By Jonathan Farrow from The Thoughtful Pharaoh
Ever wish you could see in the dark? It would make life a bit easier. No more tripping over clutter on the ground or feeling walls for a switch. Humans rely quite heavily on their sight, but some animals can make do by illuminating their surroundings with sound.
Bats are just such an animal. They belong to a privileged group of organisms including toothed whales (like sperm whales, dolphins, and killer whales) and shrews that use sound to see the world. By listening for the reflections of their high-frequency clicks, bats are able to build up an accurate picture of the world around them. The clicks are often too high for humans to hear, sometimes reaching as high as 110 kHz (human hearing generally goes from 20Hz-20kHz). This amazing superpower is called echolocation but not all bats have it. Most microbats (usually small, insect-eating, with proportionally large ears) can echolocate using their throat to produce clicks, while megabats (larger, fruit-eating, with large eyes) usually can’t. Like most rules in biology, though, these distinctions aren’t universal. Some megabats have evolved echolocation by way of specialized nose structures and others are smaller than big microbats.
So now that you’ve been acquainted with the notion of echolocation and the bat family tree, let’s start talking about some neat things that bats can do with their special ability.
Since echolocation is dependent on a bat receiving and interpreting the reflections of sound, it is particularly susceptible to interference. The biggest source of interference is the bat itself. Bats produce some of the loudest sounds in nature and have some of the most sensitive ears to register the reflections that come back hundreds of times quieter. Imagine revving up a Harley Davidson and putting a traffic cone on your ear to hear someone whispering across the room. It would probably hurt if you did those things at the same time. You’d be too rattled by the revving to be able to listen to the whisper. Bats avoid this by temporarily disconnecting their ears as they shriek, then quickly reconnecting them in time to hear the echo.
One particular species of bat, the Mexican free-tailed bat (Tadarida brasiliensis), has been recently observed messing with its competitors’ signals. By emitting a special signal right when another bat is about to catch an insect, the bats make each other miss. It’s the bat equivalent of yelling “PSYCH!” when someone is about to shoot a free-throw. Unlike the obnoxious friend though, the bat version actually works. The bats’ success rate drops by about 80%. It’s such an effective strategy that two bats will even hang out near each other, jamming each others’ signals every time one swoops in for a bug, until someone gives up.
The same species of bat that jams also lives in close proximity to natural gas fields in New Mexico. Some of the rigs have compressors that emit a constant, loud noise that can interfere with echolocation calls. For the Mexican free-tailed bats, whose normal calls fall within the same frequency range as the compressors, the loud wells are avoided when possible. The bats have also begun to change their calls, making them longer and in a more restricted range of frequencies. This strategy would make the calls more easily distinguishable from the background din and marks the first time human-made noise has been shown to interfere with bat life.
We know that humans can’t hear a lot of what the bats are “saying” when they are building up a sonar picture because our ears aren’t sensitive to the right frequencies. This makes sense because, for the vast majority of humans, it really doesn’t matter what the bats are saying. It’s a whole other issue if you’re a moth about to be eaten. There’s a lot of (evolutionary) pressure to hear the bats coming in order to avoid getting eaten. Some noctuids, a rather large family of moths, have evolved bat-sensing ears that warn the insect of impending disaster. If the bat is far enough away, the moth will make a break for it, otherwise it will just start flying erratically in random directions to try and make the bat miss. The Pallas long-tongue bat (Glossophaga soricina) still manages to get a meal by using only ultra-high-frequency, low intensity calls to find moths and by going silent on approach. This stealth mode doesn’t trip the moth’s defences.
For more information on echolocation and bats, check out:
The Bat Conservation Trust, a UK charity devoted to all things bat
This Scientific American article about how echolocation works
This study about Mexican free-tailed bat jamming
This study about Mexican free-tailed bat adapting
This study about Pallas long-tongue bat sneaking