Chimpanzees detect ant-inhabited dead branches and stems: a study of the utilization of plant-ant relationships in the Mahale Mountains, Tanzania.

Chimpanzees in the Mahale Mountains of Tanzania consume several species of stem- and branch-inhabiting ants throughout the year, without tools. Those ants are cryptic species, and it was unknown how to find them constantly. There has been little research on how the chimpanzees locate these ants. In this study, I use behavioral observations of the chimpanzee predators and surveys of the ant fauna and plants across different habitats to test the hypothesis that chimpanzees use plant species as a cue to efficiently locate ant colonies in litter units (dead parts of the plant). Ants were found to be associated with live plants and with spaces within litter units which provide nesting places. Such ant-plant litter relationships were not necessarily as strong as the mutualism often observed between live plants and ants. The proportion of available litter units inhabited by ants was 20 %, and litter units of three plant species (Vernonia subligera, Dracaena usambarensis, and Senna spectabilis) were well occupied by ants in the home range of the chimpanzees. The ant-inhabited ratio in chimpanzee-foraged litter units was higher than that in the available units in the home range. Chimpanzees fed more often on Crematogaster spp. than on other resident ants and at a higher rate than expected from their occurrence in the litter units. Above three plant species were well occupied by Crematogaster sp. 3 or C. sp. 18. It is concluded that chimpanzees locate ants by selecting litter units of plant species inhabited by ants.