Let's suppose regular humans (same intelligence and pretty much same features) but they usually have 3-6 babies at a time.

To me, many breasts seem like an evolutionary disadvantage. Something you want to have as little of as possible, especially if you're a biped (they mess with the center of gravity).

The only advantages to have n breasts I see is:

  • It's quicker to feed your "litter", reducing your vulnerability/not-hunting time.
  • That you can be sure that, without requiring supervision on your behalf, at least n of your children will be regularly fed. It (usually?) happens that for puppies > n, some puppies will go undernourished.

But that's not a problem for an intelligent animal, is it? Time is not as important, because they are not as vulnerable while feeding (they have safe villages) and they have the sufficient organizational skills to make sure every child gets his fair share. This is why I think there would be an evolutionary advantage in having less. But I may be completely wrong!

So what do you think? Given a world were hominids (maybe all primates) have bigger litters, do you think they would have several (more than 2) breasts?

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    $\begingroup$ Disclaimer: This question is inspired by my comment in: worldbuilding.stackexchange.com/questions/84914/… I think this question is similar but way more focused on this specific feature, so it's different enought, but the community is free to close it as dupe if it disagrees $\endgroup$ – xDaizu Jun 30 '17 at 12:50
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    $\begingroup$ Number of teats and size of litter are features which evolve together. It's not like one determines the other, they are automagically co-adapted by natural selection. In most species the mammary glands are tiny when not in use; humans are an exception, with human female breasts being always conspicuous. Great apes (including humans) have altricial babies which need to be carried around by the mother; this is the main reason why one baby is the norm. I'd say that carrying around 3 to 6 altricial babies would be quite difficult. $\endgroup$ – AlexP Jun 30 '17 at 13:18
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    $\begingroup$ Given a world where hominids/primates had initially started with bigger litter sizes, natural selection would've quickly weeded out the species until it had small litter sizes. That's simply because it's much easier to raise 1 child up safely and teach 1 child the required skills to survive than it is to manage large numbers of them. This change is almost essential in order to even have villages - Otherwise, you'd be too busy handling babies to do things like build a village. $\endgroup$ – Aify Jun 30 '17 at 13:28
  • $\begingroup$ @Aify: Or possibly lead to different survival strategies. I quite like the narrative possibilities of a race of tree-root dwelling hominids that burrow for survival and have little regard for infant mortality. $\endgroup$ – Joe Bloggs Jun 30 '17 at 16:20
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    $\begingroup$ Humans still carry around the genes for multiple rows of teats. I don't remember the exact details or the source so no solid data here. A quick google says it is called Polymasitie(milkglands) or Polythelie(Nipples). The only anecdotal evidence I can offer is: I have a second row of higly underdeveloped nipples just below the fold line of the chest area. $\endgroup$ – Fourpaws Jul 3 '17 at 13:13

In favor of yes is the "one-half rule" as proposed in the linked study; abstract pasted below. Animals with only one offspring mostly have 2 nipples (humans, whales, elephants). Animals with more offspring have more. I found this analysis of nipple / offspring ratio in rodents, where there are a lot of species to compare.

http://www.pnas.org/content/83/13/4828.full.pdf Mammary number and litter size in Rodentia: The "one-half rule" (Cricetidae/Muridae/Sciuridae/lactation/reproduction) AVERY NELSON GILBERT* DepartmentsofPsychologyandBiology,UniversityofPennsylvania,Philadelphia,PA 19104 Communicated by Leo M. Hurvich, March 7, 1986

ABSTRACT Litter size and mammary number in the mammalian order Rodentia show a significant positive correlation. Mean litter size is typicaliy one-half the number of available mammaries, while maximum litter size approximates mammary number. Similar relationships are found in the families Muridae, Cricetidae, and Sciuridae. The relationship of litter size to mammary number is significantly different between the arboreal and terrestrial squirrels, and between the hystricomorph and nonhystricomorph rodents. Mammary number may have operated as a selective constraint on litter size over evolutionary time.

Considering that there are other pressures tending to constrain humans to one offspring at a time (chief among which is that human offspring remain dependent for a long time) maybe the same pressure operated on primates during our evolutionary history.

In favor of a no answer is the example of pygmy marmosets. Most primates have one offspring at a time and have 2 nipples. Pygmy marmosets always have twins, but like other (all larger) primates still have just 2 nipples. They work around this by caring for offspring as a group, although it was not clear to me if a female which had not recently been pregnant could start lactating to support a little sister or niece. If the 2 nipple thing is rooted deep in the primate lineage and if there is an easy way to sidestep reduced fitness for the multiple children and a mother with 2 nipples (e.g. female relatives help nurse) then humans would stay as they are.

