Why is it more common to see two heads on some species like turtles or snake than it is to see a two headed shark or lion?
In: Biology
Development is *extremely* complicated. It’s very, very sensitive to all sorts of conditions, and if even just one small thing isn’t quite right, something can go wrong. The body also has very little means of correcting problems that occur early on, so the body continues to develop with that problem incorporated. A particularly big contributing factor is temperature. Animals that give live birth have internally regulated temperature, largely speaking – the environment is constant and predictable. In humans, the baby only gets cold if the mother dies. In sharks, water tends to be a pretty constant temperature, and when it does fluctuate it fluctuates in predictable ways that the animal can evolve to deal with.
Temperature fluctuates a lot more in egg-laying development, especially if the eggs aren’t buried or incubated by a parent animal. Turtles and snakes both lay eggs then fuck off, which means eggs are much more subject to temperature change and this can induce unusual effects.
It should also be noted that the vast majority of two headed foetuses will simply die due to malfunction of something during development. Turtles and Snakes are unusual in this regard, in that they’re much better at surviving having two heads, and various other congenital defects. Two headed animals are seen more often in turtle and snake species simply because they’re less likely to die as a result of it.
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