Does fear modulate defensive and offensive types of maternal attack in mice?

Abstract

Swiss CD-1 lactating mice show a different pattern of attack toward intruders of differing sex, displaying defensive attack against the male (bites on the head and ventrum associated with fear) and offensive attack against the female (bites on the back and flanks with no elicitation of fear). This dichotomy may reflect diverse functions of maternal aggression: the attack toward males (the more infanticidal gender in laboratory strains) has been interpreted as a counterstrategy to infanticide, whereas the attack toward females may serve to establish a social hierarchy or to space rivals of the same sex. In terms of proximal mechanisms, fear may be a key factor involved in the modulation of the different patterns of attack. In Experiment 1 we compared the pattern of attack of lactating females in Swiss CD-1 and Wild mice toward male and female intruders in relation to fear components of behavior of the attacking dams. Results showed that in Swiss mice, male intruders were attacked with a defensive type of attack accompanied by high levels of fear, whereas female intruders did not elicit fear in the attacking animal but were attacked with an offensive pattern. In Wild mice, both types of intruders were attacked with a defensive pattern; notwithstanding, fear was evident only toward male intruders. This suggests that fear is not totally responsible for the expression of the defensive type of attack. To test the hypothesis that defensive attack toward male and female intruders may be related to the infanticidal potential of the intruder, Experiment 2 examined levels of infanticide in both male and female Swiss CD-1 and Wild mice. Swiss female mice showed virtually no infanticidal behavior, whereas Swiss males and both sexes of Wild mice showed similarly high levels of infanticide (55%–75%). From a game theory perspective, the defensive pattern of maternal attack toward female intruders in Wild mice is discussed as “extreme” defense of a high value resource and thus, functionally, a competitive form of aggression. Aggr. Behav. 26:193–203, 2000. © 2000 Wiley-Liss, Inc.