Infanticide, the killing of young offspring, is observed across many species, including the common rat. For a female rat (dam), this act is not driven by malice but serves as a biological mechanism to manage reproductive investment in response to immediate physiological or environmental pressures. It represents a pragmatic strategy that prioritizes the mother’s survival and the overall success of her genes. The motivations behind this behavior are diverse and include maternal responses to stress and scarcity, as well as the distinct reproductive strategies of non-parent adults.
Environmental Stress and Disturbance
A primary trigger for maternal infanticide is the perception of a hostile or unstable environment. Female rats are highly sensitive to external disturbances around their nesting area, interpreting them as a severe threat to their safety and the survival of their litter. Stressors such as sudden, loud noises, frequent and intrusive handling of the nest, or the physical relocation of the cage can induce high levels of stress hormones, which disrupt normal maternal care.
The mother may conclude that the nesting site is too dangerous for her vulnerable pups to survive and will terminate the litter to cut her losses and conserve her energy. Unfamiliar scents, particularly from other male rats or human handling, can also signal danger, leading to elevated corticosterone levels that suppress nurturing behaviors. In this scenario, the dam essentially foregoes the current reproductive attempt to increase the probability of a successful one later, when conditions are more secure.
Survival Needs and Resource Scarcity
Maternal infanticide can also be a direct consequence of a deficit in the mother’s own physiological resources. The intense demands of gestation and lactation, especially for large litters, require a significant increase in the dam’s intake of protein, calories, and essential minerals. A lack of sufficient food, or a diet deficient in specific nutrients like calcium, can impair the mother’s ability to produce milk necessary for the pups’ development.
When faced with this resource limitation, the dam may consume one or more pups to reclaim vital nutrients and energy. This act of cannibalism allows her to recover protein and fat, which can be immediately reinvested to ensure her own survival or to support the milk production for the remaining, stronger offspring. This optimizes the limited resources to maximize the survival rate of at least a portion of the litter.
Culling Unhealthy or Weak Pups
A motivation for maternal infanticide is the selective removal of non-viable offspring, known as culling. The mother is adapted to identify pups that are stillborn, deformed, or significantly weaker than their littermates shortly after birth.
By consuming the unhealthy or struggling pups, the dam conserves resources like milk, warmth, and time that would otherwise be wasted on individuals with a low probability of survival. This strategic culling ensures that the mother’s limited energy is concentrated on the most robust individuals, thereby increasing their growth rate and overall chances of reaching independence.
Infanticide By Non-Parent Rats
Not all infanticide is maternal; often, the perpetrator is a male or an unfamiliar female. In male rats, infanticide is a recognized reproductive strategy, particularly when the male is unrelated to the pups. Killing a litter causes the mother to cease lactation and accelerates the return of her reproductive cycle, bringing her back into estrus.
This immediate return to fertility allows the male to mate with the female sooner and sire his own litter. The behavior is typically inhibited in males who have recently copulated with the female, as a chemical signal emitted by the pregnant female during gestation acts to suppress the infanticidal drive in the father. Unrelated males lack this inhibition and are driven by competition to eliminate the genetic material of rivals and ensure that the next investment of resources is directed toward their own offspring.