In the fission-fusion dynamics of tree-dwelling bats, frequent roost switching maintains social bonds among colony members. It is hypothesised that the ritualised swarming or rallying behaviour of bats near a roost signals its location and that this transmission of information prevents the disintegration of the social group. We investigated swarming co-occurrences (i.e. the joint occurrence of two bats at the roost during swarming) in a maternity colony of Daubenton’s bats, Myotis daubentonii, using automated PIT tag readers that monitored bat activity at the entrances of tree roosts from midnight to sunrise. Over three summer seasons, we recorded 4736 dyadic co-occurrences in 5-second time windows by 259 individuals, with high variation in the number of swarming events and mates per an individual. Using a multi-membership multilevel model we found that dyads involving juveniles, particularly juvenile–juvenile and juvenile–adult pairs, showed significantly higher co-occurrence rates than adult–adult dyads. Relatedness had minimal overall effects but influenced co-swarming when juveniles were involved, indicating kin-biased interactions early in life. The swarming index, used to identify individuals that preferentially initiate co-swarming, showed that old females have more swarming events recorded relative to possible interactions detected, suggesting they might be more likely to serve as signallers whereas juveniles are more likely to function as receivers. The exchange of information in tree-dwelling bat species is particularly important for sharing knowledge about their numerous roosts. The swarming behaviour of these bats at dawn consists of various visual displays, including multiple landing trials before entry, and broadband vocalisations. However, the function of swarming remains unclear and there is no knowledge about the interactions between swarming individuals. The frequency with which two Daubenton’s bats appear at the roost entrance within a short period of 5 s was used as an indication of co-swarming. We investigated how individual characteristics, such as genetic relatedness or age, may influence this behaviour, which is assumed to involve decision-making. We show that age plays a dominant role in shaping swarming co-occurrences; however, the effect of relatedness emerges in juvenile contexts. We suggest that dawn swarming may be one of the mechanisms that promote long-lasting social bonds between colony members.
Ružinská et al. (Wed,) studied this question.