The sugar found in floral nectars may be sweet, but it comes at a price. Birds that feed on nectar can consume a huge amount of fluid compared to their body weight. And the more dilute the nectar, the higher the volumes ingested. Although the birds derive nutrients and energy from nectar, they have to get rid of the large amounts of water taken in. Failing to do so can have devastating consequences. Palestine sunbirds have somehow overcome the problems of life on liquid diet, so Todd McWhorter and an international team of collaborators decided to examine how they dispose of their excessive water intake (p. 3391).
In the kidney, water is filtered out of blood by specialized structures called glomeruli, and some of the eliminated water is later reabsorbed in the nephron and collecting duct. The researchers set out to test how these processes respond to water intake in Palestine sunbirds. Although following the birds around and measuring their nectar intake is difficult, McWhorter and his colleagues came up with an ingenious solution to the problem. They discovered that the birds adjust the amount they consume according to the concentration of sucrose solutions they are fed: the more dilute the solution, the higher the volumes ingested. In this way, the team could vary the bird's water intake and measure the rates of renal filtration and reabsorption.
McWhorter explains that when the team began investigating this nectarivorous bird's approach to fluid management, it was thought that renal filtration changes according to water status; decreasing in response to water shortage, but increasing only moderately as the birds take on water. But this was based on ideas developed for birds that do not regularly cope with a large intake of water. McWhorter and his team also knew that when the birds are on dilute diets, water is shunted through the gut without being absorbed. So, how would Palestine sunbirds' kidneys cope?
The team found that renal filtration is not exceptionally sensitive to water loading in sunbirds; it increased only slightly in response to a dramatic decrease in sucrose concentration. On the other hand, the fractional water reabsorption - a measure of the proportion of the eliminated water that is reabsorbed by the kidney - dropped significantly when the birds were on the most dilute diet. The sunbirds' kidney responds to the elevated water levels by decreasing reabsorption, rather than by raising the filtration rate.
The team also found that the glucose and osmotic concentrations in the final excreted fluids were significantly lower than those in the ureteral fluids released by the kidney. Because the gut and urinary tracts of birds join at the cloaca, the researchers conjecture that the dietary water that shunts through the gut might have diluted the ureteral fluids. They conclude that Palestine sunbirds deal with large amounts of water intake by not absorbing it in the first place.
From an economical standpoint this makes sense, as eliminating water by increasing renal filtration rate can be energetically costly for birds. Sugar and other metabolites lost during filtration may only be retained by reabsorption, possibly overwhelming the kidney's ability to prevent solute loss. But how the gut could absorb nutrients without taking in dietary water is still a mystery, as the two processes normally come hand in hand.
- © The Company of Biologists Limited 2004