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Since mammalian frugivores generally choose to eat ripe fruit in which ethanol concentration ([EtOH]) increases as the fruit ripens, we asked whether ethanol acts as an appetitive stimulant in the Egyptian fruit bat, Rousettus aegyptiacus, and also studied the effects of ethanol on their skin temperature (T(s)). We hypothesized that the responses of fruit bats to dietary ethanol are concentration dependent and tested the predictions that the bats' response is positive, i.e., they eat more when [EtOH] in the food is in the range found in naturally ripe fruit, while it negatively affects them at higher concentrations. We also tested the prediction that in winter, even when availability of fruit is low and thermoregulatory costs are high, ingestion of ethanol by fruit bats is low because assimilated ethanol reduces shivering thermogenesis and peripheral vasodilation; these, alone or together, are detrimental to the maintenance of body temperature (T(b)). In summer, captive bats offered food containing 0.1% ethanol significantly increased consumption over food with no ethanol; they did not change consumption when food contained 0.01, 0.3, or 0.5% ethanol; but significantly decreased consumption at higher levels of ethanol [EtOH], i.e., 1 and 2%. In winter, captive bats ate significantly less when their food contained 0.1% ethanol than when it contained 0, 0.3, or 0.5%. During summer, freshly caught bats ate significantly more ethanol-containing food than freshly caught bats in winter. Skin temperature (T(s)) in Egyptian fruit bats decreased significantly at an ambient temperature (T(a)) of 12 °C (winter conditions) after gavage with liquid food containing 1% ethanol. The effect was clearly temperature-dependent, since ethanol did not have the same effect on bats gavaged with food containing 1% or no ethanol at a T(a) of 25 °C (summer conditions). In conclusion, ethanol may act as an appetitive stimulant for Egyptian fruit bats at low concentrations, but only in summer. Bats are deterred by food containing [EtOH] corresponding to that in overripe, unpalatable fruit (1 and 2%). Furthermore, during winter, Egyptian fruit bats are deterred by ethanol-rich fruit, possibly due to the potential thermoregulatory consequences of ethanol consumption.

Ethanol, a potential toxin for vertebrates, is present in all fleshy fruits and its content increases as the fruit ripens. Previously, we found that the marginal value of food for Egyptian fruit bats, Rousettus aegyptiacus, decreases when its ethanol content exceeds 1%. Therefore, we hypothesized that, if ingested, food containing >1% ethanol is toxic to these bats, probably causing inebriation that will affect flight and echolocation skills. We tested this hypothesis by flying Egyptian fruit bats in an indoor corridor and found that after ingesting ethanol-rich food bats flew significantly slower than when fed ethanol-free food. Also, the ingestion of ethanol significantly affected several variables of the bats' echolocation calls and behavior. We concluded that ethanol can be toxic to fruit bats; not only does it reduce the marginal value of food, but it also has negative physiological effects on their ability to fly competently and on their calling ability., (Copyright 2010 Elsevier B.V. All rights reserved.)

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.

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.

The sugars in fleshy fruits provide a rich source of energy to frugivorous animals. However, these carbohydrates also serve as a substrate for alcoholic fermentation by yeasts, ethanol being the main by-product of this process. Ethanol ingestion via frugivory thus occurs in a diverse assemblage of invertebrate and vertebrate taxa, including numerous species of birds. We tested the roles of ethanol as an odor cue for resource location by adult yellow-vented bulbuls (Pycnonotus xanthopygos) and as a possible appetite stimulant in feeding trials with artificial food. We hypothesized (1) that the odor of ethanol does not serve as a food-locating cue in diurnal frugivorous passerine birds, and predicted that the choice of food source and the mass of food eaten by such birds will not be influenced by the odor of ethanol. We further hypothesized (2) that food intake in passerine birds is affected by ingestion of ethanol according to its concentration [EtOH], and predicted that food intake will follow a bell-shaped curve in relation to [EtOH]. In accord with hypothesis (1) and its prediction, we found that the odor of ethanol did not affect food preferences, in either ethanol-naïve or ethanol-experienced yellow-vented bulbuls, when presented at concentrations found in naturally ripe fruit (0.0-1.0%); this suggests that the odor of ethanol is not a food-locating cue for the bulbuls. Hypothesis (2) was partially supported, namely at low [EtOH] (0-3%), food intake was constant and at high [EtOH] (3%) food intake decreased, following only the right half of the predicted bell-shaped response. Ethanol-naïve birds showed no preference towards any [EtOH] presented in two-way choice trials. However, daily food intake in ethanol-experienced bulbuls in single option trials decreased by an average of 36% when the artificial food contained the highest tested concentration of ethanol (3.0%). We suggest that decreasing food intake when food ethanol concentration is relatively high may be a means of avoiding intoxication and is related to the ethanol-metabolizing ability of the bird.

Frugivorous bats from the Old and New World use odor cues to locate and assess fruit condition. We hypothesized that Egyptian fruit bats (Rousettus aegyptiacus) use as odor cues those volatile compounds that increase in emission rate as fruit ripens. We examined whether the smell of fermentation products may indicate the degree of ripeness to fruit bats. We analyzed volatile compounds in the headspace (the gas space above a fruit in a closed container) of dates (Phoenix dactylifera) and rusty figs (Ficus rubiginosa), both of which are consumed by fruit bats, to elucidate which compounds originate from fermentative pathways and to determine which change in emission rate during ripening. Ethanol, acetaldehyde, and acetic acid were the only volatile compounds detected as products of fermentation in both fruits. In dates, emission rates of these compounds increased during maturation, whereas in rusty figs, they decreased or remained constant. Methanol, although not a fermentation product, increased in emission rate during ripening in both fruits. We found that R. aegyptiacus was neither attracted nor deterred by the smell of methanol at any of the concentrations used. Although the odor of ethanol emanating from food containing concentrations similar to those found in ripe fruit did not attract the bats, at relatively high concentrations (> or =1%), the smell of ethanol deterred them. Thus, ethanol at high concentrations may serve as a signal for bats to avoid overripe, unpalatable fruit.