Abstract
Male house mice (Mus domesticus) from four
replicate lines selectively bred for high voluntary wheel-running behavior were
compared with four random-bred control lines with respect to dominance, testis
size, and plasma testosterone level. Behavior was measured with a tube apparatus
in which focal mice encountered a standard opponent from an inbred strain, and
positions of mice were scored over a 10-min period; the test was replicated the
following day. Blood samples were taken from undisturbed mice 1 week prior to
testing (baseline condition) and immediately after the first tube test; plasma
testosterone was measured by enzyme immunoassay with chromatography. As compared
with control lines, mice from selected lines tended to be smaller in body mass,
to have larger testes, and were significantly less likely to advance towards
their opponent during the second tube-test encounter. However, no significant
differences in either baseline or postencounter testosterone levels were
detected. Significant differences in body mass, relative testis size, position
during the first tube-test encounter, and baseline testosterone were found among
the replicate lines within linetype, which indicates founder effects, random
genetic drift, unique mutations, and/or multiple responses to selection. At the
level of individual variation (residuals from nested analysis of covariance
models), an inverse relationship between baseline testosterone and advancing in
the tube test was observed, and the relationship was stronger during the second
test day. This unexpected result may reflect an alternate coping strategy.
Copyright 2002 Elsevier Science, Inc.