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First published online December 2, 2005
Journal of Experimental Biology 208, 4679-4687 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01948
Testosterone has opposite effects on male growth in lizards (Sceloporus spp.) with opposite patterns of sexual size dimorphism
1 Graduate Program in Ecology and Evolution
2 Department of Animal Sciences, Rutgers University, New Brunswick, NJ
08901, USA
* Author for correspondence at present address: The Ohio State University, Department of Evolution, Ecology and Organismal Biology, Columbus, OH 43210, USA (e-mail: cox.541{at}osu.edu)
Accepted 20 October 2005
Sexual size dimorphism (SSD) has received considerable attention from evolutionary biologists, but relatively little is known about the physiological mechanisms underlying sex differences in growth that lead to SSD. Testosterone (T) stimulates growth in many male-larger vertebrates, but inhibits growth in the female-larger lizard Sceloporus undulatus. Thus, opposite patterns of SSD may develop in part because of underlying differences in the hormonal regulation of male growth. In the present study, we examined the effects of T on male growth in two sympatric congeners with opposite patterns of SSD (S. virgatus: female-larger; S. jarrovii: male-larger). During the mating season, yearling males of both species have higher plasma T levels than females, but whereas yearling males of S. virgatus grow only half as fast as females, yearling males of S. jarrovii grow more quickly than females. Thus, we hypothesized that T inhibits growth in yearling S. virgatus males, but promotes growth in yearling S. jarrovii males. In support of this hypothesis, we found that castrated (CAST) males of S. virgatus grew faster than castrated males given T implants (TEST). In contrast, TEST males of S. jarrovii grew faster than CAST males. Our results provide the first direct evidence for opposite effects of T on male growth in closely related species with opposite patterns of SSD. We speculate that growth inhibition by T reflects an energetic trade-off between growth and reproductive investment, and propose that such `costs' of male reproduction may help explain the evolution of female-larger SSD in Sceloporus.
Key words: body size, growth, hormone manipulation, proximate mechanism, reproductive investment, sexual size dimorphism, Sceloporus jarrovii, Sceloporus virgatus, testosterone
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