Humans have long been fascinated by animals that detect the world through sensory systems that we lack, such as dolphins and porpoises, which navigate by echolocation. However, many of these animals, which dwell in coastal waters, are at increasing risk from human activity. Ursula Verfuß from the University of Tübingen explains that many die entangled in fishermen's nets; `either they don't perceive the nets or they don't perceive them as a threat,' she says. Knowing that harbour porpoises residing in the German Baltic are particularly at risk, Verfuß and her colleagues Lee Miller, Peter Pilz and Hans-Ulrich Schnitzler were curious to know how the animals perceive the world around them. Having already discovered that the porpoises navigate by reducing the interval between echolocating clicks as they approach a landmark, the team decided to find out how the animals adjust their sonar as they close in on a tasty snack(p. 823).

Verfuß travelled to the Fjord&Bælt centre in Kertminde,Denmark, to work with two young porpoises, Freja and Eigil. According to Verfuß, the porpoises were very cooperative and fun to work with, but working in the open was a challenge. `We couldn't film for days after a storm because the water was disturbed and another time the enclosure was full of jelly fish,' Verfuß recalls. However, when conditions were good, one of the porpoises' trainers threw a brook trout into the porpoises' enclosure while Verfuß filmed the animals and recorded their echolocation clicks as they homed in on the victim.

According to Verfuß, the porpoises speeded up after hearing the splash and eventually rolled onto their backs in the final moments before sucking the fish down. And when Verfuß repeated the experiments with Freja's eyes covered, she slowed a little but still caught the fish. The porpoises could hunt successfully by echolocation alone, but how were they using echolocation to locate the hapless fish?

Analysing the porpoises' click sequences, the team realised that the pursuit could be broken down into three phases: search, initial approach and final approach. During the search phase, the porpoises seemed to lock their navigation system onto a local landmark and gradually reduced the time intervals between clicks as they closed in. But as soon as the animals spotted the trout, the click pattern changed. The porpoises maintained a constant average click interval as they drew closer to the victim until they were close enough to pounce. Then they rapidly accelerated their click rate and reduced the click interval to a few milliseconds, just like echolocating bats, which produce a buzz of clicks in the moments before capturing an insect.

Verfuß admits that she hadn't expected the porpoises to switch to an almost constant click interval during the initial approach phase. She had thought that the animals would continue shortening the click interval, sending clicks out as soon as the echo returned, speeding up the echolocation process as they approached the trout. However, Pilz suggested that maybe the porpoises switch to a constant click interval to confuse their targets. Verfußexplains that some fish have good hearing, and could be alerted to the porpoises' approach by the accelerating clicks. According to Verfuß, it is possible that by switching to a constant average click rate, the porpoises lull their prey into a false sense of security, making it easier to sneak up and gulp them down.

Verfuß, U. K., Miller, L. A., Pilz, P. K. D. and Schnitzler, H.-U. (
). Echolocation by two foraging harbour porpoises (Phocoena phocoena).
J. Exp. Biol.