Thermoregulation by big brown bats (eptesicus fuscus): ontogeny, proximate mechanisms, and dietary influences

Abstract

I studied the thermoregulatory behaviour of big brown bats (Eptesicus fuscus) of different ages in the field, investigated adaptations associated with thermoregulation (i.e., degree of insulation, clustering behaviour, hair development, and changes in metabolic rate), and examined potential dietary influences (i.e., fatty acid content of milk and handling-time of beetles) on the use of torpor. By extrapolating laboratory measurements of metabolic rate at given skin temperatures and ambient temperatures, I estimated metabolic savings associated with torpor in the field for bats based on skin temperature and ambient temperature measurements alone. On a daily basis, estimated energy expenditure of individual bats in the field was 5-19% lower than if they had remained normothermic over 24 h. Morphological, behavioural, and physiological factors may contribute to energy expenditure or energy savings from the use of torpor. For example, clustering within the roost further reduced expenditure. Laboratory measurements of skin temperature and metabolic rate indicated that clustering by older juveniles and adults reduced metabolic expenditure by up to 52%. Alternatively, by going torpid at low ambient temperatures, metabolic savings up to 98% were realized by bats during metabolic trials. Unlike older juveniles, early prevolant juveniles appear incapable of staying warm when their mothers are away from the roost during foraging. The body of E. fuscus was fully furred by 7 days of age, coinciding with fully developed thermoregulatory capability. Because early prevolant juveniles did not maintain normothermic body temperatures at low ambient temperatures when stimulated to do so, it appears as though they are physiologically incapable of staying warm, rather than being capable of staying warm but choosing not to. Early prevolant juveniles may not realize any energetic benefits associated with clustering, and it may be more beneficial for them to roost alone. Changes in milk composition during lactation corresponded to changes in thermoregulatory ability of juvenile E. fuscus. Handling time of beetles by volant juveniles was significantly different from that of adults, corresponding with a reduction in the use of torpor by early volant juveniles. These data suggest that diet may play a significant role in limiting use of torpor.