Differences in gorilla nettle-feeding between captivity and the wild: local traditions, species typical behaviors or merely the result of nutritional deficiencies?

Behavioral and cognitive studies on captive apes often pay little attention to the specific environmental conditions of their study subjects. A recent report form Byrne et al. (Anim Cogn doi:10.1007/s10071-011-0403-8, 2011), comparing nettle-feeding techniques between captive and wild gorillas, claimed to document the strongest evidence yet to come from any great ape that observational learning of a skilled conspecific can allow social learning and culture in gorillas. An earlier study with similar findings placed emphasis instead on the many similarities and claims for species typical behavior, thus a genetic hypothesis instead of a cultural hypothesis. This commentary aims at formulating a third environmental hypothesis based on path-dependent behavioral differences owing to different diet and availability of nutritional resources of wild and captive gorillas. Captive diet provides gorillas with a much lower concentration of fibers. Gorillas are hindgut fermenters, and this deficit of natural fermentation of fibers may impact their health and their behavior in zoos. Results of Byrne et al.'s study will be discussed comparing feeding choice and availability of nutritional resources of wild and captive gorillas, showing that in captivity gorilla, motivation to consume certain food or certain plant parts may differ drastically from that of wild gorillas. This view does not intend to deny that social learning and culture may exist in gorillas, but to guide and encourage future works investigating social learning in great apes to take more accurately into account the living conditions and, when comparing populations, the possible environmental differences.