Too many men in the room? The cost of overcrowding for gregarious male cave-dwelling bats

Abstract

1. Bats are known for their gregarious social behaviour, often congregating in caves and underground habitats, where they play a pivotal role in providing various ecosystem services. Despite their ecological importance, studying bat behaviour remains an underexplored aspect of bat conservation. 2. We explore the costs and impacts of overcrowding on bat social behaviour. This study examined variations in bat behavioural patterns between two distinct groups: aggregated and non-aggregated male Rousettus amplexicaudatus, within the Monfort Bat Cave Sanctuary on Mindanao Island, Philippines. 3. We found significant disparities in the frequencies of various bat behavioural activities, particularly in terms of aggression and movement, between these two groups. Increased aggregation was closely related to negative social behaviour among bats. 4. Conversely, positive social behaviours, such as sexual interactions and grooming, were more prevalent in less crowded conditions. The disparities in bat behaviour with an apparent decline in bat social behaviour as a consequence of overcrowding, with more aggressive behaviours emerging, aligns with the ‘behavioural sink hypothesis’. 5. Our study underscores the significance of considering the quality of habitat and the availability of resources in the management and conservation of bat colonies, as these factors may reduce the occurrence of detrimental social behaviours in colonies with high population density by providing alternative habitats.