Their heads and bodies of bats have amassed an extraordinary array of adaptations that have make them lords of the night sky. Today, the thousand-plus types of bats make up a fifth of living mammal species. Richard Dawkins once described the evolution of bats as “one of the most enthralling stories in all natural history” and as of this week, the story has a clearer beginning.
The success of bats hinges on two key abilities: their mastery of flight, a feat matched only by birds and insects; and echolocation, the ability to navigate their way through pitch-blackness by timing the reflections of high-pitched squeaks. For evolutionary scientists, the big question has always been: which came first?
The ‘clawed bat’
Until now, fossil bats haven’t provided any clues for all of them show signs of both echolocation and flight. But a stunning new fossil, discovered by Nancy Simmons from the American Museum of Natural History is an exception and it provides a categorical answer to the long-running debate – the earliest bats could fly but could not echolocate.
The new creature hails from the Green River in Wyoming and is known as Onychonycteris, meaning “clawed bat”. Its fossils date back to about 52.5 million years ago and by comparing it to other prehistoric bats, Simmons found that it is the most ancient member of this lineage so far discovered. It acts as a ‘missing link’ in bat evolution, much like the famous Archaeopteryx hinted that birds may have evolved from dinosaurs.
Onychonycteris was clearly a capable flyer. Its wings are remarkably similar to those of today’s bats, with the exception of small claws at the end of its digits that modern bats lack. But the really exciting part of the new fossil is the size of its cochlea – the coiled tube that allows mammals to hear. Relative to the size of their head, all bats that use echolocation have massively enlarged cochleae and the two traits are so tightly linked that large cochleae have been used to show that other prehistoric bats were also echolocators.
Not so for Onychonycteris. The dimensions of its skull revealed that its cochlea was too small to have supported echolocation and was closer in size to those of flying foxes, large bats that favour vision over echolocation. It provides direct evidence that bats mastered the art of flying before they developed a way of navigating through the dark.
Flight or echolocation
When the flight-echolocation debate first started, the “echolocation-first” camp had the upper hand. According to this school of thought, the earliest bats used echolocation from tree perches to detect and snatch passing insects, and indeed, the most primitive of modern bats sometimes use this technique. The ancestral bats then evolved long, webbed arms and fingers to better catch their prey and eventually took to jumping after them.
However, this theory has since run into troubled times. It turns out that echolocation – which involves producing very loud bursts of sound – takes a lot of energy and it’s something that a perching bat cannot afford to keep up for long. A flying bat on the other hand, has no such difficulties. On each stroke of their wings, bats are contracting the powerful lung muscles that power their ultrasonic shouts so that if they time their flaps and squeaks correctly, echolocation is effectively free.
Other parts of the fossil provide clues about Onychonycteris‘s lifestyle. Its strong hind legs and wing claws suggest that it was an agile climber and could have scampered through branches on all fours.
The proportions of its limbs are closest to the modern mouse-tailed bats, which fly with an undulating style that alternates true flight with gliding. Onychonycteris may have also used this efficient tactic and it may have been a stepping stone from gliding to proper continuous flight.
But how could Onychonycteris have found its way through the air without echolocation to guide it? Perhaps it was a daytime flier and relied on vision to find its way around. Its descendants may have been forced to become nocturnal when the birds came to power towards the end of the dinosaurs’ reign, some 65 million years ago.
We would normally turn to Onychonycteris‘s eyes for clues for nocturnal animals that fly by sight, like the flying foxes, have large eye sockets. Unfortuantely, that will have to wait until the discovery of new fossils. The top half of the skulls that Simmons used were found crushed. While their ear cavities have already told us much about the evolution of bats but their eyes sockets will tell no tales.
Image courtesy of Nature
Reference: Simmons, N.B., Seymour, K.L., Habersetzer, J., Gunnell, G.F. (2008). Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation. Nature, 451(7180), 818-821. DOI: 10.1038/nature06549