Relationships between dominance, testosterone level and scent marking of males in a free-living root vole (Microtus oeconomus) population

Manifestations of domination: Assessments of social dominance in rodents

Social hierarchies are ubiquitous features of virtually all animal groups. The varying social ranks of members within these groups have profound effects on both physical and emotional health, with lower-ranked individuals typically being the most adversely affected by their respective ranks. Thus, reliable measures of social dominance in preclinical rodent models are necessary to better understand the effects of an individual's social rank on other behaviors and physiological processes. In this review, we outline the primary methodologies used to assess social dominance in various rodent species: those that are based on analyses of agonistic behaviors, and those that are based on resource competition. In synthesizing this review, we conclude that assays based on resource competition may be better suited to characterize social dominance in a wider variety of rodent species and strains, and in both males and females. Lastly, albeit expectedly, we demonstrate that similarly to many other areas of preclinical research, studies incorporating female subjects are lacking in comparison to those using males. These findings emphasize the need for an increased number of studies assessing social dominance in females to form a more comprehensive understanding of this behavioral phenomenon.

Female choice, male dominance and condition-related traits in the polygynous subterranean rodent Ctenomys talarum

Ctenomys talarum is a solitary and highly territorial species in which dominant males aggressively deter other males and monopolize reproductive activity. Female preference for dominant males is not easy to assess due to coercive mating by males. Hence, we aimed to answer if behavioural dominance and several condition-related traits like testosterone and cortisol levels, endoparasite load, and hematocrit volume may affect female preference when they have the opportunity to exert it. We designed a laboratory experiment using wild-caught C. talarum and employed a combined approach involving behavioural observations and the measurements of parameters of physical condition. We staged dyadic encounters between males to determine dominance index and then analyzed female preference towards tethered males (n=15) or their odours (n=18). Male dominance did not affect female preferences when odours were presented. When two tethered males were offered, females preferred those with higher dominance index. Preference of females for dominant males would mainly represent indirect benefits. Females did not show preference for males in relation to any physiological trait evaluated. Dominance was found negatively related to cortisol levels, probably avoiding the glucocorticoids-related costs, and positively related to parasite diversity, since they could tolerate it without impairing their health.

Shaking the myth: Body mass, aggression, steroid hormones, and social dominance in wild house mouse

In social mammals, the position of a male in the group's hierarchy strongly affects his reproductive success. Since a high social rank is often gained through competition with other males, selection should favour bigger males over smaller ones. We may therefore predict faster growth and/or delayed sexual maturity in dominant males. Likewise, dominants should have higher levels of testosterone, hormone important in many aspects of male dominance. Less obvious is the relationship between dominance and levels of corticosterone but generally higher concentrations are expected in subordinate individuals. We studied body growth, sexual maturation and endocrinal changes in males of two house mouse subspecies, raised in fraternal pairs. Since Mus musculus domesticus is the subspecies which dominates mutual encounters with Mus musculus musculus we predicted higher growth rate, delayed puberty and aggression, and higher testosterone and corticosterone levels in domesticus males compared to musculus. In all comparisons, no differences were found between dominant and subordinate musculus brothers. On the other hand, in M. m. domesticus, dominant males revealed a different growth trajectory and lower corticosterone levels than subordinate males but not delayed puberty and higher testosterone concentrations, thus contradicting our predictions. In inter-subspecific comparisons, musculus males matured earlier but became aggressive at the same time as domesticus males. The musculus testosterone ontogeny suggests that social positions in this subspecies remain unfixed for an extended period and that the increasing levels probably reflect prolonged hierarchy contests. It appears that the ontogeny of behaviour and physiological traits diverge cryptically between the two subspecies.

Differences in forebrain androgen receptor expression in winners and losers of male anole aggressive interactions

Size matched male green anoles (Anolis carolinensis) were paired in a neutral setting and allowed to engage in aggressive displays. Winners and losers were apparent in each pair within 90min, resulting in stable dominant/subordinate dyads. Androgen receptor (AR) expression was assessed at three time points after the initial pairing, 2h, 3 days, and 10 days in dominants, subordinates, and two groups of control males housed alone or with a female for an equal period of time. Expression was quantified in three forebrain areas that have been implicated in aggression and reproductive social behavior in this species, the preoptic area (POA), the anterior hypothalamus (AH), septal area (SEP), and ventromedial nucleus of the posterior division of the dorsal ventricular ridge (PDVRVM ). There were significant overall group differences in AR mRNA expression in the POA and AH that appeared to result from higher POA AR expression in dominant males compared to other groups, and generally lower AR expression in subordinate males. Pairwise comparison revealed that dominants' AR mRNA expression in the POA was significantly higher in the 2h and 3 day groups compared to that of subordinates, with a similar, but nonsignificant, difference in the 10 day group. Dominants had significantly higher AR mRNA expression in the AH compared to that of subordinates in the 2h group, but differences were not significant at later times. The results suggest that POA and AH sensitivity to androgens is increased in dominants compared to subordinates, and that the difference can be seen soon after the agonistic interaction establishing winners and losers.