Bat breath adds fuel to evolutionary flap.

Bat breath adds fuel to evolutionary flap

Flying consumes a lot of an animal's energy. Sending out ultrasonic pulses and analyzing their echoes can also burn up plenty of fuel. So blind bats that use sonar systems to "see" must be real energy guzzlers when they fly.

Wrong.

Echolocating bats are surprisingly energy efficient in flight, report two zoologists, who believe their finding supports a controversial theory that some bats evolved from primates.

Jonathan R. Speakman and P.A. Racey of the University of Aberdeen in Scotland studied two species from the suborder Microchiroptera -- small bats that navigate by sonar instead of sight. Armed fist-sized "microbats" use to echolate while the rest, they calculated the energy cost of sonar-navigated flight by comparing pre- and post-flight levels of oxygen in the bats' breath and carbon dioxide in the bats' blood. Surprisingly, the energy needed to fly by sonar was only slightly more than the energy of echolocating at rest, they report in the April 4 NATURE.

In a companion article, zoologist Jeremy M.V. Rayner of the University of Bristol in England suggests that a flying microbat saves energy by pumping out each ultrasonic pulse "on the back of" a wingbeat and a simultaneous exhalation of breath from the lungs.

Even so, a good pair of eyes would seem to save a bat some bother. But when Speakman and Racey went on to compare the energy costs of flight in microbats and in larger, sighted bats of the suborder Megachiroptera, controlling for the difference in body size, they discovered that microbats and megabats use approximately the same amount of energy.

Echolocation's negligible energy cost during flight, combined with its superiority over vision as a means of detecting insects, raises the question of why so few megabats have sonar rather than visual systems, Speakman and Racey assert. They say their results fit with the theory that microbats and megabats evolved separately, with microbats emerging first and megabats appearing about 30 million years later. According tol this controversial scenario, megabats probably descended from a primitive flying lemur -- which, like all primates, had made an early evolutionary commitment to a highly developed visual system.

In contrast, the standard theory -- based mostly on wing similarities -- holds that microbats and megabats evolved from nocturnal mammals of the order Insectivora, and that most megabats later lost their echolocating abilities.

Those who argue for the bats' separate evolution cite several lines of evidence. In 1986, for instance, studies of the megabat visual system turned up "about 30 very striking features which were thought to be unique to primates," says John D. Pettigrew of the University of Queensland in Australia, who directed the work. In addition, fossil records suggest that well-developed, echolocating microbats existed more than 50 million years ago, while the oldest megabat fossils are only 20 million years old and show very primitive wing formation, he says.

Pettigrew cautions, however, that the fossil records for both microbats and megabats are very sparse and that any conclusions based on these records remain "very speculative."