Aggression and anxiety in pregnant mice are modulated by offspring characteristics

We investigated aggressiveness and anxiety during late pregnancy in mice, Mus domesticus in relation to the pups' characteristics. The female's aggressive behaviour towards a male intruder and behavioural anxiety were evaluated in separate experiments during late pregnancy (days 17-18) and individual scores were correlated with the number and sex of the delivered pups. We did a preliminary experiment to compare the behavioural measures of anxiety in the classical elevated plus-maze test and in a new experimental condition designed for pregnant females (three-compartment test choice: clean, own and unknown male's bedding): the time spent in the male's compartment by pregnant females was negatively correlated with the percentage of entries and time spent in the open arms of an elevated plus-maze. This allowed us to measure anxiety twice in the same female: in dioestrus and during pregnancy. Females' anxiety increased during late pregnancy and no significant correlation was observed between individual scores. Aggressive behaviour and anxiety were affected by the number of pups the pregnant female was carrying: the larger the litter, the more aggressive and more anxious the female was towards a male's cues. By contrast, the offspring fetal sex ratio did not affect the amount of aggressive behaviour and the level of anxiety shown by the mothers to be. © 2006 The Association for the Study of Animal Behaviour.