At night, bats are awake and flying, searching for prey to feed on. However, to navigate through the darkness, bats need to rely on a special sense – echolocation.

When bats echolocate, they send out high frequency calls that bounce off objects, and travel back to the bat, which gives information on the location of the object. When targets are closer to the bat, echolocation calls are shorter in duration, and there are shorter time intervals between them. Bats can adjust their echolocation calls before take-off and during flight to determine where an object is. Evidence shows that echolocation may have evolved twice in bats, but this remains controversial. 

In order to translate the timings of their echolocation calls into distances, bats need to know the speed of sound. The speed of sound is not always the same, and changes based on surrounding humidity, temperature, and altitude. Bats are widely distributed over large geographic ranges, and can travel long distances, and so any individual may experience large variations in the speeds of sounds. This observation made researchers Eran Amichai and Yossi Yovel of Tel Aviv University hypothesize that bats are not born with the knowledge of the speed of sound, but rather can learn and adjust when their environment changes.

In their recently published study, Amichai and Yovel used a unique set of experiments to test their hypothesis. They raised bats from birth in flight chambers under two different conditions: under normal air or helium-enriched (80 percent helium, 20 percent oxygen) air, where sound travels 15 percent faster. In order to get food, the bats needed to echolocate and fly to a target that was 1.3 meters away in both environments. The researchers first documented the young bats' echolocation calls before take-off and during flight to the target in the environment that they were raised in. Following this, all bat pups were switched to the opposite environment, and the experiment was repeated.

If bat pups learned the speed of sound based on experience, then bats raised in the helium-enriched environment should have learned a faster speed. This means that when these bats then flew in normal air, they would have perceived the target further than it really was, resulting in their echolocation calls being further apart. However, surprisingly, this did not happen, and bats raised in the helium-rich air were able to successfully land on the target in normal air.

When both groups of bats were in the helium-enriched air, they perceived the target that was 1.3 meters away as being closer than it was. Before take-off, there was no difference between the echolocation calls between the two groups and during flight, both groups of bats showed echolocation calls that were in short intervals. This indicated that bats from both groups misperceived the distance of the food target during take-off and flight while in a helium-enriched environment.

This contradicted what the researchers had hypothesized: It demonstrated that bat pups are born knowing what the speed of sound is and don’t learn it through experience. All bat pups, regardless of the environment they were raised in, had the same reference for the speed of sound.

Amichai and Yovel then took the experiment a step further and ran the same trials (a target 1.3 meters away) with adult bats, except in environments of helium-enriched air which produced speeds of sound that were 10 percent, 15 percent, or 27 percent faster. This range of speeds of sound were slightly beyond anything that could be found in nature, but gave them more information about how bats did – or did not – adjust to changing speeds of sound.

In all trials, adult bats perceived the target as being closer that it actually was and could not adjust for the difference in speed of sound. Adult bats also did not adjust their echolocation calls to the variation in the speed of sound over time, showing that they bats were unable to learn a different reference for the speed of sound. These results lead the Amichai and Yovel to conclude that bats perceive the world in terms of time and not space. This means that the bats in the experiment ‘saw’ their food targets as being 7.5 milliseconds away, and not 1.3 meters. This would be similar to a you or I telling a friend that a grocery store is a 30 minute drive away, opposed to instructing them that it’s 15 kilometers away.

These results lead the researchers to question why bats would be born with the knowledge of the speed of sound, especially when the speed of sound changes across environments. Newborn bat pups normally begin learning to fly between two and three weeks of age and reach independence within 6-10 weeks to learn to echolocate and hunt for insects. By being born with the knowledge of the speed of sound, bats can decrease time and energy spent on learning. This is especially important, because bats living at high latitudes need to obtain large enough fat stores to sustain them through hibernation but are often constrained by time and prey availability.

While there are benefits to being born with the knowledge of the speed of sound, alternatively this may also prevent bats from being able to adapt to rapidly changing environments. Climate change, by changing temperature and humidity in many places, could also cause drastic changes in the speed of sound in those locations. Possibly, the inability for bats to adjust to variations in the speed of sound, may affect their ability to echolocate and navigate and overall may pose a threat to many bat species.

These results give us a better understanding of how bat brains encode and perceive the world. Research in echolocation has been ongoing for over 80 years, however we are still able to discover new aspects into the complexity of sensory systems.