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Evolving between the echoes

The views expressed are those of the author and are not necessarily those of Scientific American.


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This bumblebee bat could be the smallest mammal in the world.

Isolation can be a blessing. I am most productive when I’m not connected to the web. If I’m writing in a train or plane, severed from the thoughts of others, it is easier to capture my own trails of thought and let them expand. Don’t get me wrong, my inner writer loves the internet. It’s where I go to learn about new research and get inspired by scientists and writers from all over the world. With every click I uncover a new idea or story, waiting for a mind to latch onto.

And this is why my inner writer loathes the internet. On the web it seems as if every idea has been thought of before and every story has been retold countless times. In an echo chamber filled with a thousand voices, it can be hard to find your own.

In nature life is not much different. Every animal, from the smallest bat to the largest cat, has to find a niche and voice of its own. Take Kitti’s Hog-nosed Bat, or the bumblebee bat. With a length of just 3 centimetres (1.2 in), this cute creature is a strong contender for the title of ‘smallest mammal of the world’. The bumblebee bat and occurs in two separate populations in Myanmar and Thailand. Its total natural range is restricted to a mere 2,000 square kilometres (770 square miles). For comparison: this is a region half the size of Rhode Island, and a little smaller than Luxembourg.

The natural range of the Myanmar (dark blue) and Thai (green) populations of bumblebee bat.

The Myanmar and Thai bumblebee bats are indistinguishable by eye, but not by ear. The echo calls of Myanmar bats have a somewhat higher pitch. The consequences of this difference extend beyond mere echolocation, because bats also use their echoes in personal communication. In some bat species, bats even prefer to mate with partners with similar echo calls. If the same is true for bumblebee bats, a small difference in echo frequency could have driven a wedge between the two populations in the distant past.

How so? Suppose that by chance, two groups arise within a population that each perceive the world in a slightly different way. If this small difference affects their choice of partners (such as with the bumblebee bats and their echoes, perhaps), these differences and preferences are passed on from generation to generation, become amplified and get locked in, up to the point where members of the two groups no longer recognize each other as potential mates. When they have stopped interbreeding, they have taken the first steps towards becoming different species.

Biologists have come up with several names for this process, such as ‘speciation through sensory drive’, but considering how these species-to-be ignore each other, I think ‘speciation through mutual ignorance’ is a better description. While it this makes for an attractive story, it is not the only explanation for the different echo frequencies of bumblebee bats. After all, these differences could also have evolved after the Thai and Myanmar bat populations became separated.

The Khwae Noi River (Kwai River) is part of the natural habitat of the bumblebee bat.

An international team of biologists, equipped with ultrasound detectors and mist nets, set out for the forests of Myanmar and Thailand to confirm whether bumblebee bats became isolated by ignorance or not. The team caught and released over 700 bats from Myanmar and Thailand, and took punctures of their skin or wings. A DNA analysis of these samples revealed that the smaller Myanmar population split from a larger Thai population around 400.000 years ago so. At such a coarse resolution, it is only possible to take a bat’s eye view of their evolutionary history. The researchers could only conclude that the Myanmar population likely originated from a small number of Thai bats, but not how or why they became isolated.

They therefore decided to ‘zoom in’ on the colonies that make up the Thai population, to see if they could catch isolation on the wing. Even though the Thai colonies form a continuous range, the team did find an abrupt change in echo frequency in the southern colonies. Southern calls had an increased frequency of 3 kHz, compared to the calls of bats from the north. This acoustic boundary was clear and sudden, but it wasn’t reflected in the DNA of the bumblebee bat. On average, colonies on both sides of the boundary did not differ more from each than other neighbouring colonies did.

One piece of DNA formed an exception. Bats from the north side of the echo border carried a different version than bats from the south. This stretch of DNA is located near a gene that is involved in producing hair cells in the bat’s hearing organ, so it might have played a role in the evolution of the different echoes of the Thai and Myanmar bats.

This is a CT scan of a complete skeleton of the bumblebee bat scan, provided by Digimorph.

The skewed distribution of this ‘echo location gene’ suggests that it provides an advantage of some kind. But what? Enter a second species of bat: Himalayan Whiskered Bat, or Myotis siligorensis. This bat is of a similar size and catches similar prey, but more importantly: it emits its echoes along the same bandwidth as the bumblebee bat. This could give problems if a bumblebee bat and whiskered bat would be out hunting and echoing in the same region. Their echo signals would interfere, and they would be unable to determine their distance to their prey.

The bats could avoid jamming each other’s frequencies if one of them would shift the pitch of its echo. Indeed this seems to be what happened to the bumblebee bats in the south of Thailand and in Myanmar. Recordings revealed that their caves were also frequented by whiskered bats, whereas in the north of Thailand not a whisker was seen.

The researchers conclude that it is unlikely that echolocation was the driving force behind the isolation of Myanmar and Thai bats. Given the large distance (for a bumblebee sized bat, at least) between the two populations and that Thai bats are genetically the most diverse, they suggest that a few bats were swept from Thailand to Myanmar by storm, cyclone or typhoon or perhaps one of the strong winter monsoons that occur about once every 100.000 years.

A handful of bumblebee bats, tumbling in the storm. As the winds die down, the creatures find themselves far away from home. From the woods sounds an all too familiar shriek, from an unfamiliar source. The bumblebee bats know what they must do. Time to find a voice of their own.


Images:
Bumblebee bat in cavity by Steven Wong. Image all rights reserved, used with permission.
Map of bumblebee bat range from reference
Kwai River by Eric Molina.
CT scan of bumblebee bat skeleton by Digimorph. Image all rights reserved, used with permission.


References:
Puechmaille SJ, Gouilh MA, Piyapan P, Yokubol M, Mie KM, Bates PJ, Satasook C, Nwe T, Bu SS, Mackie IJ, Petit EJ, & Teeling EC (2011). The evolution of sensory divergence in the context of limited gene flow in the bumblebee bat. Nature communications, 2 PMID: 22146392

Lucas Brouwers About the Author: Lucas Brouwers is fascinated by evolution. He writes about science on his blog and for a Dutch daily newspaper. Follow on Twitter @lucasbrouwers.

The views expressed are those of the author and are not necessarily those of Scientific American.





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