Maternal Investments

A father’s reach can only extend so far. There are some ways that mothers can take control of the fate of the embryos they are carrying without any interference from their mates. They can invest different amounts of energy into their eggs, depending on how desirable the father is. Female mallard ducks, for example, will lay bigger eggs for high‑dominance males than low‑dominance ones.

In some species, mothers can increase their reproductive success by determining the sex of their child. The most adept gender controller is the Seychelles warbler. These Indian Ocean birds live in pairs, each on its own territory. On the 70‑acre Cousin Island, there is not enough land for new warblers to be guaranteed their own territories. As a result, young females will sometimes stay at home with their parents rather than search for a mate of their own. They help build nests, defend territory, incubate eggs, and feed newborn chicks. Seychelles warblers are a help to their parents when there’s enough food for everyone. But if a warbler family must eke out a living on a low‑quality territory without much food, daughters become more of a burden than a help.

In 1997 Jan Komdeur, a researcher at the time at the University of Gröningen in the Netherlands, compared the eggs that were laid by birds living on low‑and high‑quality territories. On high‑quality territories, he found that for every male chick the warblers produced, they produced six females. On low‑quality territories, they produced only one female for every three males.

These sex ratios, Komdeur discovered, are not locked in by the genes of individual warblers. The birds can, in effect, decide how many sons and daughters they will have. Komdeur proved this by transferring some warbler pairs from Cousin Island to two other islands in the Seychelles that are uninhabited by the birds. He chose warblers that had been stuck with low‑quality territories on Cousin Island and were producing mostly sons, putting them on territories where they could find plenty of food. As soon as the warblers began breeding in their new homes, they started producing mostly daughters.

The evolutionary logic to their strategy is clear. When food is scarce, it is better to make a lot of males. They will leave the nest as soon as they can in search of mates and new territories, leaving the parents to raise new fledglings on what little food they can find. (The males may not find a new territory, and die, but that’s a risk worth taking.) When times are good, daughters make good helpers, and so a warbler mother somehow alters the balance of males and females. Exactly how the warblers choose between sons and daughters, no one yet knows, but that doesn’t take away from Komdeur’s discovery that they can indeed make the choice.