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?

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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.

I’m going to take a crack at this.

So species like sharks, lions, and such give live birth. And two headed ness is a birth defect, there are also chances that there are more defects in the offspring while its forming in uterio, causing a strain of the female’s body as it tries to carry the offspring to term.The female’s body carrying the offspring would abort the pregnancy (miscarriage) due to this strain on its resources (nutrition, water, and just strain of a difficult pregnancy). Also, I would think the offspring would die during the birth process, perhaps getting stuck in the canal, perhaps taking the female with it, and then them being eaten by scavengers would also reduce the likelihood that we would even see bones of remains.

Turtles, snakes and such are born mostly in eggs (some snakes do live births), with all the genetic martial and nourishment the offspring needs, not relying on the female’s body to “monitor” ( for lack of better word) how that offspring is doing. They are able to break open their egg, but don’t usually live long afterwards without intervention.

Snakes and turtles produce way more babies.

Female green turtles can produce 1900-2300 eggs in a lifetime.

Snakes can easily have dozens of babies in a litter and reproduce 2 times per year.

Sharks have a reproductive cycle that is 1-2 years long and very few species produce more than a dozen offspring.

Lions have a litter of only 1-6 with complex reproduction. It seems the lioness will refrain from intercourse if her litter survives but if they perish she will try to have more within a few months.

So a big part of it is just that it’s a numbers game. You also have to consider things like, if a shark that regrows teeth has to regrow teeth for 2 heads it’s spending a lot more energy (reducing its chance of survival) than a snake with a bit of extra snake on it.

Mammals are likely to miscarry unviable pregnancy. I had an animal husbandry teacher bring in a lamb fetus that had miscarried and one of its legs was developing backwards.

I agree with the other comments. The birth process also has an effect:

The egg is roughly the same shape and size even if the baby snake or turtle inside has extra heads or too many legs. It’s just as easy for the mother to give birth as it would be for a normal egg.

A live birth where the baby has an extra head or extra legs is a bigger and a much more complicated shape for a mother animal to push out of the birth canal. It’s more likely that mother and baby would die during the birth.

Mothers of deformed live-birth babies are more likely to die from it, so they don’t live to have more offspring. Mothers of egg-laying species are less likely to die, so they go on to have more babies, who may have the same defect if it’s genetic.