The Influence of Sex Steroids on the Sexual Size Dimorphism in the Red-Spotted Garter Snake, Thamnophis sirtalis concinnus

Abstract

The red-spotted garter snake exhibits adult size dimorphism in which females are the larger sex. To understand which hormones may influence differential growth in this species, growth curves and hormone profiles of estradiol-17␤ (E 2 ) and testosterone (T) were constructed in male and female neonates. Growth was manipulated via implantation of exogenous hormones and hormone antagonists. Female neonates are heavier or longer beginning at either 20 or 24 weeks of age, respectively. Although low circulating levels of E 2 and T were present in males and females from birth through 15 weeks of age, these levels were not significantly different between the sexes. Differences in the growth curves of the treated and untreated snakes were significant after 24 weeks of age. Antiestrogen produced male-like growth in females but had no effect on males. Antiandrogen had no effect on either males or females. Exogenous T reduced female growth to that observed in males, and E 2 reduced male growth. These results suggest that a basal level of either E 2 or T is sufficient in males to retain typical male growth patterns. Similar endogenous levels of E 2 appear to have growthpromoting effects in females. Endogenous T does not appear to play a role in female growth.

Testosterone has opposite effects on male growth in lizards(Sceloporusspp.) with opposite patterns of sexual size dimorphism

“…elongation) by T in any squamate reptile. Although prenatal exposure to T stimulates postnatal mass gain in the lizard Lacerta vivipara (Uller and Olsson, 2003), every other study involving lizards and snakes has found either no effect or an inhibitory effect of T on skeletal growth or mass gain (Abell, 1998a;Cox et al, 2005;Crews et al, 1985;Hews et al, 1994;Hews and Moore, 1995;Klukowski et al, 1998;Lerner and Mason, 2001;Moore, 1989, 1991;Salvador and Veiga, 2000). While these studies collectively suggest that growth inhibition by T may be prevalent in this group, several caveats beg mention.…”

“…While these studies collectively suggest that growth inhibition by T may be prevalent in this group, several caveats beg mention. First, several of the above studies reported severe mass loss and/or reduced survival among Timplanted animals (Abell, 1998a;Hews et al, 1994;Hews and Moore, 1995;Lerner and Mason, 2001), prompting some authors to acknowledge concerns over pharmacological T levels (Hews et al, 1994, pp. 110-112;Hews and Moore, 1995, p. 99;Lerner and Mason, 2001, p.223).…”

“…Testosterone (T) is commonly regarded as an anabolic steroid that promotes skeletal and muscular growth, but most of the evidence supporting this generalization comes from studies of mammals, fishes and birds with male-larger SSD (Borski et al, 1996; Fennel and Scanes, 1992a;Ford and Klindt, 1989;Gatford et al, 1998;Holloway and Leatherland, 1998;Huggard et al, 1996;Kuwaye et al, 1993;Wehrenberg and Giustina, 1992). Interestingly, T inhibits growth in some reptiles with femalelarger SSD (Abell, 1998a;Cox et al, 2005;Crews et al, 1985;Lerner and Mason, 2001). Collectively, these studies suggest that opposite patterns of SSD may develop in part because of underlying differences in the hormonal regulation of male growth.…”

Neither male gonadal androgens nor female reproductive costs drive development of sexual size dimorphism in lizards

“…Another study on Psammodromus algirus reported that individuals treated with testosterone tended to grow less than control individuals, but found no significant effect, probably due to the small sample size (Salvador and Veiga, 2000). In the snake Thamnophis sirtalis, testosterone also did not have significant effects on structural growth (SVL) in a short-term study with adult males (Crews et al, 1985), while addition of testosterone proved to be lethal in neonatal males of the same species (Lerner and Mason, 2001). Overall, it seems that testosterone supplementation in gonadally intact males can lead to detrimentally high, supra-physiological doses.…”

“…Effects on growth of female gonadal hormones have rarely been tested in squamate reptiles (but see Lerner and Mason, 2001) [for evidence in other vertebrates, see Malison et al (Malison et al, 1988) or Govoni et al (Govoni et al, 2008)], which precludes evaluating whether estrogens or progestins may affect female growth in the opposite direction in female-versus male-larger lizard species [see Cutler (Cutler, 1997) for support for the role of low levels of estrogens in regulation of growth in humans]. Testing this hypothesis will require further experimental work focused specifically on hormonal manipulations in females of male-larger and female-larger species.…”