Meeting Abstract
Torpor is an adaptive mechanism that utilizes short-term hypothermia to reduce daily energy costs via a reduction in metabolic rate. Hummingbirds are well known for their ability to enter a state of deep torpor, in which body temperature (Tb) can decrease by >30°C. Tb during torpor drops passively so that the depth of torpor is limited by nighttime ambient temperature (Ta). Climate change has caused nighttime temperatures to increase more quickly than daytime temperatures, which could reduce energy savings during torpor. Our recent nighttime metabolic data for hummingbirds has suggested that some species use regulated shallow hypothermia. Regulated shallow hypothermia, where Tb drops only a few degrees, is commonly used by many birds and mammals, and allows for some rest-phase energy savings without the physiological or ecological consequences that can occur with deep torpor. Because regulated shallow hypothermia does not require costly rewarming, its use on warm nights could result in meaningful energy savings. We used infrared thermography to track overnight changes in skin surface temperature (Ts) in blue-throated (Lampornis clemenciae; 8.0g), Rivoli’s (Eugenes fulgens; 7.5g), and black-chinned (Archilocus alexandri; 3.0g) hummingbirds in the Chiricahua Mts., SE Arizona. Deep torpor was used by 58% of all birds (n=12), with one bird (a Rivoli’s) using regulated shallow hypothermia for 6 hours. During this bout of shallow hypothermia, Ts only decreased ~8 °C below normothermic Tb, and was regulated 13-15°C above Ta. The fact that one bird used regulated shallow hypothermia may suggest that hummingbirds can regulate body temperature above their minimum set point for deep torpor. Shallow hypothermia could be a useful strategy for saving energy on the warming nights resulting from climate change.
