Why have 2 nipples/breasts?

[The Guid]

I tried googling the above question but was bombarded with more porn than when I looked up Seven of Nine...

Why do humans have 2 nipples and subsequenly women have 2 breasts? Is it some kind of desire for symmetry and balance within humans? I can imagine that one breast might not balance very well but then again if it was in the middle I would presume it couldn't upset balance that much. Is it just because 2 is better than one because you can feed more children at once but then surely 3 is better than 2? There might not be room then but surely nature could adapt women to make breasts smaller to make room for the third? But then surely...

As you can see this has been bouncing around my head and before I start looking round the Biology section in the library looking for answers on breasts which might be a bit suspect considering I am a Drama student and... well... male I wondered whether anyone here knew the answer.

[Mr Bean]

To serve two customers at the same time of course. Multiple births are very commen in mammels and the more kids your likley to have the more feeding stations your likley to have.

[Superman]

To serve two customers at the same time of course. Multiple births are very commen in mammels and the more kids your likley to have the more feeding stations your likley to have.

Also, we primates don't tend to have multiple births at once, and situations like twins are not very common. This is especially true of earlier hominids and other primates.

Is it some kind of desire for symmetry and balance within humans?

Desire from whom? I don't think our own "desires" have anything to do with our morphology. We've developed for a couple hundred thousand years, and when you talk about having two breasts, this is a charactaristic of primates in general.

[The Guid]

To serve two customers at the same time of course. Multiple births are very commen in mammels and the more kids your likley to have the more feeding stations your likley to have.

Also, we primates don't tend to have multiple births at once, and situations like twins are not very common. This is especially true of earlier hominids and other primates.

Is it some kind of desire for symmetry and balance within humans?

Desire from whom? I don't think our own "desires" have anything to do with our morphology. We've developed for a couple hundred thousand years, and when you talk about having two breasts, this is a charactaristic of primates in general.

I meant the sort of "laws of nature" - desire is the wrong word and as such I apologise. A sort of "need" was what I was going for - if something developed on one side then it develops on the other - it is somehow easier for nature to develop 2 of one thing than 1 - or at least in the cases where the thing needs to be large. For example we have 2 lungs and 2 kidneys where perhaps one might suffice.

The "two customers" at once thing occured to me and that works for me with things like cats (who, as I understand have more nipples) but I was under the impression, as stated above, that we humans are used to giving birth to one child at once - hence surely only the need for one breast.

[Superman]

You don't have to apologize. You didn't do anything wrong.

I can't answer that question directly, but I can tell you that primates are animals that tend to be highly social; and it's not uncommon to see offspring from other parents nursing on other teats. You can even see this within some cultures of our own species.

I don't really know the biology behind the drive for the development of two breasts. I wonder if any other mammals display this feature.

[Trytostaydead]

Because girls would look pretty funny with one?

[Braedley]

My first instinct is to say that the number of nipples is related to the average number of offspring per pregnancy, rounded up to the next even number. The only thing is cows have a low twinning rate (comparable to our own), yet have four nipples.

[aerius]

Because human males have 2 hands.

[Solauren]

So while one is dry and 'refueling', you have another to feed the hungry kid with

[Molyneux]

So while one is dry and 'refueling', you have another to feed the hungry kid with

That's probably part of the reason.

Because girls would look pretty funny with one?

That could be another part.

I think that the last bit is that humans are descended from creatures with more than two nipples - over time, as the average size of our broods have dwindled, natural selection's gotten rid of the "extra" mammary tissue. Two breasts is a good number for an average brood size of 1, for the "recharging" reason that Solauren gave.

As for why we have hyperdeveloped breasts in females - I have no idea. Our close relatives don't...maybe it's due to self-selection?

[wolveraptor]

So while one is dry and 'refueling', you have another to feed the hungry kid with

But then wouldn't it have the same effect to pool the sources of both breasts? The kid couldn't possibly dry an adult healthy female, and while he/she slept, the mother could replenish herself.

Honestly, having fewer breasts would be an advantage. Those things are sensitive, and when a woman trys to swing her arms or falls down, they get in the way. Having 1 central breast would make it a little easier for women to practice archery. I've heard that big boobs force you to hold the bow farther away from yourself.

[The Duchess of Zeon]

You can bind your breasts to keep them from getting the way for archery. Or to look like a man; I did that during the civil war reenactments as a dragoon for a couple seasons.

