Some creatures have gone to incredible lengths to colonise the planet's most inhospitable corners. Desert species must balance their need to conserve water with their energy demands in an environment where food and water are scarce. When protein is in short supply, birds could gain big energy savings and conserve nitrogen if they switched from a uric acid excretion system to ammonia production, resulting in ammonotely.
But ammonia is toxic unless diluted in a large volume of water, ruling this option out for most desert species, except nectar feeding birds, like hummingbirds, which excrete huge volumes of water. Five years ago, Marion Preest and Carol Beuchat in San Diego published evidence that hummingbirds could switch to ammonotely when the need arose. Berry Pinshow and his colleague Lizanne Roxburgh wondered whether nectar feeding Palestine sunbirds could do the same. But after analysing the sunbird's urine, they discovered that although the birds appeared to increase the level of ammonia excreted,the increase was only relative to the amount of uric acid they produced. Instead of changing their metabolism, the birds apparently recycled their urine to conserve essential solutes, dropping the fraction of other nitrogenous waste products in the urine to give the appearance of an ammonia increase (p. 1735).
Unlike mammals, which have evolved separate waste disposal systems for liquid and solid waste, bird's digestive systems mix urine and faeces in the cloaca. The waste is either expelled from the cloaca, or recycled by refluxing in the hindgut to conserve any remaining amino acids and salts. Palestine sunbirds survive on a poor diet of dilute nectar, supplemented with a few insects for protein. When food is scarce the birds need to conserve both energy and nutrients. Roxburgh and Pinshow wondered whether the birds responded to stress by switching to a low-cost ammonia-based excretion system or conserving their resources by recycling their waste.
Pinshow and Roxburgh collected nine birds from the gardens around their research institute in the Negev desert, and began to test the effects of salt,protein, water intake, and rising temperatures on the bird's excretion. But it wasn't enough to collect the bird's waste from the cloaca, they had to collect ureteral urine, before it became mixed with the bird's faeces! They perfected a technique where they inserted a tiny tube into the bird's cloaca to extract a few microlitres of urine as it drained from both kidney's ureters. After months of sample collection, they were ready to measure the quantities of nitrogenous waste and salts in the urine before and after it entered the cloaca.
When they looked at the ammonia the birds excreted from the cloaca compared to the levels in ureteral urine, they realised that it seemed to rise. But when they compared the ammonia levels with the concentrations of other nitrogenous waste products, ammonia hadn't increased, but the fraction of uric acid in the waste had dropped after refluxing in the hindgut, making it appear as if the proportion of ammonia had risen. The birds have opted to conserve energy by recycling their meagre resources, rather than switch to an alternative metabolic pathway.
Pinshow and Roxburgh suspect that other nectar feeding species, including the hummingbird, might also use this conservative approach to their nitrogen balance.