There is a connection between alligators and birds' eggs: blood. Blood carries heat throughout the body.
The work with alligators was to figure out how they used blood flow to get their bodies warm quickly and to retain the heat to keep the body warm for a long time.
The work with birds' eggs had a similar motivation: how could the embryo use its own blood circulation to draw heat in from the incubating parent?.
Alligators are reptiles, and often are erroneously characterized as cold-blooded. In fact, reptiles strive to keep their bodies warm just as we do. Where we use metabolic energy to do so, reptiles use blood as part of a sophisticated system of managing energy flows between the environment and their bodies. One common manifestation of this is the body that heats up more quickly than it cools. Controlling blood flow to the surface lets them pull off this trick. .
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For much of their lives, bird embryos are like reptiles: they are dependent upon an external source of heat to keep their bodies warm. In alligators, this heat source is the sun. For the bird embryo, the source is the body heat of the incubating parent.
For many years, the egg was thought of as a simple physical system of heat transfer. If the embryo made any contribution, it was from increasing body heat production as the embryo matured, I found that the embryo's developing circulation was more significant, and this raised the interesting prospect that the embryo could actively control the exchange of heat with its parent.
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