When the cicadas of Brood II burst into
open air—and into song—later this month, after living 17 years in
darkness below ground, they will have one thing on their collective,
eerily synchronized mind: sex. Though millions of humans inhabiting
the mid-Atlantic states will soon hear the insects’ incredible
racket, they’re probably unaware that what they’re hearing is an
enormous mating festival.
These particular insects have been
underground—where the sexually immature nymphs dig around, suck on
tree roots, and bide their time—since 1996. This is their
coming-out party, their debutante ball, and we all get to be a part
of it, whether we decide to make ice cream out of them, fry them into
tasty little bug nuggets, or just sit out on the porch in their
deafening hum, pondering the peculiarities of other species’
reproductive habits.
Rising en masse from the ground is
particularly spectacular way of showing that you’re ready to
reproduce, but the big majority of creatures has some signal. If
you’ve visited a zoo, you may have seen those swollen pink rumps on
female chimps, proclaiming that they’re in a period of fertility.
Other creatures release tantalizing smells or put up other signs that
say they’re open for reproductive business.
Though it’s pretty clear when humans
reach sexual maturity—one might argue that the flood of hormones
around puberty creates the societal equivalent of the cicadas’
roar—we don’t actually show when we are fertile. The days in a
woman’s monthly cycle when she has an egg slotted into position,
ready to be fertilized, are very well-hidden, even from the woman
herself. A 2007 study found that lap-dancers got significantly more
tips from male customers when they were fertile, suggesting that
there may be signs that no one is consciously aware of; that study
has spawned (so to speak) a line of research on other evidence that
women’s ovulation is somehow perceptible, but compared with most
other animals, the clues are extremely subtle. Instead, women look
like they could be fertile anytime. We don’t usually think of it
this way, but having breasts that continuously protrude, rather than
just when one is in heat or pregnant, is pretty weird in the animal
kingdom.
It’s still mysterious why humans are so
different in this respect—what our species might have gained from
keeping our reproductive status under wraps. It could be that looking
fertile all the time, even when one is not, means that males have to
stick around all the time in order to ensure that they have offspring
live to reproduce. This could mean that couples have to commit to one
another long-term, with each contributing to the raising of some very
time- and resource-hungry offspring—a good thing for infant
survival. Or it could be that uncertainty over whether a given female
is fertile means that there’s less conflict between males to mate
with her and hence less violence within a population. Or maybe that
not knowing for sure that a baby is one’s own cuts down on
infanticide, a method used by male chimps to eliminate competitors’
offspring.
At this point, there’s no consensus as
to what might have happened to make this our status quo—these are
all stories spun by scientists to help get a handle on the problem.
But they are something to consider while listening to cicadas drone
and reflecting on how funny it is to witness a distantly related
species’ grandiose fertility peak, while our own remains a secret.
Veronique Greenwood is a former staff
writer at DISCOVER Magazine. Her work has appeared in Scientific
American, Popular Science, and the sites of Time, The Atlantic, and
The New Yorker. Follow her on Twitter here.
