title: Effects of testosterone on contractile properties of sexually dimorphic forelimb muscles in male bullfrogs (Rana catesbeiana, Shaw 1802).
abstract: This study examined the effects of testosterone (T) on the contractile properties of two sexually dimorphic forelimb muscles and one non-dimorphic muscle in male bullfrogs (Rana catesbeiana, Shaw 1802). The dimorphic muscles in castrated males with testosterone replacement (T+) achieved higher forces and lower fatigability than did castrated males without replaced testosterone (T0 males), but the magnitude of the differences was low and many of the pair-wise comparisons of each muscle property were not statistically significant. However, when taken as a whole, the means of seven contractile properties varied in the directions expected of masculine values in T+ animals in the sexually dimorphic muscles. Moreover, these data, compared with previous data on male and female bullfrogs, show that values for T+ males are similar to normal males and are significantly different from females. The T0 males tended to be intermediate in character between T+ males and females, generally retaining masculine values. This suggests that the exposure of young males to T in their first breeding season produces a masculinizing effect on the sexually dimorphic muscles that is not reversed between breeding seasons when T levels are low. The relatively minor differences in contractile properties between T+ and T0 males may indicate that as circulating T levels rise during breeding season in normal males, contractile properties can be enhanced rapidly to maximal functional levels for breeding success.
authors.affiliation: Department of Biology, The University of North Carolina at Charlotte , Charlotte, NC 28223 , USA.
authors.forename: Aaron R
authors.initials: AR
authors.lastname: Kampe
authors.forename: Susan E
authors.initials: SE
authors.lastname: Peters
keywords: Bullfrog; Forelimb muscles; Sexual dimorphism; Testosterone