SUNDAY, 31 MAY 2020
It is 09:57 am, I am sitting in the library, the air is filled with the smell of old and dusty books. From the open window, I can hear birds singing, in the distance the sound of children playing. As I look up from my book, I can see the scrupulously tended garden, with its many flowers, and the freshly mowed grass. But what catches my eye is not one of the opulent sculptures. It is a small black and white bird and a black cat with white spots sitting merely arms-length apart. The prey and its predator sit next to one another in, what seems like, harmony. This sight makes me wonder: 'Why does the cat not attack? Was it just fed? Is the cat sleepy and wanting to rest? Or, is the fact that the small bird could just fly away before the cat reaches it reason enough? What would the cat do if, instead of the bird, a mouse sat in front of it?'My thoughts are interrupted by a sudden motion in the garden — the cat pounced. The bird flew off. The cat did not even come close to the bird in its pathetic attempt to catch its prey. I follow its flight further until the small bird lands on a window sill, very close to where I am sitting. As flight is so abundant, I could venture to many different places on our planet to find answers to the phenomenon. Flight is everywhere, from the north to the south, from the snowy owl in the empty tundra of the Arctic, to the birds of paradise in the species-rich rainforest, to the vultures in the hot and sparse plains of the desert and to the albatrosses in a cold and unforgiving Antarctica.
But to understand the origins of flight, I must go far, far into the past and start with the basics — what is flight? With this in mind, and no budget, I start my journey where all good journeys start — in a museum. To be precise, in the Museum of Zoology, here in Cambridge.
As I enter the Museum, I see the specimen of a goose and a flying squirrel and I wonder, can both of them fly? The answer is a simple no. Birds, such as geese, are able to fly and glide, but the flying squirrel only glides from tree to tree without being able to ‘fly’.
So what is the difference between gliding and flying? The main difference is that flight is powered through the use of a wing stroke, hence why it is often referred to as powered or active flight. Gliding, however, is passive and no stroke of the wing is required. To cover a greater horizontal distance, gliding animals usually climb trees or other heights and let themselves drop, using their parachute-like structures. Other gliding animals, such as the flying fish, move with high speed before going airborne. The last stroke of the tail fin coincides with the time their wings unfurl and the wings remain in place until they contract during landing. Although gliding animals do not use any sort of active prolongation, some can travel impressive distances. The flying fish can travel an astonishing 50 metres while completely airborne, while the flying squirrel can cover even greater distances, up to 115 metres. With many different species being able to glide but not fly the question remains; which animals can fly?
The ability to fly evolved only four different times in birds, bats, insects and the extinct Pterosaur, a member in the group Avemetatarsalia. The number of species currently living in each of these groups is extraordinary, with 1,116 bat, 9,000 bird and at least 1,000,000 recorded flying insect species, in total making up more than half of the total global number of species. In contrast, gliding has evolved separately multiple times but it does not show the same overall abundance and success as flight, as it is only seen in a minimal number of species.
The origin of flight can be traced back to shortly after the origin of trees and complex terrestrial ecosystems, which evolved around 412 Ma (million years ago). The first fossil record of a flying animal dates to ~324 Ma, which let palaeontologists estimate the first occurrence of flight as being ~406 Ma and thereby marks the beginning of the Pterygota, the winged insects. With insects starting to monopolize the air, it took another 200 Myr (million years) before the first vertebrate started to fly. In relation, humans have only been on this planet for about 2.80 to 2.75 Myr.
The origin of flight is still debated and multiple different models have been suggested to explain the evolution from being ground-bound to being able to fly. The most promising theory about the evolution of flight is the arboreal, or gliding theory. This theory predicts that powered flight evolved from the ability to glide from tree to tree and was first described by Darwin in 1859. With the evolution of flight, a whole new habitat could be explored and being able to fly gives rise to many different advantages.
Flight is truly a story of success. All eight of the fastest animals on our planet are able to fly, amongst them birds, as well as bats, with the peregrine falcon being able to reach 389 km/h in a dive flight. Other flying species such as the arctic tern, a sea bird species, travel more than any other animal. On its annual migration from the Arctic to Antarctica it will travel around 71,000 km. But what makes the ability to fly so successful?
