Sex on Six Legs_ Lessons on Life, Love, and Language From the Insect World - Marlene Zuk [48]
The Trivers-Willard effect has been demonstrated in a wide range of animals, from deer to fish to birds, and may even operate in humans. A 2008 study of 740 British mothers found that the women were more likely to give birth to a boy if their diets were more nutritious around the time they had conceived. The bias was not huge—56 percent sons among the mothers that ate the most, versus 46 percent sons among the least-nourished women—but it suggests that more than chance may play a role in the sex ratio even in our Western societies. These insights in humans wouldn't have come about if we hadn't had insects, with their wildly variable lives, as test cases.
Insects do not become pregnant, of course, although some retain the eggs inside or on the mother's body after they are produced. Furthermore, among many insects, large females are favored by natural selection because they can lay more eggs. Good conditions might be expected to cue the production of daughters, rather than sons, to take advantage of the resources necessary to manufacture a robust future mother. Indeed, among many parasitic wasps and flies, the sex ratio is biased toward daughters when the host maggot or caterpillar is large, and toward sons when the host is puny and provides less nourishment for the growing parasites.
Finally, the sex ratio in some insect species can fluctuate wildly, even over the course of a very short time. A Polynesian butterfly had only 1 percent males in 2001, due to an infection by bacteria that selectively kill male embryos. But just 5 years later, researchers found a nearly 50:50 sex ratio on some of the islands where the butterfly occurs, even though the bacteria were still present. Selection apparently acted to reduce the male-killing ability of the bacteria, probably because producing males was enormously advantageous in the highly female-biased populations.
See what you miss if you assume that Jerry Seinfeld makes a good bee?
Chapter 5
Sperm and Eggs on Six Legs
DO YOU suffer from fertilization myopia? Just when you thought you'd heard of all the latest trends in maladies, from attention deficit disorder to cyberchondria (looking up dire diagnoses online at the first sign of a sniffle, in case you didn't know), here comes a new condition to worry about. Luckily, although many of us do, in fact, show signs of fertilization myopia, it can be cured without a single infomercial-shilled medication. All that's needed is a better understanding of insect sex, which might also help us understand sex in other creatures along the way.
Fertilization myopia is a term coined by Bill Eberhard, a biologist who works in Panama and Costa Rica on a wide variety of spiders and insects. For the last twenty years or so, Bill has been intrigued—some might say obsessed—by animal genitalia and the finer details of insect mating. Despite what you might think given this predilection, he is a gracious and genial man and is married with children. He just happens to have an abiding curiosity about the natural world and an unwillingness to accept the conventional wisdom regarding mating behavior.
Until quite recently, that conventional wisdom held that once a male and female mated, from an evolutionary perspective, it was all over. Sperm had been transferred, and now all that remained was to wait for the offspring to appear and carry on their parents' genes. Fertilization was the goal, and we didn't look beyond it. Even in humans, people assumed that the exciting part was the lead-up to sex: the partner choice,