Jeaneth Magelen V. Respicio, Kier Celestial Dela Cruz, Alice catherine Hughes, Krizler Cejuela Tanalgo 

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. Studying bat behaviour remains an underexplored aspect of bat ecology and conservation despite its ecological importance.

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 variations in the incident frequencies of various bat behavioural activities, particularly regarding aggression and movement, between these two groups. The increase in aggregation was closely related to negative social behaviour among bats.

4. In contrast, sexual behaviour was significantly related to the positive behaviour of individual bats and was headed in less crowded areas. The disparities in bat behaviour with an apparent decline in bat social behaviour because of overcrowding, with more aggressive behaviours emerging, align with the ‘behavioural sink’ hypothesis.

5. Our study underscores the importance of considering habitat quality and availability of resources in the management and conservation of bat colonies, as these factors can reduce the occurrence of aggressive and negative social behaviours in colonies with high population density by providing alternative habitats.