Modern birds evolved from ground-dwelling reptiles as their increasingly refined parenting skills led them into the trees, where they could better protect their young, proposes a researcher at the University of California, Davis.
This new theory, contradicting the two leading theories on the evolution of avian flight, appears this month in the German journal "Archaeopteryx," named after a feathered fossil with both reptilian and birdlike traits.
"The evidence indicates that a whole suite of behavioral and physical traits, including feathers and wings, evolved along with improved parenting and brood-care traits," said James Carey, a UC Davis demographer and ecologist. "Once the precursors to birds began to fly, the ecological interplay of flight and parental care may have been mutually reinforced, continuing the evolution of both traits and accelerating the rate at which the physical features of the modern bird were acquired."
The origin of bird flight is a fundamental issue in avian biology and in overall evolutionary theory. Many scientists point to the fossilized specimens of Archaeopteryx as evidence that there was a transitional vertebrate species that developed during the evolution from reptilian dinosaurs to birds. Furthermore, they suggest that the development of flight may explain why birdlike dinosaurs avoided extinction.
Until now, there have been two basic theories on the origins of bird flight. The first, the arboreal theory, is a tree-to-ground model, suggesting that birds' primitive ancestors were tree-dwellers that leapt from branch to branch. Through the ages, the ability to first glide and later fly developed because gliding slowed their fall to earth when they missed a branch.
The second, the cursorial theory, is a ground-up model that suggests that birds evolved from four-legged reptiles. According to this theory, scales on these creatures' front limbs gradually developed into feathers that gave them upward thrust when they ran and eventually enabled them to fly.
Carey maintains that both of these theories have major flaws. If the arboreal theory were accurate, birds' early ancestors would more likely have been four-legged creatures that developed membranes between their front and back legs, much like flying squirrels or bats, he asserts.
And, if the cursorial theory were true, there should have been obvious useful advantages to each form that developed between the four-legged reptile and the bird, he maintains. He is not persuaded by the suggestion that the intermediate stages of this line used their feathers, first developed to conserve heat, to swat insects from the air.
On the other hand, the parental-care theory is consistent with both the physical and behavioral changes that appear to have occurred as reptiles evolved into winged dinosaurs and finally into modern birds, Carey says.
He suggests that modern birds' very early ancestors were reptiles that established and guarded their nests on the ground, much like crocodiles. Over time, these creatures developed hard-shelled rather than leathery eggs and the ability to modulate their own body temperature in order to provide a more constant environment for their developing young. Scales evolved into feathers, better camouflaging and insulating the parents.
In time, these early ancestors of birds developed more advanced techniques for caring for their young. They started to feed their young in the nest, pumping liquid food or placing small food items in their mouths. They also began to produce fewer and more dependent offspring and smaller eggs, and began nesting in bushes and then small trees to better protect their offspring from predators.
Gradually the forelimbs of these creatures became feathered and even more elongated, enabling them to better manipulate their eggs and to "parachute" from their tree nests to a soft landing. Later they would develop the ability to glide and eventually fly by flapping their wings.
Carey hypothesizes that bird beaks also evolved in the context of parental care. The beak, he suggests, serves both as a point source of food for small hatchlings in the same way the nipple is used to feed mammalian young and also as a tool for sophisticated nest construction. This concept diverges from the commonly held notion that birds evolved beaks because they weigh less than teeth and so are better adapted for flight.
He points out that flight provided these prehistoric ancestors of birds with numerous advantages including the ability to safely place their young high in trees and cliffs, maximize their food sources through seasonal migration, and supply more and higher quality food by expanding their foraging range.
He adds that the fossil record, specifically Archaeopteryx, provides ample evidence that the evolution of parental care was the main driving force behind the evolution of avian flight.
- Fossil specimens of Archaeopteryx have forelimb claws, supporting the concept that ancient bird ancestors were tree-dwellers;
- The feathers on Archaeopteryx fossils appear to be much more advanced than the creature's other birdlike traits, which is consistent with the notion that feathers evolved very early to shield the nest-sitting adults from the elements;
- The beak of Archaeopteryx is quite primitive, which is in keeping with the concept that sophisticated nest-building and feeding behaviors emerged much later in the development of parenting traits; and
- While Archaeopteryx had highly advanced feathers, its bone and muscle structure appear to have equipped it for only limited flight. This makes sense, according to the parental care theory, which asserts that flight developed long after the reptiles with their protective feathers moved into the trees.
Additionally, recent findings in Asia of feathered dinosaurs provide further evidence that feathers evolved before flight, Carey adds.
In this paper, he also discusses why flying dinosaurs with nonfeathered membrane-like wings, such as the pterosaurs, became extinct. He suggests that they perished, not because they were out-competed by birds but because they lacked the sophisticated parenting skills needed to cope with a changing environment.
Carey developed the parental-care theory of avian flight evolution while doing research supported by the National Institute on Aging, the Duke University Center for Demographic Studies and the UC Berkeley Center for the Economics and Demography of Aging.
Editor's Note: A digital image of sketches illustrating the hypothetical progression of the evolution of birds is available upon request from Patricia Bailey, News Service, 530-752-9843, firstname.lastname@example.org.