Use of Feathers By Archaeopteryx

From: rowe@lepomis.psych.upenn.edu (Mickey Rowe)

> Didn't someone (Ostrom?) once suggest archaeopterygid flight
> feathers as insect traps?

Yes. However, it's my impression that nobody takes the idea seriously any more. However, the idea has gotten a fair amount of press, so let's turn to another idea that Richard Cowen (another prominent lurker) gave us last winter and which I think could stand some more consideration. I'm only excerpting a piece of what Richard sent last January. If you want more, look through the jan95 archive (archives that old were kept by month rather than by day, so be prepared to sift through a lot...) or better yet look for Richard's book:

This is a slightly edited version of a section from "History of Life" by Richard Cowen, published by Blackwell Science, 1994. Copyright Richard Cowen.

The Display and Fighting Hypothesis

Either the arboreal or the cursorial hypothesis would work, and work much more easily, if a protobird already had long, strong feathers in the right places and already had powerful arm movements. Jere Lipps and I suggest that display was involved in the evolution of flapping flight as well as in the evolution of feathers. Display provided long, strong feathers on arms and tail. Successful display was increased by lengthening the arms, especially the hand, and by actively waving them, perhaps flapping them vigorously. Flapping in display would have encouraged the evolution of powerful pectoral muscles.

But a threat display must not be seen as an empty bluff. Fighting is the last resort. Living birds often fight on the ground, even those that fly well. Wings are no longer clawed but are still used as weapons in forward and downward smashes (steamer ducks are particularly deadly at this). Beaks and feet can be used as weapons too and are most effective when used in a downward or forward strike.

A strong wing flap, directed forward and downward, is also the power stroke that gives lift to a bird in takeoff. Lipps and I suggest that strong wing flapping is a simple extension of display flapping, encouraged by fighting behavior. Powerful flapping used to deliver forearm smashes could have lifted the bird off the ground, allowing it also to rake its opponent from above with its hind claws. The more rapidly the wings could be lifted for another blow, the more effective the fighting. This would rapidly encourage an effective wing-lifting motion that minimized air resistance, so the wing action would then be almost identical to a takeoff stroke.

A variant of our idea has also been proposed by Kevin Padian, who prefers to think of the wing stroke evolving from the arm strike used by a theropod in predation. It's not clear how this could have led to whole-body takeoff, however.

A few living birds use their wings extensively as weapons. The steamer ducks of the South Atlantic are large, powerful birds with heavy, bright orange, horny knobs on the wings of both sexes. These are used by both sexes in display and fighting. Steamer ducks (especially males) fight a lot among themselves for mates and territory, and they often kill other species of water birds, holding them by the neck and beating them to death with the wing knobs. Some species of steamer duck are flightless; in other species, the males are often too massive to fly, even though juveniles and females can fly well. Selection has favored fighting ability over flying ability for many steamer ducks. Flight is perhaps less important for them than for many birds, because they live in shoreline habitats where food is plentiful all year round.

Archaeopteryx fits our display-and-fighting hypothesis well. It was well adapted for display. Like any small theropod, it was well adapted for fighting with its teeth and the strong claws on hands and feet. Archaeopteryx did not have long primary feathers on its fingers, probably because they would have hidden the claws in display and would most likely have broken in a fight.

Did Archaeopteryx Fly?

Archaeopteryx, then, was a fierce little fast-running, displaying bird, which probably spent its life scurrying around the Solnhofen shore, hunting for small prey such as crustaceans, reptiles, and mammals. In hunting style, Archaeopteryx was probably much like the roadrunner of the dry country of the American Southwest, but its ecological setting was closer to that of a steamer duck - a shoreline with year-round food supply. Archaeopteryx did not compete in the air with the pterosaurs that are also found in the Solnhofen Limestone.

From Display to Flight

In our theory, display and fighting were simple selective agents that encouraged the evolutionary transition from small dinosaurs to birds. The idea fits with our current knowledge of the biology and behavior of living birds. Display, and fighting if necessary, is very important, even within a species. Bald eagles and frigate birds often try to rob other birds of food instead of catching prey themselves. Because the penalty for wing injury is high, many birds can be intimidated by display into giving up their catch rather than fighting to defend it.

Display and fighting in birds, whether for territory, dominance, or food, takes a lot of energy, but only for brief periods or seasons, and it provides an enormous payoff in survival and selection. Sexual display in most living birds must be done correctly, or no mating takes place. New behaviors are quick to evolve, and they are evolutionarily cheap, because they usually do not require any important morphological changes in their early stages. Bowerbirds, for example, show distinct behavioral differences in display between closely related species.

The display hypothesis suggests that a protobird gained flight behavior, anatomy, and experience at low ground speed and low height, ideal preflight training. The selective payoff for successful mastery of the flight motions gave significant advantages, even before flight itself was possible. From that point, the many advantages of flight were added to those of social or sexual competition.

Lipps and I envisage Archaeopteryx as a small, fierce predator, capable of liftoff but not true flight. Once liftoff was achieved, flapping flight quickly followed. There is no need to suggest any difficult evolutionary sequence to complete the final transition to full powered flight. In more advanced birds than Archaeopteryx, the pulley system of the shoulder evolved for quick wing upstrokes, while the wishbone evolved into a spring. The breastbone evolved as the anchor for the flight muscles. The forearms became longer, lighter, and more fragile in bone structure, becoming specialized as wings, and losing the finger claws. The feathers became more aerodynamically suited to powerful swishes through air. Meanwhile, the feet and beak became the dominant fighting weapons, as in most living birds today.

The display hypothesis for the origin of flight is particularly attractive because it demands few of the assumptions required by the arboreal or cursorial hypotheses, yet it is fully compatible with the morphology of Archaeopteryx and the biology of living birds.

NOTE: If you want to quote this hypothesis, I'd suggest that you use the phrase Cowen and Lipps in Cowen (1994), and refer to my textbook. I hope that Jere and I will be able to get the full-jargon version out in a journal some day. Meanwhile, Jere and I are responsible for the idea, and you have to blame me for the detailed wording that I prepared for my text.