Emergence and return times in a colonial, cave-dwelling bat: age and sex differences driven by reproductive cycle

The time that bats emerge, and subsequently return, from a colonial roost
determines their maximum foraging period and influences their exposure to
mortality risks. The order in which different age and sex cohorts emerge
and return reflects variation in these cohorts’ resource requirements. The
critically endangered Southern Bent-wing Bat (Miniopterus orianae
bassanii) is an Australian insectivorous cave-roosting colonial bat.
Resource limitation is hypothesised to have contributed to its decline,
but may not affect all cohorts equally. We tagged and monitored 3,462 wild
Southern Bent-wing Bats over seven years with Passive Integrated
Transponder technology. To infer resource requirements of different
cohorts over the reproductive cycle, we estimated cohort-specific peak
emergence and return times and the frequency of nocturnal returns to the
roost. The emergence and return behaviour varied with age, sex, and
throughout the annual reproductive cycle. Although adult females and males
behaved similarly during the non-breeding period (winter), females emerged
significantly (12-21 minutes) earlier and returned (27-62 minutes) later
than males during pregnancy, lactation, and weaning. Adult females were
less likely than males to be detected overnight in the maternity roost
while dependent young were present, suggesting that females prioritised
maximising foraging over nocturnal nursing. When juveniles commenced
flying, they delayed emergence until several hours after sunset (well
after adults had departed the roost). During the 40-day weaning period,
they progressively emerged earlier, such that by the end of this period
they emerged with the adults, then subsequently foraged for longer than
adults over winter. Passive monitoring of emergence and return behaviour
in colonial bats can provide valuable data to infer cohort-specific
resource requirements. Regular monitoring of a population’s emergence and
return times potentially allows for the early detection of changes in
resource requirements, and the use of PIT technology allows for the most
vulnerable cohort(s) to be identified.