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Unique evolutionary cover drives

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Professor Shane Maloney, Head of Anatomy, Physiology and Biology at the University of Western Australia, takes a closer look at mammals and birds, in particular their hair, fur or feathers. Mammals generally have fur although there are exceptions. The large mammals like the hippopotamus and elephant have skin with hair, as do humans. Those large mammals are much less likely to lose heat in the cold due to their large surface area to volume ratio. It seems those larger mammals lost their fur because there was a cost to maintaining it or they did not need it as well as a risk from overheating in hot weather. 

Small mammals lose heat more easily but as a result, in the cold weather, need fur to survive. Humans are right in the middle in terms of body size but they too have lost surface fur and most of their hair, probably because humans began to use fur and fibres from other animals to insulate themselves, and so reducing the need to grow their own. That of course would depend on the lineage of those hominoids and whether they were in cold or warm climates. Cold climates would necessitate use of animal furs, while warm ones would suit sparse hair. 

Another theory speculates whether the ice age caused this change. As the homo lineage evolved, more and more hair was lost. Indeed researchers have managed to establish the time period when humans became predominantly hairless from the genetics of pubic lice on modern humans compared to body lice in our relatives. 
Only warm blooded species - birds and mammals - have feathers or fur (or hair) as they strive to maintain their body temperatures and minimise changes in their metabolic rate in spite of the external climate. However, fossils being discovered in China show proto feathers on extinct animals that would be classified as reptiles. Birds, our modern day descendants of a branch of dinosaurs, emerged as these proto feathers developed to new levels and evolved into flight feathers. There are many theories about why feathers evolved. One is that they were a hunting mechanism for catching insects; another is that they were used for sexual display like a peacock. A combination of many contributing factors is likely. 

When it comes to range of functions and physical appearance and properties on the one individual, there is little comparison between feathers with fur and hair. Even the elegant display of the Australian Lyrebird (photo by Sonja Ross) is a pale and simple comparison with the elaborate Bird of Paradise to our North. 

Both fur and feathers are primarily made of keratin, a proteinaceous output like fingernails. Mammals make their hair by producing a proteinaceous strand which is pushed out of a follicle. Feather growth is far more complex. Flight feathers are barbs on barb which produce a crucial resistance, enabling them for use in flight. Both fur and feathers trap a layer of air which provides the insulating property for both types of coats. Both mammals and birds have muscles which can lift the hair and feathers. The muscles are at the base of each hair or feather and are used to fluff up in the cold, creating a thicker layer of insulation, be it hair or feather. Humans have lost this insulating hair coat but have kept the muscles and the effect is seen when we get goose pimples from cold (or intense arousal) for example on our forearms. Birds have better insulation than mammals when compared at a given body size but in both it is the still air that is trapped that provides the insulation, not the hairs or the feathers. Heat doesn’t travel very well through air so by trapping pockets of air in their coats, mammals and birds have exploited that physical property of air even though both hair and feathers have high thermal conductivity. 
While many mammals have an array of fur or hair types on their body, for example underfur and then stiffer guard hairs, feathers on a bird comprise a vast array of shapes, sizes and functions, quite apart from their essential role in flight and insulation. Many birds line their nests with the finest soft down that is buried between flatter more robust feathers on the body. Tiny feathers shape eyelids and cover the face, head and body to aerodynamic perfection. The thicker covering of the body not only serves as insulation and protection from sharp branches and to ward off aggressive others, but provide hardwearing cushions for roosts on branches or rocks. 
The selective pressure on birds has seen them develop physically robust flight feathers as well as downy ones that essentially insulate. The trapped air also provides a flotation device in waterbirds like ducks. Birds that dive, however, like penguins, have to exert themselves to dive in order to overcome that buoyancy. The buoyancy does help them when they swim back to the surface. Birds that are affected by oil spills lose this buoyancy, the air being replaced by oil which means the feathers are full of water, not air.

For a long time people thought that a black coat for an animal absorbed more energy from the sun than a white one. This extra energy absorption meant the animal could expend less energy to maintain its body temperature. However, further research looked at the different structures of black and white coats. White coats appear white because all wavelengths of the light spectrum are reflected back. 

Above: wide angle view of the wind tunnel Professor Maloney and colleagues use to make measurements of insulation and effects of sunlight.    
Black coats appear black because those wavelengths are absorbed into the coat. It was found that not all wavelengths were reflected back from white coats. Some were reflected forward and absorbed further into a white coat, while absorption in a black coat was superficial.The effect of wind speed proved to be another difference for these coats. Wind directly removed radiant heat at the black coat’s surface, whereas the deeper absorption of radiation in a white coat protected it from the wind. Glen Walsberg proved that at a wind speed of 0m per second, black coated pigeons absorbed more radiation than white coated ones. At just 3m per second of wind speed, however, it was the white coated pigeons that absorbed more. 
There are, of course, other selection pressures that come into play to determine an animal’s colour and that affect its behaviours, habitat and preferred foraging hours. However, this research certainly put paid to a long-held idea. Once the variables were changed from previous research, so did the scientific outcome. 
Hair, fur and feathers are important to mammals and birds and different selection pressures drove each. Hair, fur and feathers are a result of unique cover drives. 


Footnote: The only mammals that can fly do not have feathers or hair or fur on their wings. Bats are perhaps the ultimate flying machine in terms of specialisation for flight as this has cost them many of the usual functions of hands, arms and legs. They have lost all except one functional hook on their 'hands'. They must hang by their legs under branches since their legs are unable to support their weight against the pull of gravity. However they are superb in flight. Their wings are of specialised skin and are light weight, very flexible and shaped and stretched over their elongated 'finger bones'. Nancy Pallin elaborates in Bat Chat click here.

Image:Grey headed flying fox belly dipping copyright Nick Edards


Professor Shane Maloney was interviewed for A Question of Balance by Ruby Vincent. Summary text by Victor Barry. Images from Shane Maloney, Sonja Ross and Nick Edards. July 2016

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