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Hyenasor hyaenas (/haɪˈiːnəz/ hi-EE-nəz; from Ancient Greek ὕαινα, haina)[1] are feliform carnivoran mammals belonging to the family Hyaenidae (/haɪˈɛnɪdiː/). With just four extant species (each in its own genus), it is the fifth-smallest family in the order Carnivora and one of the smallest in the class Mammalia.[2] Despite their low diversity, hyenas are unique and vital components of most African ecosystems.[3]

Although phylogenetically closer to felines and viverrids, hyenas are behaviourally and morphologically similar to canids in several elements due to convergent evolution: both hyenas and canines are non-arboreal, cursorial hunters that catch prey with their teeth rather than claws. Both eat food quickly and may store it, and their calloused feet with large, blunt, nonretractable claws are adapted for running and making sharp turns. However, hyenas' grooming, scent marking, defecation habits, mating and parental behavior are consistent with the behavior of other feliforms.[4]

Hyenas feature prominently in the folklore and mythology of human cultures that live alongside them. Hyenas are commonly viewed as frightening and worthy of contempt. In some cultures, hyenas are thought to influence people's spirits, rob graves, and steal livestock and children.[5] Other cultures associate them with witchcraft, using their body parts in traditional medicine.[6]

The aardwolf can trace its lineage directly back to Plioviverrops 15 million years ago, and is the only survivor of the dog-like hyena lineage. Its success is partly attributed to its insectivorous diet, for which it faced no competition from canids crossing from North America. It is likely that its unrivaled ability to digest the terpene excretions from soldier termites is a modification of the strong digestive system its ancestors used to consume fetid carrion.[7]

The striped hyena may have evolved from Hyaenictitherium namaquensis of Pliocene Africa. Striped hyena fossils are common in Africa, with records going back as far as the Villafranchian. As fossil striped hyenas are absent from the Mediterranean region, it is likely that the species is a relatively late invader to Eurasia, having likely spread outside Africa only after the extinction of spotted hyenas in Asia at the end of the Ice Age. The striped hyena occurred for some time in Europe during the Pleistocene, having been particularly widespread in France and Germany. It also occurred in Montmaurin, Hollabrunn in Austria, the Furninha Cave in Portugal and the Genista Caves in Gibraltar. The European form was similar in appearance to modern populations, but was larger, being comparable in size to the brown hyena.[9]

The list follows McKenna and Bell's Classification of Mammals for prehistoric genera (1997)[14] and Wozencraft (2005) in Wilson and Reeders Mammal Species of the World for extant genera.[15] The percrocutids are, in contrast to McKenna and Bell's classification, not included as a subfamily into the Hyaenidae, but as the separate family Percrocutidae (though they are generally grouped as sister-taxa to hyenas[16]). Furthermore, the living brown hyena and its closest extinct relatives are not included in the genus Pachycrocuta, but in the genus Parahyaena. However, some research has suggested Parahyaena may be synonymous with Pachycrocuta, making the brown hyena the only extant member of this genus.[17]

The following cladogram illustrates the phylogenetic relationships between extant and extinct hyaenids based on the morphological analysis by Werdelin & Solounias (1991),[25] as updated by Turner et al. (2008).[26]

Hyenas have relatively short torsos and are fairly massive and wolf-like in build, but have lower hind quarters, high withers and their backs slope noticeably downward towards their rumps. The forelegs are high, while the hind legs are very short and their necks are thick and short. Their skulls superficially resemble those of large canids, but are much larger and heavier, with shorter facial portions. Hyenas are digitigrade, with the fore and hind paws having four digits each and sporting bulging pawpads.[28] Like canids, hyenas have short, blunt, non-retractable claws.[29] Their pelage is sparse and coarse with poorly developed or absent underfur. Most species have a rich mane of long hair running from the withers or from the head.[28] With the exception of the spotted hyena, hyaenids have striped coats, which they likely inherited from their viverrid ancestors.[7] Their ears are large and have simple basal ridges and no marginal bursa.[29] Their vertebral column, including the cervical region are of limited mobility. Hyenas have no baculum.[30] Hyenas have one more pair of ribs than canids do, and their tongues are rough like those of felids and viverrids.[31] Males in most hyena species are larger than females,[32] though the spotted hyena is an exception, as it is the female of the species that outweighs and dominates the male. Also, unlike other hyenas, the female spotted hyena's external genitalia closely resembles that of the male.[33]

