Postisolation aggression and olfactory cues

Olfactory contacts mediate plasticity in male aggression with variable male density

Males typically adjust their reproductive strategies based on the perceived density and relative abilities of nearby competitors. In high-density populations, repeated encounters facilitate reliable, learned associations between individuals and their relative competitive abilities. In contrast, opportunities to form such associations are limited when densities are low or in flux, increasing the risk that individuals will unintentionally engage in potentially costly interactions with higher-quality or aggressive opponents. To maximize their fitness, individuals in low-density and fluctuating populations therefore need a general way to assess their current social environment, and thus their relative competitive ability. Here, we investigate how olfactory social signals (scent marks) might perform this function. We manipulated the perceived social environment of isolated, male house mice ( Mus domesticus ) via their periodic contact with scent marks from 3 or 9 male conspecifics, or a control of no scents, over 15 days. We then paired them with an unknown opponent and examined how the diversity of recent scent contact mediated their behavior towards dominant or subordinate opponents. There was an overall pattern for increasing scent diversity to significantly reduce male mice’s aggression (tail rattling and lunging) towards their opponents, and also their willingness to engage in reciprocal investigation. Such cautiousness was not indicative of perceived subordinance, however; the diversity of recent scent contact did not affect mice’s investigation of their opponent’s scents, and some measures of aggression were greater when mice faced dominant opponents. These results suggest that house mice can use scent signals to assess their current social environment in the absence of physical interactions, modifying their behavior in ways that are predicted to reduce their risks of injury when the likelihood of encountering unknown opponents increases.

Stress adaptation and adrenal activity in isolated and crowded rats

Socially reared male control rats (Co) were compared with crowded reared rats (Cr) and isolated reared rats (I). We assayed the adaptation to moderate stress (activity and defecation rate in open-field test) and to intensive stress sensitivity to restraint ulcer), so as the basal adrenal activity measuring basal values of corticosterone (R.I.A.) and fresh weight of adrenal and thymus glands. Results show a great lack of adaptation to moderate and intensive stress in Cr and I reared rats which leads to a decrease of activity variables, an increase of defecation rates and a high sensitivity to restraint ulcers. The effect of crowding conditions show higher values than isolation. We also found, in isolated and crowded reared rats, an increase in the adrenal function with great values for the basal secretion of corticosterone and atrophy of the thymus gland, besides which, crowded reared animals showed adrenal hypertrophy.

Development of responsiveness to conspecific scent in house mice: effects of preweaning and postweaning experiences

Responses to male conspecific odors (soiled bedding) presented in an alleyway were compared among five groups of adult male albino house mice with different rearing histories. Both isolated mice that had as pups been fostered onto a rat mother and normally nursed mice that had been group housed since weaning showed reduced attraction to conspecific odor. Preweaning paternal presence, a factor known to affect the development of response to male conspecific scent in rats and gerbils, had no reliable effect with mice.

The effect of testosterone and cyproterone acetate on the concentration of gamma-aminobutyric acid in brain areas of aggressive and non-aggressive mice

Mice which had been housed in groups were introduced into the home cages of isolated mice and the aggressive defensive behaviour assessed. The grouped mice were chronically treated with testosterone, cyproterone acetate or arachis oil. The concentration of GABA was determined in 10 regions following the last behavioural assessment. The grouped mice which were treated with testosterone or with the anti-androgen, cyproterone acetate, were subjected to fewer aggressive attacks by the isolated mice thatn the grouped controls. The concentration of GABA in particular brain areas appears to depend on the degree of aggressive behaviour exhibited, the type of environment into which the mouse is introduced, the hormone treatment, and the housing conditions.