By Emily Wilson
Flight has only evolved four times in evolutionary history in birds, insects, pterosaurs and bats. Bats are the only flying mammal and have been so successful that there are more than 1,116 species worldwide! Many species use echolocation, a form of sonar, to locate insects on the wing.
So how did bats evolve these characteristics?
Hundreds of fossils have been found and many species have been identified. Despite this we know little about how bats evolved. This is because most of the fossils are either incomplete or resemble a modern day bats. Icaronycteris index is an example of this.
Originating from the early Eocene Icaronycteris index had long digits and long arms which allowed it to have powered flight. Most of its claws had been lost and it used echolocation to find food. Onychonycteris finneyi lived around the same time as Icaronycteris index but was more primitive in a number of ways. Unlike Icaronycteris index, Onychonycteris finneyi could not echolocate, this is due to its small cochlea, a component of the inner ear, which allows bat to echolocate. Onychonycteris finneyi also had claws on every digit, due to this it is thought that Onychonycteris finneyi retained the ability to climb up tree trunks.
However, these fossil do not answer how bats evolved flight but do raise questions such as; what came first? Echolocation or flight?
It It is thought that the origin of bats dates back 65 million years ago. Although there aren’t any fossils from this period there is genetic evidence to suggest bats inhabit earth even earlier! A study analysed the DNA of the short-tailed bats of New Zealand and the fisherman bats of South America. It was found that these species diverged from all other bat species 65 million years ago. This evidence confirms that bats existed 65 million years ago or earlier.
51 million years ago the climate warmed by 7 C°, this caused an increase in angiosperm plants and therefore insects. This resulted in a proliferation of bat species. The first bats are thought to be tree dwelling animals which jumped from tree to tree, scampering over branches looking for insects. However, evidence of this animal has never been found. Therefore, we can only hypothesise how flight evolved in bats. It is thought that the ancestral bat evolved membranes between its limbs and digits, similarly to flying lemurs, as it costs less energy to glide between trees and branches than to jump. Another advantage of staying in the trees is that the ancestral bat could avoid being predated upon by terrestrial predators. Over time the digits would have extended, taking the membrane with them to form modern bat wings.
What were the pressures which caused bats to become nocturnal?
When bats were thought to be evolving ~51 million years ago, birds were well established with the first bird Archaeopteryx appearing 135 million years ago. This meant that birds already inhabited the aerial niche which drove bats to inhabit the night time niche.
Modern day bats have historically been divided into two groups: Megachiroptera (fruit bats) and Microchioptera (carnivorous bats). It is thought due to their morphological and ecological differences that megabats and microbats had different ancestors. This was introduced by Petigrew et. al. (1989) as the flying primate theory. It was found that fruit bats shared a neural characteristic of the retina with primates and flying lemurs. Therefore, it was thought that these groups must have shared a common ancestor. However, through studies in genetics it was found that neither primates or flying lemurs were of close relation to mega bats. In fact, they found that Rhinolophidea, which contains British horseshoe bats, is more closely related to fruit bats than other microbat groups. This disproved Petigrew’s theory and confirms that mega and micro bats share one common ancestor. Therefore, making the terms Megachiroptera and Microchiroptera obsolete.
Clearly there is a lot more to be researched in the subjects surrounding the evolution of bats but I personally find this very exciting as there is a lot more to be discovered.
References and further reading:
Adams, R. and Pedersen, S. (2013). Bat Evolution, Ecology, and Conservation. New York, NY: Springer.
Altringham, J., McOwat, T. and Hammond, L. (2011). Bats: from evolution to conservation. Oxford: Oxford University Press.
DIETZ, C. and Kiefer, A. (2016). Bats of Britain and Europe. New York: BLOOMSBURY WILDLIFE, pp.18-20.
Simmons, N., Seymour, K., Habersetzer, J. and Gunnell, G. (2008). Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation. Nature, 451(7180), pp.818-821.