creature feature. The cicada

We humans are not equipped with the sharpest of senses — we don’t see as well as owls or hawks, we don’t hear or smell as well as dogs. Nevertheless, the information we receive through all our senses is integrated into our perceptions of any place, and when we’re outdoors, we notice different creatures through different sensory routes. In many forests in the summer, there’s one insect that you will undoubtedly hear long before you see or smell or touch it: the cicada.

The incessant and incredibly loud buzzing produced by the male cicada is a signal. Cicadas live their early lives underground, feeding on tree roots, and when they emerge in large numbers from the soil to transform into adults and mate, males and females must begin by finding each other. The males’ persistent clicking not only lets females locate them but may also give them the information they need to choose the most suitable mate. This cacophony is crucial to cicada reproduction.

For a male cicada to have any chance of fathering offspring, his song must stand out amid the din of thousands of other cicadas. He must sing through light and dark, come rain or shine. Cicadas have therefore evolved an impressive bag of behavioural and physiological tricks to optimise their signal. The most impressive of these tricks is a capacity for heating up their own bodies, or “endothermy,” an ability usually found in mammals and birds.

Most insects are ectothermic, which means that they depend on heat from the sun to raise their body temperatures before they can begin the day’s activities. But male cicadas are more finicky than most insects, and must attain a particular temperature before they can produce the exact song that alerts females of their own species to their presence. At lower temperatures, a male cicada needs to warm up by producing a sound that may alert predators to his location but will not attract a female cicada — a lose-lose proposition, if there ever was one. It’s therefore in the male cicada’s best interests to be able to raise his body temperatures independent of the sun, and he does so by co-opting the process that converts energy into movement. Insect flight muscles are particularly inefficient at turning energy into movement, but this inefficiency is fortuitous for the cicada, because energy that doesn’t become movement is lost as heat.

Cicadas are found across the world in all sorts of environments, and it isn’t surprising that different species of cicadas vary in exactly how they go about warming up. Some cicadas are endothermic only some of the time. When the sun is out, they behave ectothermically, positioning themselves in full sunlight and retreating to the shade if they get too hot. But at dusk, these cicadas become endothermic, flying frantically between trees to stay warm enough to continue singing all evening. Other cicadas are endothermic all the time, and therefore must decouple heat production from flight, else they’d need to fly constantly. Instead, they shiver imperceptibly and breathe deeply as they sing, pulsing their abdomens in and out to increase their capacity for oxygen. Both routes to endothermy allow a chorus of cicadas to sing continuously through the summer, making their raucous hum the forest’s background score.

(Ambika Kamath studies organismic evolutionary biology at Harvard University)

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