Meeting Abstract
Torpor (controlled reduction in body temperature and metabolic rate) is a key energy saving strategy for small heterotherms. In hummingbirds, past studies show exclusive use of deep torpor where body temperature declines with ambient temperature down to a species-specific minimum body temperature. Because hummingbirds have among the highest metabolic rates of all vertebrates, and store little body fat during the day, maximizing their energy savings at night by using deep torpor seems an efficient energy management strategy. However, extensive use of deep torpor could have physiological and fitness consequences, and rewarming costs, particularly on cold nights, could reduce net energy savings. Given these potential disadvantages of deep torpor, a shallower form of torpor might be a better energetic strategy in some cases. From a combination of open-flow respirometry measurements and infrared imaging (all under near-natural temperature and light cycles) on 16 hummingbird species in Arizona and the Ecuadorian Andes, we found that some species are capable of using both deep (~30°C surface-temperature depression) and shallow (8-12 °C surface-temperature depression) torpor (sometimes on the same night), while others almost never use deep torpor. When in shallow torpor hummingbirds do not exhibit the immobility that is characteristic of deep torpor, perhaps suggesting that shallow torpor offers some level physiological restoration characteristic of sleep, while continuing to provide some energy savings. Further, the increased responsiveness during shallow torpor could result in more rapid response to potential predators.
