The origin of the bats

The most recent group of tetrapods to evolve flight were a type of mammal. The bats.

The earliest known bat fossils date to around 52 million years ago. However, these early bats were very much like modern microbats in morphology, and their only really primitive features were unfused ribs, some finger features, and the retention of certain teeth absent in modern bats (Benton, 2005). Furthermore, the very early known bats were already very widespread, being found in Eurasia, North America and Australia (Long et al, 2002). This suggests bat origins go back further. A 75 million year old fossil of a noctuid moth egg in Massachussets (noctuids are a group of moths adapted to hear bat echolocation) suggests bats may have been around as long as 75 million years ago (Altringham, 2011).

Figure 1: Icaronycteris, a bat that lived in Wyoming around 52 million years ago.  Artist: unknown.

Earlier in this blog we saw how gliding played no role in the evolution of flight in pterosaurs and dinosaurs. Again, gliding played no role in the evolution of bat flight either. In a study conducted by Kevin Padian and Kenneth Dial, it was found that bat anatomy, physiology and behaviour pointed to an origin as climbing mammals who would hang upside down and fall down towards insect prey, using proto-wings to control their descent in a manner quite different to that of any glider. In time this early fluttering developed into true flight (Kaplan, 2011).

Figure 2: Baby bats cannot fly but if they fall they can flutter to slow their descent, likely a vestige of the fluttering ancestry of bats.  Photographer: Peet van Schalkwyk.

Bat wings are essentially webbed hands. These hand wings are somewhat better airfoils than the feather wings of birds and other flying dinosaurs, though not quite as good as the multi-layer wings of pterosaurs. Bats lack air sacs but compensate by having enormous lungs. Perhaps the bats' most amazing feature though is the ability of echolocation (though this has been lost in most megabats). By emitting ultrasonic pings that bounce off solid objects bats can work out the exact distance and location of said objects regardless of whether they can actually see them. Bat echolocation is remarkable in its precision. Indeed, it is around 1 trillion times more effective than our best sonar detector technologies (Levy, 1999).

Figure 3: A simple explanation of how bat echolocation works. The bat emits ultrasonic pings that bounce off solid objects. From this the bat can determine things like distance, location and, in the case of the moth shown here, which direction something is moving in.

Next post shall be exploring the diversity of these remarkable creatures.

References
Altringham, J. D. (2011). Bats: From Evolution to Conservation. Oxford University Press: Oxford.
Benton, M. (2005). The Rise of the Mammals. Quantum Publishing Ltd: London.
Kaplan, M. (2011). Ancient bats got in a flap over food. Nature.
Levy, C. K. (1999). Evolutionary Wars: A Three-Billion-Year Arms Race. W. H. Freeman and Company: New York.
Long, J., Archer, M., Flannery, T. & Hand, S. (2002). Prehistoric Mammals of Australia and New Guinea. University of New South Wales Press Ltd: Australia.

Image sources
Figure 1: Accessed May 12, 2015, from:  http://age-of-mammals.ucoz.ru/_si/2/24863285.jpg
Figure 2: Accessed May 12, 2015, from: http://farm3.static.flickr.com/2629/3965523629_c86b1ce4e2.jpg
Figure 3: Accessed May 12, 2015, from: http://paulmirocha.com/wp/wp-content/uploads/2011/11/echolocation1.jpg