Energetics of migratory bats during stopover: a test of the torpor-assisted migration hypothesis

The torpor-assisted migration hypothesis posits that migration is facilitated in bats by the use of torpor during stopover roosting periods, and predicts that at stopover, bats regulate time in torpor facultatively so that daily energy expenditure is independent of ambient roosting temperature. Energy savings can thus be directed to migratory movement. However, direct measurements of total roosting energy expenditure in relation to ambient and body temperature are lacking. We captured migratory silver-haired bats (Lasionycteris noctivagans; ∼11 g) at Long Point, ON, Canada, in spring and autumn 2016. We used quantitative magnetic resonance analysis to measure body composition change and energy expenditure over a 12 h roosting period in a ventilated incubator at 10, 17 and 25°C. We assessed the effects of season, body mass, sex and age on energy expenditure. We found that daily energy expenditure was independent of roosting temperature, and that this was achieved by flexible use of torpor. Variation in body mass at capture was driven mainly by differences in fat, and the amount of body fat was negatively related to torpor use, particularly in spring. Season, sex and age also affected torpor use and energy expenditure, notably with pregnant females being generally fatter and using less torpor than males in spring. We estimate that stopover contributes only 15-20% to the total energy costs of migration in bats compared with 70% or more in typical birds. This study provides support for the torpor-assisted migration hypothesis, and furthers our understanding of the energy budgets of migratory bats.