Orspers

Although I have started to reread the original shorter version of Poul Anderson's "Virgin Planet," it is helpful also to refer to the Author's Note at the end of the novel. It may be necessary to reread both versions in their entirety.

The women only colonists of another planet have domesticated large flightless birds, "orspers," to ride on. Such steeds are more imaginative and exotic than extraterrestrial equivalents of horses. Regular Anderson readers will remember his novel, The Day Of Their Return, in which colonists of the planet Aeneas have imported both horses and green, six-legged stathas as draught animals and the planet is also visited by the Chereionite Aycharaych whose species may be descended from large flightless birds. But that is on another planet in another history.

Although the orspers are an exotic touch, birds remain a Terrestrial kind of organism and we even have some that are large and flightless. However, the Author's Note convincingly accounts for the existence of such similar organisms on another planet and also demonstrates the amount of background thinking hidden behind any well conceived work of hard sf:

first, terrestroid biochemistry is to be expected on such an Earth-like planet;

secondly, currently life is flourishing in a mild interglacial climate;

thirdly, only birds are equipped to escape from the sudden, major environmental changes that are caused by this planet's irregular weather and turbulent geology.

Thus, although a few primitive mammals do survive on another part of the planet with stabler weather, it is the birds that have multiplied, and sometimes increased in size, whenever conditions have again become favorable. Land species are usually wiped out before they can become well-established.

Parallel evolution, of course, occurs on Earth. Fish and swimming birds and mammals are similarly streamlined. Sight and flight have such high survival value that both have evolved several times. We can expect flyers, if not birds, on other planets. The future history that contains Aeneas and Chereion also contains Ythri whose dominant species, although not birds, are winged and feathered and remain hunting carnivores even after they have become intelligent.