Sugars are complementary resources to ethanol in foods consumed by Egyptian fruit bats

Food resources are complementary for a forager if their contribution to fitness is higher when consumed together than when consumed independently, e.g. ingesting one may reduce the toxic effects of another. The concentration of potentially toxic ethanol, [EtOH], in fleshy fruit increases during ripening and affects food choices by Egyptian fruit bats, becoming deterrent at high concentrations (>/=1%). However, ethanol toxicity is apparently reduced when ingested along with some sugars; more with fructose than with sucrose or glucose. We predicted (1) that ingested ethanol is eliminated faster by bats eating fructose than by bats eating sucrose or glucose, (2) that the marginal value of fructose-containing food (food+fructose) increases with increasing [EtOH] more than the marginal value of sucrose- or glucose-containing food (food+sucrose, food+glucose), and (3) that by increasing [EtOH] the marginal value of food+sucose is incremented more than that of food+glucose. Ethanol in bat breath declined faster after they ate fructose than after eating sucrose or glucose. When food [EtOH] increased, the marginal value of food+fructose increased relative to food+glucose. However, the marginal value of food+sucrose increased with increasing [EtOH] more than food+fructose or food+glucose. Although fructose enhanced the rate at which ethanol declined in Egyptian fruit bat breath more than the other sugars, the bats treated both fructose and sucrose as complementary to ethanol. This suggests that in the wild, the amount of ethanol-containing fruit consumed or rejected by Egyptian fruit bats may be related to the fruit's own sugar content and composition, and/or the near-by availability of other sucrose- and fructose-containing fruits.