Popular Science: Testosterone does not make big fighters – at least in chameleons?
In most reptiles, sexual dimorphism is more subtle but chameleons with their vibrant colour differences and diverse bone protrusions are the exception to the rule. Horns, cranial casques and other head growths have caught the attention of scientists ever since Charles Darwin, who called them “almost monstrous deviations of structure”. Veiled chameleon (Chamaeleo calyptratus) is a species with particularly huge differences between sexes. While females grow only up to 45 cm, males can be as long as 65 cm and they have big head casques. The bigger the casque the more space for muscle attachment and it can thus provide an advantage in male-male combats.
For this study, experimental chameleons were divided into five groups – two groups of castrated males (one of which was treated with external testosterone), one group of control males and two groups of females (one of which was once again treated with external testosterone and the other one was for control purposes). All the animals were regularly measured with head length and height, height of the casque and leg length being especially carefully monitored.
The results have shown minimum differences in head and leg length between groups of one sex while the differences between sexes were significant. Despite castration, all the males were bigger than females even though one group of females got treated with external testosterone and blood levels of testosterone were comparable between the two groups (males and females) treated with it. The comparison between control males and castrated males was the most important. These two groups achieved comparable results in all measurements and they were all significantly larger than control females even despite comparably low blood levels of testosterone in control females and castrated males.
The outcome of the study supported the initial hypothesis that in chameleons male sex hormones have little influence on the ontogeny of sexual dimorphism. Castrated males reached a male-typical body size and they even grew big head casques. This led the authors to speculate that the ontogeny of sexual dimorphism of these groups of squamates is controlled by female sex hormones produced in ovaries rather than by male hormones from testicles. The hypothesis was indirectly supported by a different study done in chameleons where treating females with external testosterone led to slight growth of their head casque. The testosterone caused shrinkage of the ovaries and thus led to reduced production of sex hormones. The effect of external testosterone in females is therefore an example of defeminization (inhibition of female-typical growth) rather than masculinization (introduction of male-typical growth). The necessity of feminization in the ontogeny of sexual dimorphism was found in other reptiles as well when castrated females lacking female sex hormones evolved in the male-typical growth pattern.
In this study, scientists managed to disprove a hypothesis of the influence of male sex hormones on the ontogeny of sexual dimorphism of Veiled chameleon. Males do not need masculinization by male sex hormones from testicles to grow big and get a large head casque. Male-typical growth can therefore be a baseline from which females depart based on the influence of their sex hormones.
Bauerová, A., Kratochvíl, L. & Kubička, L. Little if any role of male gonadal androgens in ontogeny of sexual dimorphism in body size and cranial casque in chameleons. Sci Rep 10, 2673 (2020). https://doi.org/10.1038/s41598-020-59501-6