How to budget metabolic energy: torpor in a small Neotropical mammal.

Neotropical nectar-feeding bats (Glossophaginae) are highly specialized in the exploitation of floral nectar and have one of the highest mass-specific metabolic rates among mammals. Nevertheless, they are distributed throughout the tropics and subtropics over a wide elevational range, and thus encounter many extreme and energetically challenging environmental conditions. Depressing their otherwise high metabolic rate, e.g., in situations of food restriction, might be an important adaptive physiological strategy in these dietary specialists. We investigated the thermoregulatory behavior of captive 10-g nectar feeding bats (Glossophaga soricina; Chiroptera, Phyllostomidae) under variable ambient temperatures (T (a)) and feeding regimes and predicted that bats would use torpor as an energy-conserving behavior under energetic constraints. All tested animals entered torpor in response to energetic restrictions and the depth of torpor was dependent on the body condition of the animals and hence on their degree of physiological constraints. Periods of torpor with body temperatures (T (b)) below 34 degrees C were precisely adjusted to the photoperiod. The median length of diurnal torpor was 11.43 h. The lowest T (b) measured was 21 degrees C at a T (a) of 19 degrees C. Estimated energy savings due to torpor were considerable, with reductions in metabolic rate to as low as 5% of the metabolic rate of normothermic bats at the same T (a). However, contrary to temperate zone bats that also employ diurnal torpor, G. soricina regulated their T (b) to the highest possible levels given the present energetic supplies. To summarize, G. soricina is a precise thermoregulator, which strategically employs thermoregulatory behavior in order to decrease its energy expenditure when under energetic restrictions. This adaptation may play a crucial role in the distribution and the assembly of communities of nectar-feeding bats and may point to a general capacity for torpor in tropical bats.