Tuesday, July 9, 2013
The great cicada story of 2013 is almost at its end. Brood II has emerged from the ground, created quite a bit of noise in some parts of the Northeast, bred and begun to die off. Now is a good time to reflect on the insects’ incredible feat. Imagine being placed in a cave without a clock or calendar and being told to come out after exactly 17 years. Few other insects — or any species, for that matter — have such a long and precise arc of development.
Many scientists have speculated about the “why” of the cicada life cycle. Coming out together, in a huge horde, diminishes the possibility that predators could wipe out an entire generation of cicadas. There are simply too many of them.
It’s also possible that the life cycles involve prime numbers (17 years, or 13 years for some other broods) — because they might prevent predators, most of which live shorter lives, from synchronizing their own generations around the cicadas.
For example, a predator that lived for six years couldn’t rely on a bumper crop of cicadas at the beginning or ending of any generation. Data from ornithologists suggest that birds, which feed on cicadas, suffer dips in population right around the time the cicadas appear. The mass emergence of cicadas may somehow cause a die-off of their predators, although the mechanism is unknown. (Such patterns are common in nature: The snowshoe hare population, for example, rises and falls in a 10-year cycle.)
The “how” of cicada time measurement is a much more difficult question.
Since a scientist has to wait at least 13 years to observe the cicada emergence, a single researcher may only see two or three generations of a given brood in her entire career. It’s not an easy thing to study.
Entomologists have, however, learned a great deal about how insects measure smaller units of time, such as days and seasons, and those findings may be relevant to the cicada’s 17-year cycle.
“Insects have internal biological clocks,” says Colin Steel, a biologist at York University in Toronto. “They are mostly located in specialized cells in the brain, known as clock cells, used for measuring 24-hour cycles of time.” Further research has shown that clock cells are present in the brains of other animals, including humans.
Steel’s research has helped reveal the genetic basis of time measurement.
Genes in the clock cell produce proteins at varying concentrations throughout the day. The rhythmic nature of the rise and fall of protein concentration enables the animal to know the time.
Since only a few cells in the brain are responsible for time measurement, the animal needs a messenger to transmit the time to the rest of the body. There appears to be some direct communication between the clock cells and the nervous system, but more of the communication happens through hormones. We see this phenomenon not only in insects but also in mammals, which release corticosteroids into the bloodstream at varying concentrations depending on the time of day.
It’s important to note that any animal’s internal time estimate is just that — an estimate.
“Without something to synchronize the clock, it will drift further and further from the correct time of day,” explain Steel. “That’s why they’re called ‘circadian rhythms,’ from the Latin for ‘approximately a day.’ “ Even a bug has a circadian rhythm.
You’ve probably noticed that mosquitoes have an annoying tendency to come out around dusk to feed on human blood. There’s a rather obvious cue for that behavior: the setting of the sun. But researchers have shown that mosquitoes don’t need that cue. If you place a mosquito in a light-free laboratory for several days, it will still begin searching for food at about the same time. On the first day, it may emerge a few minutes late, though, on account of the inaccuracy of its internal clock. On Day Two, it will become active a few minutes later, and so on. In fact, one of the perplexing things about biological timing mechanisms is the consistency of their inaccuracy. All individuals belonging to the same species are mis-calibrated by almost exactly the same amount in each 24-hour cycle.
Of course, the error in internal clocks isn’t a problem in nature, because all animals have methods for correcting their clocks. For day-to-day rhythms, they use the rising or setting of the sun. When a mosquito perceives the sunset, it resets its clock. One of the mysteries of cicadas is how they calibrate their internal clocks. They live long periods without the benefit of the sun, yet their life cycle still progresses like clockwork.
Animals’ biological clocks don’t just assess time of day; they also perceive the time of year. When the animal’s clock cells synchronize themselves to the rising and setting of the sun, they take note of the changing length of the day during the season. The rhythm of the seasons is thus recorded in the animal’s brain. (Animals on the equator, revealingly, tend not to alter their behavior based on time of year.)
So how does all of this relate to the cicadas? As noted above, no one has figured out exactly how these incredible insects manage to measure a 17-year period so reliably. (As with all animals, the cicadas are imperfect. Some stragglers jump the gun, emerging from the ground four years ahead of schedule.) Based on his study of other insects, Steel has a theory, although he’s quick to insist it’s merely conjecture.
“I would guess that the larvae of the cicada goes through a number of different life stages in the ground,” he says, “and it seems likely that each of the life stages is controlled by a clock, and it can go into an overwintering stage at the end of each of these larval stages, according to the day length.”
It’s possible that the cicada doesn’t have to measure 17 years. All it needs to do is measure one year, which is the length of time it takes to go from one larval stage to the next. Perhaps the progression from birth to adult happens in 17 stages, until the perfected adult is ready to climb above ground, breed and die. And raise an incredible din. And if you enjoyed the insect symphony, you need only wait eight more years. The massive Brood X is set to emerge in 2021.