A new Tel Aviv University study has revealed, for the first time, that bats
know the speed of sound from birth. In order to prove this, the researchers
raised bats from the time of their birth in a helium-enriched environment in
which the speed of sound is higher than normal. They found that unlike humans,
who map the world in units of distance, bats map the world in units of time.
What this means is that the bat perceives an insect as being at a distance of
nine milliseconds, and not one and a half meters, as was thought until now.
The study was published in the Proceedings of the National Academy of
Sciences.
In order to determine where things are in a space, bats use sonar -- they
produce sound waves that hit objects and are reflected back to the bat. Bats
can estimate the position of the object based on the time that elapses
between the moment the sound wave is produced and the moment it is returned
to the bat. This calculation depends on the speed of sound, which can vary
in different environmental conditions, such as air composition or
temperature. For example, there could be a difference of almost 10% between
the speed of sound at the height of the summer, when the air is hot and the
sound waves spread faster, and the winter season. Since the discovery of
sonar in bats 80 years ago, researchers have been trying to figure out
whether bats acquire the ability to measure the speed of sound over the
course of their lifetime or are born with this innate, constant sense.
Now, researchers led by Prof. Yossi Yovel, head of the Sagol School of
Neuroscience and a faculty member of the School of Zoology in the Faculty of
Life Sciences and his former doctoral student Dr. Eran Amichai (currently
studying at Dartmouth College) have succeeded in answering this question.
The researchers conducted an experiment in which they were able to
manipulate the speed of sound. They enriched the air composition with helium
to increase the speed of sound, and under these conditions raised bat pups
from the time of their birth, as well as adult bats. Neither the adult bats
nor the bat pups were able to adjust to the new speed of sound and
consistently landed in front of the target, indicating that they perceived
the target as being closer -- that is, they did not adjust their behavior to
the higher speed of sound.
Because this occurred both in the adult bats that had learned to fly in
normal environmental conditions and in the pups that learned to fly in an
environment with a higher-than-normal speed of sound, the researchers
concluded that the rate of the speed of sound in bats is innate -- they have
a constant sense of it. "Because bats need to learn to fly within a short
time of their birth," explains Prof. Yovel, "we hypothesize that an
evolutionary 'choice' was made to be born with this knowledge in order to
save time during the sensitive development period."
Another interesting conclusion of the study is that bats do not actually
calculate the distance to the target according to the speed of sound.
Because they do not adjust the speed of sound encoded in their brains, it
seems that they also do not translate the time it takes for the sound waves
to return into units of distance. Therefore, their spatial perception is
actually based on measurements of time and not distance.
Prof. Yossi Yovel: "What most excited me about this study is that we were
able to answer a very basic question -- we found that in fact bats do not
measure distance, but rather time, to orient themselves in space. This may
sound like a semantic difference, but I think that it means that their
spatial perception is fundamentally different than that of humans and other
visual creatures, at least when they rely on sonar. It's fascinating to see
how diverse evolution is in the brain-computing strategies it produces."
Reference:
Eran Amichai, Yossi Yovel. Echolocating bats rely on an innate
speed-of-sound reference. Proceedings of the National Academy of Sciences,
2021; 118 (19): e2024352118 DOI:
10.1073/pnas.2024352118
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Plants & Animals