Their dentition is similar to that of the canid, but is more specialised for consuming coarse food and crushing bones. The carnassials, especially the upper, are very powerful and are shifted far back to the point of exertion of peak pressure on the jaws. The other teeth, save for the underdeveloped upper molars, are powerful, with broad bases and cutting edges. The canines are short, but thick and robust.[30] Labiolingually, their mandibles are much stronger at the canine teeth than in canids, reflecting the fact that hyenas crack bones with both their anterior dentition and premolars, unlike canids, which do so with their post-carnassial molars.[34] The strength of their jaws is such that both striped and spotted hyenas have been recorded to kill dogs with a single bite to the neck without breaking the skin.[35][36] The spotted hyena is renowned for its strong bite proportional to its size, but a number of other animals (including the Tasmanian devil) are proportionately stronger.[37][38] The aardwolf has greatly reduced cheek teeth, sometimes absent in the adult, but otherwise has the same dental formula as the other three species.[39] The dental formula for all hyena species is: 3.1.4.13.1.3.1

Although hyenas lack perineal scent glands, they have a large pouch of naked skin located at the anal opening. Large anal glands above the anus open into this pouch. Several sebaceous glands are present between the openings of the anal glands and above them.[29] These glands produce a white, creamy secretion that the hyenas paste onto grass stalks. The odor of this secretion is very strong, smelling of boiling cheap soap or burning, and can be detected by humans several meters downwind.[40] The secretions are primarily used for territorial marking, though both the aardwolf[7] and the striped hyena[41] will spray them when attacked.

Hyenas groom themselves often like felids and viverrids, and their way of licking their genitals is very cat-like (sitting on the lower back, legs spread with one leg pointing vertically upward). They defecate in the same manner as other Carnivora, though they never raise their legs as canids do when urinating, as urination serves no territorial function for them. Instead, hyenas mark their territories using their anal glands, a trait found also in viverrids and mustelids, but not canids and felids.[42] When attacked by lions or dogs, striped[43] and brown hyenas[44] will feign death, though the spotted hyena will defend itself ferociously.[36] The spotted hyena is very vocal, producing a number of different sounds consisting of whoops, grunts, groans, lows, giggles, yells, growls, laughs and whines.[45] The striped hyena is comparatively silent, its vocalizations being limited to a chattering laugh and howling.[46]

Mating between hyenas involves a number of short copulations with brief intervals, unlike canids, who generally engage in a single, drawn out copulation.[42] Spotted hyena cubs are born almost fully developed, with their eyes open and erupting incisors and canines, though lacking adult markings.[47] In contrast, striped hyena cubs are born with adult markings, closed eyes and small ears.[48] Hyenas do not regurgitate food for their young and male spotted hyenas play no part in raising their cubs,[42] though male striped hyenas do so.[49]

The striped hyena is primarily a scavenger, though it will also attack and kill any animals it can overcome,[43] and will supplement its diet with fruit.[50] The spotted hyena, though it also scavenges occasionally, is an active pack hunter of medium to large sized ungulates, which it catches by wearing them down in long chases and dismembering them in a canid-like manner. Spotted hyenas may kill as many as 95% of the animals they eat.[51] The aardwolf is primarily an insectivore, specialised for feeding on termites of the genus Trinervitermes and Hodotermes, which it consumes by licking them up with its long, broad tongue. An aardwolf can eat 300,000 Trinervitermes on a single outing.[7] Except for the aardwolf, hyenas are known to drive off larger predators, like lions, from their kills, despite having a reputation in popular culture for being cowardly.[43] Hyenas are primarily nocturnal animals, but sometimes venture from their lairs in the early-morning hours. With the exception of the highly social spotted hyena, hyenas are generally not gregarious animals, though the striped and brown hyenas may live in family groups and congregate at kills.[52]Spotted hyenas are one of the few mammals other than bats known to survive infection with rabies virus[53] and have shown little or no disease-induced mortality during outbreaks in sympatric carnivores, in part due to the high concentration of antibodies present in their saliva.[54] Despite this perceived unique disease resistance, little is known about the immune system of spotted hyenas,[55][56][57] and even less is known about other Hyaenidae species.

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