Sibling Relationships: Does Shared Blood Lead to Shared Love?

When one looks at the dynamics of systems and relationships in the natural world, rarely is anything simple. The same can definitely be said about sibling relationships in humans. While in an ideal world, “brotherly and sisterly affection” would no doubt be the norm, that's certainly not true in all cases. From being loving, caring, and great at working together, to more neutral relationships, to nasty feuds and drawn out legal battles, in humans we see it all. But what about in the animal world? What kind of sibling relationships do animals have and what are their consequences?

An interesting starting point is sibling rivalry. In such cases, there is competition among siblings for resources or parental care, with each individual trying to maximise its own fitness. Such behaviour is often amplified when resources are hard to come by. There are many intricate aspects to this. When there is competition among offspring to maximise individual fitness, only some individuals will ultimately succeed, due to the limited resource pool. Thus, the fitness of some offspring must reduce, an outcome in contrast to the predictions of the kin-selection theory, which dictates that individuals may act in order to promote the fitness of closely related individuals, even at the cost of their own. This is because closely related siblings often share large parts of their genome (humans share 50% of their genome with immediate brothers and sisters on average), and hence promoting sibling survival also helps individuals propagate their genes to future generations. The key here is that there is no hard and fast rule; everything is situation and context specific. In some cases, promoting individual fitness outweighs promoting kin fitness, and we observe sibling rivalry (this is especially the case as the relatedness of the siblings reduces). One example of this is the American robin (Turdus migratorius), where chicks display begging behaviour in the form of chirps. It has been shown that chicks with the more aggressive begging behaviour receive more food from their parents, and it is clear that such behaviour would become more important when food is hard to come by.

An extreme manifestation of sibling rivalry is siblicide, where the actions of certain siblings leads to the death of relatives. Such behaviour is often observed in birds, especially when resources are scarce, and may or may not be encouraged by the parents. Parent encouragement could come about when resources are limited, supposedly due to the notion that the surviving individuals have “stronger” genes, and hence focusing the limited resources on them may lead to greater fitness gains than distributing them among the “weaker” offspring. A good example of this is in the cattle egret Bubulcus ibis. They are known for laying 3 eggs per clutch, but in a staggered manner, meaning that two eggs are laid first, and the third egg afterwards. The first two eggs may also be provided with greater amounts of androgen, thus conferring to them an advantage both in terms of hormonal levels and early hatching. If competition ensues among the three chicks due to resource scarcity, the third chick may die or be killed by its older siblings, leaving the parents to focus their efforts on the first two, which have an enhanced probability of survival. It has been postulated through studies in other birds that the third egg is laid due to chances of survival in case food is abundant, or in case one of the other two eggs is sterile. Sibling aggression has also been observed in the spotted hyena, and is used as a means of establishing social ranks, with intense aggression sometimes leading to death. It should be noted however, that in the examples discussed above, sibling rivalry does not necessarily lead to sibling death, and this often only occurs in situations of scarcity. Such siblicide is referred to as facultative siblicide. In obligate siblicide, the behaviour of certain siblings nearly always leads to the death of others. However, not all forms of sibling rivalry are necessarily lethal, although the fitness of certain offspring necessarily decreases as a result of it.

At the opposite end of the spectrum, there are several examples of care and “love” amongst siblings! A prime example of this is in elephants, where older sisters assume the role of caretakers for their younger brothers and sisters when the mother isn’t around. As is well known, lion sisters often spend their lives living and hunting together, while their more unscrupulous brothers are kicked out of the pride to find their own way in life. Shrews have been known to travel together as a family. They often form lines called caravans, with the mother at the front, followed by siblings one after the other, with each individual holding on to the one in front.

I guess we can be thankful that fights with our siblings very rarely reach the dangerous levels seen in some cases in the animal kingdom, but the bonds developed among siblings in some species sure do carry over!