Patterns of maternal effort in mouse lines bidirectionally selected for aggression

The present study was designed to describe maternal behaviour and body weight regulation during lactation, and its consequences for the growth of the pups, of darns from house mouse, Mus domesticus, lines bidirectionally selected for aggression, both when caring for their own pups and when caring for cross-fostered pups. Mothers from the aggressive line (SAL) nursed and groomed their pups more, and rested less alone, than mothers from the non-aggressive line (LAL). Body weight regulation during lactation differed markedly between the two lines. SAL mothers kept their body weight at a relatively stable level until the last phase of lactation, when they lost weight. LAL mothers showed the more generally observed pattern of weight development: a steady increase in body weight, followed by a stabilization and a rapid fall during the last 3-4 days of lactation. Pup weight and growth rate were not influenced by these differences in behaviour and physiology of the darns. Maternal behaviour of SAL dams was not affected by cross-fostering. LAL dams nursed cross-fostered SAL pups more than their own pups, without reaching the nursing level of SAL darns. In neither line was body weight regulation of the dams affected by cross-fostering. The different patterns in maternal effort imply a significant difference in the early experience of genetically aggressive and non-aggressive mice. (C) 1996 The Association for the Study of Animal Behaviour