Siblicide (attributed to behavioural ecologist Doug Mock), the death of an individual by its close relatives may occur directly between siblings or indirectly across the parent-offspring relationship and is seen to have beneficial indirect results for the genetic viability of a population or direct results for the recipient individuals. Cattle Egrets, Bubulcus ibis, exhibit asynchronous hatching and androgen loading (in the first two eggs) in its 3-egg clutch. This results in more aggressive chicks with the first having a developmental head start. If food is scarce the third chick often dies or is killed by the larger siblings and so parental effort is better distributed between the remaining chicks, more likely to survive to reproduce. Thus the investment in the older chicks is safeguarded for longest. This is possibly either due to the unforeseen possibility of food abundance or the chance of sterility in one egg, something suggested by studies into the Common Grackle, Quiscalus quiscula.
In Spotted Hyenas, Crocuta crocuta, pups of the same sex exhibit siblicide more often than male-female twins. Sex ratios may be manipulated in this way and the dominant status of a female and transmission of genes may be ensured through a son or daughter which inherits this solely, receiving much more parental nursing and decreased sexual competition. Siblicidal ‘survival of the fittest’ is also exhibited in parasitic wasps, which lay multiple eggs in a host, after which the strongest larva kills its rival sibling.
The theory of kin selection may be seen as a genetically-mediated altruistic response within closely-related individuals whereby the fitness conferred by the altruist to the recipient outweighs the cost to itself or the sibling/parent group. The fact that such a sacrifice occurs indicates an evolutionary tendency in some taxa toward improved vertical gene transmission in families or a higher percentage of the unit in reaching a reproductive age in a resource-limited environment.