Why did some sharks evolve live birth?
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As we learned in the animation "Darwin's Monster", in reptiles on land, live birth often evolves to protect embryos from cold or sometimes dry conditions, since internal development allows the mother to control temperature and moisture. In water, a key pressure for the evolution of live birth in some sharks and other fish species is in overcoming the difficulty of hiding eggs from predators. Egg predators exist on land too, but in the water, egg predation is a far more difficult problem. Eggs release scent molecules that quickly disperse in water, making eggs easy to detect and track. Live birth keeps embryos inside the mother where no scent escapes, offering better protection in predator-rich environments. While land favors live birth for climate control, water favors it for stealth.
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