Keystone resource (Ficus) chemistry explains lick visitation by frugivorous bats

Background/Question/Methods

Geophagy, defined as the deliberate consumption of soil, is a widespread behavior among plant-eating animals.  A set of hypotheses ranging from physiological to social explanations has been proposed to explain this behavior. In the Neotropics, mineral licks are activity hotspots for frugivorous bats (Stenodermatinae). Bats visit licks to drink mineral-rich water accumulated in soil depressions made by geophagous mammals. Two non-mutually exclusive, mechanistic hypotheses have been proposed to explain this behavior: (1) licks are reliable sources of limiting nutrients, especially sodium; and (2) licks provide substances that render dietary toxins less harmful. We assessed the nutrient limitation hypothesis by examining the composition and nutritional content of fruits consumed by bats in conjunction with lick water chemistry. We determined the diet composition of 22 bat species and assessed their correlation with lick visitation in southeastern Peruvian Amazonia. We also determined the nutritional composition of Ficus and Piper fruits collected in the area of study and analyzed the concentrations of four key minerals, as well as nitrogen (as a measure of protein). 

Results/Conclusions

We found that most bats that visit licks belong to the subfamily Stenodermatinae (R = 0.74, P < 0.001) and are specialists on Ficus fruits (G₂ = 109.9, P < 0.001) – keystone resources in Neotropical forests. We also found that although Ficus fruits are good sources of calcium (0.8%), magnesium (0.3%), potassium (1.9%), and nitrogen (1.2%), their sodium content is limited (17.4ppm) in relation to the physiological requirement of a small mammal. In contrast, frugivorous bats in the subfamily Carolliinae, which do not visit licks on a regular basis, supplemented their fruit diets with insects, a potential source of sodium. In addition, we found that lick water has significantly higher concentrations of sodium compared to other available water sources (F_2,19 = 98.345, P<0.001). Our results strongly support the sodium-limitation hypothesis for lick use by frugivorous bats. Furthermore, they suggest a mechanistic link between bats and the terrestrial ecosystem engineers (e.g., tapirs and peccaries) that make soil-borne resources available. Because sodium is an essential nutrient for vertebrates and Ficus is a keystone resource for many animal species, our results may have broader implications for the community of frugivorous vertebrates in areas where sodium is limited. Soil licks may play a critical role as sodium sources and thus they should be considered as important conservation targets.