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Efficiency of facultative frugivory in the nectar-feeding bat Glossophaga commissarisi: the quality of fruits as an alternative food source


The efficiency of food exploitation correlates positively with the extent of dietary specialization. Neotropical nectar-feeding bats (Glossophaginae) have one of the most specialized diets among mammals, as floral nectar constitutes a sugar-rich and highly digestible but protein and fiber depleted food source. However, dietary constraints, such as a temporary scarcity of nectar, or protein demands may sometimes require the uptake of alternative food items. We investigated the influence of a diet switch from nectar to fruit on intestinal morphology, body mass, and energy budget in the nectar-feeding bat Glossophaga commissarisi and quantified feeding efficiency. We hypothesized that these nectar specialists depend on a constant supply of nectar, if they were lacking the ability for morphological and physiological plasticity in response to a fiber-rich diet. Although capable of harvesting infructescences of Piper hispidum, G. commissarisi was less efficient in extracting energy from fruits (48% digestive efficiency of total fruit energy content) than from nectar (c. 99% digestive efficiency). The intestinal morphology and organ masses did not change after bats were switched from nectar to fruits. Captive bats exhibited lower daily energy expenditures and flight activity when feeding on fruits than during nectarivory. Possibly, this may have been a deliberate regulation to balance reduced feeding efficiency, or simply the consequence of extended digestive pauses. The low digestibility of Piper, in combination with slow digestion and the bats’ inability for morphological and physiological plasticity may cause nectar-feeders to reduce their maximum energy expenditure when feeding on fruits. We argue that although fruits may substitute for nectar, they may cause restricted maximum energy assimilation compared with nectar.

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Dry mass


Body mass


Daily energy expenditure


Doubly-labelled water


Standard deviation


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We would like to thank the Organization for Tropical Studies and the Costa Rican authorities, namely Javier Guevara (SINAC) for research permissions. We thank Dagmar Viertel, Doris Krumnow and Doris Fichte for histological preparation and evaluation, Heidrun Barleben and Doreen Pick for nutritional analysis, Paula Redman for DLW analysis, Mirkka Jones for proofreading, all helpers in the field and two anonymous reviewers for improving the manuscript. The experiments complied with the Costa Rican law and were approved by the Costa Rican Ministry for Environment and Energy (MINAE). This study was financed by the DFG grant Vo 890-10.

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Correspondence to Detlev H. Kelm.

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Communicated by G. Heldmaier.

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Kelm, D.H., Schaer, J., Ortmann, S. et al. Efficiency of facultative frugivory in the nectar-feeding bat Glossophaga commissarisi: the quality of fruits as an alternative food source. J Comp Physiol B 178, 985 (2008).

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  • Glossophaga
  • Digestive efficiency
  • Frugivory
  • Nectarivory
  • Nutrient analysis