Nile crocodiles lay their eggs in nests along the bank of the river Nile. now when a crocodile egg is laid the embryo inside has no gender. their gender is determined later by the temperature of the neck. so if the egg are laid in a very warm or very cool nest the eggs will be female and if the temperature is in the middle they will be male. mother crocodiles know this so they will choose where they want to lay their their eggs depending on whether they want boys or girls. this means if their population is low on females a female will lay her eggs in a very warm nest and they'll get some more girls. how cool is that? they can regulate their own population.
now if only humans could do than!!