Aggression--from basic science to real life.

Neuropeptide cholecystokinin (CCK) regulates the adaptation of rodents in the novel environment. In the present study we analysed the behavioural changes induced by the individual housing in mice, lacking CCK(2) receptors. The wild-type (+/+) and homozygous (-/-) CCK(2) receptor deficient mice of both gender were used throughout the study. The weight gain during the 21-day isolation period and changes in the locomotor activity following the social separation were measured. The elevated plus-maze and resident/intruder tests were also performed to test alterations in the emotional behaviour. Social isolation induced locomotor hyperactivity, reduced weight gain and increased aggressiveness in the wild-type (+/+) and homozygous (-/-) male mice. In the wild-type (+/+) female mice the significant reduction of exploratory activity in the plus-maze was evident. By contrast, in female mice, lacking CCK(2) receptors, the exploration of the plus-maze was not significantly affected by the individual housing. This finding demonstrates that the social isolation does not cause anxiety-like state in the CCK(2) receptor deficient mice. Moreover, the targeted invalidation of CCK(2) receptors increased in male mice the affinity of dopamine D(2) receptors in the sub-cortical structures, whereas in female mice the increased affinity of 5-hydroxytryptamine(2) (5-HT(2)) receptors in the frontal cortex was established. The increased affinity of 5-HT(2) receptors is probably the compensatory change to the lack of CCK(2) receptors in female mice and probably reflects the reduced sensitivity of these animals to the anxiogenic manipulations. In conclusion, targeted mutation of CCK(2) receptors selectively antagonised the behavioural changes induced by the individual housing in females, but not in male mice.

Aggression↗

PubMed2004

Age at group formation alters behavior and physiology in male but not female CD-1 mice.

In the laboratory environment, rodents are usually housed in unisexual groups, which are assembled after weaning. Housing of unfamiliar subjects has been described, however, as a stressful social setting for rodents and other mammals. Aim of the present study was to evaluate whether the age at which house mice are grouped might affect their behavior and physiology. Male or female unisexual groups were formed at different ages: at weaning, i.e., before puberty (JUV); at adolescence, i.e., after puberty (AD); and controls were raised with siblings since birth (CON). Results show that age at group formation induced several behavioral and physiological alterations in males but not in females. Specifically, when compared to controls, JUV males showed higher aggression, smaller preputial gland, and a marked reduction of neophobia in the free exploratory paradigm. Fewer changes occurred in the AD males, which showed reduced neophobia in the free exploratory paradigm and, when adults, a reduction in body weight. Females were not affected by the experimental treatment. Surprisingly, the basal corticosterone assessed at the nadir was lower for both males and females JUV and AD respect to CON. In conclusion, it is clear that mixing groups at different ages has profound effects on mouse behavior and physiology.

Aggression↗

PubMed2004

Conflict over male parentage in social insects.

Mutual policing is an important mechanism that maintains social harmony in group-living organisms by suppressing the selfish behavior of individuals. In social insects, workers police one another (worker-policing) by preventing individual workers from laying eggs that would otherwise develop into males. Within the framework of Hamilton's rule there are two explanations for worker-policing behavior. First, if worker reproduction is cost-free, worker-policing should occur only where workers are more closely related to queen- than to worker-produced male eggs (relatedness hypothesis). Second, if there are substantial costs to unchecked worker reproduction, worker-policing may occur to counteract these costs and increase colony efficiency (efficiency hypothesis). The first explanation predicts that patterns of the parentage of males (male parentage) are associated with relatedness, whereas the latter does not. We have investigated how male parentage varies with colony kin structure and colony size in 50 species of ants, bees, and wasps in a phylogenetically controlled comparative analysis. Our survey revealed that queens produced the majority of males in most of the species and that workers produced more than half of the males in less than 10% of species. Moreover, we show that male parentage does not vary with relatedness as predicted by the relatedness hypothesis. This indicates that intra- and interspecific variation in male parentage cannot be accounted for by the relatedness hypothesis alone and that increased colony efficiency is an important factor responsible for the evolution of worker-policing. Our study reveals greater harmony and more complex regulation of reproduction in social insect colonies than that expected from simple theoretical expectations based on relatedness only.

Aggression↗