Winter climate affects torpor patterns and roost choice in New Zealand lesser short-tailed bats

Abstract

<p>Variation in winter conditions differentially impacts thermoregulation of endotherms, with some species using torpor as an energy-saving strategy during periods of negative energy balance. We evaluated how differences in energy balance due to winter ambient temperatures (T_a), would lead to differing torpor expression and roost choice between populations. Here, we monitored Ta and skin temperatures (T_sk) of New Zealand lesser short-tailed bats (<i>Mystacina tuberculata</i>) using temperature telemetry from an inland population from Pureora, North Island and another from Hauturu/Little Barrier Island (Hauturu), an offshore island. Relative to Pureora, all recorded Ta variables were higher in Hauturu. Hauturu bats used short torpor (<24 h) more frequently (51% of observational days) than their Pureora conspecifics (33%). Furthermore, minimum T_sk of Hauturu bats correlated positively with mean Ta and Hauturu bats were more likely to arouse on nights with warmer sunset T_a. No correlation was found for Pureora bats. Torpor bout duration did not differ between sites. On Hauturu, bats preferred thermally unstable roosts and preferentially roosted inside dead punga/silver fern-trees (<i>Cyathea dealbata</i>). As punga thermal characteristics did not differ from T_a, Hauturu bats may take advantage of warm T_a to reduce rewarming costs. We did not observe differences in any thermoregulatory variables between sexes in Hauturu. Differences in winter T_a may differentially impact populations across a latitudinal gradient. Although bat populations in warmer climates are under less thermoregulatory stress, site and climate specific adaptations appear to be employed that affect roost choice and torpor patterns.</p>