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

Swiss CD-1 lactating mice show a different pattern of attack toward intrude rs of differing sex, displaying defensive attack against the male (bites on the head and ventrum associated with fear) and offensive attack against th e female (bites-on the back and flanks with no elicitation of fear). This d ichotomy 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 involv ed in the modulation of the different patterns of attack. In Experiment 1 w e compared the pattern of attack of lactating females in Swiss CD-1 and Wil d mice toward male and female intruders in relation to fear components of b ehavior of the attacking dams, Results showed that in Swiss mice, male intr uders were attacked with a defensive type of attack accompanied by high lev els of fear, whereas female intruders did not elicit fear in the attacking animal but were attacked with an offensive pattern. In Wild mice, both type s of intruders were attacked with a defensive pattern; notwithstanding, fea r was evident only toward male intruders. This suggests that fear is not to tally responsible for the expression of the defensive type of attack, To te st the hypothesis that defensive attack toward male and female intruders ma y be related to the infanticidal potential of the intruder, Experiment 2 ex amined levels of infanticide in both male and female Swiss CD-1 and Wild mi ce, Swiss female mice showed virtually no infanticidal behavior, whereas Sw iss males and both sexes of Wild mice showed similarly high levels of infan ticide (55%-75%), From a game theory perspective, the defensive pattern of maternal attack toward female intruders in Wild mice is discussed as "extre me" defense of a high value resource and thus, functionally, a competitive form of aggression. (C) 2000 Wiley-Liss, Inc.