Nature, it seems, strategizes so species survive. That’s why a caterpillar crawling amid greenery turns into a green pupa, blending into its surroundings and lessening the chance a bird will make it breakfast. Usually, a butterfly will emerge.
And that’s why, as women age, their bodies switch from ovulating single eggs to ovulating two, increasing the chance that they will give birth to a baby healthy enough to grow up and reproduce. Occasionally, fraternal twins will be born.
Biology professor Wade Hazel has studied the “conditional strategy” that produces butterflies and babies and was among five authors of a paper published May 11 in the prestigious journal Nature Ecology & Evolution. He is set to retire June 30.
Hazel started studying butterflies as an undergraduate at Virginia Polytechnic Institute and State University and continued throughout his schooling and his 30-year career at DePauw. About five years ago, he partnered with Rick Smock, a DePauw emeritus mathematics professor, and colleagues at the University of Western Australia and the London School of Hygiene and Tropical Medicine to study how conditional strategy affected the birth rate of fraternal twins.
Fraternal twins are relatively rare, occurring in fewer than 15 out of every 1,000 births in most countries. The phenomenon that leads to fraternal twins – a woman ovulating more than one egg at a time – is common, but “the chances of one embryo making it to birth declines rapidly as women age,” Hazel said.
Nature apparently tries to make up for that. Young women rarely ovulate two eggs, but women approaching the end of their reproductive lifetime double-ovulate half the time – nature’s strategy to increase the chance of a live birth. Of course, sometimes both eggs are fertilized and mature properly, resulting in the birth of twins. Hazel and his colleagues observed that the likelihood that a woman will produce fraternal twins increases as she ages, peaks when she is in her mid-30s and then declines.
Hazel, Smock et al. explained the pattern of twinning mathematically “but then we were also able to … explain the pattern in an evolutionary sense: Why is it that this happens? And the reason it happens is because, as the probability of live birth declines as women age, the way you can keep your reproduction going is to ovulate more than one egg. It turns out that increasing double ovulation appears to be an adaptive, evolved response to that declining live birth rate.”
Explaining something that no one has ever before explained “is part of why you do science,” Hazel said. “Even if it’s just you figuring it out for yourself … it’s kind of fun. I think that’s why people like doing it, because you can figure something out. It really doesn’t matter if it’s the color of butterfly pupae or, really, how to build a house, you know? It’s the same stuff; it’s nice to accomplish something.”
He and his colleagues plan to continue their work; “I’m retiring, but I’m not going to quit being a biologist,” he wrote in a recent blog entry.
They’re intrigued by variations in the rate of twinning among countries; twins are much more likely to be born in Africa, for example, than in Asia.
“So what explains that pattern? We’ve got some ideas for that,” Hazel said, but that’s all he’ll say publicly for now.