CHAPPELL , M.A.*; REZENDE, E.L.; GOMES, F.; Univ. of California, Riverside; Univ. of California, Riverside; Univ. of California, Riverside: Energetics of voluntary running in deer mice

Energy costs of locomotion in small mammals have been extensively studied, but the cost versus speed relationship is known only for forced exercise and it is unclear if the energetics of voluntary running differ. We used deer mice (Peromyscus maniculatus) exercising voluntarily in a running wheel respirometer to ask: (1) are energy costs of voluntary running similar to those of forced running? (2) do voluntarily-attained speeds approach or exceed the maximum aerobic speed? (3) do mice running voluntarily utilize a wide range of speeds, or are certain speeds preferred? (4) does variation in speed or distance correlate with variation in resting and exercise metabolism? (5) how are voluntary running energetics affected by temperature? Most deer mice readily utilized the wheels and ran up to 19 km/night, with considerable variation in distance, running time, and running speeds. The relationship between speed and metabolism in voluntary running seems to be different from that of forced running, but all voluntary running was at metabolic rates below maximal aerobic capacity. We found positive correlations between cumulative distance and both running speed and metabolism (i.e., faster runners with higher exercise metabolism covered more distance than slower mice), but there was no relationship between resting metabolism and either cumulative distance or maximal exercise metabolism. Ambient temperature (Ta; 3 �C, 10 �C, and 24-28 �C) did not affect either mean or maximal running speeds. Deer mice partially substitute exercise heat for thermostatic heat production when running voluntarily: at low Ta, slopes for speed versus metabolism regressions were lower and intercepts were higher than for near-thermoneutral temperatures.