Why do humans have their udders on their chests, cows and goats and sheep have their udders near their groin, and the rest of the mammals usually seem to have lots of them and spread out along their belly and chest?
Regarding the “lots of them” part, most animals seem to have about the same number of teats as the usual size of the litter produced, plus a spare.
Thus horses generally have a single foal, and they have 2 teats. (Just like humans.) Similar with sheep & goats.
Whereas cats, dogs, pigs, etc. have litters of several young, and so have several teats.
(Cows seem to be an exception – they usually have only 1 calf, but have 4 teats. Possibly that’s why they are the most common dairy animal used by humans.)
I expect Colibri will be along shortly with the scientific answer, but I’ll just note that the position of teats seems to me to be related to the nursing position. Primates hold their infants so it makes more sense for the teats to be up top. Dogs, cats, & rats nurse on their sides so have rows of teats down the chest. Cows, horses, & sheep nurse standing on four legs so have their teats to the rear of the rib cage.
I’m not sure there is any particular reason for where teats are located. The ancestral pattern is to have a line of nipples along the abdomen. Some mammal species reduced the number of nipples because they weren’t needed, but there doesn’t seem to be any obvious pattern for which ones were lost, beyond the fact that evolution seems to have favoured keeping the ones at the end rather than in the middle. Beyond that, there’s no clear pattern. While ungulates tend to have lost the the nipples closest to the head, primates, bats and elephants have lost the ones closest to the tail.
I suspect the pattern is one of pure chance. Once a mammal reduced the number of offspring below 2, multiple mammaries just increased the risk of infection and made feeding more difficult. As a result any animal that was born with fewer nipples was favoured. But whether the mutation resulted in the loss of nipples from one end or the other doesn’t seem to have any obvious effect at all, and there is no reason to believe that it would.
Yes, some primates hold their infants with their while nursing, but that is the result of where the nipples are located, not the cause of it. If the nipples were located in the groin, then they would hold their young with their hind limbs, or the young would stand on the ground or any of the techniques used by other mammals. Plenty of animals from bats to squirrels to opossums are quite capable of feeding their young in the trees without holding them.
Backing up a bit, all mammals have a structure called a “milk ridge”, consisting of proto-mammary tissue, that runs from the collarbone area down the ventral surface, ending near the groin. There are two ridges, one on each side of the body. How much of this ridge develops into actual mammary glands, and where, is what varies from species to species. As has been said, animals with large litters tend to have many mammary glands and those with fewer young have fewer. This is, incidentally, why we humans occasionally have three or even more nipples. The “turn into a nipple” hormone signal is a bit too strong, or spreads a bit too far, and the next bit of the milk ridge turns into a nipple.
That’s just some of the underlying physiology. It’s nice, because on the surface, it’s hard to explain how evolution could cause the mammaries to pick up and move around like that. Knowing about the underlying structure makes the variation much easier to understand.
I am not a biologist, but it is my recollection/understanding that mammals in general have two parallel lines each containing multiple potential faucets, but in several species those lines of tissue have evolved so that only the top (e.g., humans) or bottom (e.g., cows) actually develop fully. Hence humans have the suppressed tissue-capability of owning a pair of strings of the devices starting at the familiar top position and repeating in pairs all down the frontal surface. (And cows, reciprocally, have tissue all the way up towards their frontal armpits but development is suppressed there also). Yet other critters, such as kitty cats, retain the entire paired ribbon-string as space from which nipples may manifest.
ETA: dang it Smeghead!
Which is why a being with three mammaries in a row would have to be an alien. Mammaries occur in pairs. 
It would probably be reasonable for evolution to favor teats located closer to the hind legs in prey animals like horses, sheep, deer, etc. that depend on being able to run fast to get away from predators. Udders & teats near their front legs would slow them down/reduce their maneuverability, so evolution would work against that.
This doesn’t apply to humans & other primates, who walk upright on 2 legs. And humans are more predator than prey anyway.
And to reinforce Blake’s point, if the mammary count significantly outnumber the offspring, then there’s the risk some may not be used. Not sure what happens when a mammary fills up and does not get used regularly, but I gather it’s not pleasant or healthy… hence a survival selection for “necessary and sufficient”. I don’t know how some animals ensure the appropriate rotation of available nipples when the litter count is low (i.e. cat with only two kittens?).
Mammals usually have twice as many nipples as the average litter size, with a few exceptions. And yes, their position depends on how the animal nurses, whether standing, lying down, swimming, etc.
The mammary line of an otter is closer to the backbone than the belly, so the babies can nurse while the mother swims.
The supply of milk isn’t endless – there’s only so much milk, and it takes hours for the mother’s body to refill the udder. So when one teat runs dry, the baby/babies will switch to another one that still provides milk. Thus the suckee rotates thru the available teats, and they all get used.
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How do you explain elephants?
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Why would having udders near the front legs slow an animal down any more than having udders near the rear legs?
Humans are the only living primate that regularly walks upright on two legs. The rest walk on four legs or swing using their arms. The ones that walk on four legs have no problem running despite the location of the udders. And the ones that are almost entirely dependent on their arms for swinging run counter to your argument.
And bats, elephants, hyraxes and dugongs are more prey than predator. But that doesn’t pose any problems to having anterior mammaries.
As I said above, there’s no obvious pattern here.
Most mammals have rows of mammaries that run the length of their body.
The ungulates all evolved from a common ancestor and all have posterior mammaries. So posterior mammaries evolved a total of once.
Primates, bats and afrotherians all evolved from different ancestors and have anterior mammaries. So anterior mammaries evolved at least three times.
To the extent that we can say that evolution favours either pattern, it seems that anterior mammaries provide a better solution for prey species. But quite frankly it’s probably just random chance which way a line evolves and the position of the mammaries provides no benefit of any sort.
I know nothing about them. I’ve on;ly held the lead on one once when our farrier was trimming off a broken toenail. Seemed like a nice enough animal, despite it’s pain. I don’t understand how such a nice animal can be a Republican/
It’s more a matter of mobility than speed. 4-legged animals mostly change directions starting with their front legs. And when trying to escape a predator, prey animals do a lot of quick direction changes, trying to throe off their pursuer (who is generally bigger and moving faster). Think of the films you’ve seen of lions chasing prey – the prey twists & turns like crazy trying to get away from the lion. Heavy udders between the front legs would make it harder to do those quick turns.
Do you have any evidence at all to support this? Because my intuition tells me exactly the opposite.
Most animals change direction with both pairs of legs simultaneously. Neither pair is more important.
Having weight at the rear would seem to make it harder to do this or thisthan having weight at the front.
The position of the teats must have something to do with the size and manner of locomotion of each animal. My guess:
- Cattle are tall, and they don’t like to lie on their sides, so they nurse their babies while standing. That way they can always run without delay when the herd moves on, or when a predator approaches.
- Cats and dogs are small enough that lying on their side and getting up again is not much of a logistical challenge.
- Humans and other apes carry their babies in their arms, so the teats sit higher up, to minimise the transition from carrying position to nursing position.
Did you read this thread before posting? :dubious:
In what sense are elephants prey animals? Nothing preys on elephants, except perhaps on very young calves who wouldn’t have developed mammaries yet anyway. Well, and us, but we haven’t been on the scene long enough to shape elephantine evolution like that.