December 13, 2013
Bats Rely On Vision To See Ahead, Echolocation To Catch Insects: Study
April Flowers for redOrbit.com - Your Universe Online\
Bats rule the night skies, using the power of echolocation, or reflected sound. More than 1,000 species of echolocating bats exist, compared to just 80 species of nocturnal non-echolocating birds. It seems that normal vision works in tandem with echolocation to give bats an evolutionary edge, however, no one knows exactly how.A new study, led by Arjan Boonman and Yossi Yovel of Tel Aviv University's Department of Zoology, suggests that bats use vision to keep track of where they are going and echolocation to hunt tiny insects that are invisible to most nocturnal predators. Published in Frontiers of Physiology, the study results add to our understanding of sensory evolution.
"Imagine driving down the highway: Everything is clear in the distance, but objects are a blur when you pass them," said Dr. Boonman. "Well, echolocation gives bats the unique ability to home in on small objects — mostly insects — while flying at high speeds."
The majority of a bat's feeding takes place at dusk, when insects are most active and there is still plenty of light. Vision seems better suited to these conditions than echolocation. Vision conveys more information, and more quickly, at a higher resolution. This led the researchers to a question: if, as scientists believe, vision evolved in bats before echolocation, why did echolocation evolve at all?
To answer this question, the researchers compared the distances at which the two senses can detect small objects. The team played taped calls of two species of bats in a soundproof room to estimate the range of ultrasonic bat echolocation. They recorded the way the sound bounced off four dead insects — a moth, an ant, a lacewing, and a mosquito. Because vision is hard to simulate, they extrapolated from the findings of two prior studies to calculate the distance at which bats would be able to see the same insects in medium to low light.
The research team found, even erring on the side of vision in their estimates, that echolocation was twice as effective as vision in detecting the insects in medium to low light — a difference of 40 feet away with echolocation versus 20 feet with vision. In addition, they found that echolocation is unaffected by objects in the background, while visual range is three-to-five fold worse when it has to contend with obstacles such as vegetation. Prior research has revealed that echolocation provides more accurate estimates of the distance and velocity of objects, and sometimes even of the distance of the background behind them.
Because it allows bats to track insects from farther away and with greater accuracy at peak feeding times, the study results suggest that echolocation gives bats a huge evolutionary advantage. In addition, it allows bats to continue hunting into the night, long past when their competitors are blinded by darkness.
Echolocation has some negatives as well. It is poor at detecting large objects in the distance, where vision is able to detect them at distances several orders of magnitude greater than echolocation does. This leads the researchers to believe that bats use both senses in combination — vision mostly for orientation, navigation, and avoiding large objects in the distance, and echolocation to search for small prey. They suggest that different species of bats probably combine the senses somewhat differently.
"We believe that bats are constantly integrating two streams of information — one from vision and one from echolocation — to create a single image of the world," said Dr. Yovel, also of TAU's Sagol School of Neuroscience. "This image has a higher definition than the one created by vision alone."
A large nocturnal advantage has opened up for bats with the combination of vision and echolocation. This advantage allowed them to multiply and diversify, with bats now accounting for 20 percent of all classified mammal species on Earth today. The team suggests that nocturnal birds may not have evolved their own ultrasonic echolocation for anatomical reasons. They want to continue their investigations by researching how bats integrate echolocation and vision and what the evolutionary costs of echolocation are.
Another similar study came to light just yesterday.
Researchers found that the Pallas long-tongued bat uses a stealthy form of echolocation to trick its insect prey.