Keeping it out of the family

If you marry your sister or brother, as the Pharaohs did, your offspring may inherit disorders.

Even wild creatures avoid mating with their kin. Their precautions tend to be simple ones but, according to recent research in Benidorm, storm petrels (above) have a sophisticated way of avoiding incest.

Charles Darwin married Emma Wedgwood, his first cousin. Anxious by disposition and a life-long hypochondriac, the great naturalist worried about possible consequences for his children. Fretting about things outside our control is futile but, in Darwin’s case, it was beneficial. Worries about ‘consanguinity’ focused his attention on the nature of inheritance; how traits are passed from one generation to the next became central to his theory of natural selection.

Genes had yet to be discovered but, unknown to Darwin, Gregor Mendel was engaged in ground-breaking experiments on peas at his monastery in what is now the Czech Republic. Inherited disorders are carried on ‘recessive’ genes, dormant in a family’s pedigree. Usually overridden by ‘dominant’ ones from the opposite parent, they are seldom ‘expressed’.

However, if both parents are from the same family, an undesirable recessive gene may come from both sides, with dire consequences.

’Incest taboos’ prevent us from inbreeding. Animals use other mechanisms. Young male mammals, for example, tend to leave their place of birth and seek out pastures new, while their sisters stay closer to home. Thus, siblings are less likely to encounter one of their kin when choosing a mate. In birds, it’s the females that travel most. Year-old male swallows, having spent the winter in Africa, return to the place where they fledged, while females tend to choose locations a few kilometres away. Such methods are not foolproof. Two mute swan females, which I ringed, mated with their sons and a five-year-old Oedipus drove out his father and nested with his mother. Nor is segregation according to sex always an option. Storm petrels, for example, return to their natal colonies when they are two or three years old.

There they select their partners, after which they are mated for life. If an unsuitable mate is chosen, a bird’s entire progeny is at risk. Siblings and cousins will be among the birds seeking partners at the colony. So how does a storm petrel bride or groom check out the background of a prospective partner? Sniffing out a suitable mate is normal among our furry mammal friends.

We humans use perfumes and after-shave lotions to help us win conquests. Birds, it used to be argued, are unable to detect odours. Flying creatures must be light. Good sight and hearing are much more useful to a bird than being able to smell, so the limited head-space and neural resources are devoted to making those faculties as efficient as possible.

This view prevailed until an extraordinary American woman began researching the topic.

Betsy Bang was an amateur ornithologist who prepared medical and biological illustrations for a living. She became interested in nasal organs through depicting them. Birds, she noticed, have well-developed nasal cavities and some species have particularly large olfactory lobes, the part of the brain which processes signals from the nose. She and her husband began measuring these lobes in birds.

The paper they published in the journal Nature in 1960 would overturn the traditional doctrine that birds can’t detect smell.

Tube-nosed seabirds, such as albatrosses, are particularly sensitive to odours and it’s easy to see why. Fish and marine offal are smelly. Being able to find such food from afar is a most useful faculty when you are out at sea. The storm petrel, another member of the tube-nosed tribe, also has a large olfactory lobe and, according to Francesco Bonadonna of the Evolutionary Ecology Centre in Montpellier, finding food is not its only function.

Storm petrels nest on La Isla de Benidorm, a rock sticking out of the sea 5km from beaches thronged with sunbathers. The colony there has been studied for 18 years. Extraordinarily, no related pair has ever been found. As every bird ringer knows, these burrow-nesters are smelly. So, Bonadonna asked, could they use odours in mate selection? His team collected swabs from the birds.

Placing a captured petrel in an artificial Y-shaped burrow, with a swab from the bird’s kin in one arm of the Y and that from a non-relative in the other, the bird almost invariably opted for the stranger. The birds, Bonadonna concluded, recognise the smell of a family member and give it a wide berth.


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