From Bees to birds.. Study Finds Ethanol Present in Nectar of Most Flowers Tested

Researchers from the University of California, Berkeley have identified ethanol in nectar samples from 26 out of 29 plant species examined, marking the first extensive study of alcohol presence in floral nectar. The ethanol concentrations were generally low, likely resulting from yeast fermentation of natural sugars in the nectar, with one sample containing 0.056% ethanol by weight, approximately one-tenth of a proof.

Many pollinators rely heavily on nectar for sustenance. For example, hummingbirds consume between 50% and 150% of their body weight in nectar daily. Based on these feeding patterns, scientists estimate that an Anna’s hummingbird (Calypte anna), commonly found along the Pacific coast, may intake about 0.2 grams of ethanol per kilogram of body weight each day, comparable to a human consuming one alcoholic drink.

Pollinators’ Interaction with Alcohol in Nectar

Despite the continuous ingestion of fermented nectar, pollinators like hummingbirds appear unaffected. Previous experiments demonstrated that hummingbirds willingly consumed sugar water containing up to 1% alcohol, though their interest declined when alcohol levels increased beyond that.

Scientists suggest that alcohol in nectar could subtly influence pollinator behavior. Similar to other naturally occurring compounds in nectar such as caffeine and nicotine, ethanol might have signaling or appetitive effects. Doctoral student Aleksey Maro, who analyzed the nectar alongside postdoctoral fellow Ammon Corl, noted that hummingbirds metabolize energy rapidly, so any alcohol is unlikely to accumulate in their bloodstream. However, the exact behavioral impacts remain unclear.

Robert Dudley, a professor of integrative biology at UC Berkeley, added that the rapid metabolism of ethanol by these birds likely prevents intoxication but might have other behavioral consequences related to their foraging biology.

Experimental Evidence of Alcohol Consumption and Metabolism

One of the team’s earlier studies involved placing alcohol-containing sugar water in a feeder near Dudley’s office. The results showed that Anna’s hummingbirds consumed the solution when alcohol concentrations were below 1% by volume, but visits decreased by about half when the concentration reached 2%. This suggests that hummingbirds regulate their alcohol intake and that natural nectar alcohol levels likely stay below 1%.

Another experiment led by former graduate student Cynthia Wang-Claypool detected ethyl glucuronide, a metabolic byproduct of ethanol, in feathers of several birds including Anna’s hummingbirds. This finding indicates that these birds not only consume alcohol but also metabolize it similarly to mammals. The researchers interpret these results as evidence that many animals, including human ancestors, may have evolved an alcohol tolerance or preference.

Ammon Corl emphasized that their laboratory experiments confirmed hummingbirds’ willingness to drink ethanol-containing nectar, though they avoid higher concentrations. The presence of ethanol in nectar is widespread, according to their latest study.

Comparing Alcohol Intake Among Nectar Feeders

The researchers measured ethanol levels in nectar samples using enzymatic assays and estimated daily alcohol intake for birds in their natural habitats. Due to limited data on nectar consumption, the focus was on two hummingbird species, including Anna’s hummingbird, and three sunbird species. Sunbirds, which feed on plants such as honeybush (Melianthus major) in South Africa, fulfill a similar ecological role to hummingbirds in the Americas.

The team compared estimated alcohol intake among these birds with other nectar-feeding animals including the European honeybee, the pen-tailed tree shrew, as well as fruit-eating chimpanzees and humans consuming one standard American drink daily (0.14 grams per kilogram per day). The pen-tailed tree shrew exhibited the highest estimated intake at 1.4 grams per kilogram per day, while the European honeybee had the lowest at 0.05 grams per kilogram per day. Nectar-feeding birds consumed intermediate amounts, ranging from 0.19 to 0.27 grams per kilogram per day in their native environments.

Interestingly, feeder experiments suggest Anna’s hummingbirds may consume more alcohol from fermented sugar water in artificial feeders (0.30 grams per kilogram per day) than from naturally fermented nectar in flowers.

Ongoing Research on Ethanol Adaptations

This study is part of a broader five-year project funded by the National Science Foundation aiming to collect genetic data from all hummingbird and sunbird species. The goal is to understand how these birds have adapted to challenging conditions and specialized diets, including high altitudes, sugar-rich nectar, and naturally fermented nectar.

Robert Dudley remarked that these findings imply a wide range of physiological adaptations to dietary ethanol across the animal kingdom. The responses observed in humans may not represent all primates or animals generally. He suggested that other detoxification pathways or nutritional effects of ethanol might exist for animals consuming it daily throughout their lives. Dudley emphasized that chronic exposure to ethanol post-weaning warrants further comparative biological study.