As bees and hummingbirds flit from flower to flower, greedily sipping nectar in exchange for pollination, the animals often get another treat: alcohol.
In the first broad analysis of the alcohol content of flower nectars, University of California, Berkeley biologists found detectable alcohol in at least one flower of 26 of the 29 species of plants tested. While most samples had very low levels, almost certainly from yeast fermenting the sugars in the nectar, one contained 0.056% ethanol by weight: about 1/10 proof.
While this concentration may seem minuscule, for some animals nectar is their main source of calories. Hummingbirds consume between 50% and 150% of their entire body weight in nectar every day. The researchers calculate that an Anna's hummingbird (Calypte anna), common along the Pacific coast, would consume about 0.2 grams of ethanol per kilogram of body weight per day - equivalent to a human drinking about one serving of alcohol.
The birds and the bees consume the alcohol in small doses throughout the day and appear to show no obvious effects from the spiked nectar. In fact, a previous study by the same group showed that while hummingbirds tolerate sugar water that contains up to 1% alcohol, they tend to avoid higher concentrations.
Ammon Corl/UC Berkeley
Nevertheless, other chemicals found in small amounts in nectar - nicotine and caffeine, for example - have demonstrated effects on the animals that consume it. The same could be true of ethanol.
"Hummingbirds are like little furnaces. They burn through everything really quick, so you don't expect anything to accumulate in their bloodstream," said doctoral student Aleksey Maro, who collected and analyzed the nectar with postdoctoral fellow Ammon Corl. "But we don't know what kind of signaling or appetitive properties the alcohol has. There are other things that the ethanol could be doing aside from creating a buzz, like with humans."
"There may be other kinds of effects specific to the foraging biology of the species in question that could be beneficial," added Robert Dudley, UC Berkeley professor of integrative biology. "They're burning it so fast, I'm guessing that they probably aren't suffering inebriating effects. But it may also have other consequences for their behavior."
Maro, Corl and Dudley published a paper about their findings today (March 25) in the journal Royal Society Open Science, coauthored with their Berkeley colleagues, Rauri Bowie and Jimmy McGuire, both professors of integrative biology and curators in the campus's Museum of Vertebrate Zoology.
Dudley noted that their first experiment involving birds and alcohol, conducted at a feeder outside his office window, confirmed that Anna's hummingbirds are indifferent to alcohol in sugar water if it's at low concentration (below 1% by volume) but visit the feeder half as often when the concentration is 2%.
"Somehow they are metering their intake, so maybe zero to 1% is a more likely concentration that they would find in the wild than anything higher," he said.
Ammon Corl/UC Berkeley
A second experiment, led by Cynthia Wang-Claypool, a former graduate student who worked with the research group, showed that feathers, including those of the Anna's hummingbird, contain a metabolic byproduct of ethanol, ethyl glucuronide. The implication is that they not only ingest alcohol in their diet, but they metabolize it much like mammals do. The new experiment is further evidence that birds and other animals, including our ape ancestors, evolved a tolerance for and, in some cases, a preference for alcohol.
"The laboratory experiment was showing that yes, they will drink ethanol in their nectar, though they have some aversion to it if it gets too high," Corl said. "The feathers are saying that, yes, they will metabolize it. And then this study is saying that ethanol is actually pretty widespread in the nectar they consume."
After collecting the nectar and measuring its ethanol content using an enzymatic assay, the researchers attempted to calculate the daily alcohol consumption, based on estimated caloric intake, of birds that live in the native habitat of some of these flowers. Since daily caloric intake of nectar is known for very few species, they were only able to estimate daily ethanol intake for two hummingbirds, including the Anna's hummingbird, and three species of sunbirds. Sunbirds are nectar feeders that occupy the same niche in Africa as hummingbirds do in the Americas, and the UC Botanical Garden contains several plant species from South Africa that they feed on, including the honeybush (Melianthus major).
They compared these estimates with the calculated daily alcohol consumption of two other nectarivores, the European honeybee and the pen-tailed tree shrew, as well as with fruit-eating chimpanzees and humans imbibing one standard American drink per day (0.14 grams/kg/day). They concluded that the tree shrew consumes the most alcohol in its daily diet (1.4 g/kg/day), while the honeybee consumes the least (0.05 g/kg/day). When feeding from flowers native to their home environment, the nectar-eating birds consumed about the same amount of alcohol: 0.19 to 0.27 g/kg/day.
Ironically, the hummingbird feeder study suggests that Anna's hummingbirds likely get a higher alcohol dose from fermented sugar in feeders (0.30 g/kg/day) than through fermented flower nectar.
The research was conducted as part of a larger, five-year project funded by the National Science Foundation to collect large-scale genetic data for all hummingbird and sunbird species in order to assess their genetic adaptations to various environments and food sources, including high altitude, very sugary nectar and frequently fermented nectar.
"These studies suggest that there may be a broad range of physiological adaptations across the animal kingdom to the ubiquity of dietary ethanol, and that the responses we see in humans may not be representative of all primates or of all animals generally," Dudley said. "Maybe there are other physiological detoxification pathways or other kinds of nutritional effects of ethanol for animals that are consuming it every day of their lives. That's the interesting thing - this is chronic through the course of the day, but that's a lifetime exposure post-weaning. It just means that the comparative biology of ethanol ingestion deserves further study."
