Little if any role of male gonadal androgens in ontogeny of sexual dimorphism in body size and cranial casque in chameleons

Abstract

proximate control of the development of sexual dimorphism is still hotly debated in reptiles. in some squamates, many male-typical exaggerated traits including body size were assumed to be controlled by masculinization by male gonadal androgens. We performed a manipulative experiment to test the importance of this mechanism in the development of pronounced sexual differences in body size and size of head casque in the chameleon Chamaeleo calyptratus. castrated males attained maletypical body size highly deviating from the body size of control females. ontogenetic allometries of casque size on head length revealed that sexes depart considerably in casque growth later in the ontogeny; however, castrated males still follow male-typical casque growth. paradoxically, exogenous testosterone led in females to slight increase of casque size, which might reflect interference with the feminizing effects of female gonadal hormones. The results in males strongly suggest that masculinization by male gonadal androgens during growth is not required for the development of sexual dimorphism in body size and casque size in the chameleon. the ontogeny of sexually dimorphic body size and exaggerated traits in at least some squamates is likely controlled by other proximate mechanism, possibly by feminization by ovarian hormones. Body size is one of the most prominent sexually dimorphic traits in animals. Squamate reptiles, an important group of vertebrates with a huge variability in body size, represent no exception. In many lineages of squamates, even closely related species may differ in the magnitude and also in the direction of sexual dimorphism in body size 1-4. Despite several studies, the proximate mechanism allowing this notable evolutionary plasticity in sexual dimorphism in body size is still poorly understood and several hypotheses on the proximate mechanisms responsible for the ontogeny of sexual dimorphism in body size have been suggested for squamates 2-7. The most hotly debated is the male androgen hypothesis suggesting that male growth is masculinized by gonadal androgens. Sex-specific levels of gonadal steroids are generally important for the control of sexually dimorphic traits in vertebrates 8 and the expression of many male-typical morphological, physiological and behavioural traits has been linked to the levels of male gonadal androgens present in squamates (e.g., size of copulatory organs 9 , activity of scent glands 10 , colouration 11 , odour and other sex recognition cues 12 , social behaviour 13,14). Nevertheless, the evidence that the growth of rigid, skeletal structures, for example total structural body size, is solely under the control of male gonadal androgens, is equivocal. Cox et al. 3 presented broader evidence for the bipotential effect of gonadal androgens which assumes that male gonadal androgens masculinize growth with a growth-stimulating effect in male-larger species and a growth-suppressing effect in female-larger species 2,3. This hypothesis was supported mainly by experiments on members of t...

Sex-specific growth arrest in a lizard

“… Taylor and Denardo, 2005 for similar results in an experiment monitoring growth and ontogeny of T levels in a snake). These results agree with the earlier manipulative experiments in this and other species of lizards, where castration with and without T supplementation does not largely influence the final body length in males ( Starostová et al., 2013 ; Kubička et al., 2013 , 2015 ; Bauerová et al., 2020 ). On the other hand, the start of the notable fluctuation in E 2 levels in females and the increase in ovary and oviduct size coincide well with the deceleration of their growth, which supports the experimental evidence that sexual dimorphism in lizards is controlled by ovarian hormones ( Kubička et al., 2017 ).…”

“…According to these studies, male androgens, namely testosterone (T), are responsible for male-typical growth in both male- and female-larger species through their bipotential (either stimulating or suppressive) effect on growth. However, our long-term studies did not support the main role of male gonadal androgens in the development of SSD in squamates: castrated males attained the same final body size in chameleons and both male-larger and female-larger gecko species ( Starostová et al., 2013 ; Kubička et al., 2013 , 2015 ; Bauerová et al., 2020 ). Surprisingly, exogenous T affects growth in females ( Starostová et al., 2013 and citations therein; Kubička et al., 2013 ; Cox et al., 2014 ; Bauerová et al., 2020 ).…”

“…However, our long-term studies did not support the main role of male gonadal androgens in the development of SSD in squamates: castrated males attained the same final body size in chameleons and both male-larger and female-larger gecko species ( Starostová et al., 2013 ; Kubička et al., 2013 , 2015 ; Bauerová et al., 2020 ). Surprisingly, exogenous T affects growth in females ( Starostová et al., 2013 and citations therein; Kubička et al., 2013 ; Cox et al., 2014 ; Bauerová et al., 2020 ). However, T in this case may not have the direct masculinization effect but can cause growth defeminization by interfering with normal levels of female gonadal hormones ( Starostová et al., 2013 ; Kubička et al., 2017 ).…”

“…In addition, we explored ontogeny in the circulating levels of androgens and estrogens and tested whether their ontogenetic changes correlate with the departure of male and female growth trajectories and the activity of growth plates. Previous experimental manipulations suggested that male gonadal androgens do not, while female gonadal hormones do represent sex-specific growth regulators in this species as well as in other reptiles ( Starostová et al., 2013 ; Kubička et al., 2017 ; Bauerová et al., 2020 ). However, the hormonal manipulations often have important side effects (e.g.…”

Development of male-larger sexual size dimorphism in a lizard: IGF1 peak long after sexual maturity overlaps with pronounced growth in males

“…However, most of the support came from relatively short-term studies done under the view that lizards are indeterminate growers, and there was thus no control in these studies on whether the animals already stopped growth or not. The long-term growth experiments comparing growth in castrated and control males do not support the role of male gonadal androgen neither in male-larger species, namely the gecko P. picta ( Starostová et al, 2013 ; Kubička et al, 2015 ) and the chameleon Chamaeleo calyptratus ( Bauerová et al, 2020 ), nor in the female-larger gecko Aeluroscalabotes felinus ( Kubička et al, 2013 ). It was also noted that the increase of testosterone levels in males does not coincide with the period of sexually dimorphic growth in a rattlesnake and the gecko P. picta ( Taylor and DeNardo, 2005 ; Kubička et al, 2022 ).…”