Some hawkmoths have evolved a surprising way to defend themselves from bats – they use their genitalia to produce ultrasonic signals. The sounds can disrupt echolocation, which predators use to locate their prey in the dark. A new study shows that this unusual ability has evolved independently at least seven times over the past 30 million years, reports Naked Science.
The discovery was made in members of the hawkmoth family (Sphingidae) – nocturnal butterflies known for their rapid flight and the ability of some species to hover in mid-air like hummingbirds. The study, led by entomologist Paul Maysnaker of the Florida Museum of Natural History, was published in the scientific journal Proceedings of the Royal Society B.
The scientists found that male hawkmoths produce ultrasound by rubbing specialized structures on their genitalia against the surface of the abdomen. The mechanism resembles the way crickets produce sounds by rubbing parts of their bodies together.
The researchers identified three different mechanisms for producing ultrasound. In some species, specialized scales rub against one another; in others – against tiny spines; and in still others – against small projections on the abdomen.
The resulting sounds have a frequency that the human ear cannot perceive. When the recordings are slowed down to frequencies audible to humans, they resemble rapid clicking, buzzing, or a series of short vibrations.
According to the scientists, the longest ultrasonic signals have characteristics that could make it difficult for bats to navigate. The predators use echolocation by emitting high-frequency sound pulses and analyzing the reflected signals to determine the location of insects.
Not all of the butterflies' sounds, however, are long enough to disrupt this process. The shorter signals likely serve another defensive function – they may startle an approaching predator or mimic the warning signals of other insects that are poisonous or distasteful.
Interestingly, some hawkmoths have auditory organs that allow them to detect approaching bats and respond with ultrasound. Other species lack this ability but nevertheless produce sounds when they come into physical contact, suggesting different defensive strategies.
The most interesting finding is that the mechanism was not inherited from a single common ancestor. Genetic and anatomical analyses show that different groups of butterflies independently evolved similar abilities over millions of years.
This is an example of convergent evolution – a process in which different organisms arrive at similar solutions to the same problem.
The scientists suggest that in some species, ultrasonic signals may also serve additional functions, including recognizing mates during reproduction. However, this hypothesis has yet to be tested.
The study shows how complex the relationships between predators and prey can be. Over millions of years, bats have refined their ability to hunt using echolocation, while nocturnal butterflies have developed increasingly unusual ways to evade their attacks.