Causes and Consequences of Tree-Cavity Roosting in a Temperate Bat, Chalinolobus tuberculatus, from New Zealand

Abstract Among the 79 taxa of Microchiroptera in Australasia, frequency of treecavity roosting increases as mean annual temperature decreases and latitude increases. This gradient suggests there may be significant thermal benefits to tree-cavity roosting in cold climates. We explore the causes and consequences of tree-cavity roosting during summer months in Chalinolobus tuberculatus, a species that occurs at the southern limit (highest latitude) of this gradient. Five geographically distinct populations are compared. C. tuberculatus selected the oldest and largest trees for maternity roosting and avoided roosting under bark and in caves and buildings, despite the abundance of these sites. It also selected small, well-insulated cavities that accrue significant energy conservation benefits compared with other potential roosts (the ‘‘thermal hypothesis’’). Reproductive females selected roosts that reach maximum temperatures late in the day and retain high temperatures through the night, thus benefiting nonvolant young. Productivity and survival were significantly higher in populations that selected well-insulated roosts. We propose that selection favors smaller, rather than larger, roosting group sizes in this cold, temperate climate. Smaller groups of bats that use relatively small, well-insulated cavities have higher survival rates than larger groups that use larger, less insulated cavities. C. tuberculatus formed behaviorally, though not geographically, isolated subgroups. All colonies exhibited extreme roost-site lability on a daily basis, but strong long-term philopatry among pools exceeding 100 roosts. Most roosts were used once per year but date of reuse was similar each year. Strict temporal philopatry suggests that bats do not switch roosts in response to daily variability in weather conditions. The thermal hypothesis suggests that development of grouping behavior may be an incidental response to physiological constraints on thermoregulation and reproduction. Nevertheless, social interdependence would increase the probability that clusters are large enough on any one day to be thermally beneficial and individuals could improve the reproductive success of other relatives within the group. We conclude by outlining hypotheses that could test the general applicability of findings to tree-cavity roosting bats.