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

Abstract

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 24h before to 24h 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.

Causes of death in newborn C57BL/6J mice

“…Cannibalism is common in laboratory mice and infanticide has been attributed as a cause of neonatal death in laboratory animal house facilities 43, 48 although our recent data suggests this is less common than previously reported 3 . In the present study, only one pup from 324 was clearly the victim of trauma ante-mortem sufficient to cause death by the presence of incised wounds, facial bruises with visible oedema and blood inside the stomach.…”

“…This characteristic is maintained in laboratory mice and is often exploited by housing mice in breeding trios, which allows for increases in the number of pups per cage 47 and individual pup growth 15, 16 . However, this social conformation is more prone to litter overlap 3 , the existence of a previous litter inside the cage, which is associated with higher neonatal mortality 3 .…”

High laboratory mouse pre-weaning mortality associated with litter overlap, advanced dam age, small and large litters

“…Pup mortality is often associated with infanticide [16,20], assuming that cannibalized pups were killed before they were eaten. However, our previous behavior studies suggest that infanticide precedes less than 15% of the cannibalism events [6,21] and that pups die primarily from other causes than direct killing. Litter asynchrony, which often leads to overlap, is likely to increase unequal competition for access to milk and parental care, trauma caused by trampling and stepping of newborns by the adults or the older siblings, and problems related with increased cage stocking density.…”

“…The presence of two litters in the cage has been demonstrated to increase parturition duration and affect parental behavior. Adults in trio cages with two litters were observed to care for their newborn pups a total of 20% less time (all the three adults together) than adults with one single litter in the cage [6], while parental investment is known to improve the chances of survival of young mice. In fact, C57BL/6 females which lost their litters entirely have been found to spend more time outside the nest and invested less time in building the nest prior to parturition [23], while the presence of males in cages with breeding females (CD-1) has been demonstrated to increase pup survival by facilitating maternal behavior [24].…”

“…Recently, we identified the presence of older litter mates in the cage when a new litter is born (litter overlap) as a major factor affecting pup survival [6]. In a study with 55 litters of C57BL/6 mice (n = 521 pups) housed in trios [6], a 2.3 fold increase was found in litter loss (death of the entire litter) in cages where older littermates were present, compared to trio cages with no older littermates. Litter overlap happens in both trio (two adult females and one male) and pair (one adult female and one male) housing, which are the most common configurations in mouse breeding.…”