Nature - Nazca Boobies (the killer brother)03:09

Nature - Nazca Boobies (the killer brother)


Kookaburra mothers engineer the timing of their chicks' birth to encourage a brutal battle for survival between them. One chick will fall by the wayside. The tougher brothers and sisters begin by stopping it from getting any food. As the unlucky chick gets weaker they begin attacking it. This is kept up until it's inevitable death. Siblicide pre-empts an anticipated food shortage later in the nestling period.

Siblicide is a behavior most common among birds (Mock and Parker, 1997) in which one or multiple siblings target the youngest because this sibling is the weakest and easiest to take advantage of, allowing more resources and energy to be directed toward the most fit off-spring.  This behavior is typically seen as beneficial to the population as a whole because it ensure the success of more off-spring in the long term (Mock and Parker, 1997). (In many species, siblings only share 1/4-1/2 of their genes with their siblings. Therefore, the direct benefits of surviving solo can sometimes far outweigh any indirect benefits of the sibling's success)(Drickamer, Vessey, Jakob, 2002).  Siblicide also occurs in amphibians and insects; however the process usually concludes with the consumption of their dead sibling (Dobler and Kolliker, 2010). 

Hatching Asynchrony: When chicks hatch at different times. This allows brood size to adjust for the maximum number of progeny in relation to resource availability. (Drickamer, Vessey, Jakob, 2002). 

Types of siblicide:

There are two types of siblicide: obligate and facultative.  The first results in death, whereas facultative is the frequent fighting over resources and attention but does not always lead to death.  Facultative siblicide is more common among birds, in particular, because the expenditure of energy needed to outright kill their sibling may not prove beneficial in the long term. (Mock, 1984) 

Causes of siblicide:  

Although some species demonstrate obligate siblicide before environmental factors become a catalyst, the majority of the time siblicide is a response to food and resource scarcity, limited space, or shared parental attention/altruism (Mock and Parker, 1997).  

Some evidence for these causes are: great blue herons feed their chicks large portions of food that can't be monopolized by one sibling. Great egrets feed their chicks small fish, and this makes is easier for the more dominant chick to steal the subordinate chick's food, and thus become even stronger. When there is enough food to go around, spotted hyena cubs actually become allies instead of murdering their sibling. (Drickamer, Vessey, Jakob, 2002).

Examples of species that demonstrate siblicide behaviors:

-Great egret

-Nazca and Masked boobies

-Swallow-tailed kites

-Spotted hyena

-Sand tiger shark

-European earwig

-Passalid beetle 

-Peregrine Falcon

-Bearded vultures


Dobler, R., & Kölliker, M. (2010). Kin-selected siblicide and cannibalism in the European earwig. Behavioral Ecology, 21(2), 257-263.

Drickamer, L., Vessey, S., Jakob, E. (2002). Animal Behavior (5th ed.). New York, NY: McGraw Hill. 

Ento, K., Araya, K., & Kudo, S. (2010). Laboratory observation of siblicide with hatching asynchrony in an insect with parental provisioning. Journal Of Ethology, 28(2), 405-407.

Mock, D. (1984) Infanticide, siblicide, and avian nestling mortality. Infanticide: Comparative and Evolutionary Perspectives, 3–30. 

Mock, D. and Parker, G. (1997) The evolution of sibling rivalry. Oxford University Press.

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