I’ve noticed that animals such as cows, deer, and moose have udders with 4 teats. Goats and sheep have two. I don’t know the right term to use here, but is there a division in a classification of animals that includes this, so you could know if, say, an ibex mom has two teats or four?
Just to add to confusion, don’t dogs and cats have six or eight? Probably tied to average litter size?
Not really. The number of teats is fairly flexible evolutionarily. There may be certain numbers that are characteristic of particular genera, families, or even orders, but this is not generally a primary character for classification. Note that cows are much more closely related to goats and sheep (all family Bovidae) than they are to deer or moose (Cervidae).
You’ll even occasionally get a human with more than the standard-issue two.
Sometimes it’s variable (in normal individuals, not the occasional abnormal case). Pigs may have as few as 8 or as many as 16.
Has a human ever had more than two functioning ones though? No really, I want to know!
I’m thinking more in terms of the bag between the hind legs, versus the variation in mammary quantities found along the body
During development, all mammals have what’s called a “milk line” that runs down the torso and abdomen - one on each side, actually. Teats can develop anywhere along those lines and in any number - from two up top, like us humans, to lots all along the lines, like dogs, cats, and pigs, or just a few down at the bottom, like cows and goats. That’s the reason why it’s evolutionarily flexible. Instead of having to develop a whole new structure in a new place, it’s more a matter of just flipping some switches along the line.
There does seem to be some connection here. And you can see an obvious evolutionary benefit: if your average litter is 4-6 kittens, having 8 teats is an advantage. But if a human has only 1 child 99% of the time, anything more than 1 teat = 1 spare would be a waste of resources.
It might be interesting to try to add to this table:
Species #teats usual litter size frequency
--------- -------- ---------------- --------------
human 2 1 98.92%
horse 2 1 99.99%
cow (beef) 4 1 98.9% - 99.6%
cow (dairy) 4 1 91.1% - 98.1%
cat 8 4-6 ?
I dunno. I’ve heard it said that anything over a mouthful is wasted, but perhaps having a spare can come in handy; there is something to be said for symmetry. If two teats reproduced more than one, I can see two as being an advantage. I’ve always liked them in pairs (although one pair is enough for me).
excavating (for a mind)
I’d heard of women with supernumerary nipples find them “leaking” when they’re nursing from the two, typical ones. Without stimulation, the extras stop producing milk. An outcome which is probably for the best, fetish aspect aside, because who wants to wear absorbent nursing pads up around their collarbone, or down along the bottom side of their ribcage. That’s where the human “milk line” is, the “fully developed boob on the sternum” style of the movie Total Recall isn’t possible.
You are obviously male (like me).
But I have been told by my sister, mother, & other female relatives that having 2 is verrry helpful when nursing – you switch the baby over whenever the one in use gets sore or tired or empty.
And symmetry is very important to vertebrates – nearly everything in our body comes in matched pairs.