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

The evolutionary ecology of ethanol

The consumption of ethanol has frequently been seen as largely restricted to humans. Here, we take a broad eco-evolutionary approach to understanding ethanol's potential impact on the natural world. There is growing evidence that ethanol is present in many wild fruits, saps, and nectars and that ethanol ingestion offers benefits that favour adaptations for its use in multiple taxa. Explanations for ethanol consumption span both the nutritional and non-nutritional, with potential medicinal value or cognitive effects (with social-behavioural benefits) explored. We conclude that ethanol is ecologically relevant and that it has shaped the evolution of many species and structured symbiotic relationships among organisms, including plants, yeast, bacteria, insects, and mammals.

The effect of COVID19 pandemic restrictions on an urban rodent population

Shortly after the enactment of restrictions aimed at limiting the spread of COVID-19, various local government and public health authorities around the world reported an increased sighting of rats. Such reports have yet to be empirically validated. Here we combined data from multi-catch rodent stations (providing data on rodent captures), rodent bait stations (providing data on rodent activity) and residents' complaints to explore the effects of a six week lockdown period on rodent populations within the City of Sydney, Australia. The sampling interval encompassed October 2019 to July 2020 with lockdown defined as the interval from April 1st to May 15th, 2020. Rodent captures and activity (visits to bait stations) were stable prior to lockdown. Captures showed a rapid increase and then decline during the lockdown, while rodent visits to bait stations declined throughout this period. There were no changes in the frequency of complaints during lockdown relative to before and after lockdown. There was a non-directional change in the geographical distribution of indices of rodent abundance suggesting that rodents redistributed in response to resource scarcity. We hypothesize that lockdown measures initially resulted in increased rodent captures due to sudden shortage of human-derived food resources. Rodent visits to bait stations might not show this pattern due to the nature of the binary data collected, namely the presence or absence of a visit. Relocation of bait stations driven by pest management goals may also have affected the detection of any directional spatial effect. We conclude that the onset of COVID-19 may have disrupted commensal rodent populations, with possible implications for the future management of these ubiquitous urban indicator species.

Antipredator responses toward cat fur in wild brown rats tested in a semi-natural environment

Sensitivity to predator-related cues and performance of antipredator behaviors are universal among prey species. Rodents exhibit a diverse suite of antipredator behaviors that have been examined in both field and laboratory studies. However, the results from the laboratory have not always translated to the field. While laboratory studies consistently indicate strong fear-inducing effects of cat fur/skin odors, it is unclear whether this occurs in the field with wild rats. To address this issue, we tested the antipredator responses of wild brown rats (Rattus norvegicus) to predatory (domestic cat fur) and nonpredatory (common brushtail possum fur) odor cues in a semi-natural experimental paradigm. Rats were housed in open air enclosures containing two feeding stations. Following several nights of acclimatization, the feeding stations were paired with cat fur, possum fur, or no fur. Rats spent less time at a feeding station that was paired with cat fur. Duration of time spent at feeding stations increased across consecutive test days and across hours within individual test nights, although the rate of increase within nights was lower for cat fur paired stations. This overall increase might reflect habituation of antipredator behaviors, increasing hunger, or loss of cue potency over time. We suggest that wild brown rats recognize and respond to cat fur odor cues, but their behavioral response is highly adaptable and finely tuned to the trade-off between predation risk and starvation that occurs across short temporal scales.

Adapting to alcohol: Dwarf hamster (Phodopus campbelli) ethanol consumption, sensitivity, and hoard fermentation

Ethanol consumption and sensitivity in many species are influenced by the frequency with which ethanol is encountered in their niches. In Experiment 1, dwarf hamsters (Phodopus campbelli) with ad libitum access to food and water consumed high amounts of unsweetened alcohol solutions. Their consumption of 15%, but not 30%, ethanol was reduced when they were fed a high-fat diet; a high carbohydrate diet did not affect ethanol consumption. In Experiment 2, intraperitoneal injections of ethanol caused significant dose-related motor impairment. Much larger doses administered orally, however, had no effect. In Experiment 3, ryegrass seeds, a common food source for wild dwarf hamsters, supported ethanol fermentation. Results of these experiments suggest that dwarf hamsters may have adapted to consume foods in which ethanol production naturally occurs.

