Siblicide

A Nazca Booby (Sula granti) with a chick and egg. When the second egg hatches, any siblings present will almost certainly kill their younger brother or sister.

Siblicide (attributed by behavioural ecologist Doug Mock to Barbara M. Braun) is the killing of an infant individual by its close relatives (full or half siblings). It may occur directly between siblings or be mediated by the parents. The evolutionary drivers may be either indirect benefits for the genetic viability of a population or direct benefits for the perpetrators. Siblicide has mainly, but not only, been observed in birds.

The word is also used as a unifying term for fratricide and sororicide in the human species; unlike these more specific terms, it leaves the sex of the victim unspecified.

Examples

Cattle Egrets, Bubulcus ibis, exhibit asynchronous hatching and androgen loading in the first two eggs of its 3-egg clutch. This results in the older chicks being more aggressive and 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 distributed between the remaining chicks, which are hence more likely to survive to reproduce. The extra "excess" egg is possibly laid either due to exploit the possibility of elevated food abundance (as seen in the Blue-footed Booby (Sula nebouxii) or due to the chance of sterility in one egg, something suggested by studies into the Common Grackle, Quiscalus quiscula and the Masked Booby, Sula dactylatra.

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.

See also

• Parent-offspring conflict
• Fratricide, the killing of a brother
• Sororicide, the killing of a sister
• Infanticide (zoology), a related behaviour
• Nazca Booby (displays obligate siblicide)
• Sibling abuse
• Sibling rivalry

References

• Alcock, J. (1998). Animal Behaviour. 6th Ed. Sinauer Associates.
• Smith RL & Smith TM (2001). Ecology and Field Biology. 6th Ed. Benjamin Cummings.

Further reading

• Michalski, R. L., Russell, D. P., Shackelford, T. K., & Weekes-Shackelford, V. A. (in press). Siblicide and genetic relatedness in Chicago, 1870-1930. Homicide Studies. Full text

• Russell, D. P., Michalski, R. L., Shackelford, T. K., & Weekes-Shackelford, V. A. (2007). A preliminary investigation of siblicide as a function of genetic relatedness. Journal of Forensic Sciences, 52, 738-739. Full text