Siblicide

Siblicide is the act of one sibling killing another within the same family unit. It occurs in a variety of species and can influence brood size, survival, and population dynamics. In biology, siblicide is classified as obligate when the death of a sibling is typical under the species' breeding conditions, and as facultative when killing is not guaranteed and depends on environmental factors such as food availability and parental provisioning.

Mechanisms include competition for limited resources, aggression between nestmates or embryos, and developmental timing. In many birds, asynchronous hatching creates a size and energy gap between older and younger siblings; when resources are scarce, the larger chick may attack and kill the smaller one, allowing the stronger to be fed. In other species, even if both siblings survive initial growth, later competition can lead to lethal outcomes. In elasmobranchs like the sand tiger shark, embryos may feed on their smaller siblings while still in the uterus, a phenomenon known as adelphophagy.

Ecological and evolutionary explanations suggest siblicide can optimize parental investment by ensuring that at least one offspring survives to adulthood in environments with unpredictable resource pulses. It can also stabilize brood sizes in species with high egg numbers. Siblings that die often do not reflect malice but an adaptive response to resource limitation and life-history strategy. Humans infrequently, and typically under unusual circumstances, observe siblicide among close relatives.