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Although turtles have a limited control over their internal body temperature, they do respond to temperature differences in complex ways. These include things like basking in the sun to raise their temperatures, responding to the cold by hibernating, and even determining the sex of their offspring via the ambient temperature that the eggs develop as. So, it's not a shock to find out that turtles start physically responding to temperature differences before they even hatch from an egg. What is surprising is that these responses occur before there's much in the way of a skeleton or nervous system to support it.