This giraffe and I have the exact same number of vertebrae in our necks: seven!

Nope, sorry.

The correct answer was: The Three-toed Sloths have more neck vertebrae.

You knew this was a trick question right? The giraffe has the exact same number of vertebrae in its neck as you: 7! In fact, all mammals have 7 vertebrae except for two notable exceptions: the sloths and the manatees.

The Three-toed Sloths (genus Bradypus) usually have 9 vertebrae! (Sometimes 8). The weird thing is, the Two-toed Sloths (genus Choloepus) have fewer vertebrae then the rest of the mammals (5 to 7!)

Manatees also have fewer neck vertebrae: 6.

Why do I say “usually”? Because the number varies from individual to individual. There is a high rate of mutation in the number of neck vertebrae present in sloths, but unlike most species, selective pressure doesn’t tend to weed them out. (Meaning they don’t usually die before reproducing as would happen in most species.)  For example, mutations in the number of neck vertebrae are fairly common in humans (around 7%) but due to the complications caused by them, nearly all die before adulthood.

It is believed that because of the unusually slow metabolic rate and low activity levels of the sloths and manatees, these mutations have less negative effect on their lives, allowing them to survive and pass them on!


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