In the evolutionary arms race between hosts and parasites, hosts benefit from the ability to detect and avoid parasites, while parasites benefit by evading host detection. Given the broad diversity of parasites transmitted via the fecal-oral route, feces may be a sufficiently reliable indicator of parasite exposure risk for hosts to detect and avoid, rather than cues of the parasites themselves. In this talk, I will discuss my recent efforts to address two important unknowns in this context. First, research on humans finds that disgust motivates avoidance of parasite-associated cues, e.g. fecal material, but the literature disagrees about whether other animals have disgust. Second, avoidance of feces-contaminated resources has associated costs, such as lost opportunities for nutrients or water, yet how these costs and benefits are balanced remains unclear. In paired studies of captive lemurs at the Duke Lemur Center and water resource-limited wild lemurs in Kirindy Forest, Madagascar, I conducted choice experiments between clean and feces-contaminated (but disinfected) water sources to determine whether lemurs avoid ingesting feces, and whether this response is influenced by water availability. In a second experimental study in captive lemurs, I investigated whether feces avoidance in lemurs is consistent with definitions of disgust in humans.
Results/Conclusions
In the study of preferences for clean versus feces-contaminated water, the captive lemurs preferred clean (40 drinking events) to contaminated water sources (3 drinking events) (p<0.0001, binomial test). In wild lemurs, experiments that introduced water sources into the habitat had marked impacts on ranging patterns, highlighting the importance of water in this arid environment. Despite these severe water limitations, however, experiments with wild lemurs showed a similar level of preference for clean water (35 drinking events) rather than feces contaminated water (5 drinking events; p<0.0001, binomial test). At naturally occurring water sources, fecal contamination did not deter lemur water consumption, suggesting that lemurs’ behavioral immune systems are plastic to environmental conditions. In the second study, lemurs perform behaviors when rejecting feces that are consistent with distaste or toxin rejection behaviors. That behaviors are conserved across these two contexts is consistent with conceptualizations of the human disgust response, suggesting that non-human primates may have a disgust response that is evolutionarily continuous with that of humans, contrary to current dogma in psychology that disgust is uniquely human. Collectively, these findings raise the possibility that parasites may induce a “landscape of disgust” on their hosts.