Sunday, January 7, 2001		Nature

BATS are master-hunters of the dark. Different species of this mammal hunt in different ways. Some listen to sounds created by prey while others rely on their own sound to detect food. The long-eared bat, named for having extraordinary large ears, is one of the ‘gleaner’ bats that has such sensitive hearing that it can detect the sound of an insect’s wingbeats. Many others adopt different strategies . They do not listen for the sounds made by the prey; rather, they produce their own, very high-frequency sounds and listen for the echoes that bounce back, a behaviour known as echolocation.

A small insect-eating bat, flying about in no kind of hurry, ensures that it does not bump into anything by producing pulses of echolocation sound 5-20 times a second. If an echo bouncing back from prey is received, the pulse-rate goes up to, say, 50 times a second. As the bat homes-in on its target, jigging rapidly this way and that, it needs even more detailed information about size, speed and direction of the prey and increases its sound emissions to a buzz containing 200 or more pulses per second. It is critical for the bat to have this data for its manoeuvres in the dark must be more precise. To help gain even more information, the bat can adjust its call frequency. Many of the small bats, known as ‘FM’ bats (which stands for frequency modulation), sweep their echolocation sounds downwards at the end of each pulse. In doing so they extend the band width of their signal and gain extra information about the whereabouts of the target.

The greater horseshoe bat does it differently. It is one of the ‘doppler’ bats, and any motorist caught speeding by doppler radar will understand how effectively it works. This prey location system operates, not by looking at the time taken for the echolocation signal to bounce back, but by comparing the frequency of the returning signal with that of the outgoing. A bat knows what frequency it sent out, but the frequency coming back will be a little different if the prey is moving with respect to the bat — a phenomenon known as the Doppler Effect. In short, if the target is moving away, the frequency of the returning signal drops, but if the prey is coming towards the bat, the frequency goes up. The difference in frequency between the transmitted signal and the echo is, therefore, a means of measuring the relative motion of the predator and prey: an amazing calculation for a little bat to be able to do.

Doppler bats, like the greater horseshoe, have a less frantic hunting tactic than the noctule. It remains stationary, sometimes at the roost site, rotating its head and scanning the surrounding area with a narrow beam of sound, much like a ground-to-air-missile radar. When a contact is made, the bat flies out, catches it and returns to the launch site.

Some bats, like the European pipistrelle, make use of both methods depending on the circumstances. It is the commonest of the British bats and is the one usually seen flying just above ones head in summer evenings. At the start of a hunt, the bat needs to survey as large an area as possible and can do so with a long, low-frequency signal from which it can receive back a useful ‘Doppler’ signal. This gives it good information about the speed of the target and the direction it is heading but little about its actual distance from the bat. So, having made contact, the bat switches to the ‘FM’ system, emitting increasingly faster pulses of frequency sweeps until the target insect is caught.

Most of the small bats hunt by echolocation using very high-frequency signals (ultrasounds), but there are some that break the rules. These are the ‘gleaners’. The pallid bat of North America is a gleaner. Instead of emitting its own sounds, it listens for the sounds made by the prey. Moths and other flying insects ‘warm-up’ before take-off. They must raise the temperature of their flight muscles before they can fly efficiently and the pallid bat homes-in on the wing vibrations.

Another rule breaker lives in the Central American rainforest, but it ignores the rich variety of jungle insects in favour of tiny frogs. The fringe-lipped bat is fond of mud-puddle frogs, and it has discovered that the location of its favourite prey can be found by listening to the mating calls. The bat navigates through the forest using normal high-frequency echolocation, but on picking up the calls of the frog, it switches off, homing-in on the lower-frequency calls of the frog. The fringe-lip sweeps down, grabs the frog in its mouth and heads for home.