As hinted at in my last post, I’ve just returned from a weekend trip to Guassa, a cold but lush region of the Ethiopian highlands. While most of IHO’s research focuses on a single, highly specialized primate species (if you are reading this, you belong to that very species), I’ll use this blog post to provide a little background on one of our most interesting relatives. I spent the weekend in Guassa observing gelada baboons, and thanks to their passive nature, I was able to get up close and personal with one of Africa’s most amazing primates.
The gelada is among the rarest living primate species. The only place they are observable in the wild today is in a few of the cool and high altitude afro-alpine mountains of Ethiopia (although their close relatives were once more widely distributed across Africa, as will be discussed later). Their hair is generally a mix of brown, black, and gold, and their face is black with the exception of light pink eyelids. There is a reddish patch of skin on their chest and the tone varies depending on reproductive stage, dominance, and age (they are sometimes called the “bleeding heart monkey”). They are almost completely terrestrial and spend most of their time (up to 70 percent a day) feeding on the ground. Geladas are the only graminivorous, or grass-eating, primate, and 90 percent of their diet consists of high-protein grasses found at altitudes greater than 1700 meters. Unlike some hoofed mammals (like giraffids and bovids), geladas lack a specialized multi-chambered ruminant stomach and instead rely on bulk-feeding and extensive chewing with specialized cheek teeth to digest a diet so high in cellulose.
The living gelada, Theropithecus gelada, is the single relictual (i.e., a remnant) member of a previously widespread group. The closest living relatives of Theropithecus are Papio (baboons) and Lophocebus (baboon-mangabeys), and Perelman and colleagues found that the last common ancestor of Theropithecus and Papio lived in Africa about four million years ago (mya) based on a well-resolved molecular phylogeny of living primates (a phylogeny is the depiction of evolutionary relationships among a group of organisms; “molecular” indicates the phylogeny was constructed using genetic data vs. anatomical or fossil data). The date of origination suggested by the genetic evidence compares favorably with the fossil evidence, as the earliest species of Theropithecus are found in Koobi Fora, Kenya at 3.94 mya and in the Middle Awash of Ethiopia at 3.9 mya.
There are many fossil species of Theropithecus, most of which were much larger than the living gelada. For example, T. oswaldi (2.5-0.25 mya) was a particularly large species, with males weighing over 85kg or 187 lbs in the more specialized Pleistocene representatives (males of the living gelada weigh 20kg or 44 lbs at most). The Hadar species of Theropithecus, T. darti, is almost certainly an early representative of what would eventually become the T. oswaldi lineage. Thus two small ancestors, T. darti and Australopithecus afarensis (Lucy’s species), living side by side at Hadar three million years ago would probably go on to give rise to two of the largest primate species, T. oswaldi and Homo sapiens (humans), respectively.
Interestingly, in another parallel with the human lineage, while Theropithecus initially evolved in Africa it is clear that the group subsequently dispersed into Eurasia during the Pleistocene, as fossils of Theropithecus have been found in Israel and India. Although, these two primates were not alone in their journey out of Africa, as their fellow travelers included species of Oryx (a large antelope), Hippopotamus (hippos), Kolpochoerus (a suid closely related to living African pigs), and Crocuta (spotted hyenas), all of which are of African origin like Theropithecus and Homo.
Okay—that’s enough digressing for now. There’s still more to do in my remaining two weeks in Addis.
Until next time,