Why Do Cicadas Stay Underground for So Long?

Summary

The video explains the unusual life cycle of periodical cicadas, especially the North American species that emerge in enormous, synchronized groups every 13 or 17 years. It presents several possible explanations for these long cycles, emphasizing that no single explanation is definitive.

Cicadas and their life cycle

Cicadas are true bugs in the order Hemiptera, not locusts. They feed on plant sap and generally do not threaten crops in the way locusts can. Most cicada species have shorter, overlapping life cycles, so some are active every year.

Periodical cicadas of the genus Magicicada are different. Of the seven known species, four emerge every 13 years and three every 17 years. They are found in the eastern United States.

A female lays eggs in slits in tree bark. After hatching, the young cicadas fall to the ground and burrow underground, where they feed on root sap and pass through five stages of development. When the soil reaches the right temperature, usually in spring or early summer, mature nymphs emerge, climb a vertical surface, and shed their exoskeletons. The adults then harden their bodies and wings, mate, and lay eggs.

Adult cicadas live only about four to six weeks. Males attract females with loud, species-specific songs, while females signal their interest by flicking their wings. Emergence is so synchronized that cicadas can reach extraordinary densities. Their sheer numbers help ensure that many survive, even though predators consume large quantities.

Broods and historical records

Periodical cicada populations are grouped into “broods” based on where and when they emerge. The video says entomologist C. L. Marlatt organized them into numbered groups in 1898. Many of Marlatt’s proposed broods were never observed, and some are considered extinct. The video reports that 15 broods are currently known.

One of the largest, the Great Eastern Brood, spans several eastern states and the District of Columbia. The subtitles refer to it as “Crust 10,” apparently an automatic-caption error for “Brood 10.” Historical records of cicada emergences date back more than 300 years.

Why emerge in huge numbers?

Predator saturation may explain why cicadas emerge all at once. Their numbers can overwhelm birds, mammals, and other predators: predators may eat many cicadas without consuming enough to eliminate the population. The video compares this strategy with bamboo, which can produce vast numbers of seeds during synchronized flowering.

Why 13 or 17 years? The prime-number hypothesis

Predator saturation helps explain why cicadas emerge in large groups, but not why their cycles are specifically 13 or 17 years. Evolutionary biologist Stephen Jay Gould proposed that prime-number life cycles reduce overlap with predators’ population cycles.

Because 13 and 17 are prime numbers, they share few common factors with shorter cycles. For example, a 10-year cicada cycle could repeatedly align with predators whose cycles last 1, 2, 5, or 10 years. A 13-year cycle would align with such cycles much less often.

Biomathematician Glenn Webb modeled cicada cycles of 10, 12, and 15 years. In the model, those cycles synchronized more readily with predator populations, and the cicadas declined or disappeared. The modeled populations with 13- and 17-year cycles remained stable.

One suggestion for why the cycles are so long is that the interval makes predators that specialize in cicadas less likely to persist through multiple generations while waiting for the next emergence. These arguments are hypotheses, not settled explanations.

Other proposed explanations

• Avoiding competition between broods: Different prime-number cycles might help overlapping populations avoid emerging at the same time and competing for resources. A 13-year and a 17-year population would coincide only once every 221 years. The video cites a simultaneous emergence in 2015 as an example of such a rare overlap.
• Maintaining genetic adaptations: Cicada expert Lou Sorkin proposes that the cycles may be tied to genetics and adaptation to cold. If cicadas adapted to remain underground longer interbreed with cicadas lacking that trait, the adaptation could be diluted. Separate cycles might help keep populations apart.

The video suggests these explanations could complement predator avoidance: interbreeding might create offspring with less predictable cycles, potentially making them more vulnerable to predators.

Threats and uncertainties

The video describes Massospora, a parasitic fungus that infects cicadas, alters their behavior, and spreads its spores through other cicadas. Despite this, it has not caused a major population-wide decline.

Climate change is presented as a more serious potential threat because cicadas use soil temperature to time their emergence. The video warns that earlier or shifting emergences could cause broods to overlap more often, potentially increasing interbreeding and disrupting their long cycles.

Overall, the video concludes that periodical cicadas’ timing may help them evade predators and preserve distinct populations, but scientists have not established one definitive reason for the 13- and 17-year cycles.

Speakers and sources mentioned

• Narrator/host: Unnamed; provides the explanation and closing remarks.
• Stephen Jay Gould: Evolutionary biologist whose essay on bamboo, cicadas, and prime-number cycles is cited.
• Glenn Webb: Biomathematician at Vanderbilt University, credited with modeling cicada and predator population cycles.
• Lou Sorkin: Cicada expert at the American Museum of Natural History, whose genetic and climate-adaptation explanation is discussed.
• C. L. Marlatt: Entomologist credited with organizing periodical cicadas into broods.