Animals, including siblings, compete for resources such as food, territory, and potential mating partners. In animal sibling rivalry, individuals compete for parental care or limited resources, which can sometimes result in siblicide. Sibling rivalry occurs in many different forms. Siblings may compete for resources in a prenatal and/or post-birth environment. The degree of rivalry varies, ranging from a low level of violence in non-aggressive to the killing of kin in siblicide.
When there are multiple offspring in a single brood, the potential for sibling rivalry arises due to competition for food and parental attention. Natural selection may favor behaviors that allow an individual offspring to gain more resources, even if the behavior decreases a sibling’s fitness. Competition for food and resources can be seen in many bird species. For example, blue-footed booby (Sula nebouxii) siblings often exhibit aggression towards each other, with older chicks pecking at younger chicks. This behavior increases when there are food shortages, indicating more intense competition. In other bird species, siblings compete for food through manipulation of parental behavior rather than direct aggressive acts. Increased parental attention may mean more food for the offspring, favoring the development of begging behavior in nestlings. American robin (Turdus migratorius) chicks compete for food provided by their parents through louder and more prominent cheeps or other vocalizations, with the most food given to chicks exhibiting the most intense begging behavior.
Sibling rivalry may not seem to align with the kin selection theory, which predicts that altruistic behaviors may evolve if inclusive fitness benefits (including those of relatives) from such behaviors outweigh the costs. Theoretically, helping relatives would allow individuals to spread genes related to their own. However, some species may show sibling rivalry when the fitness costs outweigh the benefits of helping relatives. Interestingly, sibling relatedness can influence degree of rivalry. Canary nestlings are more selfish and competitive if other nestlings are less related. When offspring beg for more food from their parents, they also are “competing” with their future siblings by decreasing the fitness of parents, reducing their ability to invest in future offspring. This is known as interbrood rivalry, which can lead to parent-offspring conflict.
Not all forms of sibling rivalry in animals involve direct aggression or death of a sibling. This is not an extremely aggressive form of rivalry; however, it still results in reduced sibling fitness.