Sex and species differences in plasma testosterone and in counts of androgen receptor-positive cells in key brain regions of Sceloporus lizard species that differ in aggression

Abstract

We studied neuroendocrine correlates of aggression differences in adults of two Sceloporus lizard species. These species differ in the degree of sex difference in aggressive color signals (belly patches) and in aggression: S. undulatus (males blue, high aggression; females white, low aggression) and S. virgatus (both sexes white, lower aggression). We measured plasma testosterone and counted cells expressing androgen receptor-like immunoreactivity to the affinity-purified polyclonal AR antibody, PG-21, in three brain regions of breeding season adults. Male S. undulatus had the highest mean plasma testosterone and differed significantly from conspecific females. In contrast, there was no sex difference in plasma testosterone concentrations in S. virgatus. Male S. undulatus also had the highest mean number of AR-positive cells in the preoptic area: the sexes differed in S. undulatus but not in S. virgatus, and females of the two species did not differ. In the ventral medial hypothalamus, S. undulatus males had higher mean AR cell counts compared to females, but again there was no sex difference in S. virgatus. In the habenula, a control brain region, the sexes did not differ, and although the sex by species interaction significant was not significant, there was a trend (p = 0.050) for S. virgatus to have higher mean AR cell counts than S. undulatus. Thus hypothalamic AR cell counts paralleled sex and species differences in aggression, as did mean plasma testosterone levels in these breeding-season animals.

Sex, androgens, and whole-organism performance in an Australian lizard

“…Plasma androgens did not differ between male and female E. quoyii, although there was substantial variation within each sex. A lack of androgen differences between the sexes has been observed in other taxa (Ketterson et al, 2005), including lizards (Hews et al, 2012). Ketterson et al (2005), provides evidence that testosterone is higher in females of sexually monomorphic bird species, and, although the data in lizards are limited, comparisons between two Sceloporus species that differ in female aggression and degree of sexual dimorphism have revealed similar findings (Hews et al, 2012).…”

Sexual selection predicts brain structure in dragon lizards

“…In a pairwise comparison between two sister species of voles ( Microtus ) in which one species is monogamous and the other is promiscuous, the MPON was sexually dimorphic only in the species under strong sexual selection (Shapiro et al ., ). In a similar comparison between two fence lizards ( Sceloporus ), only the species under strong sexual selection had sexually dimorphic aromatase‐expressing cell counts in the MPON and VMN (Hews et al ., ). Based on these studies, we propose that strong sexual selection increases sexual dimorphism in brain regions that control sexual activity, as these regions have a direct impact on reproductive success, the ultimate target of sexual selection.…”

Sex hormones and the development of sexual size dimorphism: dihydrotestosterone inhibits growth in a female-larger lizard (Sceloporus undulatus)

“…In S. undulatus, sex differences in growth and color development between males and females are likely through sex differences in circulating androgen levels, similar to what occurs in A. sagrei, but sexual dimorphisms may also arise in part through sex differences in androgen-receptor densities. Hews et al (2012) and Moga et al (2000), for example, reported that while both male and female S. undulatus had androgen receptors present in regions of the brain, males had higher receptor densities and higher circulating testosterone than their female counterparts.…”