Sorry I haven’t been being terribly faithful with these “Meet a …” things, but I’ve been otherwise engaged for the past week or so. Our very first Meet a Contemporary is at once easy and hard to compose a profile on: Easy because it’s an orangutan and you can find about anything you want on it, but hard because it’s an orangutan and anyone who’s ever watched a nature show knows all the basics already, so it might be a bit difficult to make this interesting.
Meet, the Sumatran orangutan (or orang-utan for our Commonwealth readers).
A P. abelii in the type of suspensory posture typical of the orangutans.
First, let’s start with a little taxonomy. All of the apes share the Superfamily Hominoidae, which is divided into the families Hylobatidae (gibbons) and Hominidae (great apes). Dispensing with the Linnean terminology, the Hominidae is basically divided into African and Asian branches with a common ancestor calculated at ~12 mya. Orangutans (notice the absence of a “g” at the end of the name; “orangatang” irritates me to no end) constitute the Asian branch. Thus, though they live alongside gibbons, they are more closely related to us, chimps, and gorillas, but both chimpanzees and gorillas are more closely related to us than either are to orangutans. Sumatran orangutans are those individuals isolated on the island of, surprise, Sumatra, and which many primatologists feel to constitute a seperate species which shared a common ancestor with the Bornean orangutans 1.5 million years ago (all of this classification is up in the air to some extent. See the wikipedia article for an overview of the changing attitudes toward these relationships).
A good cladogram of the Hominoidae
Orangutan movement has been called “quadramanous,” one of those queer, not quite accurate, but illuminating Victorian terms. They use all four limbs to spread their substantial body weight amongst several branches at once. This allows all but the heaviest males to spend much of their time in the trees, making orangutans the largest arboreal animals on earth. Orangutan diet is classically ape: fruit, fruit, more fruit, and what ever else they can get their hands on. No primate can eat only fruits; they need proteins from either animal matter or leaves. Orangutans grab leaves as they forage and will eat birds eggs or small animals as they encounter them. Sumatran orangutans are particularly fond of the pith and bark of trees as well.
What I’d really like to get into is orangutan social behavior. Orangutans, the Sumatran variety in particular, practice a form of social organization known as noyau. One big male patrols a large territory within which are the multiple smaller territories of as many females as the male can defend. Other, smaller males, who lack the fatty cheek patches of the territorial males, roam freely and mate with any females who will have them (and occasionally some who won’t). All forage solitarily through most of the year.
My rather politically incorrect diagram of a noyau arrangement
This social organization is accompanied by an extreme degree of sexual dimorphism, with females much smaller than males. More unusual still is the inter-male dimorphism, with two distinct male body types: territorial and free-ranging. This social organization is a primitive one not only for primates, but for mammals. As orangutans almost certainly evolved from ancestors with the more typical mutli-female social groups of the other higher primates, this primitive social organization offers something of a mystery.
A big male. The smaller males look much like the P. abelii at the top of this entry.
One hypothesis which is gaining ground of late holds that orangutans lived more like other primates in the recent past before humans entered their range and collapsed their numbers. The noyau arrangement might be an easy “default” for forest mammals whose more elaborate societies collapse. It serves the reproductive interests of both males and females admirably. Females are able to forage for themselves and their offspring relatively free from harassment by and competition from males other than their resident. Resident males have the possibility of a relative reproductive monopoly over as many females as they can guard, though roving males get a place at the table too as the big resident is not always able to watch his entire territory. It’s a very similar arrangement to that of the lowly Southeast Asian treeshews, among the most primitive of all mammals and once considered to be good analogues for the Cretaceous ancestors of the Primates. I find that parallel interesting.
There’s a myriad of other facets of orangutan research I could go into if my interests were of such a persuasion, but as I’m an anthropologically-minded sort of fellow I’ll drop my analysis at social structure and sexual politics. I hope the red ape of Sumatra has been an agent of wholesome enlightenment for you.
References
Emmons, Louise H. Tupai: A Field Study of Bornean Treeshrews. 2000. U of California P.
Fleagle, John G. Primate Adaptation and Evolution. 2nd ed. 1998. Academic Press.
“Orang-Utan (Pongo pygmaeus).” Primate Behavior. May 27, 2007. http://www.theprimata.com/pongo_pygmaeus.html
Rowe, Noel. The Pictorial Guide to the Living Primates. 1996. Pogonias Press.
