Thursday, 4 December 2014

Non-Echolocating Bats Actually Echolocate Using Wing Clicks

Spectacled flying fox doesn't possess echolocation
Spectacled flying fox (Pteropus conspicillatus)
A non-echolocating species
Credit: Mnolf
Contrary to what most people think, bats are not blind. The truth is that all one-thousand something bat species can see. Most people also think that since bats are blind they rely on their echolocation to get around. Again a mistake, since many bats don't possess echolocation.

For example, most species of Megabats [Suborder: Megachiroptera] have to rely exclusively on their vision. Or that's what we thought up to now..

In a discovery that overturns conventional wisdom about bats, researchers have found that non-echolocating Old World fruit bats -another name for Megabats- actually do use a rudimentary form of echolocation.

What is even more surprising is that they don't use vocalizations at all. Instead, the clicks they emit to produce the echoes that guide them through the darkness are produced by their wings, although the researchers don't yet know exactlyhow they do it.

"I was surprised by the fact that all of the fruit bats we recorded clicked and by the fact that clicks are produced by the wings. Arjan and I still find that hard to believe." said Yossi Yovel of Tel Aviv University in Israel.

Yovel and postdoctoral fellow Arjan Boonman received their first hint about fruit bats using echolocation from a friendly man on a bus in Indonesia who told them about a species of bat that clicked with its wings. As further confirmation, Boonman discovered a single old paper about a fruit bat with wings that clicked, but it wasn't clear whether those clicks served a particular purpose.

Rather than look for that one in earlier-described species, Yovel suggested something else: "Why not check other fruit bats?"

The research team selected a total of 19 wild individuals representing three species of fruit bat and different parts of the evolutionary family tree and discovered that all of them produced audible clicks with their wings.

"We did all we could to prove it wrong, including sealing the bats' mouths and anesthetizing their tongues, but nothing stopped them from clicking, except for when we interfered with their wing flaps." said Yovel.
Further study revealed that two of the three species increased their clicking rate by a factor of three to five or even more when placed in a dark tunnel, strongly suggesting that the clicks are a natural behavior for the bats.

Tests of the animals' ability to find their way in the dark showed that the fruit bats do have echolocation abilities, although to a much smaller degree compared to echo-locating bats. The fruit bats constantly crashed into thick cables, but they could readily learn to discriminate between larger objects, like an acoustically reflective black board versus a similar-looking sheet of cloth.

Even with large objects, however, the fruit bats didn't exactly come in for a smooth landing, suggesting that their ability is rather rudimentary in comparison to that of bats that rely on clicks vocally produced by their larynxes.

According to Boonman and Yovel, the findings are interesting in light of earlier suggestions that echolocation may have evolved initially for bats to identify and avoid crashing into large objects such as cave walls.

The new discovery in fruit bats offers insight into how this sophisticated ability in other bats may have evolved over time, although it is unlikely that the laryngeal clicks of those other bats evolved directly from fruit bats' wing clicks.

Actually, Yovel believes, that is more possible for echolocation to have independently evolved many different times.

"When we study extant species of echolocating bats, we see a developed sensory system that has been adapted and improved over millions of years of evolution. The rudimentary echolocation of the fruit bat is one example of how the first types of echolocation may have evolved." said Yovel.

Highlights of the study
  • Old World fruit bats were found to use a rudimentary form of echolocation
  • This unknown type of sonar employs short clicks the bats produce with their wings
  • The sonar is used to detect large surfaces and is unsuitable for smaller ones, like wires
  • Rudimentary sonar may evolve easily, and fossils must be assessed differently

Large flying fox, an old world fruit bat classified as non-echolocating

- Boonman, A., Bumrungsri, S., & Yovel, Y. (2014). Nonecholocating Fruit Bats Produce Biosonar Clicks with Their Wings Current Biology DOI: 10.1016/j.cub.2014.10.077

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