BIOLOGISTS ANSWER AGE-OLD QUESTION
by Whit Gibbons
August 11, 2003
is "the egg." The question is, "Which came first, the chicken
or the egg?"
Birds appeared on Earth during the Jurassic Period, which began about
180 million years ago, and chickens followed several million years after
that. However, eggs of various sorts had already been around on the evolutionary
stage hundreds of millions of years before birds, let alone chickens,
ecological and evolutionary theories and discoveries revolve around eggs.
Exceptions are what make evolutionary ecology exciting, and the mechanisms
for giving birth have many exceptions. Some animals are live-bearers,
which means they do not lay eggs. Live-bearers, including humans, rattlesnakes,
and mosquito fish, actually have eggs, but the embryos develop inside
the mother instead of in a shelled egg outside the body.
vary greatly in how they carry out the process of development and birth.
The level of nourishment given to an embryo by the mother during development
varies from a little or none to complete, depending on the type of organism.
Thus most mammals provide a continual and direct supply of food to the
developing baby through a placenta before birth whereas the embryos of
most live-bearing reptiles obtain their proteins and energy for growth
from an attached yolk sac while inside the mother. Embryonic development
in many live-bearing reptiles is similar to that of an embryo developing
inside a shelled egg.
are live-bearers, with two exceptions, both of which live in Australia.
Duckbilled platypuses and spiny anteaters, also called echidnas, actually
lay eggs. The mothers stay with their eggs, which number two to four for
the platypus and one for the spiny anteater. The babies drink milk that
the mother produces in a gland and that flows out onto her body.
some groups, such as turtles and crocodilians, lay eggs and are never
live-bearer. Most snakes and lizards also lay eggs, but approximately
20 percent are live-bearer. Biologists have determined that early reptiles
were egg layers but that some have evolved to a point that they give birth
directly after the young hatch inside the body. To appreciate the variation
among egg-laying reptiles, consider that some lay eggs that take more
than a year to hatch whereas others lay eggs that hatch within a week.
Evolutionary biologists assume that the shortened incubation periods of
the eggs in some species are indicative of how some reptiles became live-bearer.
Thus, eggs that hatch shortly after being laid must have undergone extensive
development inside the mother. Hence, delaying egg laying for a few more
days would create a situation in which an egg shell was no longer necessary
because the young would complete development inside the mother’s
which include frogs, salamanders, and the tropical wormlike creatures
known as caecilians, lay eggs, but the eggs have no shells. The array
of behaviors associated with how amphibians, especially tropical frogs,
care for the eggs is intriguing: some females carry their eggs on their
back; others lay their eggs in trees so that the tadpoles fall into water;
others actually swallow the eggs and keep them in their stomach until
the young hatch and come out their mouth. Unfortunately, the gastric brooding
frog of Australia that did this last trick is now extinct. True to the
rule of exceptions, a few frogs, salamanders, and caecilians give live
birth without laying eggs.
so many unusual modes of egg laying and live-bearing that complete books
can be written about them. Some marine catfish keep their eggs and babies
in their mouth, and the big freshwater fish known as a bowfin or dogfish
lay eggs in a nest guarded by the male, who stays with and protects the
baby fish after they begin to school.
to mammals, reptiles, amphibians, and fish, all birds lay eggs, thus being
an exception among the major vertebrate groups. Bird eggs of course are
associated with some captivating parental behaviors, but eggs were already
associated with some fascinating behaviors with other organisms long before
chickens and other birds ever got here.
I would be less admiring of porcupines if my vehicle or outbuildings were
subject to their depredations. But I don't think so.
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