Why is it more common to see a two headed snake or turtle but rare to see a two headed lion or shark?
In: Biology
Conjoined human twins generally don’t survive the first few days of their life (if they aren’t born stillborn in the first place). The reason for this is that humans have big brains and a fast metabolism. Most conjoined twins only have one set of organs and those organs cannot support the energy demands or waste products produced by two brains. In the rare instances where conjoined twins do survive its because they don’t just have two heads – they also have two sets of their internal organs. In essence, they’re not *really* a “two headed human” but, rather a pair of upper human bodies sharing a set of legs.
Other warm blooded animals have smaller brains than humans but the problem of having two heads with one set of internal organs remains the same – a single heart, liver, set of lungs, or set of kidneys just doesn’t have the capacity to support two warm blooded brains. As a result, nearly all two headed warm blooded animals die from metabolic insufficiency either before or shortly after birth.
If a two headed, warm blooded animal does survive the first few days of its life then it has another problem. All warm blooded animals rely on their parents to care for them to some extent. However, animals generally kill or abandon deformed babies. This isn’t so much a problem with humans (which is why you do occasionally get a two headed human) or cold blooded animals (which generally do not care for their young).
Two headed, warm blooded animals cannot survive to adulthood without a lot of outside intervention. People do occasionally intervene and there are historical instances of two headed farm animals that survive to adulthood. But those are just as rare as two headed humans (which is to say, very).
Reptiles have very small brains and a very slow metabolism. The amount of energy that a reptile’s brain uses, relative to its body size, is much lower than than of a warm blooded animal. Unlike adding a second brain to a warm blooded animal, adding a second brain to a reptile body doesn’t increase the overall workload for the body’s internal organs by very much. Because a second brain won’t overwhelm the organs in a reptile’s body, reptiles that are born with an extra head generally don’t just die right away from metabolic insufficiency. They still *might* die if the second head otherwise prevents them from functioning, which gets to fish –
You don’t see fish with two heads because fish rely on their body shape to swim. Adding a second head to a fish disrupts that shape and prevents them from being to move in a straight line (or at all in many cases). Fish that can’t move also can’t survive.
You don’t see two headed insects because insect bodies are very, very different from other animals and there just isn’t really any space for a second head, so an insect body that tries to grow two heads just ends up as a nonfunctional lump of biomass.
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Different reproductive strategies. Snakes and turtles make more babies to ensure survival. Apex predators make one or two and invest time and care to ensure survival. The higher frequency of reproduction larger numbers of offspring makes random mutations and congenital mistakes more frequent as well.
I’m also going to hazard another guess as to why more of these mutations make it past the first few days of life in animals like snakes and turtles: higher order predators require more organization in hunting, speed, and other physical abilities to catch live, moving prey. Could it be that animals that tend to graze on plants or prey on eggs/immobile baby animals have an easier time adapting to these mutations?
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