Meeting Abstract
Controlled nighttime hypothermia (reduction in body temperature and metabolic rate) has been described in many bird and mammal species. In hummingbirds, all past studies show exclusive use of deep torpor where body temperature drops with ambient temperature down to a minimum body temperature that seems to be species-dependent. As hummingbirds normally have among the highest metabolic rates of all vertebrates, maximizing their energy savings at night by maximizing the use of deep torpor seems an efficient energy management strategy. However, two potential disadvantages of deep torpor are higher risk of predation, and less time to gain the restorative advantages of sleep. Indeed, not all hummingbird individuals or species use deep torpor every night. While carrying out torpor measurements using nighttime respirometry experiments, we found evidence to suggest that some hummingbird species use a ‘shallow’ form of controlled hypothermia, where they drop their body temperature by only a few degrees on some nights, and use full torpor on other nights. Use of shallow controlled hypothermia was also supported using thermal imaging at night to record hummingbird surface temperatures. From this combination of respirometry and infrared imaging (all under near-natural conditions) on 16 hummingbird species in Arizona and the Ecuadorian Andes, we found that some species seem capable of using both shallow and deep torpor. If hummingbird torpor is on a continuum with sleep, this could have useful implications for our understanding of how hummingbirds balance the use of controlled hypothermia for energy savings with the need for restorative sleep.
