Es Paul

Litter composition affects parental care, offspring growth and the development of aggressive behaviour in wild house mice

Male mice pups reared with female sibings only are known to develop into more aggressive adults than males reared with males only. The purpose of this study was to examine whether differences in experience of sibling competition and/or parental behaviour might underlie these differences in adult behaviour. The parental care and development of eight litters containing four male pups (MM litters) and eight litters containing one male and three females (MF litters) was compared. As expected, MF males developed into faster attackers than MM males. A variety of measures of sibling competitive and aggressive behaviour, both in the home cage and during tests of competition for food, failed to provide any evidence that experience of success in competitive encounters played a prominent role in the development of individual differences in subsequent aggressiveness. On the other hand, clear differences in parental behaviour and offspring growth were detected as a result of the experimental manipulation. Mothers of MF litters provided more maternal care and appeared to wean their offspring later than mothers of MM litters. Despite this, MF pups grew more slowly than MM pups, suggesting that their high levels of sucking behaviour reflected high milk demand, perhaps due to a low milk supply, rather than high milk intake. One interpretation is that mothers of litters with a female-biassed sex ratio (MF mothers) were in poorer condition than those with a male-biased sex ratio (MM mothers), and consequently provided less adequate nutrition for their pups. Undernutrition during early life is known to result in the development of more aggressive individuals, perhaps by increasing competition for nipples and thereby promoting a more active/competitive mode of behaviour in later life.

Parental care, sibling relationships and the development of aggressive behaviour in two lines of wild house mice

Distinct behavioural strategies for dealing with environmental and social challenge are known to exist in the adults of a variety of mammalian species, but little is known about the developmental bases of these strategies. In an attempt to start to fill this gap in knowledge, the present study set out to describe and compare the development of males from two lines of wild house mice which are known to differ in their aggressive behaviour and response to challenging situations. Adult males from one line (Short Attack Latency = SAL line) are more aggressive, show less sensitivity to changes in their environment and behave in a more internally controlled, routine-like way, than do males from the other line (outbred = Control line). The parental care and development of eight litters of four male pups from each line was observed. Paternal care did not differ between the two lines, but SAL pups received higher levels of nursing and general maternal care than did Control pups, and they appeared to be weaned later. Despite these differences, SAL pups grew more slowly than Control pups, suggesting that their high levels of sucking behaviour were a reflection of high milk demand, perhaps due to a low milk supply, rather than high milk intake. SAL pups were also delayed in a number of measures of early behavioural development relative to Control pups. The apparent early retardation in certain measures of SAL pup behavioural development disappeared at the end of the parental care period. From day 32 onwards, SAL pups began to show higher levels of aggression towards each other than did Control pups, and also showed a reduced reactivity to tests involving changes to the home cage environment ; a characteristic of adult SAL males. As expected, SAL males developed into faster attackers than Control males and, in SAL litters, intra-litter variation in attack speed was strongly influenced by preceding experience of sibling aggression. The fastest attackers in each litter were more successful in fights occurring in their litter than their slowest attacking siblings. No such relationship was observed in the Control litters where levels of inter-sibling aggression were low. The results point to the potential significance of maternal care behaviour and pup fighting behaviour in the development of the fast-attacking phenotype. One hypothesis is that inadequate nutrition of young SAL pups, mediated through the mother, promotes increased competition for access to the mothers nipples and predisposes the pups to develop into more active/competitive individuals. Subsequent sibling-sibling aggression appears to influence intra-litter variation in adult attack latency.