Pup mortality in laboratory mice

Successful mouse breeding is a crucial part of providing animals for research. However, loss of single pups or entire litters after birth is a relatively common problem. Determining how pups die is crucial for the understanding of mortality, but the scientific literature does not provide a clear picture of pup mortality and the reason why pups die is still poorly understood. The overall aim with this thesis was to investigate the causes of pup mortality in laboratory mice, focusing on maternal behaviour and the effect of housing environment. Specifically the aims were to investigate if litter loss was higher in primiparous females (study 1), if female mice actively killed their pups and if there were any differences in behaviour between females that lost the litter shortly after birth and females that successfully weaned their litters (study 2), and how the conditions for nest building influenced nest building and pup survival (study 3). In study 1 (paper I), breeding data from mice of the strains C57BL/6 and BALB/c were used. An effect of strain but no effect of parity on litter mortality was found. In study 2, C57BL/6 females were housed in four different treatments with different amounts of nesting material and cage furnishment. Behaviours of females whose litter died were observed in detail from birth of the litter until the litter died (paper II). No evidence that females actively killed their pups was found. In paper III, both females that lost their entire litter shortly after birth and females that successfully weaned their litter were observed from 24h before to 24h after parturition. Litter loss was associated with females showing less nest-building behaviour before parturition, more parturition-related behaviours and more time outside the nest. In the last study (paper IV) females were housed in four treatments with different amount of nesting material and structure present or absent. Females given a larger amount of nesting material built more dome shaped nests of higher quality. In summary, this thesis does not support the assumption that female mice actively kill their offspring. Pregnant females should be given a large amount of nesting material to enable nest-building behaviour. Further, monitoring females around time for parturition should be considered to detect problematic parturitions