Meat-Eating in Sumatran Orangutans

Caught in the act. Mother and infant Sumatran Orangutan sharing their kill, a slow loris.
Photo from NewScientist.

Well, well, well ... it seems like we have another great ape species who can't stay on a vegetarian diet. I mean, who in their right mind would want be an obligate herbivore right? Right? Actually ... contrary to popular belief, orangutans are not strictly frugivores (fruit eater). They also subsist on plant parts (leaves, buds, flowers, bark, sap, etc) and the occasional bird eggs and insects. Thus it came as to no surprise for me that some orangutans have acquired taste for meat. A varied diet is a good diet. A flexible diet in response to scarce food resources is an even better diet! Yes, I'm looking at you Giant Panda. Eat something else will ya? Edit: A rep from Giant Pandas did confirm that they do eat something else.

Sumatran Orangutans (Pongo abelii) are not the first great ape to have a taste for meat. Chimpanzees (Pan troglodytes schweinfurthii) from Gombe Stream National Park were observed hunting and eating red colobus monkeys (Colobus badius tephrosceles) by Dr. Jane Goodall while savannah chimpanzees (Pan troglodytes verus) in Fongoli have been observed hunting and eating lesser bushbabies (Galago senegalensis) by Dr. Jill Pruetz.



A new paper published in International Journal of Primatology, Behavioral, Ecological, and Evolutionary Aspects of Meat-Eating by Sumatran Orangutans (Pongo abelii), by Hardus et al. (2012) describes 3 rare cases of meat-eating by a mother and infant dyad Sumatran Orangutans. This pair was observed eating slow lorises (Nycticebus coucang), which the authors posit that meat-eating is a fallback strategy when food source (ripe fruits) are scarce. 
The authors also touched on very fundamental yet intriguing questions about meat-eating and hominid evolution in this paper. What are the minimum time on chewing is necessary for an adult female Australopithecus africanus to reach its daily energy requirement when subsisting partially on raw meat?  How did meat became a substantial component of hominid diet?

A. africanus needs 1202–1507 calories per day for their daily energy requirement. By using the chewing rate of P. abelii as a model, the authors estimate that A. africanus have to chew about 2 hours on raw meat to achieve 25% of their caloric needs while the remaining 75% were subsisted from easier to chew food sources such as leaves and insects.  Raw meat does not seem to be an efficient way to satisfy caloric needs but seemed to be a "fallback" resource when other food source are scarce. As to when meat became a substantial component of hominid diet, it is probably after the "discovery" of fire by Homo erectus. Cooked meat are easier to chew and provides more nutrient than from raw meat. The rate of mastication also decreases because most fibers within raw meat are broken down after it's cooked.