IN THE undergrowth of a New Zealand forest, something stirs. A small, fuzzy animal is scurrying over tree roots and through leaf litter, foraging for insects and fruit. It scuttles with an odd gait, as if on stilts. Is it a mouse? A bird? No, it鈥檚 a bat. The New Zealand lesser short-tailed bat, or pekapeka-tou-poto, to be precise.
Bats first took to the skies about 52 million years ago, and they have stayed there ever since. Among the world鈥檚 1300 or so species, not one of them is flightless. Most can鈥檛 even walk very well, which is why many of us would be surprised by the behaviour of the pekapeka-tou-poto, a bat as comfortable on the ground as it is in the air.
But exactly why there are no flightless bats is an evolutionary mystery. The other great group of flying vertebrates, birds, have evolved to be flightless multiple times globally. They often do so on remote islands, such as those of New Zealand, where there is little danger from ground-based predation (at least until humans come along 鈥� roast dodo anyone?). In these circumstances, flightlessness is a good adaptation because flying is energetically costly.
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As the world鈥檚 most terrestrial bat, the pekapeka-tou-poto has long seemed key to explaining the curious absence of flightless bats. But research over the past 20 years has revealed a surprise: many other species of bat can also walk. Some can even run. So, what are they doing on terra firma, and can studying them help us work out why evolution has failed to produce a bat that can鈥檛 fly?
Bats are exquisitely adapted for flight, as is clear to anyone who has marvelled at their aerial acrobatics. Take them out of the skies, however, and it is a different story. Most bats are very bad at crawling. Leaf-nosed bats, ghost-faced bats, horseshoe bats and others are .
Some 50 years ago, bat biologist Terry Vaughan suggested a reason. He argued that . He realised that the rare confident walkers 鈥� which include the pekapeka-tou-poto (Mystacina tuberculata) as well as the common vampire bat (Desmodus rotundus) 鈥� have strong hip bones and legs that look a little like ours, in that they have three fully developed long bones: the femur in the upper leg and the fibula and tibia below. In all other bats, the fibula is a tiny stump that is attached only to the ankle, which presumably leaves these bats weak at the knees.
It is clear today just how well the pekapeka-tou-poto and the vampire bat can walk on their sturdy legs. Back in 2006, , then at Cornell University in New York, and his colleagues . Their study confirmed that both species have a curious but effective walking style, tucking their wings in and moseying along on their backwards-facing hind limbs and leathery wrists 鈥� an arrangement that gives their appendages a stilt-like appearance.
Running bats
The study also revealed that the vampire bats have a secret skill: they can run. When the treadmill speed reached roughly 0.6 metres per second, the bats broke into a gait that the researchers described as 鈥渂ounding鈥�, leaping from all four limbs at the same time (also called pronking). Imagine having a vampire bat bounding towards you, like a bat out of hell. It was thought at the time that vampires and pekapeka-tou-potos were the only two species of bat to be adept walkers. But we have learned more recently that a number of other species, including some with weaker hips and legs, are also powerful, if ungainly, walkers.
at Arizona State University has led some of this research into the walking bats. A few years ago, he had a pest problem at work. He tried using ground-mounted sticky traps to catch the pests 鈥� and frequently caught big brown bats (Eptesicus fuscus), one of the most common bat species in the Americas. 鈥淭hey were on the ground for some reason or another,鈥� he recalls.
Earlier this year, Jones and his colleague at the University of Kansas explored further. They stuck some big brown bats in a sandbox with a clear plastic covering and found to their surprise that the bats were ambling around happily and not even attempting to fly.
鈥淭hey did really well on the ground,鈥� says Jones. The study suggested that weak legs might not be the obstacle to walking that we had assumed them to be 鈥� an idea that fits with additional evidence. In 2019, for instance, researchers at Moscow State University and the Severtsov Institute of Ecology and Evolution, also in Moscow, tested five species of weak-legged bat 鈥� the common noctule (Nyctalus noctula), the parti-coloured bat (Vespertilio murinus), the pond bat (Myotis dasycneme), Daubenton鈥檚 bat (Myotis daubentonii) and Nathusius鈥檚 pipistrelle (Pipistrellus nathusii) 鈥� .
But why would these expert flyers lower themselves to our level? As with flightless birds, bats that can walk probably gained the ability because it carries advantages for them. Common vampire bats, for instance, sneak up on prey on the ground and may . The pekapeka-tou-poto鈥檚 ancestors, meanwhile, clearly found they could make a good living scurrying around foraging for pollen, fruit, nectar and invertebrates (especially flightless crickets called wetas that are endemic to New Zealand). Today, pekapeka-tou-potos spend about 40 per cent of their foraging time on or under the ground.
Whether walking carries benefits beyond feeding or lower energy costs isn鈥檛 yet clear, largely because biologists haven鈥檛 spent much time studying walking bats. 鈥淭here鈥檚 so little work on the terrestrial locomotion of bats. There鈥檚 probably a lot more going on than has previously been suggested,鈥� says Jones.
What little we do know indicates that bats and birds may evolve to spend more time on the ground for different reasons. For birds, it is sometimes an absence of ground-based competitors that seems to encourage the evolution of flightlessness. But at the University of New South Wales in Sydney is unconvinced that this model also applies to bats. In 2009, she and her colleagues described a 26-million-year-old fossilised creature discovered in South Australia that is . Australia had no shortage of ground-based mammals 26 million years ago, so this adaptation to also travelling on foot can鈥檛 have been driven by lack of competition and predation. Exactly what pushed the ancient bat to begin walking is still unclear.
Why bats can fly
The differences between birds and bats might also help explain why we have yet to find evidence that any bat, past or present, ever evolved to be flightless. Jones points out that bat anatomy is very different from that of birds. 鈥淭heir whole body, essentially, from their head down is built to be a flying apparatus. Their wings are comprised not only of their forelimbs, but also of their hindlimbs.鈥� This means it is probably more challenging to remodel a bat body into a fully flightless form.
What鈥檚 more, with the way humans have changed the world in recent centuries, the prospects of a flightless bat evolving in the future seem pretty remote. For instance, the rats and cats introduced to New Zealand by humans regularly prey on the pekapeka-tou-poto. Not only does this help explain why the species is now , it has also upped the selection pressure on the bats to stay airborne, where they hunt successfully for winged insects.
But never say never to the idea of truly flightless bats. 鈥淚 don鈥檛 think it鈥檚 impossible,鈥� says Jones. 鈥淚f somebody came out with a fossil from an island of a bat that lost flight, I wouldn鈥檛 be, you know, completely shocked.鈥�
No bats were hurt in the making of this story, though some got very tired running on a treadmill.
Graham Lawton always prefers to walk
