Scientists have found another piece in the puzzle of how echolocation in bats evolved, approaching solving an evolutionary mystery of decades.
All bats, except bats in the family Pteropodidae (also called flying foxes), can “echolocate” using high-pitched sounds to navigate at night.
An international study led by us, published on March 5, 2021 a Current biology, has shown how the ability for sophisticated echolocation not only evolved several times in bat groups, but also that May evolved into fruit bats.
The remarkable sounds of bats
To navigate by echolocation, bats make high-frequency calls to the larynx (voice box) and emit them through the nose or mouth. These calls, usually made at higher frequencies than humans can hear, echo objects and bounce.
From this comment, bats can extract 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 did not spread to fruit bats. So far, they have struggled to reach a consensus.
Some evolutionary biologists think that fruit bats could echo like their modern counterparts, but at some point they lost that ability. Others propose that fruit bats never acquired this trait and that they evolved several times into different groups of bats.
Embryos help unravel an evolutionary mystery
Discovering the history of bat echolocation has always been a difficult task. There are more than 1,400 species of bats, representing about a quarter of all mammal species on Earth. As such, they present a remarkable range.
However, bat fossils are especially scarce and fragmented. Scientists do not have the specimens needed to reconstruct the evolutionary history of 65-million-year-old bats.
In addition, genetic information from current echolocating bat species has not helped us much in understanding how the sonar-like system actually works.
We took a different approach. Instead of focusing on bat genes or fossils, we examined the very early development of the bones in the ears and throat.
Evolutionary studies have shown that if a group of species ends up losing a trait that their ancestors possessed, not all aspects of the trait are completely lost. In contrast, the trait often begins to develop in the early stages of life, but does not progress.
Thus, if echolocation was present in the common ancestor of all bats, we would expect modern fruit bats to show some trace of development in the development of the ear and throat.
Our research group, which included biologists from Hong Kong City University, Tokyo University, and the Vietnam Academy of Science and Technology, studied hundreds of specimens of bat embryos from around the world.
We used a modern imaging method to digitally reconstruct the soft tissue structure of embryos with microscopic details. We compared fruit bats with echolocating bats and also non-echolocating mammals, such as mice.
Amazing results
Our analysis revealed that fruit bats were indistinguishable from mammals that did not echo in all aspects of the early development of ear bones.
There were also no characteristics similar to those observed in bats that had a sophisticated ability to echolocate. In other words, there was no evidence to suggest that fruit bats were able to hatch.
This raised several questions for us. Does this mean that the common ancestor of all bats did not have the echolocation skills provided to future bats? This is a possibility.
Alternatively, this common ancestor could only have had a very primitive version of echolocation. If so, it may have looked and seemed surprisingly different from what we see in today’s sophisticated echolocators.
Unfortunately, we cannot know for sure which is correct. Pteropods have the most incomplete fossil record of all bat lineages, so we cannot study how the bones of the ears changed over time.
Confirming previous theories
Our team also found that the two large groups of sophisticated bat echolocators, Rhinolophoidea and Yangochiroptera, have different patterns of ear and throat development. This suggests that they evolved their sonar independently.
This conclusion also fits with the latest views of bat genome sequencing, which indicate that if the ancestor of all bats echolocated, it would probably be a type of primitive echolocation, not the skillful laryngeal equolocation found in modern bats.
The next step will be to combine development analysis information with bat genomic data.
By studying how genes related to bat hearing are expressed during early development, we could find out whether fruit bats completely erased a primitive echolocation system present in an ancestor, or if it was ever there. 
Camilo López-Aguirre, PhD student, UNSW and Laura AB Wilson, Professor at the National University of Australia.
This article is republished from The Conversation under a Creative Commons license. Read the original article.