Ah, spring, when the meadows come alive with the sweet trilling of grasshopper come-ons. The bow-winged grasshopper attracts females with a very specific song—so specific, in fact, that it is the only thing that distinguishes it from related species in the field. In quiet, Alpine grasslands, this system works like a charm. But as urban development encroaches on more of the grasshopper’s habitat, city noise is getting in the way of the grasshoppers getting it on.
Scientists have already determined that some species of birds, mammals, and frogs change their mating calls in response to urban noise. In a new study in Functional Ecology, scientists report that urban-dwelling grasshoppers, as well, have responded by changing parts of their tune to a higher frequency—one more easily differentiated from traffic.To attract a female bow-winged grasshopper, the male rubs a comb-like structure on his hind leg against a vein that protrudes from his front wing. A female, if sufficiently impressed, will call back and invite him over. If these precisely tuned messages get lost in the din of nearby traffic, female grasshoppers may not hear, recognize, or properly gauge a male’s mating call. This is bad news for the survival of the species.
It makes sense, then, that grasshoppers would adapt to the surrounding noise. But they seem to do it differently that other animals do, the researchers found. Urban animals in other studies changed the volume, pitch, or timing of their calls in order to be heard in noisy environments and reverted back to their normal calls in quieter conditions. But urban grasshoppers called at a higher pitch than their rural counterparts all the time, even in a quiet lab. This means that the change is not just behavioral. It suggests that they have actually evolved to call higher as a result of selection pressure from their aural environment.
This is the first time that the mating call of an animal species has shifted this way to deal with city noise, but it is unlikely to be the last, as scientists learn more about the ways in which animals adjust to life in an increasingly urban world.
Image courtesy of Quartl via Wikimedia Commons