From resource to female defence: the impact of roosting ecology on a bat’s mating strategy
Linus Günther , Marlena D. Lopez , Mirjam Knörnschild, Kyle Reid , Martina Nagy, and Frieder Mayer
With their extraordinary species richness and diversity in ecological traits and social systems, bats are a promising taxon for testing socio-ecological hypotheses in order to get new insights into the evolution of animal social systems. Regarding its roosting habits, proboscis bats form an extreme by occupying sites which are usually completely exposed to daylight (e.g. tree trunks, vines or rocks). This is accompanied by morphological and behavioural adaptations to remain cryptic in exposed day roosts. With long-term behavioural observations and genetic parentage analyses of individually marked proboscis bats, we assessed its social dispersion and male mating strategy during day and night. Our results reveal nocturnal male territoriality—a strategy which most closely resembles a resource-defence polygyny that is frequent also in other tropical bats. Its contrasting clumped social dispersion during the day is likely to be the result of strong selection for crypsis in exposed roosts and is accompanied by direct female defence in addition to male territoriality. To the best of our knowledge, such contrasting male mating strategies within a single day–night cycle have not been described in a vertebrate species so far and illustrate a possible evolutionary trajectory from resource-defence to female-defence strategy by small ecologically driven evolutionary steps.
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Social organization in day and night roosts of the Proboscis Bat (Rhynchonycteris naso)
Marlena Lopez , Martina Nagy
Our understanding on how roosting habits of bats influence their social organization and mating systems is scarce. Here we attempted to fill in some of these gaps by examining the social organization of the Proboscis bat at La Selva Biological Station, Costa Rica, both during the day and at night. Day roosts of these bats are very exposed and bats are visually and behaviorally cryptic during the day but not at night. We thus predicted that the social organization would differ between the day and the night roost of this species. Behavioral observations of individually banded bats show that there were 2 social groups in Cabina 5 during the day using only a small part of the potential roosting space and that virtually all members showed high fidelity to the day roost. In contrast, we found bats to be using a much larger part of the potential roosting space at night and to have much lower fidelity; only 38% of the males present in the day were regularly present at the night roost and our results provide evidence of five male territories in the night roost. Our study suggests that R. naso might have a resource-defense polygyny rather than a female-defense polygyny as previously thought. Our finding thus indicates that female choice probably plays a much larger role in Proboscis bats than previously anticipated. Observing the social structure at night allowed us to understand a major feature of the social organization of this bat species that had been missed before and has essential consequences for interpreting the mating system.