Hyena


Hyenas or hyaenas are feliform carnivoran mammals belonging to the family Hyaenidae. With just four extant species, it is the fifth-smallest family in the order Carnivora and one of the smallest in the class Mammalia. Despite their low diversity, hyenas are unique and vital components of most African ecosystems.
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.
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. Other cultures associate them with witchcraft, using their body parts in traditional medicine.

Evolution

Origins

Hyenas originated in the jungles of Miocene Eurasia 22 million years ago, when most early feliform species were still largely arboreal. The first ancestral hyenas were likely similar to the modern African civet; one of the earliest hyena species described, Plioviverrops, was a lithe, civet-like animal that inhabited Eurasia 20–22 million years ago, and is identifiable as a hyaenid by the structure of the middle ear and dentition. The lineage of Plioviverrops prospered and gave rise to descendants with longer legs and more pointed jaws, a direction similar to that taken by canids in North America.
Hyenas then diversified into two distinct types: lightly built dog-like hyenas and robust bone-crushing hyenas. Although the dog-like hyenas thrived 15 million years ago, most became extinct after a change in climate, along with the arrival of canids into Eurasia. Of the dog-like hyena lineage, only the insectivorous aardwolf survived, while the bone-crushing hyenas became the undisputed top scavengers of Eurasia and Africa.

Rise and fall of the dog-like hyenas

The descendants of Plioviverrops reached their peak 15 million years ago, with more than 30 species having been identified. Unlike most modern hyena species, which are specialised bone-crushers, these dog-like hyenas were nimble-bodied, wolfish animals; one species among them was Ictitherium viverrinum, which was similar to a jackal. The dog-like hyenas were numerous; in some Miocene fossil sites, the remains of Ictitherium and other dog-like hyenas outnumber those of all other carnivores combined. The decline of the dog-like hyenas began 5–7 million years ago during a period of climate change, exacerbated by canids crossing the Bering land bridge to Eurasia. One species, Chasmaporthetes ossifragus, managed to cross the land bridge into North America, being the only hyena to do so. Chasmaporthetes managed to survive for some time in North America by deviating from the endurance-running and bone-crushing niches monopolized by canids and developing into a cheetah-like sprinter. Most of the dog-like hyenas had died off by 1.5 million years ago.

Bone-crushing hyenas

By 10–14 million years ago, the hyena family had split into two distinct groups: dog-like hyenas and bone-crushing hyenas. The arrival of the ancestral bone-crushing hyenas coincided with the decline of the similarly built family Percrocutidae. The bone-crushing hyenas survived the changes in climate and the arrival of canids, which wiped out the dog-like hyenas, though they never crossed into North America, as their niche there had already been taken by the dog subfamily Borophaginae. By 5 million years ago, the bone-crushing hyenas had become the dominant scavengers of Eurasia, primarily feeding on large herbivore carcasses felled by sabre-toothed cats. One genus, Pachycrocuta, was a mega-scavenger that could splinter the bones of elephants. Starting in the early Middle Pleistocene, Pachycrocuta was replaced by the smaller Crocuta and Hyena, which corresponds to a general faunal change, perhaps in connection to the Mid-Pleistocene transition.

Rise of modern hyenas

The four extant species are the striped hyena, the brown hyena, the spotted hyena, and the aardwolf.
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.
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.
The spotted hyena diverged from the striped and brown hyena 10 million years ago. Its direct ancestor was the Indian Crocuta sivalensis, which lived during the Villafranchian. Ancestral spotted hyenas probably developed social behaviours in response to increased pressure from rivals on carcasses, thus forcing them to operate in teams. Spotted hyenas evolved sharp carnassials behind their crushing premolars, therefore they did not need to wait for their prey to die, and thus became pack hunters as well as scavengers. They began forming increasingly larger territories, necessitated by the fact that their prey was often migratory, and long chases in a small territory would have caused them to encroach into another clan's turf. Spotted hyenas spread from their original homeland during the Middle Pleistocene, and quickly colonised a very wide area from Europe, to southern Africa and China.
The eventual disappearance of the spotted hyena from Europe has traditionally been attributed to the end of the last glacial period and a subsequent displacement of open grassland by closed forests, which favoured wolves and humans instead. However, analyses have shown that climate change alone is insufficient to explain the spotted hyena's disappearance from Europe, suggesting that other factors – such as human pressure – must have played a role. This suggests that the events must be seen within the broader context of late-Quaternary extinctions, as the late Pleistocene and early Holocene saw the disappearance of many primarily large mammals from Europe and the world.
Expansion or duplication of the olfatory receptor gene family has been found in all 4 extant species, which would have led to the evolution of the more specialised feeding habits of hyenas.
Expansion in immune-related gene families was also found in the spotted hyena, striped hyena and brown hyena, which would have led to the evolution of the scavenging in these species. Mutations and variants were also found in digestion-related genes. One of these digestion-related genes has variants also related to enhanced bone mineralisation, while other have also a role in inflammatory skin responses.
In aardwolves, expansion of genes related to toxin response were found, which would have led to the evolution of the feeding of termites Trinervitermes in this species. Mutations and variants in genes related to craniofacial shape were also found. Another gene is related to protective epidermis function.

Genera of the Hyaenidae (extinct and recent)

The list follows McKenna and Bell's Classification of Mammals for prehistoric genera and Wozencraft in Wilson and Reeders Mammal Species of the World for extant genera. 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, although they are generally grouped as sister-taxa to hyenas. However, as of 2024 many experts consider them members of Hyaenidae. 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.
  • Family Hyaenidae
  • * Subfamily Incertae sedis
  • ** †Tongxinictis
  • * †Subfamily Ictitheriinae
  • ** †Herpestides
  • ** †Plioviverrops
  • ** †Ictitherium
  • ** †Thalassictis
  • ** †Hyaenotherium
  • ** †Miohyaenotherium
  • ** †Lycyaena
  • ** †Tungurictis
  • ** †Protictitherium
  • * Subfamily Hyaeninae
  • ** †Palinhyaena
  • ** †Ikelohyaena
  • ** Hyaena Early Pliocene to Recent of Africa, Late Pliocene to Late Pleistocene of Europe, Late Pliocene to recent in Asia
  • ** Parahyaena
  • ** †Hyaenictis
  • ** †Leecyaena
  • ** †Chasmaporthetes
  • ** †Pachycrocuta
  • ** †Adcrocuta
  • ** Crocuta
  • * Subfamily Protelinae
  • ** †Gansuyaena
  • ** ''Proteles''