As a practical matter, archery is a skill developed after our body form was fixed to what it is now, so obviously the female body will not have a physical shape which is developed for archery.

And, as for having two breasts--the structure of the torso on a mammal is designed to support breasts in a female off to each side, usually in rows for animals which have large numbers of multiple live births. So the torso would have to change rather considerably in a female to support a single large central breast and such a change is inefficient to evolution.

Two breasts has the following advantages:

1. Multiple children from one birth, while uncommon, happens often enough that a second breast is advisable as a backup for the average human female to maximize her reproductive success.

2. Minimal changes required in evolution from that of other mammals which have two rows of larger numbers of breasts; keep the same position as if in rows, but we've evolved away the additional breasts as unnecessary as the number of children to a pregnancy have reduced. Evolution is efficient and doesn't do unnecessary things.

3. In primates, which are highly group-oriented and cooperative creatures, if one mother has dry breasts or dies, another lactating mother can feed those children with her "extra" breast, improving the overall success of the group.

[Setesh]

So while one is dry and 'refueling', you have another to feed the hungry kid with

That's probably part of the reason.

Because girls would look pretty funny with one?

That could be another part.

I think that the last bit is that humans are descended from creatures with more than two nipples - over time, as the average size of our broods have dwindled, natural selection's gotten rid of the "extra" mammary tissue. Two breasts is a good number for an average brood size of 1, for the "recharging" reason that Solauren gave.

As for why we have hyperdeveloped breasts in females - I have no idea. Our close relatives don't...maybe it's due to self-selection?

Its a natural selection trick to make them more attractive by giving them a more fertile apperance. In most primates breast swell only during the time they actualy have kids to feed. In us the mammary area stores fatty tissue, so large breasts trigger 'fertile' and 'well fed/healthy' instincts.

Larger breasts are also a more recent development, as one of my biology teachers put it big boobs are a symtom of wearing clothes.

[Akhlut]

Why two and not one? Because all nipples are part of a pair. Most other mammals have more than two but always an even number.

Why not more than two? Because humans, on average, have less than 3 children per birthing.

As for why we have hyperdeveloped breasts in females - I have no idea.

Perhaps it was a selection strategy to keep a male in a strong relationship with a female, since most other primates don't develop any breast tissue until they're pregnant. If he didn't know whether or not she was pregnant (and she was capable of getting pregnant any time of the year) until it was pretty certain she was going to have a baby, he might stay around longer and help her out to make certain she was. Just a thought.

[GrandMasterTerwynn]

I tried googling the above question but was bombarded with more porn than when I looked up Seven of Nine...

Why do humans have 2 nipples and subsequenly women have 2 breasts? Is it some kind of desire for symmetry and balance within humans? I can imagine that one breast might not balance very well but then again if it was in the middle I would presume it couldn't upset balance that much. Is it just because 2 is better than one because you can feed more children at once but then surely 3 is better than 2? There might not be room then but surely nature could adapt women to make breasts smaller to make room for the third? But then surely...

Humans, like most other animal life on Earth, exhibits exterior bilateral symmetry. Thus, everything we are going to have more than one of, by and large, will occur in multiples of two. Two eyes, two ears, four limbs, etc, etc. So if humans were to have more breasts, we'd go up from two to four or six.

However, sometime in our murky past, our evolutionary ancestors adopted the "live a long time, and lavish lots of attention on few offspring" reproduction strategy, as opposed to the "live fast, die young, and crank out hundreds of offspring in the hopes that some of them survive to reproduce" strategy that smaller animals take. Those animals have two, three, or four pairs of nipples, as they have multiple offspring for each mating success. However, humans and other primates can only reliably produce a single offspring of nursing age at a time. This was also likely true of our earlier ancestors, since there was little selective pressure to keep around the extra sets of nipples.

So, as a result, we're left with the minimum number of breasts that we, as mammals can get away with . . . which is two.

[Zero]

It probably has nothing to do with humans at all, but some previous point in our evolution where 2 nipples were more necessary. It's probably stayed that way because of sexual selection; women would look funny with only one. This is, of course, a shot in the dark, a complete guess, but I'd think it isn't necessarily that far off. Many creatures can retain traits of their previous evolutions that they never use. Don't some kinds of bugs have wings, but no wingflaps, or ability to use them at all?

[SAMAS]

You can bind your breasts to keep them from getting the way for archery. Or to look like a man; I did that during the civil war reenactments as a dragoon for a couple seasons.

