LITTER COMPOSITION AFFECTS PARENTAL CARE, OFFSPRING GROWTH AND THE DEVELOPMENT OF AGGRESSIVE-BEHAVIOR IN WILD HOUSE MICE

Male mice pups reared with female siblings 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-biased 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.