Boozing Treeshrews: Heavyweight drinkers in small packages
Although political and moral stances on alcohol use have gone back and forth over the decades, anthropological studies suggest that low-level alcohol ingestion has been an important factor in primate evolution. Alcohol is associated with ripe, nutrient-rich fruits, in addition to functioning as an appetite stimulant to quickly take advantage of ephemeral food resources (Dudley 2002). As highly mobile herbivores, primates could have benefited these alcohol-producing plants through pollination and wide seed dispersal capabilities, leading to important mutualisms and possible co-evolution between angiosperms and mammals.
This type of relationship is not limited to primates. Mammals such as bats (Sanchez et al. 2004), rodents (Milton 2004), and even elephants (Morris et al. 2006) have all been shown to indulge (or overindulge) in fermented fruit whenever it is available (classic video of drunken African wildlife here). Mutual trade-offs could have led to the reciprocal adaptations necessary for co-evolution, in which plants incorporated more and more alcohol into their nectar and/or fruit, while mammals became increasingly able to tolerate and metabolize the alcohol, an adaptation allowing them to exploit fruiting plants and function as important pollinators and dispersers.
Wiens et al. (2008) investigated the relationship between the Bertram palm (Eugeissona tristis) and its vertebrate pollinators, with a special focus on the pentailed treeshrew ( Ptilocercus lowii ) in Malaysia. The authors were interested in elucidating the role that alcohol exposure has had in mammalian evolution, in addition to determining whether fruits and/or flowers with high alcohol content benefit a plant by attracting mammalian pollinators.
First, the authors investigated the phenology of the Bertram palm. They counted flowers and recorded features of the inflorescences, determining that the plants typically produce about half male and half hermaphroditic flowers, which bloom in synchrony and follow a unique succession cycle, with a very long nectar production phase followed by one or two days of pollen exposure, then a long period without nectar, with the cycle repeating all year round.
To assess the effects of the mammals on the plant, the authors conducted a bagged inflorescence experiment which excluded vertebrates but allowed insects to access the palm flowers. The plants that were isolated from mammals had a 50% lower fruit set than controls, suggesting that these animals are significant pollinators of the Bertram palm. In addition, inflorescences were monitored during the night with cameras, and at least 80% of inflorescences were visited by at least one mammal each night. Analysis of feeding patterns showed that pentailed treeshrews and slow lorises spent more time feeding from the Bertram palms than from any other food source.
Having gathered data on the basic ecology of the palm and the mammals of interest, Wiens et al. were primarily interested in the role of alcohol in this mutualism. They found that the flower buds contained complex yeast communities, with many species previously unknown to science. The authors tested alcohol concentrations in the nectar, and found that nectar inside the bud contained up to 3.8% alcohol (roughly equivalent to beer), while levels of alcohol freshly exuded nectar were lower, and “older” exuded nectar even lower. In order to determine how much alcohol was being consumed by the palm’s mammalian visitors, they used ethyl glucuronide (EtG) in hair samples as a biomarker. They tested several species, and found that the pentailed treeshrews had the highest EtG concentrations by a significant margin. If adjusted for body size, the alcohol levels indicated by EtG analysis of pentailed shrew hair would be enough to result in inebriation in humans.
While the methods in this study were thorough and covered many aspects of the mutualism in question, the authors acknowledge two basic assumptions. The first assumption is that the animals are not showing a behavioral preference for buds with higher or lower alcohol content. The authors did not determine whether buds with higher than average alcohol levels in their nectar are more or less successful at attracting pollinators.
Second, it is assumed that nectar which is removed by an animal is replaced with nectar with equivalent alcohol concentrations. This assumption could potentially prove problematic in the face of the phenological data which the authors report: freshly exuded nectar has higher alcohol concentrations than nectar which has been outside the bud for more than half an hour. The amount of time it takes a flower to replenish its nectar once it has been fed upon, which would be useful information, is not reported. However, treeshrews were shown to forage on the palm for an average of 136 minutes, which is longer than it takes for the nectar to “age.” This means that if they prefer lower alcohol levels, their feeding bouts are long enough to wait for nectar to “age” after being replenished by a flower.
The palm flowers were found to house unique communities of yeast, which facilitate fermentation. Further studies could investigate how much variation there is in yeast communities between flowers. It seems possible that one flower could have significantly higher yeast content and/or activity, and it would be interesting to determine whether these “superbrewers” are more or less attractive to mammalian pollinators. If so, increased fruit set from visitation by the animals could facilitate selection for higher alcohol content, but only if some heritable trait was the cause of the increased yeast activity.
While the results of this study are significant and intriguing, I wonder whether alcohol is the only substance attracting the mammals to this palm species. The authors did not do a detailed nutritional analysis of the nectar, so we do not know if there are other compounds present—such as special amino acids or lipids—which could make this species especially desirable for small mammals.
The authors are highly interested in the role of alcohol ingestion in primate evolution, and point to the treeshrew as an important model because of its basal position on the evolutionary “branch” leading to primates. What, however, is the significance of the fact that the loris—which also feeds upon the Bertram palm— shows markedly elevated EtG levels, even though they are not quite as high as those of treeshrews. Is this because the loris is less specialized on the palm, or because it metabolizes the alcohol differently, or some other explanation? The loris is more closely related to hominid primates than the treeshrew, so it seems that the comparison between an extremely basal species and a more derived one is a topic deserving of further discussion, in the context of a the evolutionary implications of this plant-animal mutualism.
Lastly, how costly is it for the plant to produce this nectar? The yeast communities perform the fermentation, but is there any metabolic cost to the plant itself for playing host to the brewery? The unusual nectar production cycle—with over a month of available nectar, followed by a longer period of “cutting off the tap” makes this a fascinating system. What happens to those yeast communities when nectar is unavailable? Cost-benefit analysis is important when studying co-evolution between plants and animals. While insect pollinators get most of the coverage in discussion of plant-animal mutualisms, this fascinating study showed that vertebrates are effective pollinators for the Bertram palm, and hopefully it will inspire further studies would be useful to provide more detailed insights into the dynamics of this mutualism.
Dudley, R. 2002. Fermenting fruit and the historical ecology of ethanol ingestion: is alcoholism in modern humans an evolutionary hangover? Addiction 97: 381-388.
Milton, K. 2004. Ferment in the family tree: does a frugivorous dietary heritage influence contemporary patterns of human ethanol use? Integrative and Comparative Biology 44: 304-314.
Morris, S., D. Humphreys, and D. Reynolds. 2006. Myth, marula, and elephant: an assessment of voluntary ethanol intoxication of the African elephant (Loxodonta Africana) following feeding on the fruit of the Marula tree (Sclerocarya birrea).
Sanchez, F., C. Korine, B. Pinshow, and R. Dudley. 2004. The possible roles of ethanol and the relationship between plants and frugivores: first experiments with Egyptian fruit bats. Integrative and Comparative Biology 44: 290-294.
Wiens, F., Zitzmann, A., Lachance, M., Yegles, M., Pragst, F., Wurst, F., von Holst, D., Guan, S., & Spanagel, R. (2008). Chronic intake of fermented floral nectar by wild treeshrews Proceedings of the National Academy of Sciences, 105 (30), 10426-10431 DOI: 10.1073/pnas.0801628105