Molecular biology is changing the way we approach behavioral genetics. Some studies have identified complex behaviors that are strongly influenced by a single gene. One well-understood instance of a single gene’s effect on behavior comes from the fruit fly Drosophila.

In Drosophila, there are two different alleles of the foraging (for) gene—for^s and for^R—that are common in populations. These two alleles have different effects on the behavior of Drosophila larvae. The gene encodes an enzyme expressed in the brain that affects neuronal activity and alters behavior. In the absence of food, both “sitter” (for^s) and “rover” (for^R) larvae move about in search of food. In the presence of food, however, sitters barely move, feeding on the patch on which they find themselves, whereas rovers move extensively both within a patch of food and between patches (Fig. 45.7a).

Both forms are present in natural populations (Fig. 45.7b); typically, 70% are rovers, 30% are sitters. That both strategies are present in populations suggests that both are adaptive because, if one strategy were markedly inferior, we would expect natural selection to eliminate it. Furthermore, studies have shown that rovers are selected for in crowded environments in which there is an advantage to seeking new food sources, and sitters are selected for in less crowded ones because they take maximum advantage of their current food source. In this case, variation at a single gene affects a complex behavior in fruit flies.

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The foraging gene is present in other insects as well, including honeybees, suggesting that it has been evolutionary conserved. Honeybees with low levels of for expression in their brain tend to stay in the hive, whereas those with high levels of for expression are likely to be foragers (Fig. 45.8).

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Studies of the North American vole, in the genus Microtus, have also revealed how genes influence behavior (Fig. 45.9). The prairie vole is monogamous: Couples pair bond for life, meaning that they mate only with each other. By contrast, the prairie vole’s close relative, the montane vole, is promiscuous: A male mates and moves on, and a female over her lifetime produces litters by several different males. What differences underlie these different behaviors?

Hormones and their receptors provide an answer. In all mammals, antidiuretic hormone (ADH) controls the concentration of urine (Chapter 41). The receptor for ADH is not only expressed in the kidney, but also in the brain, where it affects behavior. Both the monogamous prairie vole and the promiscuous montane vole produce ADH, but the hormone receptors in the two species are different. The American neuroscientist Thomas Insel analyzed the genes that encode the ADH receptor in the two vole species. He found a difference in the region of the gene that determines under what conditions and in what cells it is expressed. Because of this difference, the distribution of ADH receptors in the brain is different in the prairie vole and in the montane vole.

Does the difference in gene expression explain the different sexual behaviors of the two species? The answer seems to be yes. Insel and his colleague Larry Young inserted the prairie vole ADH receptor gene, complete with its regulatory region, into a laboratory mouse (a promiscuous species, like the montane vole). Interestingly, Insel and Young observed a marked change in the mouse’s behavior. Rather than mating with a female and then moving on to another mate, the transgenic male mouse pair bonded with the female. In short, the addition of a single gene affected a complex behavior such as pair bonding, making the otherwise promiscuous mouse more likely to pair bond.