In drought, zebra finches wring water from their own fat

Thirsty zebra finches “drink” their body fat. The songbirds are the first birds shown to get through a day without water by breaking down adipose tissue to stay hydrated, says evolutionary physiologist Ulf Bauchinger.

Two earlier tests of deprived birds summoning water from their tissues report that birds rely on protein. But zebra finches (Taeniopygia guttata) coped with one-day droughts in the lab not by breaking down such tissues as muscle but with the safer choice of metabolizing fat, say Bauchinger, Joanna Rutkowska and their colleagues at Jagiellonian University in Kraków, Poland. In comfortable temperatures and humidity, the little birds (averaging 13.5 grams in weight) produced about 0.444 grams of water metabolically. That boost would have taken large amounts of fleshy moist protein, equivalent to one-third the mass of their flight muscles or three times the mass of their hearts, the researchers say online August 31 in the Journal of Experimental Biology.

“Exciting,” says Alexander Gerson of the University of Massachusetts Amherst, whose own work has shown birds taking the protein route. Gerson’s interest in animals deriving water by metabolizing body parts traces to research on migratory birds surviving several thousand kilometers of flight across the Sahara. His wind-tunnel tests of five-hour flights in dry air suggested that birds were fueling their flight with energy from fat reserves but were supplementing with water produced by breaking down protein. 

What deprived birds do when they’re not migrating, however, might involve different trade-offs. But Gerson’s work with house sparrows kept from water still showed evidence of metabolizing proteins.

Unlike house sparrows, zebra finches have an evolutionary history of life in dry places, such as arid Australia. To see their water-management techniques, the researchers in Poland created total food and/or water shortages for lab birds just doing mundane finch things in cages instead of crossing a desert. 

All the birds reached the end of their bad day without signs of dehydration, the researchers found. But 12 birds deprived of food and water showed more total fat loss than another 12 birds allowed to drink but not eat. Parched finches had 42 percent less fat than birds that had access to drinking water. Measures of lost lean tissue, including protein-rich muscle, barely differed.  

Other bird species might respond to water shortages in the same way, Rutkowska speculates. Her test method differs a bit from the sparrow work. Gerson muses that zebra finches, with arid lands in their native range, might have different thresholds for metabolizing fat versus protein than house sparrows do.

For humans, Rutkowska says she gets asked about implications for dieting. Her answer: Sorry, no evidence of miracle shortcuts here.