Scientists have found another piece in the puzzle of how echolocation evolved in bats, getting closer to solving a decades-long evolutionary mystery.
All bats – except for the fruit bats of the Pteropodidae family (also called flying foxes) – can ‘echolocate’ by using high-pitched tones to navigate at night.
An international study led by us, published March 5, 2021 in Current Biology, has shown how the ability for advanced echolocation has not only evolved multiple times in groups of bats, but also that it never evolved in fruit bats.
The remarkable sounds of bats
To navigate using echolocation, bats produce high-frequency calls in their larynx (larynx) and send them out through their nose or mouth. These calls, usually made at frequencies higher than humans can hear, echo off objects and bounce back.
From this feedback bats can obtain information about the spatial and textural properties of their environment.
For three decades, scientists have been trying to understand how echolocation evolved in bats and why this adaptation didn’t extend to fruit bats. So far they have struggled to reach a consensus.
Some evolutionary biologists think fruit bats could one day echolocate like their modern counterparts, but at some point they lost this ability. Others propose that fruit bats never acquired this trait at all and that it evolved several times in different bat groups.
Embryos help unravel an evolutionary mystery
Uncovering the history of bat echolocation was always going to be a difficult task. There are more than 1,400 bat species, accounting for about a quarter of all mammal species on Earth. As such, they come in a remarkable range.
Bat fossils, however, are notably scarce and fragmented. Scientists lack the specimens needed to reconstruct the 65 million-year evolutionary history of bats.
Also, the genetic information of the current echolocating bat species has done little to help us understand how the sonar-like system actually works.
We have taken a different approach. Rather than focusing on bat genes or fossils, we examined the very early development of their ear and throat bones.
Evolutionary studies have shown that if a group of species lose a trait that its ancestors possessed, not all aspects of the trait are completely lost. Instead, the trait often begins to develop in the very early stages of life, but it does not develop.
So if echolocation was present in the common ancestor of all bats, we would expect modern fruit bats to show some developmental trace of this in their ear and throat development.
Our research group, which included biologists from the City University of Hong Kong, University of Tokyo and the Vietnam Academy of Science and Technology, studied hundreds 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 echolocating bats and also non-echolocating mammals, such as mice.
Striking results
Our analysis revealed that fruit bats were indistinguishable from non-echolocating mammals in all aspects of their early ear bone development.
There were also no features comparable to those seen in bats that do have advanced echolocation capabilities. In other words, there was no evidence to suggest that fruit bats could ever have echolocated.
This raised a number of questions for us. Does this mean that the common ancestor of all bats did not have the echolocation skills that future bats acquired? This is one possibility.
Alternatively, this common ancestor may have had only a very primitive version of echolocation. If so, it may have looked and sounded strikingly different from what we see in today’s advanced echolocators.
Unfortunately, we are not sure what is correct. Pteropodids have the most incomplete fossil record of all bat lines, so we cannot study how their ear bones have changed over time.
Confirmation of previous theories
Our team also found that the two main groups of advanced bat echolocators, Rhinolophoidea and Yangochiroptera, have different patterns of ear and throat development. This suggests that they developed their sonar independently.
This conclusion also ties in with the latest insights from bat genome sequencing, which indicate that if the ancestor of all bats echolocated, it was likely some sort of primitive echolocation – not the agile laryngeal echolocation found in modern bats.
The next step will be to combine insights from developmental analysis with bat genomic data.
By studying how the hearing-related genes of bats are expressed during early development, we were able to find out whether fruit bats completely erased a primitive echolocation system in an ancestor, or if it ever existed. 
Camilo López-Aguirre, PhD Candidate, UNSW and Laura AB Wilson, Senior Lecturer, Australian National University.
This article has been republished from The Conversation under a Creative Commons license. Read the original article.