Urban grasshoppers are changing their tune.
According to a new paper in Functional Ecology, males that dwell by busy roads boost the bass of their courtship songs to be heard above traffic.
Previous research has shown that human-made sounds affect the calls of birds, whales, and frogs. This study is the first to show that insects aren't immune.
Ecologist Ulrike Lampe and her colleagues at Bielefeld University in Germany rounded up 188 male bow-winged grasshoppers (Chorthippus biguttulus)—half from quiet places, half from roadside spots—and exposed them to a female grasshopper. When the road warriors "sang" their two-second-long courtship song by rubbing their hindlegs against their front wings, they increased the lower frequencies.
Their country cousins did not. Lampe says the bass boost helps males be heard over the din of traffic, which could be disturbing the species' call-and-response mating rites. The fact that these males sang loudly in a quiet lab environment, she adds, suggests that the change is "not a spontaneous behavioral adaptation to noise" but a long-term effect.
Lampe doesn't know if other insect species are evolving similarly. But she suspects that other types of human-made noise—from places like construction sites, airports, and train stations—would have a similar effect on grasshoppers.
Bow-winged grasshoppers are found throughout northern and central Europe. They vary in size (from 1.5 to 2 centimeters) and color (from green and brown to red and purple). Males "sing" by rubbing their hind legs against their front wings, producing a broadband signal. Most of their song occurs in a range the human ear can't hear. (See bug pictures.)
"We can distinguish between the extremes, though," says Lampe. "If we have one grasshopper that produces songs with very high frequencies and one that produces songs with [roughly] 1 kilohertz lower frequencies, we can hear the difference."