Social environment as a cause of litter loss in laboratory mouse: A behavioural study

Perinatal mortality is a widespread problem in laboratory mouse breeding and is often manifested by the loss of the entire litter within the first days of life. High mortality is an animal health and welfare concern that violates the 3R principles of reduction and refinement. High pup mortality is often considered normal in mouse breeding and the underlying causes are poorly understood. This study investigated the role of the social environment on pup survival and parental behaviour in C57BL/6 mice. Before parturition, multiparous females were allocated to three treatments: single-housing (SH, n=54) or group-housing in trios of two females and a male, with group-housing further divided into trios with (GH1, n=20) or without (GH0, n=35) the presence of another litter. Mouse behaviour was recorded from 24 h before to 24 h after parturition. Mouse breeding performance (offspring survival and weight) was analysed using logistic and generalized linear models, and behaviours using logistic and multivariable linear models. We confirmed previous findings of high mortality rates for all treatments, but the highest rates were found in GH1 where half of the litters were lost entirely. Social environment had an impact on breeding performance. On the one hand, the presence of adult cagemates in GH0 did not affect litter survival nor pup body weight at 20 days. Adult cagemates shared the work of breeding by performing nest building before and after litter birth, and parental care after birth. GH0 and GH1 dams were less frequently seen performing nest building before and after parturition than SH dams. GH0 and GH1 dams were also less frequently seen inside the nest and performing parental behaviour after parturition than were SH dams. On the other hand, the risks for litter and pup loss were dramatically increased by 2.3 and 1.8 times, respectively, in GH1 compared to GH0. Parturition lasted longer in GH1 than in GH0 treatment. While dam behaviour did not differ between GH1 and GH0, GH1 adult cagemates spent less time performing parental care than did GH0 cagemates. Plausible reasons for the higher mortality in the presence of another litter are insufficient parental care, competition for milk access, poor nest quality, and crushing of newborns by older pups in the nest. This study indicates that being born in a cage where there is already an older litter, so-called reproductive asynchrony, is a major risk factor for litter loss.