Bat Evolution

By Shannon Currie

Bats represent one of largest and most diverse radiations of mammals, illustrating the marvel of evolution. They are the only mammals capable of true powered flight, possess an intricate and sophisticated body structure, and are outnumbered only by rodents for total species amongst mammals. Yet, the story of bat evolution has been a source of contention for many years. Answers have been hard to come by and this great debate has been spurred along by a lack of fossil evidence. Ancient bats, very similar to their modern counterparts, were small and delicate and likely lived in tropical climates where decomposition is quick. This means that for a bat to become fossilized it needed to have died in a place where it was covered in sediment almost immediately, shielding it from scavengers and sources of decay. The fossils that have been discovered demonstrate very few structural differences to modern day bats, complicating the issue. What is agreed upon by all evolutionary biologists is that bats form a class of mammal all their own.

To appreciate how distinct bats are and understand their evolution we must look at their defining traits; their wings and the ability of most bats to echolocate. Bats underwent a unique suite of morphological modifications that enabled them to fly and echolocate and distinguished them from other mammals. Their wings are formed from elongated forearm and finger bones that support a thin wing membrane, giving them incredible manoeuvrability and a distinct form of flight. Echolocation was permitted by the alteration of three critical bones in the ear and throat. For example, the cochlea (structure in the inner ear) is enlarged relative to other skull structures, which makes bats better able to detect, and discriminate between, high-frequency sounds. It is clear that both these features contributed significantly to the diversification of bat species. However, this generated the important question- which came first, echolocation or flight?

Over the last decade three competing hypotheses have been in play. The flight-first theory suggests that bats developed flight as it facilitated speedy foraging, with echolocation following as a more accurate way to catch prey at night. In contrast, the echolocation-first hypothesis posits that protobats hunted aerial prey from perches using echolocation and later developed flight from gliding to return to these perches more easily. The final theory proposes that the two processes evolved simultaneously. This theory arose after studies showed that it is energetically very costly to echolocate when stationary and that cost of echolocation becomes negligible when in flight as the flight muscles help to pumps the lungs. Until recently, fossil evidence had been unable to definitely rule out any of these hypotheses.

Evolutionary biologists are always in search of the ‘missing link’; the fossil specimen that shows an intermediate stage of animal evolution. In the evolution of birds the discovery of the fossil Archaeopteryx improved our knowledge of the transition of modern birds from their reptilian ancestors. In the 1960’s a new bat fossil was discovered in the Green Lake Formation of Wyoming. Icaronycteris index was found to be 52.5 million years old and was the oldest bat fossil on record. For a long time I. index was considered very important in the study of bat evolution as it possessed a distinguishing feature from younger specimens. This bat retained a claw on the second digit of the wing, thought to be a remnant from terrestrial ancestors. However, the most significant thing about this fossil was that it mirrored extant species in almost every recognisable chiropteran characteristic. It possessed features suggestive of an insectivorous diet, full powered flight, and the ability to echolocate. Although an important fossil, unfortuantely I. index did not successfully complete the search for a transitional species.

It wasn’t until 2008 that a new fossil species was identified. Onchonycteris finneyi was also found in the Green Lake Formation and shows both ancient and modern bat features. As its name suggests (Latin for clawed bat) O. finneyi can be clearly distinguished from I. index and modern bats as it posses a claw on each digit of the wing, left over from its terrestrial ancestor. This specimen also shows proportionately longer hind limbs and shorter forelimbs than all other bats. In fact, their limb proportions are comparable to other arboreal mammals such as sloths and gibbons, suggesting that they may have evolved from animals with similar forms of locomotion. The arrangement of its wings suggest that O. finneyi could fly, however its wing ratio indicates it most likely flew in a combination flapping/gliding motion, supporting the idea that bats evolved from gliding ancestors. Finally, O. finneyi provides an answer to the question of echolocation versus flight. The ear and throat of this fossil lacks the modifications required for echolocation, indicating that bats could surely fly before they began echolocating.

DNA evidence has shown that all 26 families of bats (19 extant, 7 extinct) had evolved before the end of the Eocene period, 33.5 million years ago. This suggests a rapid diversification of bat species following the evolution of echolocation and likely coincided with a rise in global temperature and significant diversification of plant and insect species. New evidence has also been discovered to suggest how bats were able to take to the skies, developing wings from their hands. Morphometric analyses show that the length of the third, fourth and fifth digits, relative to body size, has not changed over course of bat evolution. This suggests that the bat wing may have evolved abruptly and very quickly. One study suggests that this swift evolution could be due to the alteration of a single gene during development. Comparing the development of bats and mice, scientists were able to pinpoint the time period during embryonic development when the critical forelimb digits elongate. With this information they predicted that the difference between limb development in these two species relates to a gene that codes for a growth factor. This gene is called bone morphogenetic protein 2 (Bmp2) and was found to be expressed 30% more in the forelimbs of bats than in mice. This intriguing insight may help to explain why we have had such difficultly finding further intermediate fossil specimens linking bats to their small mammalian ancestors.

Even with these new pieces of the puzzle, the complete story of bat evolution remains a mystery. The identity of the closest mammalian relative to bats is still unknown. They have been placed in the ancient group known as Laurasiatheria; however this group is large and very diverse. The modern relatives of these creatures include such varied animals as carnivores, whales and shrews! Nevertheless, the original Laurasiatherians were likely small insectivorous animals that walked on all fours. With advances in technology and rapid developments in scientific investigation, hopefully we will soon be able to elucidate a clearer picture of how these amazing creatures evolved to fill the night skies.

© 2012 Organization for Bat Conservation