Flying is the most energy efficient way of transporting a unit of mass on land, with only swimming being overall the most efficient. This makes traveling a certain distance less energy costly than walking or running and can considerably increase the dispersal radius. Flying offers the opportunity to migrate between land masses, as flying animals are not restricted to one location. The expansion into three-dimensional space causes an increase of species richness as various habitats can now be accessed. Furthermore, the ability to fly increases the chances of escaping from a predator as well as feeding and breeding, and finding resources without the threat of land predators. While the list of the benefits of flight is nearly endless, nevertheless, everything comes with a cost.
The ability to fly comes with a high metabolic cost such that many flying animals, e.g. the hummingbird, consume their weight in food on a daily basis. Furthermore, being airborne increases the likelihood of being spotted as potential prey, and resources, which could otherwise be used for reproduction and growth, must be allocated to the development of navigation abilities, especially in migratory bird species. With the evolution of vertebrate flight and examples of the first birds and other early-avians, many associate feathers with the ability to fly. The Archaeopteryx, the first recognised bird-like dinosaur, is often taken as an example of an animal which has feathers but cannot fly (Note: experts are debating if the Archaeopteryx was able to fly or not. Update: Only recently researchers at the University of Cambridge discovered the Wonderchicken, a species now known as the oldest modern bird.)
Not all vertebrates which are able to fly have feathers, in fact only birds and their early ancestors do. But why do some flying animals have feathers while others do not? The answer is not so simple and took years of research. Nonetheless, ground-breaking work revealed that feathers evolved independently from the ability to fly. It was nearly 30 million years after the origin of the feather (~230Ma) before the first vertebrate was able to fly. This first specimen showed feather-like structures, which were more complex than hair but simpler than more evolved feathers which show very advanced branching structures. Pterosaurs were the first vertebrates to show active flight and also the first to show feather-like structures. Recent studies surmise these feathers were used for tactile sensing, signalling, thermoregulation, and aerodynamics and were not yet suitable for the use of flight. Only through molecular modifications were these early feathers made suitable for flight, and it is suggested that they coevolved with the ability to fly. But feathers are not the only factor which determines the ability of an animal to fly. For instance, all Pterosaurs only possessed wings without modified feathers but the Pterosaur species Quetzalcoatlus northropi was the largest flying animal that ever lived. Its body mass was an astonishing 250 kg and it had a wingspan of an impressive 15.5 metres. Q. northropi was not the only giant of its time. Some time before the first vertebrate was able to fly, insects of enormous size evolved due to the lack of airborne vertebrate predators and higher levels of oxygen in our atmosphere. During the time of historical “insect gigantism”, around 300 Ma, the largest insect ever recorded, the dragonfly Meganeura monyi, could grow to be enormous with a wing span of 70 cm.
Although these giants are long lost to our planet, we can still see some of the ancestors of the ancient insects and birds which lived at the same time as the dinosaurs. Furthermore, the success story of flight continued, as more flying species, birds, bats, and insects alike, evolved.
Nowadays, flying species contribute to more than half of our species richness but due to climate change these fantastic species, which took millions of years to evolve, are in great danger. Migratory birds are especially impacted by the rising temperatures. Many of these birds migrate in early spring to higher latitudes to nest and breed in the temperate and polar regions. As larvae and other insects are a typical food source for young bird hatchlings, the synchronisation of the larval peak abundance and chick hatching time is crucial for their survival. As the poles are warming, new green foliage emerges earlier in the year, leading to an earlier peak of larval abundance. Due to this, the perfect synchronisation between bird migration and food availability is pushed into disequilibrium. As the mismatch progresses, experts predict populations will plummet, as the gap grows until it is completely mismatched.
The first studies investigating climate change were conducted more than ten years ago but have led to fairly limited actions. Since the ability to fly has come a long way and has fascinated many, it would be a shame to let millions of years of evolution go to waste. Finding solutions to climate change is, therefore, the necessary next step to continue this story of success.
— I blinked and noticed a movement outside. The small bird had flown off —
Felicitas Pamatat is an MPhil student in Zoology at King's College. Artwork by Clara Munger.
