Infanticide, the act of actively killing an infant, is a potential cause of mouse pup mortality in breeding facilities. Perhaps because it is an incomprehensible cruel behaviour for humans, it has led to the popular belief that it is the early disturbance of female and pups by employees that makes females kill and cannibalize their pups. Yet, although a stressful environment might exacerbate the phenomenon, infanticide is a natural and relatively frequent behaviour in wildlife.
As presented in a previous article, cooperation for breeding provides a lot of benefits such as shared caring effort, enhanced thermoregulation and group defence against intruders. Besides the benefits, communal nesting also creates opportunities for cheating and exploitation by some individuals.
Mice can use various diabolic tactics to improve their own reproductive success such as harassment, unequal share of resources or infanticide.
Infanticide of congeners’ pups allows at biasing the relative contribution to the communal litter in its own favour and thus increase the reproductive success. Indeed, the victim female will have more time and energy to take care of the pups of the diabolic female. Victims will continue to care for the alien pups perhaps due to physiological constraints that prevent them from immediate shutdown of care behaviour, such as milk production.
New born infanticide usually happens under asynchronous breeding, when a litter born after the other one. Some may think that it is because mice cannot discriminate their own offspring from alien pups of the same age, reducing infanticide to asynchronous breeding events only. But it is probably not the reason since mice seems able to discriminate their own pups from alien pups of the same age. This is typically the female that is still gestating that sometimes kills pups of the other female before she gives birth herself. Hence an alternative explanation would be that maternal hormones arising after parturition may prevent females from infanticide, indicating why females kill alien pups only before their parturition. This conflict mainly concerns congener females since well-established males in a group have no reason to kill offspring that have his genes. In practice, the breeding synchronisation of group-housed mice is a good strategy to improve colony efficiency since it reduces the risk of infanticide while providing the undoubtable benefits of communal nesting.
It is also claimed that even solitary females sometimes kill their own pups. Such belief is not surprising since pups are often not found when the cage of the singly-housed female is inspected from one day to another, or they are found partly eaten. Yet, there is little probability that a mother would kill her own progeny. Rather, dam may eat pups that are already dead.
Cannibalism, the act of eating dead animals of its own species, is considered as an adaptive behaviour and this behaviour has been selected several times in various species across evolution.
In this context, cannibalism may allow lactating mice to conserve nutrients in this energy-consuming period. It may also prevent dead pups from attracting predators with their decomposition smell as well as avoid inducing microbiological contamination.
These findings don’t mean that nothing should be done to reduce infanticide nor the actual rate of infanticide is “normal”. A considerable amount of mouse breeding facilities face to high pup mortality rates (and probably sometimes due to infanticide). Everything must be done to decrease pup mortality and improve productivity in order to reduce the total amount of raised animals for research. However, understanding the natural behaviour of our animals and discriminating it from what is not normal are the first steps towards the resolution of issues. In addition, controlled captive environments have the advantage to provide safe conditions that can allow at improving productivity and reducing mortality, as opposed to rude wildlife conditions. This may motivate research centers to always try to decrease the number of used animals. Future studies are needed in order to determinate if infanticide is exacerbated by actual captive conditions and if and how it could be reduced.
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