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
You don’t see two-headed fish not because they aren’t born (I’ve seen several two headed trout fry while working at a trout hatchery) but because they can’t swim effectively. Being unable to swim means they rarely last long. Two headed turtles and snakes can still move…they are still not likely to live long in the wild, but can make it fairly well in captivity depending on the nature of the division.
Two headed mammals do happen although they also often don’t last long. Mammals have many fewer offspring than reptiles and because they have pregnancies rather than eggs, offspring with developmental problems often don’t make it to full term. This means you have fewer babies and fewer of those have multiple heads. Also mammals have higher energetic requirements which probably makes it harder for two headed animals to get enough to eat and breathe and generally make it harder to function. This may be why you rarely see polycephaly in birds, too, though I can’t confirm that.
Anyway, the main reason you’ve never seen a polycephalic lion is probably that there are so few lions, and especially so few lions in captivity, that such a rare occurrence has never been observed. Polycephaly is rare, people don’t get to see all that many lion cubs, so the odds of spotting one with polycephaly are low. But it’s been observed in cats (people observe a huge number of kittens every year, making the odds of finding it much higher) so it could certainly happen in lions. People don’t observe that many baby sharks either, so that probably helps explain the rarity of known polycephalic sharks.
Here’s an article which has an image of polycephaly in a cat and a shark.
https://www.abc.net.au/news/science/2018-10-05/two-headed-animals-causes-and-how-common-are-they/10337640
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