Extra eyes and ears provide security from predators, knowledge of food sources can be shared and more mating partners are available. However, there are also downsides. With brothers and sisters living cheek by jowl, ‘consanguinity’ may occur; incest isn’t recommended.
Health defects will appear when particular recessive genes are expressed. A problem gene may be carried, but unexpressed, for generations, over-ruled in each individual by a dominant one from the other parent. These genes ‘run’ in families so, with incestuous relationships, there is an increased risk that copies of a rogue one will be inherited from both sides. The danger is greatest among closely-knit social groups where daughters stay at home well into adulthood.
Young male mammals, becoming independent of their parents, generally move away from the area where they were born. Their sisters tend to stay closer to home. With birds it’s the opposite; the females are the adventurous ones; the sons stay put. Even migrants have separation strategies. A young swallow, having spent his first winter in Africa, will ‘pant to the place from whence at first he flew’, as Goldsmith said about the hare.
A study in Westfalia, Germany, found male swallows tended to nest within a few hundred metres of where they themselves hatched, while their sisters settled a few kilometres away. Such strategies help keep siblings apart at mating time, so that incest is avoided. The pharaohs married their sisters, causing mayhem to their dynasties. Many of the crowned heads of 19th century Europe were insane or feeble-minded, through breeding too close to their kin. To discourage dangerous liaisons, we evolved ‘incest taboos’, prohibiting sexual relations ‘within the forbidden degrees of kindred’ as the old Catechism put it.
However, how do other, less sophisticated, social animals cope with the problem? One such species, the mountain gorilla, seems to have adopted a strategy very similar to our own, according to research just published.
Linda Vigilant, of the Max Planck Institute, has been studying the behaviour of gorillas in Rwanda. The world’s largest primate, made famous by David Attenborough’s groundbreaking TV documentary of 1979, lives in an extended family, ruled by an ‘alpha’ male. The other males defer to him and he does most of the mating. By analysing DNA collected from droppings, Vigilant’s team has identified the parents of 97 gorillas belonging to four troops which have been studied continuously for decades. About 72% of the youngsters, they found, were fathered by the alpha male. As he gets older, and the number of subordinate males increases, this percentage falls.
With gorillas and chimpanzees, it’s the females rather than the males which disperse. Between 50%to 60% of the alpha males’ daughters, monitored by Vigilant, moved to another troop to find mates. The others remained with their natal troop. The parentage of five cubs born to stay-at-home daughters was determined. All of them proved to be offspring of sub-ordinate males; none had been sired by the alpha-male, their grandfather. Clearly, gorillas are able to avoid father-daughter incest. How to they manage to do so?
Although female gorillas are much smaller than males, they seem to call the shots when it comes to mating. Daughters, it seems, can recognise their fathers. At any rate, they indulge in a form of age discrimination by refusing the advances of older studs. The dominant males in turn, prefer older females.
But not all inbreeding is avoided; brothers and half-brothers are still regarded as fair game for mating. Of 79 offspring tested, nine were the result of relationships between siblings.
- Linda Vigilant et al. Reproductive competition and inbreeding avoidance in a primate species with habitual female dispersal. Behavioural Ecology and Sociobiology. May 2015