Clearly, if men didn't have nipples, to demonstrate their theoretical membership in the La Leche League, we could only identify them as mammals by their hairiness. And where would that leave bald guys? What are they, reptiles?

There are some male mammals without nipples, a fact I was alerted to by Aristotle, who wrote "Such, for instance, is the case with horses, some stallions being destitute of these parts."

Since Aristotle's medical facts were sometimes a bit wobbly -- he said cabbage cures hangovers -- I called an equine veterinarian. "I have never seen a stallion with nipples," she declared flatly. "And I have looked around down there." As far as I know, she's never seen a bald stallion, either, so that's how they avoid being called reptiles.

The veterinarian pointed out that a mare's two nipples are located toward the tail end of the body, as opposed to the chic head-end location in humans. This, she daintily hinted, might be why stallions don't exhibit nipples. "There's no room."

These shocking facts sent me on a quest for other data on animal nipples or, as medical types have long preferred to say, mammae. Male nipples? Mammae masculinae. (If you need to be even more obscure you can also call a nipple a mamilla or a thelium.)

My mother, when I told her of my research, may have been hinting that there were more hard-hitting stories I could be working on by bringing up the folk analogy "as useless as tits on a boar hog." My research appears to indicate that boar hogs do in fact have tits. Which they are not known to use.

Not only do male platypuses not have nipples, neither do females. The milk simply flows out through pores and is licked up by baby platypuses. And while platypuses are not actually categorized as reptiles, you'll notice that people are always talking about how "primitive" they are and making fun of their noses.

I would have assumed that nipples were only available in even numbers had I not learned that female possums, for example, have between seven and 25 nipples. The delightful Virginia opossum, which inhabits the middles of American roads and highways, usually has 13, efficiently arranged in an open circle with one in the center. This information should not tempt you to snicker and point the next time you see a possum: They also have 50 teeth.

Most mammals, however, stick to even numbers of nipples, and often the males get to have them too. In addition to boar hogs, dogs, cats, all primates and many other animals feature the mamma masculina.

It seems that human embryos develop mammary tissue before they bother to check on whether they're going to be male or female and start modifying the basic plan with surges of this or that hormone. After only a few weeks, milk ridges form -- two stripes of tissue that start in the armpits, curve out over the chest, go straight down the stomach and then veer in toward the groin, ending somewhere high on each thigh. Later the milk ridges regress to some extent, usually leaving us with just two nipples.

Quite a few people end up with an extra, or supernumerary nipple somewhere along the trail of the milk ridge, however. (One man had five.) Sometimes they can't be mistaken for anything but a nipple, and other times they look like a mole. In fact, many people with supernumerary nipples don't know they have them until some officious and informative person starts examining their moles. Extras often run in families -- Darwin cites two brothers who each had a supernumerary nipple. Anyone who thinks that's weird should immediately leave the room and go check his or her torso for moles. How do you know you're not head-to-foot extra nipples and we've all just been too polite to mention it?

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