Why don’t mammals have more than 4 limbs?

-a cheetah in action captured by Mark Dumont

Approximately one month ago I wondered why there were no mammals that had more than 4 limbs. I wondered whether they were somehow optimal for the challenges faced by mammals. I was thinking about the Cheetah in particular: would it run faster if it had an additional pair of limbs?

Unsure about how to proceed I posted the question on the biology StackExchange where it generated some interest. The accepted answer by a zoology researcher could be summarized in this way:

“So basically, terrestrial vertebrates have four legs because they evolved from a fish ancestor that had four members that were possibly used as legs (that could “easily” evolved into legs). The explanation is as simple and basic as that. You can have a look to the diversity of terrestrial vertebrates here (click on the branches).”

Unsatisfied with this answer, I asked another zoologist from Cambridge University, Jenny Clack, who also happens to be an expert on tetrapod evolution and she gave me a similar answer:

“If four appendages are optimal, it’s because they were optimal for swimming before tetrapods became terrestrial. Clearly four are not always optimal for tetrapods since many of them lose them, but it would take a major Hox gene duplication – probably deleterious if not fatal – for tetrapods to get more.”

It appears that nobody has tried to do computer simulations or analyses of any kind to determine whether 6 legs may be better or worse for mammals that have particular constraints due to their habitats and functional purposes. After more research I have come to this conclusion. But, after attempting to transform the question into a falsifiable hypothesis(Are 4 legs optimal for certain/all mammals?) I have realised that there are quickly serious difficulties.

Even if we considered a special case of this question: “Are 4 legs optimal for a 100 kg carnivorous savannah mammal?” there are serious challenges. To begin with, there are many important factors that aren’t well understood:
a) type of vascular system used
b) type of respiratory system
c) other variables
d) dynamic relationship between these components

Given the limited understanding of leading researchers of the biological components of land mammals, I realised that doing significant analysis of this problem is basically impossible.

This is the first time I’ve encountered a scientifically interesting problem that can’t lend itself to any kind of reasonable analysis.

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