Scientists have found another piece to the puzzle of how echolocation evolved in bats, moving closer to solving a decades-old evolutionary mystery.
All bats, except the fruit bats of the Pteropodidae family (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 in Current biology, has shown how sophisticated echolocation ability not only evolved multiple times in bat groups, but also never evolved into fruit bats.
The remarkable sounds of bats
To navigate using echolocation, bats make high-frequency calls in their larynx (voice box) and emit them 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 extract information about the spatial and textural properties of their environment.
For three decades, scientists have tried to understand how echolocation evolved in bats and why this adaptation did not spread to fruit bats. So far, they have struggled to reach consensus.
Some evolutionary biologists think that fruit bats were once able to echolocate like their modern counterparts, but at some point they lost this ability. Others propose that fruit bats never acquired this trait in the first place and that it evolved several times in different bat groups.
Embryos help unravel an evolutionary mystery
Discovering the history of the echolocation of bats has always been a difficult task. There are more than 1,400 species of bats, representing about a quarter of all mammalian species on Earth. As such, they come in a remarkable range.
However, bat fossils are remarkably rare and fragmented. Scientists lack the specimens needed to piece together the 65-million-year-old evolutionary history of bats.
Furthermore, the genetic information of bat species that are echolocated today has done little to help us understand how the sonar-like system actually works.
We take a different approach. Rather than focus on the genes or fossils of bats, we examine the early development of the bones of the ear 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. Instead, the trait often begins to develop early in life, but does not progress.
So if echolocation were 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, the University of Tokyo, and the Vietnam Academy of Science and Technology, studied hundreds of bat embryo samples from around the world.
We use a modern imaging method to digitally reconstruct the soft tissue structure of embryos in microscopic detail. We compared fruit bats with echolocator bats and also with non-echolocator mammals, such as mice.
Our analysis revealed that fruit bats were indistinguishable from non-echolocating mammals in all aspects of early ear bone development.
There were also no characteristics that were similar to those observed in bats that have a sophisticated echolocation ability. In other words, there was no evidence to suggest that fruit bats had ever been able to echolocate.
This raised several questions for us. Does this mean that the common ancestor of all bats did not have the echolocation abilities that are granted to future bats? This is one possibility.
Alternatively, this common ancestor could have had only a very primitive version of echolocation. If so, it may have looked and sounded surprisingly different than what we see in today’s sophisticated echolocators.
Unfortunately, we cannot know for sure which one is correct. Pteropodids have the most incomplete fossil record of all bat lineages, so we can’t study how their ear bones changed over time.
Confirming previous theories
Our team also found that the two main groups of sophisticated bat echolocators, Rhinolophoidea and Yangochiroptera, have different patterns of ear and throat development from each other. This suggests that they developed their sonar independently.
This conclusion also fits with the latest insights from bat genome sequencing, which indicate that if the ancestor of all bats was echolocated, it was probably some kind of primitive echolocation, not the clever laryngeal echolocation found in modern bats.
The next step will be to combine the insights from developmental analysis with genomic data from bats.
By studying how hearing-related genes in bats are expressed during early development, we could find out if fruit bats completely obliterated a primitive echolocation system present in an ancestor, or if it was ever there.
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.