Meeting Abstract

P3-130    Is Surface Body Temperature a Good Indicator of Metabolic State in Hummingbirds during Torpor? Blackwell, ER*; Powers, DR; Shankar, A; George Fox University, Newberg, OR; George Fox University, Newberg, OR; Lab of Ornithology, Cornell University, Ithaca, NY eblackwell19@georgefox.edu http://www.dpowerslab.com

Hummingbirds are well known for their use of torpor to conserve energy at night. In deep torpor hummingbirds lower their body temperature (T_b) to just above ambient temperature (T_a) and decrease their metabolic rate to an extremely low level. Skin surface temperature (T_s) has been used as a proxy for T_b to track nighttime metabolic state in hummingbird torpor studies. However, the link between T_s and metabolic rate during torpor has not been directly measured. In this study we made simultaneous measurements of T_s using infrared thermography and nighttime metabolic rate using open-flow respirometry in Anna’s hummingbirds (Calypte anna, 4.5 g) subject to natural light and T_a cycles to determine if T_s can indeed be used to track metabolic rate in hummingbirds. We predicted that T_s would be a good proxy for metabolic state and could be used to determine when hummingbirds transition into torpor and when deep torpor occurs. Some hummingbirds we monitored used torpor and some stayed normothermic throughout the night. Torpid T_s stayed 1-3°C above T_a, while normothermic T_s was 32-37°C regardless of T_a. We found that T_s and metabolic rate closely track each other, changing in a parallel manner over the entire duration of our experiments across metabolic states. Transition into torpor occurred over a 20-30 minute time period, regardless of T_a or time of night. Minimum T_s during deep torpor varied depending on T_a while minimum metabolic rate was temperature independent, potentially indicating a decoupling between metabolic rate and T_b. The ability to use T_s as a proxy for T_b is important because infrared imaging of hummingbird body surfaces is far less invasive than any protocol for directly measuring T_b or for measuring metabolic rates, and therefore perhaps results in torpor measurements that better reflect those of free-living hummingbirds.