EVOLUTIONARY BIOLOGISTS ANSWER AGE-OLD QUESTION

by Whit Gibbons

August 11, 2003

The answer 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, even appeared.

Many intriguing 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.

Live-bearer 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.

All mammals 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.

Among reptiles, 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 body.

Most amphibians, 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.

Fish have 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.

In contrast 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.

Perhaps 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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