Age and brood defence in male CRL:NMRI BR laboratory mice, Mus musculus domesticus

Abstract

According to Lack's principle, older mice should invest more in their offspring than younger ones. I tested this principle with pairs of CRL:NMRI BR mice protecting their young against an intruder, an unfamiliar same-stock male or female. The intruder was presented within a small wire-mesh cage in an extension, a cage the same size as the pair's home cage. The dam, her mate and the young in the nest were present in all tests. Although both parents had full access to the intruder, only the male interacted with it. Young breeding pairs and young intruders were 6 months old, the old pairs and old intruders 12 months. Each breeding pair was tested with both young and old male intruders. Older males defended their pups more vigorously than younger ones. The old males showed a higher preference to stay near (to interact with) the opponent; they also showed more tail rattling and biting attempts than the young ones. This indicates that older males take greater risks in defending their offspring than younger males. Older intruders also elicited a stronger brood defence in older males and a weaker response in younger ones.