Feeding efficiency of two coexisting nectarivorous bat species (Phyllostomidae: Glossophaginae) at flowers of two key-resource plants

Abstract

Animals should maximize their energy uptake while reducing the costs for foraging. For flower-visitors these costs and benefits are rather straight forward as the energy uptake equals the caloric content of the consumed nectar while the costs equal the handling time at the flower. Due to their energetically demanding lifestyle, flower-visiting bats face particularly harsh energetic conditions and thus need to optimize their foraging behavior at the flowers of the different plant species they encounter within their habitat. In flight cage experiments we examined the nectar-drinking behavior (i.e. hovering duration, nectar uptake, and the resulting feeding efficiency) of the specialized nectar-feeding bat Hylonycteris underwoodi and the more generalistic Glossophaga commissarisi at flowers of two plant species that constitute important nectar resources in the Caribbean lowland rainforests of Costa Rica and compared nectar-drinking behavior between both bat species and at both plant species. We hypothesized that the 1) specialized bat should outperform the more generalistic species and that 2) bats should generally perform better at flowers of the nectar-rich flowers of the bromeliad Werauhia gladioliflora than at the relatively nectar-poor flowers of the Solanaceae Merinthopodium neuranthum that has an extremely long flowering phase and therefore is an extremely reliable nectar resource, particularly for the specialized Hylonycteris. While we did not find substantial differences in the feeding efficiency of the generalist G. commissarisi, we observed an increased feeding efficiency of the specialized H. underwoodi at flowers of the nectar-poor M. neuranthum. This suggests that familiarity and ecological importance are more important determinants of the interaction than just morphological traits. Our results demonstrate that in addition to morphology, behavioral adaptations are also important drivers that determine the fitness of nectar-feeding bats. Both familiarity with and the ecological importance of a resource seem to contribute to shaping the interactions between pollinating bats and their plants.

Fig 1. The resident nectar-feeding bat guild and two key nectar resources.

The two resident nectar-feeding bat species, Glossophaga commissarisi (A) and Hylonycteris underwoodi (B), and the pendulous flower of Merinthopodium neuranthum (C) and the horizontally oriented Werauhia gladioliflora (D) at La Selva Biological Station, Costa Rica. When visiting a flower of M. neuranthum, bats (here H. underwoodi) raise their head to the flower above and lap the nectar (E).

Fig 2. Experimental setup in the flight cage.

H. underwoodi drinking from the horizontally oriented flower of W. gladioliflora presented in an artificial inflorescence (A) and from the downward-hanging flower of M. neuranthum (B). Both the IR-light-sensor (B) and the reflector (A) are visible in the background.

Fig 3. Comparison of nectar-drinking behavior of the two nectar-feeding bat species at the two resource plants.

(A, B) Hovering duration, (C, D) extracted nectar volume, (E, F) standardized nectar extractions efficiency. 9 individuals of H. underwoodi performed 383 drinking events at flowers of M. neuranthum and 5 individuals accomplished 612 drinking events at flowers of W. gladioliflora. For Glossophaga commissarisi, we observed 5 individuals with 272 drinking events at flowers of M. neuranthum and 6 individuals with 653 drinking events at flowers of W. gladioliflora.