Attributes of Social Organisation and Male Singing Behaviour in the Lesser Short-Tailed Bat, Mystacina tuberculata

Abstract

Bats (Mammalia: Chiroptera) exhibit an extraordinary diversity of social structures and behaviours. Social groupings vary in size from solitary individuals to upwards of a million, and almost every known mammalian breeding strategy is represented. Bats have demonstrated the ability to recognise individual members of a colony and to preferentially associate with or avoid other individuals. This degree of social complexity is frequently accompanied by sophisticated communicative abilities, including intricate vocalisations. Recent work has indicated that a number of bat species use courtship songs that are at least equivalent in output and complexity to those of songbirds. Of these, the New Zealand short-tailed bat (Mystacina tuberculata) has one of the highest song-output rates ever recorded. I investigated the social and singing behaviour of M. tuberculata using a variety of field and analytical methods. M. tuberculata are a lek-breeding species, in which males perform sustained courtship songs to attract females. It is likely that there are honest signals of male attributes and resource-holding potential inherent in the display. The aim of my study was to investigate what those signals might be. I used a playback experiment to assess behavioural responses of M. tuberculata to modified courtship song, examining the influence of duty cycle (proportion of time spent singing) on song attractiveness. While the number and duration of bat passes recorded during song playbacks exceeded those recorded during noise or silence, there was no observed effect of duty cycle on song attractiveness, nor was there an effect of playback site or the identity of the singer. To further investigate the possible role of duty cycle, I used temperature telemetry to record energetic expenditure by males during song bouts. The mean energetic cost of singing exceeded the cost of day-roosting normothermy, but I found no correlation between duty cycle and energy expenditure over short time-frames. I then analysed the syntactic arrangement of recorded songs, and discovered that significant patterning exists in the construction of syllables and phrases. Individuals exhibited both high syntactic complexity and considerable variation in song structure. To complement song-based chapters, I analysed roost entrance data to determine whether M. tuberculata form long-term associations – a known indicator of social and communicative complexity. Network analysis revealed that both sexes maintain long-term associations that are independent of roost-preference and can persist for at least 7 years. Aside from courtship song and echolocation, mechanisms of information-transfer in M. tuberculata are unknown, but the persistence of long-term social bonds at maternity colonies implies that these roosts are of high social importance. It further implies a role for non-song communicative modes in relationships between day-roosting individuals.