Question

In most vertebrates, full siblings share as many genes with each other, on average, as each one does with either parent. One might predict that serious conflict between siblings should be rare and altruistic behavior should be the norm, given that siblings share the same "genetic interests." As anyone who has ever had a brother or sister knows, this is not always the case. In fact, in a number of bird species, notably eagles and other raptors (birds of prey), females usually lay two eggs; soon after hatching, the larger chick pushes the other chick out of the nest, or otherwise attacks it and tries to kill it. (This behavior is known as Cainism, after the Bible story of Cain and Abel.) Generate a hypothesis to explain this behavior in evolutionary terms. Can you apply this hypothesis to human behavior?

Answer 1

The term "cainism" in ornithology decribes the killing of a younger sibling by an older. The name is based on the Old Testament story of the fratricide between Cain and Abel. The use of the term in the ornithological literature is not uniform. Many authors refer to any killing of siblings as cainism. More specifically the term is used only for those species in which the killing is inborn. To distinguish the latter species from the other, hence the term mandatory/obligatory cainism is often used.

The killing is done by pushing away from food and / or by direct attacks. Usually the dead body is not eaten, but it is often fed by the parent birds. While with many species of birds the nest killing of siblings during malnutrition is common, the act of killing is independent of the food situation in species with obligatory cainism. Mandatory cainism is similar to the inherent nest clearing behaviour of the Cuckoo and can be also triggered experimentally, for example by presenting a white cloth ball to the young bird. The attacks of the older towards the younger siblings usually begin with the day of hatching of the latter. Most of the younger birds survive only a few days. The parents do not hamper these attacks, it has been repeatedly observed that the older young bird continued his attacks even when a parent bird would feed him. The roles of "Cain" and "Abel" are determined solely by the sequence of hatch, in experimental transposition of "Abel" to a younger sibling, this "Abel" immediately took over the role of "Cain" and attacked the younger nestling. In further experiments on Verreaux's eagle eagles, the aggressiveness of the siblings lasted at least until the completion of the major plumage growth. The nest of the species with obligatory cainism usually consists of two eggs. That's why if both eggs hatch, only the older nestling survives.

One possible explanation for the phenomenon of Cainism may well be that the species is at an evolutionarily intermediate stage of transition from two-egg to one-egg clutches. For unknown reasons, and in contrast to earlier times, the raising of one young per breeding season is today sufficient to maintain the species. At a later stage in the course of its evolution this species, which at present lays a second egg clearly smaller than the first – one could regard this as vestigial – may well confine itself to laying no more than one egg per clutch. There are several hypothesis about such behaviour:
- food amount hypothesis
- prey size hypothesis
- brood size hypothesis
- challenge hypothesis;

Sibling conflict is common across a wide variety of species, including humans. It is an expected process because offspring compete for dominance as well as food resources (most common in nonhuman species) and also for parental attention, money, and other personal resources in the case of human children. While most conflict in humans has no long lasting effects, under some circumstances, sibling conflict can turn violent and even result in death, what is commonly referred to as siblicide. While siblicide is quite rare in humans, from an evolutionary perspective, we would expect it to be influenced by some of the same factors as less intense sibling conflict. Such factors include sex of sibling, resource availability, birth spacing, and relatedness.

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