No abstract

Peripheral immune responsiveness in male laboratory mice was reduced by infection with the trichostrongyloid nematode Heligmosomoides polygyrus. Responsiveness was also lower among high-ranking (aggressive) males regardless of infection status. Reduced responsiveness in both infected animals and high rankers was associated with elevated serum corticosterone concentration (a potential immunodepressant) and was compounded among high-ranking males by subsequent high aggressiveness. As in previous experiments, only low rankers modulated testosterone secretion in relation to current immunocompetence and corticosterone concentration. The lack of any downregulation of aggression in response to parasite-induced immunodepression contrasted with previous results using antithymocyte serum and may be due to the more localized nature of immunodepression during H. polygyrus infection. However, the additional increase in corticosterone concentration resulting from exposure to female odour and destabilized aggressive social relationships did result in downregulation of aggression among high rankers and of testosterone among mice generally, suggesting that modulation rules of thumb are at least partly dependent on the proximate cues associated with immunodepression.

Social status in randomly constituted groups of male CFLP mice was predictable from early suckling behaviour and rate of weight gain in natal litters. High-ranking males were those that had suckled on more anterior teats and gained weight more quickly. Rank was not predicted by any measures of sibling interaction or hormone (testosterone, corticosterone) concentration. Aggressiveness in eventual high-rankers was associated negatively with the proportion of males in the litter at birth and the amount of maternal attention received. Aggressive social relationships within natal litters did not predict polarized rank relationships in randomized groups. Nevertheless, while still in their natal litters, and in the absence of aggressive rank relationships, eventual rank categories showed the same difference in modulation of testosterone concentration in relation to current immunocompetence (low-rankers modulating, high-rankers not), as has repeatedly been found in randomized groups by earlier studies. The role of maternal condition in determining rank-related life-history development in male mice is discussed.

In a previous study, male laboratory mice experimentally immunodepressed with anti-thymocyte serum (ATS) showed changes in behaviour (aggression, general locomotory activity, and sleeping) and testosterone that are consistent with decision-making being modulated adaptively with respect to immunocompetence. We tested this idea further by repeating the experiment with the addition of female odours (soiled sawdust) to the home cages of males following ATS/control treatment. We predicted that, in the presence of cues suggesting reproductive opportunity, immunodepressed males would trade off potential immunity costs by failing to modulate behaviour. This expectation was borne out in that ATS-treated mice showed no change in aggression, locomotory activity, mounting, or sleeping relative to control animals, and mice overall showed differences in behaviour in the expected direction compared with a previous study in which female odours were not presented. However, despite the lack of difference in behaviour between ATS and control treatments, there was still evidence of a degree of behavioural modulation in relation to measures of immunocompetence.

. Modulation of behaviour and testosterone concentration in immunodepressed male laboratory mice (Mus musculus). PHYSIOL BEHAV 61(6) 907-917, 1997.-Recent ideas suggest that current immunocompetence may act as a constraint on behavioural and physiological decisions, where these risk imposing an additional burden on immune function. We tested this in the context of time budgeting and the secretion of the potentially immunodepressive hormones testosterone and corticosterone, by treating adult male CFLP laboratory mice with antithymocyte serum (ATS) to depress thymus-mediated immune function. In comparison with males given a naive rabbit serum (NRS) vehicle control, ATS-treated mice showed a reduction in serum testosterone concentration, aggressive behaviour, and general activity, and maintained time spent sleeping, relative to pretreatment levels. Behaviours that differed between treatments correlated with measures of immunodepression (reduction in relative thymus weight or serum total IgG concentration), but relationships with behavioural changes were independent of those with testosterone. There was little evidence that changes were affected by social status. The results are discussed in the context of the adaptive modulation of immune function and physiological and behavioural decision-making. ᭧ 1997 Elsevier Science Inc.