Scientists have found another piece in the mystery of how echolocation evolved in bats, and came closer to solving a decade-long evolutionary mystery.
All bats – apart from the fruit bats of the family Pteropodidae (also called flying foxes) – can “echolocate” by using a loud noise at night.
An international study led by us, published on March 5, 2021 in Current biology, showed how the ability for sophisticated echolocation not only developed several times in groups of bats, but also that it never developed in fruit bats.
The remarkable sounds of bats
To navigate using echolocation, bats make high-frequency calls in their larynx (voice box) and send it out through their nose or mouth. These calls, usually with higher frequencies than people can hear, echo objects and bounce.
From this feedback, bats can extract information about the spatial and textural properties of their environment.
Scientists have been trying for three decades to understand how echolocation developed in bats and why this adaptation was not to fruit bats. So far, they have struggled to reach consensus.
Some evolutionary biologists think that fruit bats could resound like their modern counterparts, but at some point lost this ability. Others suggest that fruit bats never acquired this trait in the first place and that it developed several times in different bat classes.
Embryos help unravel an evolutionary mystery
Discovering the history of bat echolocation would always be a difficult task. There are more than 1400 bat species, which make up about a quarter of all mammal species on earth. As such, they come in a remarkable range.
However, bat fossils are particularly rare and fragmented. Scientists do not have the samples needed to reconstruct the 65 million year evolutionary history of bats.
The genetic information of today’s reflective bat species has done little to help us understand how the sonar-like system actually works.
We followed a different approach. Instead of focusing on bat genes or fossils, we investigated the very early development of their ear and throat bones.
Evolutionary studies have shown that if a group of species eventually loses a trait that its ancestors possess, not all aspects of the trait are completely lost. Instead, the trait often begins to develop in the very early stages of life, but does not progress.
Thus, if ecolocation were present in the common ancestor of all bats, we would expect modern fruit bats to show a trace of this in their ear and throat development.
Our research group, which included biologists from City University of Hong Kong, the University of Tokyo and the Vietnam Academy of Science and Technology, studied hundreds of samples of bat embryos from around the world.
We used a modern imaging method to digitally reconstruct the soft tissue structure of the embryos in microscopic detail. We compared fruit bats with reverberating bats and also non-reproducing mammals, such as mice.
Striking results
Our analysis revealed that fruit bats could be distinguished from non-echolocating mammals in all aspects of their early development of the ear bone.
There were also no features similar to those observed in bats, but sophisticated echolocation was. In other words, there was no evidence to suggest that fruit bats could ever resound.
This raised several questions for us. Does this mean that the common ancestor of all bats does not have the skill to provide future bats? This is a possibility.
Alternatively, this common ancestor would have had only a very primitive version of echolocation. If so, it may have looked and sounded remarkably different from what we see in today’s sophisticated echo locators.
Unfortunately, we can not know for sure what is correct. Pteropodides have the most incomplete fossil record of all bats, so we cannot study how their upper legs have changed over time.
Confirmation of previous theories
Our team also discovered that the two main groups of sophisticated bat echolocators, Rhinolophoidea and Yangochiroptera, have different patterns of ear and throat development. This indicates that they developed their sonar independently.
This conclusion is also in line with the latest insights from the order of bat genome, suggesting that if the ancestor of all bats did echo, it was probably some kind of primitive echolocation – not the dexterous laryngeal echolocation found in modern bats. .
The next step will be to combine insights from developmental analysis with genomic data from bats.
By studying how the hearing-related genes of bats are expressed during early development, we were able to determine whether fruit bats completely obliterated a primitive echolocation system found in an ancestor, or that it was ever there. 
Camilo López-Aguirre, PhD candidate, UNSW and Laura AB Wilson, senior lecturer, Australian National University.
This article was published from The Conversation under a Creative Commons license. Read the original article.