Abstract:
Nectarivores face a constant challenge in terms of water balance, experiencing water loading or dehydration when switching
between food plants or between feeding and fasting. To understand how whitebellied sunbirds and New Holland honeyeaters
meet the challenges of varying preformed water load, we used the elimination of intramuscular-injected [14C]-L-glucose and 3H2O
to quantify intestinal and renal water handling on diets varying in sugar concentration. Both sunbirds and honeyeaters showed
significant modulation of intestinal water absorption, allowing excess water to be shunted through the intestine when on dilute
diets. Despite reducing their fractional water absorption, both species showed linear increases in water flux and fractional body
water turnover as water intake increased (both afternoon and morning), suggesting that the modulation of fractional water
absorption was not sufficient to completely offset dietary water loads. In both species, glomerular filtration rate was independent
of water gain (but was higher for the afternoon), as was renal fractional water reabsorption (measured in the afternoon). During
the natural overnight fast, both sunbirds and honeyeaters arrested whole kidney function. Evaporative water loss in sunbirds was
variable but correlated with water gain. Both sunbirds and honeyeaters appear to modulate intestinal water absorption as an
important component of water regulation to help deal with massive preformed water loads. Shutting down glomerular filtration
rate during the overnight fast is another way of saving energy for osmoregulatory function. Birds maintain osmotic balance on
diets varying markedly in preformed water load by varying both intestinal water absorption and excretion through the intestine
and kidneys.
