Scientists have found another piece in the puzzle of how bat evolution has evolved, approaching the solution of an evolutionary mystery of decades.
All bats – except fruit bats in the Pteropodidae family (also called flying foxes) – can “echo” using loud sounds to navigate at night.
An international study led by us, published on March 5, 2021 in Current biology, showed how sophisticated echolocation capacity has evolved not only several times in bat groups, but also that not evolved into fruit bats.
The remarkable sounds of bats
To navigate using echolocation, bats make high-frequency calls in the larynx (voicemail) and emit them through the nose or mouth. These calls, usually made at higher frequencies than people can hear, resound from objects and go back.
From this feedback, bats can extract information about the spatial and textural properties of the environment.
For three decades, scientists have been trying to understand how bat evolution has evolved and why this adaptation has not spread to fruit bats. So far, they have struggled to reach a consensus.
Some evolutionary biologists believe that fruit bats may once have settled like their modern counterparts, but at some point they lost this ability. Others suggest that fruit bats never acquired this trait in the first place and that it evolved several times in different groups of bats.
Embryos help unpack 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 come in a remarkable range.
However, bat fossils are especially rare and fragmented. Scientists lack the specimens needed to reconstruct the 65 million-year-old evolutionary history of bats.
Also, the genetic information of today’s green bat species has not done much to help us understand how the sonar-like system actually works.
We took a different approach. Rather than focusing on bat or fossil eyelashes, we examined the very early development of the ear and neck bones.
Evolutionary studies have shown that if a group of species ends up losing a trait that their ancestors had, 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.
So, if echolocation was present in the common ancestor of all bats, we would expect modern fruit bats to show a trace of their development in the development of the ear and neck.
Our research group, which included biologists from the University of Hong Kong, the University of Tokyo and the Vietnamese 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 embryos in microscopic detail. We compared fruit bats with echolocated bats and also non-echolocating mammals, such as mice.
Striking results
Our analysis showed that fruit bats are indistinguishable from mammals that do not echo in all aspects of their early ear bone development.
Also, there were no similar features to those observed in bats that have a sophisticated echolocation capacity. In other words, there was no evidence to suggest that fruit bats ever managed to eco-locate.
This raised several questions. Does this mean that the common ancestor of all bats did not have the echolocation skills offered to future bats? This is a possibility.
Alternatively, this common ancestor could have had only a very primitive version of echolocation. If so, it may have looked and sound strikingly different from what we see in today’s sophisticated eco-locators.
Unfortunately, we cannot know for sure which is correct. Pteropods 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 major groups of sophisticated bat echolocators, Rhinolophoidea and Yangochiroptera, have different patterns of ear and neck development. This suggests that they evolved their independent sonar.
This conclusion also fits with the latest insights into bat genome sequencing, which indicates that if the ancestor of all bats echoed, it was probably some kind of primitive echolocation – not the difficult laryngeal echolocation found in modern bats.
The next step will be to combine the ideas from the analysis of development with the genomic data of bats.
By studying how hearing-related bat genes are expressed during early development, we could find out if fruit bats completely erased a primary echolocation system present in an ancestor or if it was ever there. 
Camilo López-Aguirre, PhD student, UNSW and Laura AB Wilson, Lecturer, Australian National University.
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