A note on behavioural response to a novel arena in lactating mice highly selected for litter size

In an earlier experiment it was found that adult non-reproductive virgin female mice from a line selected for 91 generations for high litter size at birth (S-line; about 20 pups per litter) showed similar or more locomotion activity in four novelty behavioural tests (open-field, runway, maze and social confrontation) than females from a non-selected control line (C-line; about 10 pups per litter). The present short study aimed at investigating behavioural differences in the two lines during lactation in two of these four previously used tests. To investigate within line the effect of litter size on displayed behaviour, half of the litters where standardised, when larger than eight pups, to eight pups per litter. Twenty females from each line and standardisation level (80 animals in total) where subjected once around 2 weeks in lactation to the open-field and the runway test. Females from both lines crossed a similar number of squares in 60 s; S-line females crossed absolutely and relatively more squares in the centre of the open-field than C-line females. Females of the S-line ran faster than C-line females in the runway test. The results suggest that behavioural characteristics as measured in the two behavioural tests are consistent over the two physiological stages (mature non-reproductive and lactating). Standardisation level did not influence any of the measured behavioural traits significantly. Non-reproductive females from the former study appeared to run considerably faster in the runway than lactating females of the present study. Also considerably less non-reproductive females appeared to return to the startbox or did not enter the runway at all than lactating females. This suggests a novelty-induced increase in emotionality that is opposite to a generally observed physiologically induced low-anxiety state during lactation. Responsible for these results may be the fact that lactating females were tested directly after separation from their litter and in the vocal presence of their pups. Future research is needed to investigate this further.