layer between the FLB and CCCB presents three resorption lines (Fig. 5I), suggesting that the periosteal growth of the FLB layer took place after the endosteal formation of CCCB (Enlow, 1963 de Ricqlès et al., 1991). A fourth resorption line separates the FLB from the outer PFB, which also presents signs of periosteal resorption, suggesting a succession of multiple resorption events, which took place after the deposit of the outer PFB on top of the FLB (Fig. 5I). This resorption is likely to have entirely eroded the FLB layer in the rest of the section. On the anterior side, the PFB layer is thick, but scalloped it contains a few small longitudinal canals, and at least one LAG is visible (Fig. 5J). The angles formed by the posterior ridge in the proximal midshaft present CCCB with a very dense and irregular vascular pattern (Figs. 5H and 5K) the outer PFB layer is almost completely resorbed (Fig. 5H), and many longitudinal secondary osteons can be observed (Fig. 5K). The PFB in general only has a few longitudinal canals and radial anastomoses (Fig. 5L). In the lateral extension of the third trochanter, the CCCB presents a pattern of very elongated, circumferentially-oriented osteons (Fig. 5M), matching the side of the trabecular network in the underlying spongiosa (Figs. 5M and 5C).
The proximalmost sections have an ovoid shape, with a prominent anterior extension corresponding to the anterior crest, running from the proximal epiphysis to the mid-diaphysis (Le Gros Clark & Sonntag, 1926 Fig. 6A). The more distal sections present a more rounded shape, although the cortex is still slightly thicker on the anterior side than in the rest of the section (Fig. 6B). The medullary cavity is reduced and flattened on its anteroposterior axis, with only a few disorganized trabeculae (Figs. 6A and 6B). The inner cortex consists of CCCB with a mostly circular vascular pattern and a few secondary osteons (Fig. 6C). In the mid-cortex, vascularization becomes more reticular, with many radial and oblique anastomoses (Fig. 6D), along with more prominent Haversian remodeling (Fig. 6E). Many secondary osteons are aligned along the limit between the CCCB and the outer cortex (Fig. 6F), marked by a thin layer of PFB with at least one resorption line (Fig. 6F).
The outer cortex, as in the femur, consists of two distinct layers. On the lateral and medial sides, only a relatively thin layer of PFB, with at least two LAGs, is present—likely because of periosteal resorption, since the CCCB layer ends externally with a resorption line (Fig. 6G). On the anterior and posterior sides, however, the two layers are preserved. The inner one consists of mostly remodeled FLB with mainly small longitudinal osteons (Fig. 6H) on the posterior side, where this layer reaches its maximum thickness, many radial anastomoses can be observed in some sections (Fig. 6I). The outermost layer is almost avascular PFB (Fig. 6J), with periosteal edges showing signs of erosion on the whole section (Fig. 6J). Dense patches of secondary osteons can locally pervade the outer cortex up to its periosteal edges (Fig. 6K), particularly in the anterior crest, which is almost entirely remodeled in the proximalmost sections where it is most prominent (Fig. 6L). In NMBF 12311, an outer bulge of bone is present on top of the outer layer of PFB on the posterolateral side, separated from it by another resorption line (Fig. 6M), and consists almost entirely of dense HB (Fig. 6N). The few visible patches of primary bone consist of PFB (Fig. 6N), suggesting that the original PFB layer was much thicker than in the rest of the section, and has been almost entirely resorbed except for that bulge.
Sections present a roughly circular outline, with a flattened edge on the posterior side, and do not vary much in shape through the midshaft compared to other bones in the sample (Figs. 7A and 7B). The medullary cavity is very small, with a diameter inferior to the cortical thickness in all sections (Figs. 7A and 7B). Cortical thickness is relatively constant, except for the posterior side of the cortex, where periosteal resorption has occurred, and become more pronounced in the distal midshaft (Figs. 7A and 7B). A few isolated trabeculae protrude in the medulla, without forming any conspicuous pattern (Fig. 7C). CCCB is the main type of bone in the inner and mid-cortex (Fig. 7D) it ranges from a loose circular pattern in the innermost cortex (Fig. 7D) to a more irregular vascular pattern, with oblique anastomoses and radial primary osteons (Figs. 7D and 7E). Numerous small secondary osteons are present (Figs. 7E and 7F). In the outer cortex, a layer of woven bone (WB) with small longitudinal and radial canals tops the CCCB on the medial and anterior sides, with a resorption line separating the two (Fig. 7G) in some sections, these small canals are osteons with concentric lamellae, and the WB thus becomes FLB (Fig. 7E). Many large secondary osteons are present locally in the outermost cortex, mostly at the limits between the four main sides (i.e., anterior, posterior, medial, and lateral), but the underlying primary bone matrix is still visible (Figs. 7H and 7I). The outermost cortex consists of a layer of almost avascular PFB, with a highly resorbed periosteal outline, separated from the CCCB (or FLB, depending on the side) layer by a resorption line (Fig. 7J). Extensive periosteal resorption is observed in all sections: the outer PFB layer is at its thickest on the anterolateral side (Fig. 7K), whereas the posteromedial side has a scalloped outline and the PFB layer is almost completely resorbed (Fig. 7L). Growth marks are absent.
Whether you’re a safari novice or a complete wildlife addict, you simply cannot beat seeing the great wildebeest migration in Kenya or Tanzania. The sights, sounds and smells of such a vast number of wildebeest and zebra on the move, followed every step by hungry predators, is quite remarkable. The famous river crossings offer the extraordinary spectacle of thousands of anxious animals channelled into tight crossing points, dodging the huge Mara crocodiles waiting in their path. It’s an ancient battle of life and death, and one of the best wildlife shows on earth.
Although it has the appearance of an evolutionary calamity, the wildebeest is one of Africa’s most successful inhabitants, especially in the Serengeti ecosystem that straddles the Kenya / Tanzania border. This is where the famous migration occurs, and where herds approaching two million head wander between the southernmost parts of the Serengeti National Park, the Ngorongoro Conservation Area, and the Masai Mara away to the north.
The migration follows a vague pattern year by year which tends to be influenced by rainfall and food availability. The herds, accompanied by several hundred thousand zebra, congregate on the short grass plains in the south just after Christmas to give birth and then make their way north via the west of the ecosystem towards Lake Victoria. During the journey they may travel 40 miles in one day in a particular direction and then do the same in the opposite direction the following day. This is often the case during the rainy season when thunderstorms may be very localised but also quite heavy.
This footage captures the dramatic scene as thousands of wildebeest gather before plunging into the Mara River, crossing from the Serengeti in Tanzania to the Masai Mara in Kenya. Video courtesy of Nomad Tanzania.