Except in the case of a few exceptionally hardy species, a regular water supply is essential for the survival of most creatures. ‘In birds, water shortage impairs reproductive investment and may set limits to potential habitat, flight range or flight altitude during migration’, say Joanna Rutkowska, Edyta Sadowska, Mariusz Cichoń and Ulf Bauchinger from Jagiellonian University, Poland. However, some animals under extreme exertion can supplement their water supply with water generated as they consume their own tissues: proteins can release 0.82 g of water for every gram of tissue consumed, while fats liberate 1.1 g of water. However, it was less clear how birds that are simply going about their day-to-day business manage water production. Bauchinger explains that despite the greater potential for fat to release metabolic water, protein had been considered the main source of water as protein molecules are always associated with bound water that would be released as the proteins were broken down. Curious to find out more about how birds generate metabolic water, Rutkowska and colleagues put zebra finches on a diet.
Allocating the birds to one of three groups (unlimited access to food and water, access to water alone and access to neither) for 24 h, the team then measured the animals’ metabolic rates before analysing the composition of their bodies to find out how the birds had managed their metabolism while fasting. Impressively, despite having deprived a third of the birds of water for a day, all of the birds were equally well hydrated. However, their body composition varied dramatically, with the birds that had no access to food or water losing almost 0.4 g fat, relative to the fasting birds that could still drink, which lost only 0.047 g fat. And when the team converted the fat loss into water generated, the water-deprived birds were able to generate an impressive 0.444 g of water by simply burning their own fat.
Considering how much protein from body tissues the birds would have to consume to match the water produced by fat, the team points out that the starving water-deprived birds would have to metabolise a potentially life-threatening amount of protein, equivalent to three times the mass of their own hearts or one-third of their flight muscle mass. ‘Although protein catabolism would be probably spread over different organs, this exemplifies that catabolism of such an amount of protein would severely constrain tissue function, not only during fasting but also when food intake is re-established’, the team says.
So, water availability is essential for fasting birds to maintain their fat stores and the team says, ‘We revise currently established views [that protein catabolism is the main source of metabolic water] and propose fat serves as the primary source for metabolic water production’.