The reproductive benefits of maintaining a high body temperature (Tb) are well understood, but costs may be prohibitively high. Many small mammals raise offspring in insulated nests, burrows, or roosts, and may form communal maternity colonies, all of which are behaviors that reduce the costs of maintaining Tb. However, some temperate-zone bats are solitary and raise young in exposed roosts. Little is known about how these species maintain energy balance in the face of high thermoregulatory costs, and whether they stay warm invariably for reproductive benefits, or use a more adaptive thermoregulatory strategy. We studied Tb patterns in response to foraging conditions, weather, and pup age in free-ranging adult and juvenile hoary bats (Lasiurus cinereus). Lactating females regularly entered torpor, although its use was most extensive following poor foraging conditions, during inclement weather, and when pups were young. Juvenile L. cinereus appeared capable of staying warm from 3 days of age, but used torpor throughout development, gradually decreasing its use as they grew. Our results indicate that for this species, torpor is important throughout lactation, and torpor use changes in response to individually specific trade-offs between energetic cost and developmental benefit. We suggest that the costs of active thermoregulation during early development of L. cinereus pups outweigh the benefits of staying warm, and an extended growing season (as a result of their migratory nature) is more conducive to using torpor. Finally, we suggest that torpor use during lactation is dynamic and adaptive. Fine-scale changes within reproductive stages need to be considered, rather than broadly assessing thermoregulatory behavior among stages.
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