Fruit aromas in mature fleshy fruits as signals of readiness for predation and seed dispersal

The dispersal of seeds away from parent plants seems to be the underlying selective force in the evolution of fleshy fruits attractive to animals. Secondary metabolites, which are not essential compounds for plant survival, are involved in the interaction of fleshy fruits with seed dispersers and antagonists. Plant volatile organic compounds (VOCs) are secondary metabolites that play important roles in biotic interactions and in abiotic stress responses. They are usually accumulated at high levels in specific plant tissues and organs, such as fleshy fruits. The study of VOCs emitted during fruit development and after different biotic challenges may help to determine the interactions of fleshy fruits not only with legitimate vertebrate dispersers, but also with insects and microorganisms. A knowledge of fruit VOCs could be used in agriculture to generate attraction or repellency to pests and resistance to pathogens in fruits. This review provides an examination of specific fruit VOC blends as signals for either seed dispersal or predation through simple or complex trophic chains, which may also have consequences for an understanding of the importance of biodiversity in wild areas.

Behavioral actions of alcohol: phenotypic relations from multivariate analysis of mutant mouse data

Behavioral studies on genetically diverse mice have proven powerful for determining relationships between phenotypes and have been widely used in alcohol research. Most of these studies rely on naturally occurring genetic polymorphisms among inbred strains and selected lines. Another approach is to introduce variation by engineering single-gene mutations in mice. We have tested 37 different mutant mice and their wild-type controls for a variety (31) of behaviors and have mined this data set by K-means clustering and analysis of correlations. We found a correlation between a stress-related response (activity in a novel environment) and alcohol consumption and preference for saccharin. We confirmed several relationships detected in earlier genetic studies, including positive correlation of alcohol consumption with saccharin consumption and negative correlations with conditioned taste aversion and alcohol withdrawal severity. Introduction of single-gene mutations either eliminated or greatly diminished these correlations. The three tests of alcohol consumption used (continuous two-bottle choice and two limited access tests: drinking in the dark and sustained high alcohol consumption) share a relationship with saccharin consumption, but differ from each other in their correlation networks. We suggest that alcohol consumption is controlled by multiple physiological systems where single-gene mutations can disrupt the networks of such systems.

Ethanol ingestion affects flight performance and echolocation in Egyptian fruit bats

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.

Drinking and flying: does alcohol consumption affect the flight and echolocation performance of phyllostomid bats?

Background

In the wild, frugivorous and nectarivorous bats often eat fermenting fruits and nectar, and thus may consume levels of ethanol that could induce inebriation. To understand if consumption of ethanol by bats alters their access to food and general survival requires examination of behavioural responses to its ingestion, as well as assessment of interspecific variation in those responses. We predicted that bats fed ethanol would show impaired flight and echolocation behaviour compared to bats fed control sugar water, and that there would be behavioural differences among species.

Methodology/Principal Findings

We fed wild caught Artibeus jamaicensis, A. lituratus, A. phaeotis, Carollia sowelli, Glossophaga soricina, and Sturnira lilium (Chiroptera, Phyllostomidae) sugar water (44 g of table sugar in 500 ml of water) or sugar water with ethanol before challenging them to fly through an obstacle course while we simultaneously recorded their echolocation calls. We used bat saliva, a non-invasive proxy, to measure blood ethanol concentrations ranging from 0 to >0.3% immediately before flight trials. Flight performance and echolocation behaviour were not significantly affected by consumption of ethanol, but species differed in their blood alcohol concentrations after consuming it.

Conclusions/Significance

The bats we studied display a tolerance for ethanol that could have ramifications for the adaptive radiation of frugivorous and nectarivorous bats by allowing them to use ephemeral food resources over a wide span of time. By sampling across phyllostomid genera, we show that patterns of apparent ethanol tolerance in New World bats are broad, and thus may have been an important early step in the evolution of frugivory and nectarivory in these animals.

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.