Filial cannibalism is the act of an animal consuming its own offspring, while infanticide is the killing of young by a non-parent of the same species. To human observers, these behaviors appear shocking. However, biologically, these actions are adaptive strategies evolved to optimize reproductive success or ensure the parent’s immediate survival. Understanding these behaviors requires examining the cost-benefit calculations driven by natural selection.
Resource Scarcity and Parental Survival
The primary driver of filial cannibalism is the immediate need for nutrient recycling and energy conservation. This behavior is triggered when a parent faces extreme environmental or physiological stress, such as starvation or dehydration. Consuming some or all of the young allows the parent to recoup invested energy and nutrients, redirecting them toward the parent’s own survival or future breeding attempts.
The survival of the adult is the best guarantee for future reproductive success, making the current litter expendable under severe duress. This is often observed in fish with paternal care, where the male guards the eggs but may consume a portion of the clutch to meet his high metabolic demands while nest-guarding. In some fish like the fantail darter, the male consumes a fixed number of eggs to cover the energy costs of protecting the remaining brood. If the clutch size is too small, the cost of guarding may outweigh the benefit, leading the male to consume the entire clutch to invest in a larger, more viable future breeding attempt.
Rodents, such as mice and hamsters, may resort to filial cannibalism when stressed or protein-deficient, prioritizing self-preservation. A long-tailed sun skink mother may eat her own eggs if she senses a persistent threat of snakes near the nest. This action allows her to recover the energy she invested, enabling her to attempt reproduction later when the threat of predation is lower. This strategy is a reproductive hedge, salvaging a failed investment to fund the next one.
Optimizing the Litter: Culling Sick or Weak Offspring
Infanticide and filial cannibalism function as a form of triage, where parents selectively remove certain offspring to maximize the survival chances of the remaining young. This strategy optimizes the overall fitness of the litter by ensuring the strongest individuals receive all available resources. Parents often remove young that are genetically compromised, diseased, or significantly weaker than their siblings.
The removal of sick or infected young, such as eggs showing signs of fungal infection in the Japanese giant salamander, prevents the illness from spreading to the rest of the clutch. This parental manipulation safeguards the health of the entire brood and improves their chances of reaching maturity. In species that produce large litters, like domestic cats, pigs, or rodents, the mother’s ability to produce milk or provide care may be physically limited.
When resources like maternal milk are insufficient to support all offspring, the mother may consume the smallest or weakest individual. This conserves the limited resources for the stronger, more viable siblings, reducing competition and raising the average health of the remaining litter. The consumed offspring also provides a nutritional supplement to the mother, helping her maintain the physical condition necessary to continue nursing the healthier young.
Accelerating Reproduction: Male Infanticide Strategies
A fundamentally different motivation for infanticide, one not driven by hunger or parental care, is the male’s strategy to accelerate his reproductive timeline. This sexually selected infanticide involves a male killing the dependent young of a rival to gain an immediate reproductive advantage. This behavior is most common in species where males compete intensely for access to multiple females and where the female’s reproductive cycle is tied to the presence of an infant.
The mechanism relies on lactational amenorrhea, a biological state where nursing hormones prevent the mother from ovulating and becoming fertile. By eliminating the nursing infant, an unrelated male prematurely ends the mother’s lactation. This termination of the nursing period forces the female to return to estrus, or sexual receptivity, much sooner than she would have naturally.
This strategy is observed in social mammals like lions and Hanuman langurs. When a new male lion takes over a pride, he kills the cubs sired by the previous dominant males. This action ensures that the females become fertile quickly, allowing the new male to mate with them and begin siring his own offspring without waiting for the cubs to be weaned.
In primates with long periods of infant dependency, the female’s reproductive investment is substantial, making the risk of infanticide high due to prolonged infertility while nursing. The male killer benefits by eliminating a rival’s genes and shortening his waiting time to pass on his own genetic material. The female, having lost her reproductive investment, quickly mates with the infanticidal male to minimize the time until her next successful birth.