Litter composition influences the development of aggression and behavioural strategy in male Mus domesticus

The differences in behavioural strategies coping with social and environmen tal challenges in male house mice have been reported. In this study, we des cribe the influence of litter composition on the readiness of male mice to engage a social (intruder) and non-social (unfamiliar object/area) challeng e, as well as on their flexibility in behaviour. For this purpose wild-type house mice selected for long attack latency and litter compositions of 1 m ale + 5 females, and 6 males were used. Data obtained during the period in which littermates were housed together (until a subadult age) are referred to as short-term effects. Data obtained in adulthood (at least 4 weeks afte r separation from their siblings) are indicated as long-term effects. The small difference in maternal care between dams caring for 1 + 5- and 6 + 0-litters, had no consequence on the bodyweight development of the pups. However, indications for a slower locomotor development in 1 + 5- than in 6 + 0-males were found. Aggressive interactions within litters in an undisturbed situation were les s frequent in 1+5-litters compared with 6 + 0-litters. An unfamiliar intrud er, however, was faster attacked by 1 + 5- than by 6 + 0-males. In the long -term, there was no significant difference in attack latency towards an unk nown intruder between 1 + 5- and 6 + 0-males. Though, there was within time a remarkable difference in the shortening of attack latencies. It was much more pronounced in 6 + 0- than in 1 + 5-males. The same contradiction between short- and long-term consequences of litter composition on subsequent behaviour was found in the response to challengin g non-social events. In the short-term. 1 + 5-males were more prone to inve stigate an unfamiliar object than 6 + 0-males. In adulthood, however, 1 + 5 -males were much more reluctant to investigate an unfamiliar area than 6 0-males. Several previous studies demonstrated a narrow interrelation between readin ess to engage a challenge and rigidity in behaviour. However, this study sh ows that adult 6 + 0-males, which are very prone to engage a challenge, are more flexible in their behaviour, measured as response to a change in a Y- maze, than the more reluctant 1 + 5-males. It can be concluded that litter composition has a pronounced influence on t he development of aggression and behavioural strategy. Differences in the r ate of behavioural maturation might underly the differences in adult behavi our.