If you saw your lunch step off your plate, you might lose your appetite completely. If you happen to be a neotropical bat, though, a walking meal might just be a good way to find your next meal. Some bats use echolocation to detect their food, and movement might signal to a bat where they can find a tasty insect instead of a leaf. One katydid species – a grasshopper relative found in Central America – often finds itself next on bat menus and walking is not the only form of movement that could give these potential victims away. Grasshoppers produce loud chirrups to attract mates with lavish movements that echolocating bat predators could tune into. But, some insects produce mating calls with remarkably little movement by quietly vibrating their abdomens. These whispers may have developed as a channel that eavesdropping bats are less likely to spy on. Nevertheless, the diet of one particular type of bat, Micronycteris microtis, is largely made up of these quiet katydids. A team of researchers at the Smithsonian Tropical Research Institute in Panama, with researchers from Dartmouth College and The Ohio State University, USA, set out to assess whether or not M. microtis are tuning in to the insects’ movements or their discrete chirrups in order to locate the quiet prey.
The team designed a flight cage, which they allowed wild-caught bats to explore until they found dead katydids in specific locations around the cage. Next, they assessed the bats’ attraction to different types of movement by presenting them with a choice between two toy crickets, which resembled the quiet katydids, that were still, ‘walking’ or ‘whispering’ for a mate. To mimic walking, the experimenters attached the toy to a fishing line and gently slid it 15 cm. To mimic the katydid's mating call, they recorded the wild insects and replayed the calls through a mini vibrating machine attached to the toy, so that it would buzz at the same frequencies. The experimenters then recorded which toy the bat approached with its sonar beam from multiple angles.
Every bat chose a walking toy over a still toy, suggesting that although the bats can identify prey that is not moving – like the dead bugs they initially retrieved – they are more attracted to movement. To verify that their results were not due to random chance, the experimenters then simulated how 5000 arbitrary bat choices would play out to compare against their observations and found that the bats’ preference for moving toys, either walking or buzzing, was unlikely to be due to chance alone. The team also observed that the bats spent more time approaching moving prey, whether it was walking or buzzing. The longer time spent scanning with their sonar beam allows the bats to gather more information and build a better idea of where the prey is and where it is headed.
Although the bats might not be eavesdropping on the insects’ private communication channel to locate a tasty snack, movement alone is sufficient to betray the softly spoken creatures, potentially sending the females unwittingly right into a searching bat's belly as they approach a whispered romantic serenade.