Scientists have found another part in the puzzle of how echolocation developed in bats, moving closer to resolving a decades-long evolutionary mystery.
All bats — apart from the fruit bats of the family Pteropodidae (also known as flying foxes) — can “echolocate” by using high-pitched sounds to navigate at night.
An international study led by us, published in Current Biology, has shown how the capability for sophisticated echolocation evolved multiple times in groups of bats and never evolved in fruit bats.
To navigate utilizing echolocation, bats generate high-frequency calls in their voice box (larynx) and emit these through their mouth or nose. These calls, normally made at higher wavelengths than humans can hear, echo off objects and bounce back.
From this bouncing back of the sound, bats can extract information about their surroundings’ textural and spatial properties.
For over thirty years, scientists have tried to understand how echolocation evolved in bats and why this adaptation didn’t extend to fruit bats. So far, they’ve struggled to reach a consensus.
Some evolutionary biologists think fruit bats could once echolocate like their modern counterparts but at some point lost this capability. Others propose fruit bats never obtained this trait in the first place and that it evolved several times in various bat groups.
Uncovering the history of bat echolocation was always going to be a challenging task. There are over 1,400 species of bat, making up about a fourth of all mammal species on Earth. As such, they come in a striking range.
However, bat fossils are distinctly fragmented and scarce. Scientists lack the specimens needed to restore the 65-million-year evolutionary history of bats.
Also, today’s echolocating bat species’ genetic information has done little to help us understand how the sonar-like system actually works.
We took a different approach. Rather than focusing on fossils or bat genes, we examined the very early development of their throat and ear bones.
Evolutionary investigations have shown that if a group of species loses a trait its ancestors held, not all aspects of the trait are entirely lost. Instead, the attribute often starts to develop in the very early stages of life but doesn’t progress.
So if echolocation were present in all bats’ common ancestor, 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, crammed through hundreds of bat embryo specimens from all around the world.
We used a modern imaging method to digitally reconstruct the embryos’ soft tissue structure in microscopic detail. We compared fruit bats to echolocating bats and also non-echolocating mammals, such as mice.
Our analysis revealed fruit bats were indistinguishable from non-echolocating mammals in all aspects of their early ear bone development.
There were also no features similar to those observed in bats with sophisticated echolocation capability. In other words, there was no evidence to suggest fruit bats would ever have been able to echolocate.
This raised several questions for us. Does this mean the common ancestor of all bats didn’t have the echolocation skills afforded to future bats? This is a possibility.
Alternatively, this common ancestor might have only had a very primitive version of echolocation. If so, it may have looked and sounded strikingly different to what we see in today’s sophisticated echolocators.
Unfortunately, we can’t know for sure which is correct. Pteropodids have an incomplete fossil record of all bat lineages, so we can’t study how their ear bones changed over time.
Our team also discovered that the two major groups of sophisticated bat echolocators, Rhinolophoidea and Yangochiroptera, have different ear and throat development patterns to one another. This suggests they evolved their sonar independently.
This conclusion also fits in with the latest insights from bat genome sequencing, which indicate that if all bats’ ancestors did echolocate, this was likely some primordial echolocation — not the deft 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 could find out whether fruit bats completely erased a primitive echolocation system present in an ancestor or whether it was ever there at all.