Theory Sez Flight Evolution Linked To Parental Care

[Richard W Travsky <rtravsky@uwyo.edu>]

http://www.sciencedaily.com/releases/2002/01/020130073659.htm

Excerpts:

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."

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... The
second, the cursorial theory, is a ground-up model that suggests that
birds evolved from four-legged reptiles. ... 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.

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. ...

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 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. For example:

* 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.