Learning to listen: how some vertebrates evolved biological sonar

Science News, May 14, 2005 by Sid Perkins

The aggressor swoops low over the treetops, piercing the night with a barrage of sonar pulses and searching for telltale data bouncing back. Some prospective targets perceive the ultrasound, take evasive action, and escape. Others, the unwary ones, are fair game. When the prowling aerialist senses the faint echoes bouncing off one of these prey, he turns toward the target, quickens his chirp rate, and homes in for the kill.

This isn't a duel between modem fighter pilots, but an aerial battle that's been raging nightly for millions of years. It's bat versus insect. Bats are members of one of the most diverse groups of mammals, and the echolocation capability that enables some bat species to detect, track, and catch insects on the wing--even ones as small as mosquitoes--is a crucial part of bats' success.

Sonar use has evolved independently among widely disparate groups of creatures. For aquatic mammals, such as porpoises and whales, the sequence of adaptations that led to echolocation is well preserved in the fossil record of their ancestors. But no such trail exists for bats, a group whose oldest known remains indicate that echolocation was already in use.

In the handful of bird species that use sonar, the origin of that ability is even murkier. Some echolocating species have close relatives that apparently possess the anatomical means to echolocate but don't use it, implying that avian echolocation is a behavior that some species simply haven't learned. For insights into how echolocation evolved in birds and bats, scientists are turning to DNA, a modern source of information about ancient biological relationships.

Although tiny bats and toothed whales may seem to be as different as night and day, they do have something in common. A few species of their respective prey can detect high-frequency sonar and have developed a variety of techniques that increase their odds of escape and survival in the ever-escalating arms race of evolution (see sidebar; Biological Arms Race).

LISTEN UP Sonar systems on modern submarines were inspired by the principle behind biological sonar: Send out a short burst of sound and then listen for the echo. The direction from which the echo arrives and the time it takes to come back reveal a target's location and distance. In general, the higher the frequency of the emitted sound, the better echolocation works: High frequencies correspond to short wavelengths, and the rules of physics require short wavelengths to detect small objects.

For whales, the evolution of efficient biological sonar took about 30 million years. Around 50 million years ago, mammals known as pakicetids--the land-dwelling ancestors of modern whales--foraged in the rivers and streams of what is now Pakistan (SN: 9/22/01, p. 180). Those creatures, like most land mammals, could hear well in air but poorly underwater, says Zhe-Xi Luo, a vertebrate paleontologist at the Carnegie Museum of Natural History in Pittsburgh.

Fossils of pakicetid descendants that lived during the next 10 million years show a gradual improvement in their hearing underwater. During that period, the role of the outer ear in funneling sound to the middle ear was minimized, and the lower jawbone became the animal's main sound receptor.

Members of these aquatic-adapted species weren't echolocators, however, because they didn't have structures in their breathing passages that would enable them to make high-frequency sounds, says Luo. Those sound-generating organs, which in modern whales are chambers in the nasal passages, evolved later, during a period when nostril position changed from far forward on the nose in older species to high on the head in more-recent ones, he notes.

Then, a little more than 30 million years ago, the whale family tree split into two major lineages. One branch, the toothed whales, today includes porpoises, killer whales, and sperm whales. This branch evolved organs to produce high-frequency chirps and inner ear structures to detect them. By 18 million years ago, the ancestors of today's dolphins had an ear structure that suggests that they could echolocate as well as their modern relatives can.

If only the fossil record for bats' progenitors were as rich or as revealing as is that of the whales. The oldest bat fossils, belonging to an extinct lineage, were unearthed from rocks about 54 million years old, but the creatures that they represent aren't dramatically different from living bats, says Mark S. Springer, an evolutionary biologist at the University of California, Riverside.

Hallmark features of these creatures include the elongated fingers that support the wing membranes and the extensive coiling of bony structures in the inner ears, a sign that they were capable of detecting the high-frequency chirps used in echolocation. The few bat fossils unearthed to date don't include the soft tissues, such as sound-producing organs, that could show whether the creatures had the ability to produce high-frequency sounds. But some fossil bats' stomach contents, which include large numbers of flying insects, strongly suggest that the ancient animals could echolocate.