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If other animals have more than 2 nipples, then why do humans have only 2 nipples?

Even my pet dog has more than 2 nipples.

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    $\begingroup$ There is a condition called supernumerary nipples where people have extra nipples. This is an evolutionary holdover from when we needed a lot of nipples. $\endgroup$
    – Danielle Love
    Commented Dec 1, 2014 at 4:05
  • $\begingroup$ Well, for one things we don't often need them. There are times where humans have more than twins... But these aren't terribly common. $\endgroup$
    – L.B.
    Commented Feb 13, 2015 at 4:37
  • $\begingroup$ The post Why don't mammals have more than 4 limbs? is related, not by its subject but by the nature of the question.The top answer very much makes a similar point as @Ryan's answer but give some more information. $\endgroup$
    – Remi.b
    Commented Mar 12, 2018 at 16:55

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Questions of the type, "why does organism X have feature A?" invite teleological (just so) explanations which are difficult to substantiate. For example, the number of teats on a cow are difficult to explain in terms of providing milk to humans!

We should look to evolutionary history to explain human traits, not "just so" stories. The simple answer is that humans are primates, and primates have similar bodies and lifestyles. If there's a single pressure that lead to the number of teats in primates, it's likely the arboreal niche, not the mean litter size. Indeed, litter size is usually a plastic trait, even in primates, and is certainly not an accurate predictor of the number of teats.

There are no strict restrictions on nipple numbers in humans; they can occur anywhere on the milk line. So humans tend to have two nipples because that's what primates tend to have, because of our evolutionary history.

A better answer than this one would locate a candidate common ancestor to the primate and non-primate lineages and make reference to the embryonic development of the mammary tissue.

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    $\begingroup$ The 6 nippled lemurs certainly temper your claim that "[a]ll primates have two nipples." I would think you'd clean up your answer before challenging mine. $\endgroup$
    – Ryan
    Commented Nov 30, 2014 at 4:13
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    $\begingroup$ @anongoodnurse Evolution does not think sense. I agree with Ryan's answer but some explanation can be provided for the loss or gain of teats. For this, one has to make a comparison between primates, other mammals and the common ancestor of all mammals, as already pointed out by Ryan. $\endgroup$
    – WYSIWYG
    Commented Nov 30, 2014 at 7:20
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    $\begingroup$ @WYSIWYG - I agree with you that evolution doesn't think or plan. However, "because it didn't confer a survival advantage" is usually acceptable. A dog with two teats and 9 puppies does not seem advantageous to me. Pigs and cows had a many-nippled ancestor. Cows clearly lost them for a reason. I'm comfortable that my answer has truth involved somewhere. $\endgroup$
    – anongoodnurse
    Commented Nov 30, 2014 at 10:24
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    $\begingroup$ @anongoodnurse it looks like you are assuming number of nipples are the primary factor of milk production $\endgroup$
    – Macond
    Commented Dec 1, 2014 at 8:55
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    $\begingroup$ @anongoodnurse - or should we assume that every baby should be fed simultaneously? like nipples are some kind of oxygen masks? I simply don't understand that haste to associate every feature to reproductive success. Just because it seems convenient, doesn't mean that it is a product of an optimization process. $\endgroup$
    – Macond
    Commented Dec 1, 2014 at 13:08
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Your pet dog needs more than two nipples because she can have up to 11 or more puppies. If she had two nipples, most of them sound starve.

Twins are rare enough in humans. Two nipples are fine, and fit with bilateral symmetry. All primates have two nipples. So do many animals with one, two, and rarely three offspring at a time. Elephants: 2 teats, 1 calf (usually). Goats: 2 teats, 2 kids.

Why a cow has four teats, I'm sure I don't know, but I'm sure that's what made them valuable as a milk source to man.

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    $\begingroup$ someone mentioned to me that not all primates have two nipples. Is that right? And is there some evidence that the number of teats is correlated with the litter size? That makes sense, I'm just wondering if there is evidence to this end. $\endgroup$
    – Ryan
    Commented Dec 1, 2014 at 23:34
  • $\begingroup$ There is in rodents at least ncbi.nlm.nih.gov/pmc/articles/PMC323835 $\endgroup$
    – John
    Commented Oct 16, 2018 at 2:00

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