Dominance, plasma testosterone levels, and testis size in house mice artificially selected for high activity levels.
Physiology & Behavior 66:in press.
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
Copyright 2002 Elsevier Science, Inc.