LEAF-SWALLOWING BY CHIMPANZEES - A BEHAVIORAL ADAPTATION FOR THE CONTROL OF STRONGYLE NEMATODE INFECTIONS

Swallowing whole leaves by chimpanzees and other African apes has been hypothesized to have an antiparasitic or medicinal function, but deta iled studies demonstrating this were lacking. We correlate for the fir st time quantifiable measures of the health of chimpanzees with observ ations of leaf-swallowing in Mahale Mountains National Park, Tanzania. We obtained a total of 27 cases involving the use of Aspilia mossambi censis (63%), Lippia plicata (7%), Hibiscus sp. (15%), Trema orientali s (4%), and Aneilema aequinoctiale (11%), 15 cases by direct observati on of 12 individuals of the Mahale M group. At the time of use, we not ed behavioral symptoms of illness in the 8 closely observed cases, and detected single or multiple parasitic infections (Strongyloides fulle borni, Trichuris trichiura, Oesophagostomum stephanostomum) in 10 of t he 12 individuals. There is a significant relationship between the pre sence of whole leaves (range, 1-51) and worms of adult O. stephanostom um (range, 2-21) in the dung. HPLC analysis of leaf samples collected after use showed that thiarubrine A, a compound proposed to act as a p otent nematocide in swallowing Aspilia spp., was not present in leaves of A. mossambicensis or the three other species analyzed. Alternative nematocidal or egg-laying inhibition activity was not evident Worms o f O. stephanostomum were recovered live and motile from chimpanzee dun g, trapped within the folded leaves and attached to leaf surfaces by t richomes, though some were moving freely within the fecal matter sugge sting that the physical properties of leaves may contribute to the exp ulsion of parasites. We review previous hypotheses concerning leaf-swa llowing and propose an alternative hypothesis based on physical action .