Tj. Mcwhorter et Cm. Del Rio, Food ingestion and water turnover in hummingbirds: How much dietary water is absorbed?, J EXP BIOL, 202(20), 1999, pp. 2851-2858
Hummingbirds are specialized nectarivores that feed on dilute solutions of
sugars with trace amounts of amino acids and electrolytes. Their diets cont
ain excess water that, if absorbed, must be eliminated. It has been hypothe
sized that in hummingbirds only a small fraction of this dietary water may
be absorbed in the intestine. Here, we report the results of experiments de
signed to examine the relationship between nectar intake and water turnover
in hummingbirds. Our results also allow the estimation of water absorption
across the intestine and therefore test the hypothesis that ingested water
in hummingbirds passes largely unabsorbed through the gastrointestinal tra
ct. We found that fractional and total water turnover increased linearly wi
th water ingestion. At low sucrose concentrations, food intake rates betwee
n four and five times body mass per 12 h were not unusual. A simple mass-ba
lance model suggested that 78 % of ingested water was absorbed in the gastr
ointestinal tract and hence must be processed by the kidneys. However, frac
tional water absorption was variable and did not appear to be correlated wi
th food or water intake parameters, Our results do not lend support to the
hypothesis that the bulk of dietary water passes through the intestine unab
sorbed. Although hummingbird kidneys appear well suited to excrete large vo
lumes of dilute urine, rates of energy assimilation in hummingbirds may be
constrained by excess water elimination when these birds are feeding on nec
tars with a low sugar concentration.