Sibling aggression and brood reduction: a review

Siblicide may be "facultative" or "obligate". When food resources provided by the parents are insufficient to rear a whole litter successfully, dominants may kill their subordinate siblings, either directly by physical damage, or indirectly through enforced starvation. This phenomenon is termed "facultative siblicide" and occurs in a wide range of bird species and at least one mammalian species. In contrast, when the lowest ranking sibling is routinely killed by its dominant brood mate or littermate this is called "obligate siblicide", and seems to affect in particular large, long-lived species characterized by intense competition for breeding sites. This "obligate siblicide" has intrigued researchers for decades, trying to find an evolutionarily satisfactory explanation for this extreme behaviour. We review all scientific literature concerning sibling aggression published in the last 66 years. A bibliography search resulted in 104 references during the last 66 years, where birds represented 88% of the total published papers. Eleven hypotheses have been formulated for explaining these results, finding that siblicide is a complicated behaviour not controlled only for parents or chicks or environmental changes, but for a whole range of factors. These hypotheses have been identified and discussed according to actual supporting data. The relationship between food resources and brood reduction was widely documented; a sustained increase in food availability led to a highly significant decrease in both frequency and intensity of aggressiveness of the older chick towards its younger sibling in facultative species, also finding similar results in obligate siblicidal species. These results would suggest siblicide is an adaptive behaviour. This review tends to show that there is probably more than a single cause behind this behaviour. We conclude that more aspects must be considered in the design of future studies in order to understand the potential evolutionary sense of aggressive behaviour among siblings, especially those concerning food allocation decisions by parents.