Male weasels decrease activity and energy expenditure in response to high ambient temperatures.

The heat dissipation limit (HDL) hypothesis suggests that the capacity of endotherms to dissipate body heat may impose constraints on their energy expenditure. Specifically, this hypothesis predicts that endotherms should avoid the detrimental consequences of hyperthermia by lowering their energy expenditure and reducing their activity in response to high ambient temperatures (T(a)). We used an extensive data set on the daily energy expenditure (DEE, n = 27) and the daily activity time (AT, n = 48) of male weasels (Mustela nivalis) during the spring and summer breeding season to test these predictions. We found that T(a) was related in a "hump-shaped" (i.e. convex) manner to AT, DEE, resting metabolic rate (RMR) and metabolic scope (the ratio of DEE to RMR). These results support the HDL hypothesis because in response to warm Tas male weasels reduced their AT, DEE, and RMR. Although the activity and energy expenditure of large endotherms are most likely to be constrained in response to warm Tas because they are less able to dissipate heat, our results suggest that small endotherms may also experience constraints consistent with the HDL hypothesis

Estimates of model coefficients (± standard error) for the effect of body mass, mean ambient temperature (T_a) and habitat type on metabolic scope (DEE/RMR) in male weasels.

<p>Estimates of model coefficients (± standard error) for the effect of body mass, mean ambient temperature (T_a) and habitat type on metabolic scope (DEE/RMR) in male weasels.</p

Effect of ambient temperature on activity time and daily energy expenditures.

<p>The quadratic effect of average daily T_a (ambient temperature) on (A) AT (daily activity time; hours/day) adjusted for the effect of habitat; (B) DEE (daily energy expenditure; kJ/day) adjusted for the effects of body mass and AT, in male weasels. Broken line – fitted using parameters from mixed model (mean response), solid line - fitted by linear quantile regression (for <i>tau</i>  =  0.95).</p

Estimates of model coefficients (± standard error) for significant predictors of AT (daily activity time; hours/day) and DEE (daily energy expenditure; kJ/day) in male weasels, derived from a linear mixed-effects model (random effects were weasel ID and year).

<p>T_a – mean daily ambient temperature (°C).</p

Effect of ambient temperature on resting metabolic rate and metabolic scope.

<p>The quadratic effect of mean T_a (ambient temperature) on (A) RMR (resting metabolic rate; kJ/day) adjusted for the effects of body mass, and (B) metabolic scope (DEE/RMR), in male weasels. Broken line – fitted by linear least-square regression (mean response), solid line - fitted by linear quantile regression (for <i>tau</i>  =  0.95).</p