Terrestriality in bats

A number of bat species have independently evolved terrestrial locomotion. Some, such as vampire bats (Desmodontinae), move with a sort of hopping motion. Others, such as New Zealand lesser short-tailed bats (*Mystacina tuberculata*), simply walk with a peculiar, sprawling gait. However, bats are so well-adapted for flight that their anatomy has changed so dramatically that, if it ever happened, a flightless lifestyle would require very different skeletal anatomy. This raises two questions:

1. What is stopping bats from achieving full flightlessness?
2. What would a fully flightless bat look like?

Unlike most of this wiki's articles, this page will detail exactly how terrestrial bats have evolved the way they have, which ecological and evolutionary pressures have driven them to do so, and what a flightless bat would actually look like, as an exercise in speculative evolution.

## Terrestrial bat lineages

There are two main bat lineages which indulge in terrestrial locomotion. There are the mystacinid bats of New Zealand and the true vampire bats of South America.

The Mystacinidae is believed to have originated during the Oligocene, with fossils from this time being known from Australia, indicating the migration of at least one lineage from there to New Zealand. These are capable of folding their wings into specialized cartilage folds when not in use, preventing them from getting in the way while walking. Vampire bats, on the other hand, use a unique sort of bounding gait, allowing them to jump, walk, and even sprint on all fours. This appears to have developed separately from mystacinids.

## Impetus

Bats which display marked terrestrial locomotion tend to do so not as a result of general ecological pressures, but as a way of obtaining food. Mystacinids, for example, hunt in small groups on the ground, so it can be argued that they hunt on land due to its efficiency. Vampire bats do so for stealth, as walking up to their victims is stealthier than flying directly at them, explaining why they, too, have adapted for terrestrial locomotion. Finally, extinct mystacinids such as *Vulcanops* were adapted to omnivory. These species are all capable of flight, given that it is a valuable antipredator defence mechanism, and allows for a quick retreat, as well as the ability to reach difficult-to-access resources.

## Why are there no flightless bats?

Bats as a whole have been specialist fliers for more than 52.2 million years, and this can be seen in their anatomy. Every living bat species has modified hindlimbs and pelvises, due to the fact that their legs are part of their wings. Whereas the same was true with the extinct pterosaurs, the details are very different. The hind legs of a bat are actually rotated at between 90° and 180° in relation to a normal mammal, meaning that their knees point either directly outwards or backwards. This obviously makes terrestrial locomotion difficult for them, and even in species which have developed terrestrial locomotion, the forelimbs are the driving force, rather than the hind limbs. For this reason, the strongest locomotor muscles are positioned in the hind limbs, and this is especially obvious in species such as vampire bats. Likewise, a flightless bat would have limb anatomy very different to any other mammal.

As stated above, bats primarily evolve terrestrial tendencies as a response to behavioural and dietary requirements. For a fully-flightless bat to evolve, simply being on a predator-free island would not be enough. There are already bats living in environments which are mostly predator-free, such as the Hawaiian hoary bat, which have not evolved such lifestyles. There has to be some other motivation, though exactly what this would be isn't entirely clear. This is as good a time as any to move onto the most speculative section.

## What would a flightless bat look like?

This section will speculate on what a fully terrestrial bat may look like, including formulating a detailed evolutionary history for it. For a flightless bat to evolve, the environnment would need to be exactly right. Let's say we have an island off the coast of South America, where–by means of natural rafting–a population of small-to-medium-sized herbivorous rodents have been established. Let's assume for a moment that somehow, some of the big predatory bats of South America, such as a small group of spectral bats, found their way to the island. This hypothetical individual would find itself in an environment with no predators and a steady supply of whatever prey items are available. This likely would not lead to terrestriality straight away, as spectral bats often attack from above and carry their kills to roosts, so there would need to be some other impetus.

As stated above, this hypothetical island would have no predators, thus decreasing the need for flight as an antipredator defence. Let's assume for a minute that while there is some flying prey, the biggest and most attractives prospects are found on the ground. This may spur the bat's flying ability to decrease further, though this species would by no means be flightless. Its main system of locomotion would likely be similar to a true vampire bat at this stage, using its powerful forelimbs to bound towards prey. It wouldn't matter how fast the bat was actually going, as long as it was faster than its chosen prey. An important thing to note is that at this stage, the bats would *not* have lost their eyesight. Bats do not have bad eyesight by any means, and echolocation on the ground would be a very inefficient mechanism, so it's safe to assume that if anything, their visual acuity would only improve. A strong sense of hearing would certainly help, though, even if it would not involve echolocation.

After a few million years, we would be left with a slightly larger bat with increased terrestrial tendencies, improved eyesight–likely with binocular vision–and hearing which functions more like that of a typical predatory mammal. However, it's here that we run into a major issue. Bats' hindlimbs are still heavily modified, and would be mostly useless for a cursorial, raptorial bat, which leaves two possibilities. Either the bat would atrophy its legs and switch to a purely bipedal gait, or it would be restricted to a certain method of locomotion. Seeing as hindlimbs are an integral part of the wing structure, the author personally doubts that they would atrophy. At this point, we've pretty much reached a dead end. Without some mental gymnastics and unnecessary environmental pressures being piled on indefinitely, it seems hard to actually envision a flightless bat after all!

All in all, perhaps the question to ask is not what a fully flightless bat would look like, but what a large, predatory bat with terrestrial tendencies would look like. And that, the author would like to think, has been answered here.

## Citations

* https://koryos.tumblr.com/post/156829326111/why-are-there-no-flightless-bats
* https://en.wikipedia.org/wiki/Vampire\_bat
* https://www.nature.com/articles/news050314-9
* http://www.koryoswrites.com/nonfiction/the-pekapeka-tou-poto/
* https://zslpublications.onlinelibrary.wiley.com/doi/10.1111/jzo.12538