Shakespeare was ahead of us, as usual. In A Midsummer Night’s Dream, Oberon tells Puck how Cupid’s arrow fell upon a white flower (the pansy), turning it purple, and that now the juice of this flower
…on sleeping eyelids laid
Will make or man or woman madly dote
Upon the next live creature that it sees.
Puck duly fetches a pansy and Oberon wreaks havoc with the lives of those sleeping in the forest, causing Lysander to fall in love with Helena, whom he has previously scorned, and causing Titania to fall in love with Bottom the weaver wearing the head of an ass.
Who would now wager against me that I could not soon do something like this to a modern Titania? Admittedly, a drop on the eyelids would not suffice. I would have to give her a general anaesthetic while I cannulated her medial amygdala and injected oxytocin into it. I doubt even then that I could make her love a donkey. But I might stand a fair chance of making her feel attracted to the first man she sees upon waking. Would you bet against me? (I hasten to add that ethics committees will – should – prevent anybody taking up my challenge.)
I am assuming that, unlike most mammals, human beings are basically monogamous, like prairie voles, and not promiscuous, like montane voles. I base this assumption on the testicle-size argument enunciated in chapter 1; on the ample evidence from ethnography that, though most human societies allow polygamy, most human societies are still dominated by monogamous relationships; and on the fact that human beings usually practise some paternal care – a characteristic feature of the few mammal species that live as social monogamists.21 Furthermore, as we have liberated human life from economic and cultural straitjackets, such as arranged marriage, we have found monogamy growing more dominant, not less. In 1998 the most powerful man in the world, far from treating himself to a gigantic harem, got into trouble for having an affair with one intern. The evidence for long-term, exclusive (but sometimes cheated-on) pair bonds as the commonest pattern in human relationships is all around you.
Chimpanzees are different. Long-term pair bonds are unknown, and I predict that they have fewer oxytocin receptors in the relevant parts of their brains than human beings, probably as a result of having shorter gene promoters. The oxytocin story lends at least tentative support to William James’s notion that love is an instinct, evolved by natural selection, and is part of our mammal heritage, just like four limbs and ten fingers. Blindly, automatically and untaught, we bond with whoever is standing nearest when the oxytocin receptors in the medial amygdala get tingled. One sure way to tingle them is to have sex, although presumably chaste attraction can also do the trick. Is this why breaking up is hard to do?
Having oxytocin receptors does not make it inevitable that somebody will fall in love during his life, nor predictable when it will happen, or with whom. As Niko Tinbergen, the great Dutch ethologist, demonstrated in his studies of instincts, the expression of a fixed, innate instinct must often be triggered by an external stimulus. One of Tinbergen’s favourite species was the stickleback, a tiny fish. Male sticklebacks go red on the belly in the breeding season, when they defend small territories in which they build nests, which attract females. Tinbergen made little models of fish and caused them to ‘invade’ the territory of a male fish. A model of a female elicited the courtship dance of the male, even if the model was astonishingly crude; so long as it had a ‘pregnant’ belly, it excited the male. But if the model had a red belly, it would trigger an attack. It could be just an oval blob with a crudely drawn eye but no fins or tail: still it was attacked just as vigorously as if it were a real male rival – so long as it was red. One of the legends of Leiden, where Tinbergen first worked, is that he noticed his sticklebacks would threaten the red post-office vans that drove past the window.
Tinbergen went on to demonstrate the power of these ‘innate releasing mechanisms’ to provoke an instinct in other species, notably the herring gull. Herring gulls have yellow beaks with a bright red spot near the tip. The chicks peck at this spot when begging for food. By presenting newborn chicks with a series of models, Tinbergen demonstrated that the spot was a powerful releaser for the begging action, and the redder it was the better. The colour of the beak or the head of the bird mattered not at all. So long as it had a contrasting spot near the tip of the bill, preferably in red, it would elicit pecking. In modern jargon, scientists would say that the chick’s instinct, and the adult’s beak spot had ‘co-evolved’. An instinct is designed to be triggered by an external object or event. Nature plus nurture.22
The significance of Tinbergen’s experiments was to reveal just how complex instincts could be, and yet how simply triggered. The digger wasp he studied would dig a burrow, go and catch a caterpillar, paralyse it with a sting, bring it back to the burrow and deposit it with an egg on top, so that the baby wasp could feed on the caterpillar while growing. All of this complex behaviour, including the ability to navigate back to the burrow, was achieved with almost no learning, let alone parental teaching. A digger wasp never meets its parents. A cuckoo migrates to Africa and back, sings its song and mates with one of its own species without as a chick ever seeing either a parent or a sibling.
The notion that animal behaviour is in the genes once troubled biologists as much as it now troubles social scientists. Max Delbruck, pioneering molecular biologist, refused to believe that his colleague at Caltech Seymour Benzer had found a behavioural mutant fly. Behaviour, he insisted was too complex to reduce to single genes. Yet the idea of behaviour genes has long been accepted by the amateur breeders of domestic animals. The Chinese started breeding mice of different colours in the seventeenth century or earlier and they produced a mouse called the waltzing mouse, famous for its dance-like gait caused by an inherited defect in the inner ear. Mouse breeding then caught on in Japan in the nineteenth century and thence spread to Europe and America. Some time before the year 1900 a retired schoolteacher in Granby, Massachusetts, by the name of Abbie Lathrop, took up the ‘mouse fancying’ hobby. Soon she was breeding different strains of mice herself in a small barn adjoining her property and selling them to pet shops. She was especially fond of what were by then known as Japanese waltzing mice, and she developed several new strains. She also noticed that some strains got cancer more often than others; picked up by Yale University, this hint became the basis of early studies of cancer.
But it was Lathrop’s link to Harvard that uncovered the link between genes and behaviour. William Castle of Harvard bought some of her mice and started a mouse laboratory. Under Castle’s student Clarence Little, the main mouse laboratory moved to Bar Harbor, Maine, where it still is – a giant factory of inbred mouse strains used in research. Very early on, the scientists began to realise that different strains of mice behaved in different ways, too. Benson Ginsburg, for instance, found out the hard way. He noticed that when he picked up a mouse of the ‘guinea-pig’ strain (named for its coat colour), he often got bitten. He was soon able to breed a new strain that had the coat colour but not the aggressive streak: proof enough that aggression was somewhere in the genes. His colleague Paul Scott also developed aggressive strains of mice, but bizarrely, Ginsburg’s most aggressive strain was Scott’s most pacific. The explanation was that Scott and Ginsburg had handled the mice differently as babies. For some strains, handling did not matter. But for one strain in particular, C57-Black-6, early handling increased the aggressiveness of the mouse. Here was the first hint that a gene must interact with an environment if it is to have its effect. Or, as Ginsburg put it, the road from the ‘encoded genotype’ the mouse inherits to the ‘effective genotype’ it expresses passes through the process of social development.23
Ginsburg and Scott both later went on to work with dogs, Scott proving by crossing experiments between cocker spaniels and African basenjis that play-fighting in puppies