Ethanol concentration in food and body condition affect foraging behavior in Egyptian fruit bats (Rousettus aegyptiacus)

Abstract

Ethanol occurs in fleshy fruit as a result of sugar fermentation by both microorganisms and the plant itself; its concentration [EtOH] increases as fruit ripens. At low concentrations, ethanol is a nutrient, whereas at high concentrations, it is toxic. We hypothesized that the effects of ethanol on the foraging behavior of frugivorous vertebrates depend on its concentration in food and the body condition of the forager. We predicted that ethanol stimulates food consumption when its concentration is similar to that found in ripe fruit, whereas [EtOH] below or above that of ripe fruit has either no effect, or else deters foragers, respectively. Moreover, we expected that the amount of food ingested on a particular day of feeding influences the toxic effects of ethanol on a forager, and consequently shapes its feeding decisions on the following day. We therefore predicted that for a food-restricted forager, ethanol-rich food is of lower value than ethanol-free food. We used Egyptian fruit bats (Rousettus aegyptiacus) as a model to test our hypotheses, and found that ethanol did not increase the value of food for the bats. High [EtOH] reduced the value of food for well-fed bats. However, for food-restricted bats, there was no difference between the value of ethanol-rich and ethanol-free food. Thus, microorganisms, via their production of ethanol, may affect the patterns of feeding of seed-dispersing frugivores. However, these patterns could be modified by the body condition of the animals because they might trade-off the costs of intoxication against the value of nutrients acquired.

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

“…The odour and taste of ethanol may be used by Egyptian fruit bats to identify overripe, unpalatable fruit (Sánchez et al, 2008;Sánchez et al, 2006). Thus, another explanation for our results is that the bats might perceive fruit rich in ethanol and sucrose as food with lower levels of toxins than fruit rich in ethanol and fructose or glucose.…”

“…Thus, it is unclear whether there are differences between the sexes for the effect of DEE on the marginal value of sugar-containing foods in the presence of ethanol. In addition, in a previous experiment, designed to determine the effect of [EtOH] on the marginal value of artificial food in male Egyptian fruit bats, the effect of DEE was also significant (Sánchez et al, 2008). This experiment was done in summer, thus the effect of DEE, and its interaction with [EtOH], on the marginal value of food for the bats may also be related to season.…”

“…The fact that birds apparently do so suggests that they have an advantage over bats when high quality, ripe fruit is scarce. This may be because when ripe fruit is hard to find, frugivores may be in relatively poor body condition and are, therefore, more likely to take the risk of ingesting ethanol-rich fruit (Sánchez et al, 2008).…”

“…Because we previously found that daily energy expenditure (DEE) affects GUD in Egyptian fruit bats kept in outdoor cages (Sánchez et al, 2008), we assessed the possible influence of DEE here as well. Since the cages were protected from the sun and wind, we assumed that the effects of direct solar radiation and of convection were negligible, and used air temperature, T a , to estimate the metabolic rate of resting (day) and active (night) bats with the equations of Noll (Noll, 1979) (see below).…”

“…Since the cages were protected from the sun and wind, we assumed that the effects of direct solar radiation and of convection were negligible, and used air temperature, T a , to estimate the metabolic rate of resting (day) and active (night) bats with the equations of Noll (Noll, 1979) (see below). We observed that our bats were active for about 11·h per night and, of that, they were in flight for some 8·min (Sánchez et al, 2008). Therefore, to estimate DEE, we assumed that the bats rested for 13·h during their daytime, inactive phase, rested for 10·h 52·min in their night-time, active phase, and flew for 8·min at a metabolic rate 14 times the active resting rate (Thomas, 1975).…”