Summary
Mammals frequently use nectar as a supplementary food, while a predominantly nectarivorous lifestyle with morphological specializations for this feeding mode is rare within the class. However, Neotropical flower-visiting bats largely depend on nectar resources and show distinct adaptations to a nectar diet. Glossophagine bats form local guilds of 2-6 species that may differ distinctly in skull morphology. It is still unknown how and to what extent this morphological diversity influences the efficiency of nectar extraction and hence also resource partitioning within the local bat guild. As foraging behaviour is a key factor for niche partitioning of coexisting species, we compared nectar extraction behaviour and efficiency at different flower depths among sympatric bat species with different degrees of morphological specialization (Glossophaga soricina, Leptonycteris yerbabuenae, Musonycteris harrisoni). In flight cage experiments with artificial flowers all species showed at deeper nectar levels a decrease in the amount extracted per visit and an increase in time spent hovering at the flower, indicating increased energetic cost when foraging on longer-tubed flowers. The lowest nectar extraction efficiency (g/s) was found in the small G. soricina and the highest in the largest species L. yerbabuenae. However, when considering also the different energy requirements of the different-sized bat species, the morphologically most specialized Musonycteris harrisoni showed consistently the highest foraging efficiency. Our data suggest that the long rostrum and tongue of the extremely specialized M. harrisoni are probably not evolved for monopolization of coevolved deep flowers, but allow efficient access to the broadest range of the local chiropterophilous flower community.
