Vegetarian orang-utans eat world's cutest animal

When fruit is scarce, try chomping on a slow loris. That seems to be the strategy adopted by the normally vegetarian orang-utans, which have been spotted knocking the small primates out of trees and killing them with a bite to the head.

Sumatran orang-utans (Pongo abelii) get almost all their nutrients from fruit and other plant products, but there are a few isolated reports of them eating meat (American Journal of Primatology, vol 43, p 159). Madeleine Hardus of the University of Amsterdam in the Netherlands and colleagues have now observed three more cases, bringing the total to nine.

In 2007 Hardus was tracking two orangs in the canopy above her ? a female called Yet and her infant Yeni ? when Yet abruptly changed direction and approached a slow loris (Nycticebus coucang). She knocked it out of the tree, crashed down to the ground, bit the stunned loris's head, then carried the body back into the tree to eat it. When Yeni begged, she was allowed to share the meat. The great apes each chomped on opposite ends of the dead primate, sharing it between them like lovers might a strand of spaghetti.

Searching through the scientific literature, Hardus found detailed studies of six orang-utan hunts. All stunned their prey before eating it, which Hardus thinks may be to avoid being bitten. Slow lorises are unique among primates in that their saliva is toxic.

All the documented hunts took place when there was little fruit available, which may push the apes to meat-eating, says Hardus.

By contrast, chimpanzees hunt more when fruit is abundant, perhaps because it doesn't matter if they waste energy on a failed hunt.

The sample is unavoidably small, but the data have been thoughtfully analysed, says Richard Wrangham of Harvard University.

Only five individual orang-utans have been observed hunting. Yet has so far been caught in the act four times ? three times by Hardus, and once by another researcher ? making her the best documented hunter.

In other accounts, the apes stumbled upon their prey, but Yet systematically changed direction and headed straight for the loris, which Hardus says may be because she has learned to smell them. Because a few cases have been documented within a 40-kilometre range, all using the same killing method, she thinks it may be a cultural behaviour, passed from orang-utan to orang-utan.

Journal reference: International Journal of Primatology, DOI: 10.1007/s10764-011-9574-z

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