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periodically, often for migratory or mating purposes. Some groups have
distinct hierarchies, and certain actions such as a pecking order in birds are
undertaken to establish and maintain social order. Grooming among primates
establishes very elaborate family and group dynamics. Similar bonding
activities and hierarchies can be seen in large cats such as lions and wild
dogs. Seasonal group behavior is not uncommon among animals; stags will
gather to fight over female deer, and black grouse regularly gather together
in the spring at the same communal display area known as a
lek
. When
mating instigates group activities, normal modes of behavior are sometimes
abandoned for other, often more violent conduct. Some animals even change
their physical appearance—for instance, growing brighter plumage or long,
sharp horns.
Birds
Birds are not the only animals that have taken to the air and developed the
ability to fly. Many insects and some mammals also find it useful to get around
in this way. In the greater evolutionary time scale, birds have taken to the air
relatively recently. The earliest fossilized record of what is considered to be a
modern bird, Archaeopteryx, shows that it flew around 145 million years ago.
The fossil shows clear evidence of the asymmetrical nature of its feathers. This
is a very significant factor because it is the asymmetrical nature of the feather
that is needed to form a seamless wing, which in turn is essential for powered
flight. The wings of Archaeopteryx also show the vestiges of three fingers,
complete with hooked claws that would be ideal for clinging to branches and
assisting climbing and perching. Its jaw, while extended like a modern bird's
beak, is full of short, sharp teeth; this jaw, combined with a spine that extends
into a bony tail, demonstrates the reptilian ancestry of Archaeopteryx.
Most modern birds have lost all these traits, but the Hoatzin
Opisthocomus
hoazin
is a modern bird living in the forests of South America that has retained
very similar small claws on its wings. These claws enable it to move around in
the trees, much as its earlier ancestors did in order to feed on a diet of leaves.
Long before Archaeopteryx took to flying, insects ruled the skies, first
colonizing the air around 350 million years ago. With no airborne competitors,
they grew to sizes much larger than their modern-day equivalents. There is
evidence of dragonflies with a wingspan of a meter plus. Dinosaurs also had
their representatives that took to flying as a preferred mode of locomotion.
The earliest pterosaurs are thought to have existed around 200 million years
ago, though there is a great deal of conjecture as to why they developed
in order to take to the air. The largest of the earliest flying dinosaurs were
around the size of a modern seagull. The size and shape of the dinosaur wing
suggests that the mode of flight was largely dependant on soaring, in much
the same way as modern gulls soar over the ocean waves. There is even some
evidence that supports the notion that these creatures existed, also rather like
modern seagulls, on a diet of fish. Later, pterosaurs developed and became
