Subspecies-specific response to ACTH challenge test in the house mouse (Mus musculus).

Long term changes in hormonal levels of small and rare animal species can be effectively monitored by non-invasive methods such as immunoenzymatic analysis. These methods generally analyze metabolites instead of the hormone itself and thus do not invade the organism. However, they can be influenced by many factors, so before they are used, they need to be validated. For this purpose we used the "ACTH challenge" test based on stimulating the animal's adrenocortical activity and measuring the consequent increase of the level of its glucocorticoid ("stress") hormones. Specifically, we measured concentrations of fecal glucocorticoid metabolites of two house mouse subspecies, Mus musculus musculus and M. m. domesticus. Using polyclonal antibody we investigated the mice's naturally occurring circadian fluctuation and their reaction to the adrenal stimulation. This study confirmed that the selected method is suitable for analysis of fecal corticosterone metabolites in the wild house mouse. More importantly, we revealed a subspecies-specific stress response at the level of corticosterone production: while the significant effect of ACTH was proved in both subspecies, a notable adrenocortical reaction was also elicited by injecting the saline solution in M. m. domesticus. Our results thus highlight the importance of considering potential cryptic variation within the species under study when designing projects on adrenal stress hormone assessments.

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.