In fact, the very existence of male nipples is such a conundrum that "Why do men have nipples?" wracked up an average of 22,000 monthly Google searches in 2016. It appears that we are just as clueless as Erasmus Darwin (grandfather to Charles), who pondered the very same question back in the 18th century.
However, while there might not be a purpose as such, there is a biological reason why men have nipples. And it all comes down to embryonic development in the womb.
In the first few weeks after conception, the male and female embryo follow the exact same path of development. It is not until the sixth to seventh week of gestation that reproductive organs start to develop and the fetus begins to differentiate by sex. (Specifically, this is when a gene called SRY starts to take effect. The SRY gene is basically an instruction manual for the production of the sex-determining region Y protein.) Once the testes have formed in male fetuses – around the nine-week mark – the body starts to generate testosterone and the fetus begins to display more and more sex-specific biological characteristics.
But by this point, it is too late. At least as far as nipples are concerned because the mammary glands begin to develop in the very first few weeks of gestation and before the SRY gene has a chance to kick in. This means that while male nipples may be smaller than female nipples, they do still exist.
And while that's all well and good, the problem is that it doesn't explain why men have retained such a seemingly useless body part – if it's biologically useless, why haven't men evolved to go without?
It could be that they do serve a purpose, just one that we do not quite understand. That purpose could be sexual (like women, many men can enjoy nipple stimulation) or social (though extremely uncommon in most societies today, men in the Aka Pygmy tribe in central Africa are known to nurse their babies). Or, perhaps, even biological. Despite that Meet the Parents quote, men have been known to lactate, though this is usually in response to starvation or hormone imbalances and is very, very rare. (That is to say, if it happens to you, get it checked out ASAP.)
However, the most likely explanation, scientists say, is that male nipples aren't detrimental so removing them just isn't a priority, evolutionarily speaking. Like wisdom teeth, the appendix, and Darwin’s point, the male nipple is just another quirk of evolution.