Biology Index
By Richard Bruce BA, MA, and PhC in Economics

The Evolution of Feathers Warmth, Show, Flight A Just So Story

It is common for biologists to construct stories about how things evolved. These are frequently called just so stories. They are speculation. This is an just so story, but it is more speculative than many because I am not a biologist. You might think of this as a mental exercise, that I share, because in the Internet age sharing is free.

Feathers evolved first for warmth, then display, then gliding, and finally flight. Of course birds still use them for warmth, display, and gliding. As each new function was added, the previous ones usually remained.

Birds are dinosaurs. In the past we would have said birds are descended from dinosaurs. I believe, along with Robert Bakker Ph.D. and many other paleontologists that all dinosaurs were warm blooded. So birds are descended from warm blooded animals.

So feathers evolved among warm blooded non-avian dinosaurs to retain heat.

We can imagine how this might happen in small steps. If the early proto feather stuck our from the body a little they might trap insulating air.

It is perhaps harder to imagine feathers evolving purely for display. Fuzz that stuck up a little does not sound like an attractive or impressive display.

But once the feathers had evolved for warmth, it is easier to see how long feathers could evolve for display. The long feathers would not have to have a particular shape to be showy, so it would be easier to go from warmth feathers to show feathers than warmth feathers to gliding and flight feathers. If warmth feathers became a little longer they would probably create more drag and impede gliding and flight rather than help it.

Once long display feathers had evolved, often covering shorter warmth feathers, which today we would call down, we can easily imagine them evolving to feathers that assist gliding. As non-avian dinosaurs evolved into avian dinosaurs, which we call birds, they almost certainly went through a period of gliding like the flying squirrels, flying lemurs, gliding lizards, and at least one species of gliding frog.

As the dinosaurs evolved from gliding to flight, the feathers fully evolved into flight feathers.

Of course, this is not entirely new. It is the normal theory that feathers evolved first for warmth. We even have evidence for this in the fossils.

Furthermore, I believe we have found evidence for downy warmth feathers, and even display feathers on non-avian dinosaurs that clearly could not fly or probably even glide, and perhaps could not even climb trees.

Furthermore, it seems likely that insulation was a necessary step before flight. It is hard for a large animal to fly, or even glide. It is hard for a small animal to keep warm, particularly without insulation. As lizards, and at least one species of frog glide we know that cold blooded animals can glide and they do not have insulation. But birds, and bats when they are flying maintain a temperature temperature of 40 degrees Celsius, 104 degrees Fahrenheit.

It is interesting that both groups seem to fly at the same temperature. I could be wrong on this. My source says that birds typically maintain a temperature of about 44 degrees Celsius, 111 degrees Fahrenheit when in high activity. I do not know what high activity is, I am assuming it is fast flying, but it could be all flying.

Nevertheless, extended powered flight is likely to be difficult to impossible for vertebrates, which are generally larger and heavier than insects, unless the vertebrate is warm blooded. So the first flying dinosaurs had to be small and warmblooded which generally means they needed insulation.

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Last updated September 23, 2014