Summary
We studied house mice (Mus domesticus) that had been artificially
selected for high activity to test the hypothesis that a high capacity
for energy assimilation in cold-exposed endotherms could evolve as a correlated
response to selection for
increased locomotor activity. After 10 generations of selection for
increased voluntary wheel-running, mice from four selected lines ran 75
% more wheel revolutions per day than did mice from four random-bred, control
lines. The maximum cold-induced rates of food consumption (Cmax;
mean 10.6 g day-1) and energy assimilation (Amax;
mean 141 kJ day-1) were not significantly higher in the selected
than in the control mice. However, in cold-exposure trials, mice from the
selected lines maintained body mass better than did mice from the control
lines. Cmax and Amax were positively
correlated with the amount of wheel-running activity measured before cold-exposure
and also with the rates of food consumption measured when the mice had
access to running wheels. In females at least, the correlation was significant
not only among individuals but also among adjusted means of the replicate
lines, which suggests the presence of a positive genetic correlation between
the traits. Thus, despite the lack of a significant difference between
the selected and control lines in maximum rate of food consumption, the
remaining results conform to the hypothesis that a selection for increased
locomotor activity could be a factor behind the evolution of the ability
to sustain activity and maintain energy balance during prolonged cold-exposure,
as occurred during the evolution of mammalian and avian endothermy.
Copyright 2001 the Society for Experimental Biology