Science Sunday: The Diversity of Flight
As I promised a fortnight ago, the series is continuing at loftier heights. Today, we’re going to tackle flight.
Flight is an especially interesting characteristic because it hasn’t evolved just once; there isn’t any common ancestor that we can point to as the one who ties all flying animals together. If we exclude gliding over short distances, true flight is something that has evolved at least four times over the course of history, in insects, birds, pterosaurs, and last but not the least bats – the only flying mammals. It’s not a given that flying should be a popular evolutionary strategy for getting around. It’s fairly expensive in terms of energy, and learning how to fly is not a risk-free endeavor. However, once a species has attained flight, it seems to be unstoppable.
Birds are generally believed to have descended from dinosaurs, and the common ancestor of all living birds lived around 70 million years ago. They are the largest group of terrestrial vertebrates, and not surprisingly a very diverse group. Common for all birds is that their forelegs have been modified for flight, and are coated in feathers. The modified forelegs are usually hollow, to make the birds as lightweight as possible. Birds also have a lot of muscles in their wings, providing them with lots of lifting power. The specific shape of the wing varies greatly between different bird species, to accommodate different styles of flight. Birds that fly slowly generally have wings that are much longer than they are wide. Such high-aspect ratio wings are important for birds that glide or soar a lot, making it possible to slowly survey for food, and plunge very fast towards the ground or the sea. Other birds have relatively short wings – like songbirds, which are more practical in forests and other areas with dense vegetation. These are just two of the most common forms, and there are many smaller and bigger variations. Even though there are many bird species, there is another group of fliers that is even more diverse, namely insects.
There are many, many different species of flying insects, so I’m going to generalise a bit and not get lost in the swarm of information on insect flight. Unlike the other groups of fliers, insects usually have two pairs of wings, that stick out from the second and third segment of their midsection. The wings are an extension of the insects’ midsection, and are usually flat and strengthened with several veins. Insects have many different ways to fly, but these can generally be classified as either direct or indirect flight. Species that use direct flight often have muscles attached directly at the wing base. When the muscles move the wing base down, the wings will move up, and vice versa. However, most insects use indirect flight, were the muscles are instead attached to their midsection, and the wing pairs are linked. The movement of these muscles causes the midsection of the insect to deform, and thereby the wings to move, as they are a part of the midsection. The prevalence of indirect flight can be explained as it enables a more finely controlled flight; insects with indirect flight can both hover and fly backwards. Still, the origin of insect wings has not been properly explained, which is understandable as insects do not fossilise well. There are several theories that try to give an answer, suggesting that they have evolved from gills or other modified appendages. Insects developed flight around 400 million years ago, and were thereby the first animals to reach the skies – and the only invertebrates so far.
As mentioned above, bats are the only mammals that have true sustained flight. Bat wings are in fact modified forelegs, where the fingers are elongated so as to give a stabilised structure. The thin, leathery membrane that extends between the fingers constitute the wings. Bat wings are more flexible than those of birds, but at the same time more prone to tearing. From fossil evidence, we know that bats have been flitting around for at least 50 million years, and they have been quite successful at it. There are over 1200 bat species, which is about a fifth of all mammalian species in the world.
Pterosaurs were flying reptiles that lived contemporary to the dinosaurs. Contrary to what some believe, pterosaurs were not the ancestors of birds, or even that closely related to them. Their wings consisted of a complex membrane – complete with blood vessels and muscles – stretching from their bodies, and out to the fourth finger bone. This is not completely unlike the structure of bat wings, except that only the fourth finger bone was severely extended. It’s hard to tell exactly how successful the pterosaurs were, as they aren’t around to tell us anymore. What we do know is that they ruled the skies for over 150 million years, and that they were a diverse group. The smallest species had a wingspan of as little as half a metre, while the big ones had a wingspan of up to eleven metres. The species Quetzalcoatlus is believed to be one of the largest flying animals of all time, and its name is, appropriately, from the Mesoamerican serpent god Quetzalcoatl.
I’ll touch a bit upon gliding too, as it has likely been a stepping stone between locomotion on the ground and flight for those groups that master flight today. There are many more different kinds of animals that have evolved gliding than the three groups of animals that utilise true flight, like amphibians, fish, and mammals other than bats. Some animals use gliding as a strategy to escape predators. This includes species like the neon flying squid, which can thrust itself out of the water and glide up to 30 metres through the air. Many others seem to have developed gliding for convenience more than anything else, especially animals living in forests with high trees. The different species of flying squirrels will much rather glide to the nearest tree than take a detour via the ground, simply by jumping from a current tree and extending the flap of skin that stretches all the way from ankle to wrist. This makes it easier both to get to a new food source, and to avoid possible predators.
To sum up, there aren’t that many groups of animals that have evolved true flight, but those that have are some of the most diverse and successful species that exist. Carving a niche in the skies may indeed prove very useful, providing both relief from predators and better access to prey. But as the pterosaurs can attest to, there aren’t any guarantees against extinction.