Manoeuvring through the air that is firing off ultrasound to track your every move seems daunting, but the praying mantis has evolved an impressive acrobatic trick to evade their big brown bat predators. Employing its single ear, tuned to the bat's frequency, the mantis performs a sudden rapid dive in mid-flight when the ultrasound hits its body. Scientists have studied this defence technique for some time but they knew nothing about the counter-attacks that big brown bats might launch to catch up with the escape artists. To find out more about the bat's response to the insect's evasive action, Kaushik Ghose, then a graduate student at the University of Maryland,and his colleagues Cynthia Moss and Kari Bohn teamed up with insect escape experts Jeffrey Triblehorn and David Yager to film and record big brown bats pursuing diving praying mantises(p. 693).
Capturing the aerial dogfight in a laboratory flight room equipped with high-speed infrared video cameras and ultrasonic microphones, Ghose recalls that `it was tricky to get both animals into the air at the same time'. However, once a chase was established, the big brown bats emitted a characteristic ultrasound pattern that the researchers used to identify pursuit sequences and analyse the animals' movements.
In previous experiments with deafened mantises that could not dive, Ghose had discovered that big brown bats use a `parallel navigation' strategy where lines drawn between the bat's and insect's positions are always parallel,regardless of the animals' individual trajectories. Ghose explains that this strategy allows the bat to constantly adjust its own movements to those of the mantis. He adds that this is the same strategy used by guided missiles and allows the bat to minimise the time it needs to catch up with unpredictably moving targets. `We still don't know how the bat does it, but we think it locks its sonar beam to the target and then keeps its head steady in relation to the mantis,' Ghose explains.
So what happened when the mantises dived suddenly? The team found that a quarter of the bats did not manage to follow the insect into the dive and changed their ultrasound emissions to a pattern suggesting they were no longer interested in the mantis. They had given up completely.
However, in the remaining cases the bats dived too, closely tracking the mantis, although none of them managed to successfully catch their prey. When plotting lines between the animals as they both plummeted, the researchers found that the lines drawn between the two animals also remained parallel during the dive, suggesting that the diving bats use the same parallel navigation strategy that they appear to use during dive-free pursuits.
Despite the lab bats' lack of success, Ghose suspects that bats in the wild would eventually be able to catch the mantis. He attributes the lab bats'failure to intercept the diving insects to the constraints of the experiment:`the flight room is not very high and the mantis often ended up on the ground so the bat pulled up to avoid smashing itself on the floor,' he explains. But out in the open, with more height, a bat that managed to follow the mantis into the dive may well have better luck.