Ever since Darwin published The descent of man, and selection in relation to sex, people have been puzzled about the forces that drive sexual differences in body size. But as Robert Cox and Henry John-Alder point out in this issue of The Journal of Experimental Biology `relatively little is known about the proximate physiological mechanisms underlying sex differences in growth'. While males are large and females small in many vertebrate species, a few species buck this trend; female Sceloporus virgatus lizards are larger than their males. Knowing that testosterone promotes growth in many species, the team suspected that testosterone actually inhibits growth in S. virgatus males. Cox and John-Alder decided to test the effects of testosterone on S. virgatus males and compare the steroid hormone's effects on Sceloporus jarrovii lizards, whose males are larger then their females(p. 4679).

Collecting juvenile males of both species from the Chiricahua Mountains in Arizona, the team divided both species into three further groups; one that was castrated and received a testosterone implant, another that was castrated and received a placebo implant and a control group that was not castrated and received a placebo implant. Having measured each male's length, the team released the males back into the wild, leaving them for just over 40 days before recapturing them again.

Sure enough, the castrated S. virgatus males that didn't receive testosterone were larger than the males that were exposed to testosterone. On the other hand, the castrated S. jarrovii males that carried placebo implants were smaller than the castrated males whose implants carried testosterone. Testosterone inhibits growth in S. virgatus while stimulating it in S. jarrovii males.

But why does testosterone have the opposite effect on S. virgatusmales than S. jarrovii males? Cox and John-Alder suspect that reproduction may be an energetically costly process for the S. virgatus males, so they may have traded a smaller stature for their higher reproductive costs.

Cox, R. M. and John-Alder, H. B. (
). Testosterone has opposite effects on male growth in lizards(Sceloporus spp.) with opposite patterns of sexual size dimorphism.
J. Exp. Biol.