Hyenas and Prehistory

Most of our blog readers know that hyenas are strange animals, but have you ever wondered how hyenas became hyenas?  How they evolved throughout time to be so unique today?  Which environmental pressures forged their variety of adaptations? Well today is your lucky day because you are about to find out. 

Today, there are only 4 extant species of hyenas in the family Hyaenidae: Spotted Hyena, Brown Hyena, Striped Hyena, and Aardwolf.  However, many more species have persisted throughout the ages.  The ancestors of modern hyenas were arboreal creatures similar to civets and mongoose, which lived in the jungles of western Eurasia during the Early Miocene, 22 million years ago.  One of the earliest described species of these arboreal hyenids is Protictitherium, grouped into this family by dentition and structure of the middle ear which it shares with modern hyenas. Protictitherium occupied omnivore and insectivore niches in these ecosystems rather than the megafauna predators and scavenging niches that hyenas occupy today.  Utilizing its retractile claws, Protictitherium prowled the branches of trees foraging on fruit and hunting small mammals, birds, and insects.

Protictitherium and other early arboreal hyenas likely resembled this Malay civet
(C: Nick Garbutt, naturepl.com and Arkive.org)

The descendents of Protictitherium developed longer legs and more pointed jaws, giving rise to a wide array of terrestrial, nimble, dog-like hyenas that radiated across greater Eurasia during the Middle Miocene.  One of the most common species during this time was Ictitherium viverrinum.  In fact, it was so prevalent that in some Miocene fossil sites, it and other dog-like hyenas outnumbered all other carnivores combined.  The early dog-like hyenas like filled the niches of extant foxes, jackals, and coyotes.   Between 11-7 MYA, in the Late Miocene, Hyaenidae achieved its peak diversity during a second radiation of dog-like hyenas, known commonly as “hunting hyenas” to paleontologists.  This group was generally larger than the previous radiation, their limbs had more cursorial ability, and extended dentition allowed for specialized cutting and slicing of meat.  This dentition in particular reduced any hypothetical bone-crushing capability in these hunting hyenas, thus these species were on a radically different evolutionary course than the species which exist today.  The Late Miocene was also the temporal period when hyenas colonized the African continent. 

Drawing of the coyote-sized I. viverrinum, by Mauricio Anton (Turner & Anton, 2004).

Between 7-5 MYA, Hyeanidae experienced a series of drastic extinctions that corresponded with strong climate change and Canidae species crossing the Bering land bridge from North America.  The hardest hit were the civet- and dog-like hyenas, in fact, the only species surviving from these groups today is the dog-like Aardwolf.  Scientists believe the Aardwolf escaped extinction by evolving to forage on termites – an underexploited niche in Carnivora.  Another hyena to escape extinction was Chasmaporthetes ossifragus, which was the only hyena to cross the Bering land bridge into North America.  C. ossifragus managed to persist for some time in North America by diverging from the cursorial and bone-crushing niches dominated by canids on this continent and evolving into a cheetah-like sprinter.  The evolution of bone-crushing hyena specialists directly preceded the second radiation of hyenas.  These bone-crushing genera survived the appearance of canids in Eurasia and flourished to become the dominant scavengers of Eurasia by around 5 MYA.  One behemoth that prospered during this time was Pachycrocuta brevirostris, a 220 kg, lion-sized hyena which scavenged on the large herbivore carcasses felled by sabre-toothed cats and was capable of cracking the bones of the largest pachyderms.  Pachycrocuta were robust hyenas with a stocky build and possessed none of the cursorial elements present in extant bone-cracking hyenas.  Interestingly, some paleontologists believe P. brevirostris prevented expansion of the early hominid species into Europe through significant niche overlap (Madurell-Malapeira et al., 2015).  Unfortunately, with the decline of large herbivores and sabre-tooth cats during the late Pleistocene, such a large body-size had a negative impact on these hyenas and they gave way to the smaller Hyena, Parahyena, and Crocuta genuses of modern day. 

Here's one of those typically unrealistic-yet-very-beautiful prehistoric animal panoramas (due to density of animals) displaying P. brevirostris competing with some species of large sabre-toothed cat for a Megaloceros.  This is a good illustration the shear size of Pachycrocuta 
(Credit: http://prehistoricanimal.blogspot.co.ke/2010/07/famous-is-force-that-have-jaws-of.html).

The four extant hyena genera originally evolved in Africa during the Pleistocene and took refuge here during glacial periods.  Impressively, spotted hyenas dispersed out of Africa around 3.7 MYA and colonized virtually all of Europe, Asia, and Africa – save the wettest jungles of sub-Saharan Africa. Astonishingly, spotted hyenas remained in Western Europe as recently as 12,000 years ago; only receding from the continent as forest habitats returned and canids out-competed them.  These ranges aren’t surprising, given a few key adaptations.  Ancestral spotted hyenas may have resorted to the sociality of clans due to increased pressure from competitors at carcasses, allowing them to overpower larger singular scavengers at kills.  Spotted hyenas also developed sharp carnassials (cutting and slicing meat) behind their bone-crushing premolars – an adaptation not possessed by their larger, bone-crushing ancestors.  Increased cursorial capabilities, along with bi-specialized dentition, allowed spotted hyenas to hunt living prey in packs as well as scavenge on dead animals when prey availability was limited.  

Well, there you have it, the story of the hyenas from beginning to present.

Sources:

Madurell-Malapiera J, Alba DM, Espicgares MP, Vinuesa V, Palmqvist P, Martinez-Navarro B, and Moya-Sola S.  Were large carnivorans and great climatic shifts limiting factors for hominin dispersal? Evidence of the activity of Pachycrocuta brevirostris during the Mid-Pleistocene Revolution in the Vallparadis Section (Valles-Penedes Basin, Iberian Penninsula). Quarternary International, August 8^th 2015.