As a practical matter, archery is a skill developed after our body form was fixed to what it is now, so obviously the female body will not have a physical shape which is developed for archery.

And, as for having two breasts--the structure of the torso on a mammal is designed to support breasts in a female off to each side, usually in rows for animals which have large numbers of multiple live births. So the torso would have to change rather considerably in a female to support a single large central breast and such a change is inefficient to evolution.

Two breasts has the following advantages:

1. Multiple children from one birth, while uncommon, happens often enough that a second breast is advisable as a backup for the average human female to maximize her reproductive success.

2. Minimal changes required in evolution from that of other mammals which have two rows of larger numbers of breasts; keep the same position as if in rows, but we've evolved away the additional breasts as unnecessary as the number of children to a pregnancy have reduced. Evolution is efficient and doesn't do unnecessary things.

3. In primates, which are highly group-oriented and cooperative creatures, if one mother has dry breasts or dies, another lactating mother can feed those children with her "extra" breast, improving the overall success of the group.

There you have it, folks! Straight from the horseman's mouth! *ahem*

[Lord of the Abyss]

My first instinct is to say that the number of nipples is related to the average number of offspring per pregnancy, rounded up to the next even number.

I've heard that theory before; IIRC most mammals follow it.

As for why we have hyperdeveloped breasts in females - I have no idea. Our close relatives don't...maybe it's due to self-selection?

Some theories I've heard :

One :They are evolved to resemble buttocks, which are a male turn on.

Two : They are evolved to confuse men whether or not a woman is pregnant ( along with the hidden female fertility cycle in humans ). This prevent men from knowing if a child is theirs ( and killing those that aren't ), as well as making it hard for them to be sure when she is pregnant and leaving ( mission accomplished ! )

Three : Fertility display, a way of saying "I have enough fat to sustain a pregnancy" by concentrating it in a relatively convenient spot.

Four : Sexual selection; men like them, so they get larger as women with bigger ones get more and better men. This also helps explain why different groups have different breasts; culture A men like pointy breasts, so the girls develop those; Culture B men like rounded ones, so the ladies evolve those.

Personally, I suspect there is more than one reason, probably many.

[Broomstick]

Honestly, having fewer breasts would be an advantage. Those things are sensitive, and when a woman trys to swing her arms or falls down, they get in the way. Having 1 central breast would make it a little easier for women to practice archery. I've heard that big boobs force you to hold the bow farther away from yourself.

Honestly, having fewer testicles would be an advantage. Those things are sensitive - look what happens when a man take a blow to the crotch - and when a man swings his legs or falls down they get in the way. Having 1 central testicle would make it a little easier for men to get around. I've heard that big balls causes problems for men riding horses and bicycles.

In other words - big boobs aren't nearly as bothersome as men think (or hope) they are.

[Gil Hamilton]

I think we have two simply because of bilateral symmetry, lifeforms like us do things in plurals as a general rule or along the center line if done singly.

As for why not have more pairs; as stated, how many boobs could we possibly need? We don't tend toward more than one child at a time. Species that have common multiple litters have more; dogs for instance have eight teats.

[Gil Hamilton]

I think we have two simply because of bilateral symmetry, lifeforms like us do things in plurals as a general rule or along the center line if done singly.

As for why not have more pairs; as stated, how many boobs could we possibly need? We don't tend toward more than one child at a time. Species that have common multiple litters have more; dogs for instance have eight teats.

Ghetto Edit: Dogs have 10, not 8. I mistyped because two of them aren't really accessable.

[Gil Hamilton]

Honestly, having fewer testicles would be an advantage. Those things are sensitive - look what happens when a man take a blow to the crotch - and when a man swings his legs or falls down they get in the way. Having 1 central testicle would make it a little easier for men to get around. I've heard that big balls causes problems for men riding horses and bicycles.

In other words - big boobs aren't nearly as bothersome as men think (or hope) they are.

This is why man kind invented the jock strap. You've got to batten down the hatches, dontcha know.

[chaoschristian]

As you can see this has been bouncing around my head

<snicker>

[Fleet Admiral JD]

My thought was that there were two so you had an Emergency Backup. For example, if breast cancer forces you to remove a breast, you'd have a second in case of that. Just a thought, I don't have evidence to back it up.

[DPDarkPrimus]

One for each hand, obviously!