Alcohol Discrimination and Preferences in Two Species of Nectar-Feeding Primate
I am interested in primate behavioral ecology and, more specifically, the ecological and evolutionary importance of alcohol. Ethanol holds a significant amount of calories and is often associated with sugars in fermented fruit or nectar. This underutilized food source could provide a selective advantage to those species that can utilize it.
Within the alcohol dehydrogenase family (enzymes that metabolize ethanol) class IV (ADH4) is the first enzyme to encounter and metabolize ethanol in the body. ADH4 is inactive in most primate species, yet it is both active and 40 times as efficient (due to the A294V mutation) in two distantly-related primate taxa: the aye-aye (Daubentonia madagascariensis) and the African apes (humans, chimpanzees, bonobos, and gorillas).
The aye-aye is a nocturnal lemur endemic to Madagascar that has a unique anatomy (e.g., skeletal middle finger, elongated incisors, large ears) adapted for percussive and probative foraging for beetle larvae in tree bark. However, larvae do not naturally ferment and are an improbable source of alcohol, so it would seem that the mutation to the ADH4 is most likely spurious and not functionally adaptive. However,aye-ayes also have a proposed mutualism with the traveler's tree (Ravenala madagascariensis), spreading its pollen in exchange for its high-sucrose nectar, which most likely ferments into an alcoholic solution. The hypothesized consumption of alcoholic nectar in aye-ayes is compatible with the finding of a mutated ADH4 gene and may indicate that it has an ecologically adaptive function. Natural selection may have favored a behavioral preference for alcohol in aye-ayes and the ancestor of African apes and humans.
The slow loris (Nycticebus coucang), a nocturnal primate endemic to Southeast Asia, is known to consume fermented nectar from the bertam palm (Eugeissona tristis). Although the protein sequence of its ADH4 is currently unknown, its interaction with fermented nectar highlights the same selective pressure on ADH4 and its potential effect on dietary preference.
Accordingly, we conducted multiple-choice feeding experiments with two independent aye-ayes and one slow loris in which we provided each animal with an array of nectar-like solutions containing sucrose-water and varying volumes of ethanol. We observed frequency of probing (insertion of finger into solution and subsequent licking) and mass consumed with aye-ayes and only mass consumed with the loris as a measure of preference. Both probing and mass measurements indicate that the aye-ayes were able to significantly discriminate between solutions and that they had a significant preference for higher alcohol concentrations over lower ones. These results may suggest that the ADH4 mutation is a functional adaptation and serves a role in aye-aye behavior. We found a similar preference for high alcohol concentrations in the slow loris.
The convergence of the ADH mutation in two separate primate lineages, the aye-aye and the African apes, may be due to a convergence of selective pressures in each of their environments. This may suggest that dietary alcohol was present in the diet of African apes and hominin species sometime in their evolutionary histories, potentially indicating feeding behaviors and food sources. Future studies may include detection of ethanol in fermented nectar in the traveler's tree and sequencing of the ADH4 protein in the slow loris.
The research process has been important in terms of pursuing my curiosities. I have gained valuable experience in scientific inquiry, experimental design and construction of procedures, grant acquisition, execution of procedures, data analysis, and manuscript writing. This research has brought me to the Duke Lemur Center, where I carried out the procedures for the research, made meaningful relationships with staff and other researchers, and had the privilege to work hands-on with amazing animals in a stimulating environment.
In 2016, we published our research in Royal Society Open Science and presented at the joint meeting of the International Primatological Society and the American Society of Primatologists in the summer of 2016 and at the Duke Lemur Center's 50th Anniversary Celebration in the fall of 2016. Media coverage includes National Geographic, BBC, New Scientist, Popular Science, Seeker, and Smithsonian Magazine.