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    $\begingroup$ I have a real life example supporting what you say in H. sapiens, in addition to rats. When my wife was pregnant, we went to courses to help us survive the rigors of the first few weeks post-childbirth. They covered topics like nursing. The teacher explained a great deal about the matter, and then asked if anyone in the class was having twins or triplets. One lady raised her hands. The teacher told her to come by later, because there was a whole corpus of additional things she needed to know to manage breastfeeding twins that the rest of us didn't have to worry about. Twins are hard! $\endgroup$ – Cort Ammon Jun 30 '17 at 15:08

Women breast are per se a disadvantage, as you point out. The females of other mammals grow them only during breastfeeding, because energetic-wise is not smart to have and maintain that unused reserves of fat all the year round.

Humans do it because it brings some advantages (I read this about 20 years ago on Scientific American), since they mask the fertile period. In this way the male has to be around the female constantly and is more engaged in nursing the offspring, together with the constant receptivity (while in other mammals the developed breasts are a clear sign of "no mating here", and mating is often limited to give time window). And also, considering the risk related to delivery, masking the fertile period is wise.

So, in case of bigger offsprings, it would be better to increase the milk production of the two breasts, rather than putting more effort in growing an additional pair.

As said above, growing and maintaining breasts is an energetic expense to carry on from puberty to death, while breastfeeding is an effort "limited" to the time after birth.


Yes, they would have to have more than two breasts.

For one thing, breast-milk is not homogeneous. Colostrum is the very first milk, very easy to digest and specific to newborn infants. Then, as the child matures, milk develops into two parts - fore-milk, the first milk from a full breast, is thin and watery with vitamins and proteins. And also hind-milk, the last milk from a near-empty breast, which is thick and creamy and heavy on fats. In short, the composition of the milk changes over a feeding session, and over a child's growth, likely to prioritize which nutrients are available and address which are needed. Nor are these necessarily discrete substances, it is much more likely a gradation for production and letdown of one to another depending on the child(ren)'s needs - making it harder to equally subdivide each type of milk when feeding multiple infants.

This makes scheduling or rotating feedings much more complicated. Multiple babies rotating between two breasts will likely get differing levels of nutrition, differing amounts of calories, even different volumes, depending the order each eats in a feeding session. And it would be easy for some children to get left behind, undernourished, if there's any problem feeding or if the scheduling or rotating is even slightly off balance - those eating first getting nutrients so the second feeders are malnourished, those eating later getting the bulk of fats and calories so those eating first are starved. On the other hand, each breast has its own supply of each kind of milk (one reason switching breasts when feeding is a thing), so having at least one breast per child should help to minimize that kind of unequal division - though more would be useful backup for when the children need more than one breast's worth of milk.

For another thing, infants eat every two or three hours, can take as long as an hour for a single feeding in some circumstances - and can have marathon feedings, preceding a growth spurt, with feedings every 20 to 50 minutes. Rotating feedings would mean spending twice as much time on feeding, not to mention the extra amount of time it would take to completely refill the breast (to make sure both kinds of milk are produced). And with all the other demands on the parent's time doubled, well, the first few months are already commonly held to be a time of sleep deprivation and frantic activity (cleaning, soothing, keeping warm and dry, self-care), needing to double the frequency or duration of feedings is likely not going to help at all.

An intelligent animal can manage, I believe, as you said (as we do, when multiple kids are born) - but it would be a real and nontrivial effort and require a great deal of support from others. And those others would likely have their own to tend, as this is a normal and not extraordinary circumstance, limiting the amount of assistance each could reasonably expect to get. Also, time may be somewhat less important for having safe and organized villages as opposed to being vulnerable and alone, but it is still quite important - especially since more time per litter would be needed generally for childcare, and more calories would be needed to produce milk (thus more time to forage, prepare food, eat, rest, etc).

It is possible to balance out the feedings so each gets their fair share, but it is a great deal of work - and it is equally possible to miscalculate, to under-nourish one or more of the litter and those so neglected will be much less likely to survive. I would expect that such balancing would be much, much harder further back in evolutionary history - with the effect that either those with smaller litters would out-compete those with more, as their kids are healthier, or else more breasts would evolve.

A third nipple is a possible mutation already, if it gave a concrete advantage I would expect it to spread broadly and possibly eventually a mutation of a fourth nipple may appear, or more, until the number meets or exceeds expected children per litter (to ensure each has a chance at equal nourishment). And each breast may be smaller, or some or all of them may be dormant, flat and not engorged, when not actively breastfeeding a litter. This happens in other animals, so it seems a likely solution if extra breasts would negatively impact the center of balance.

  • $\begingroup$ Indeed, one or more supernumerary nipples occasionally do grow with such development as to require excision, in humans and other animals. $\endgroup$ – can-ned_food Aug 2 '17 at 6:31

One possibility I don't see mentioned is that men could develop the ability to nurse. While there are apparently no examples of male lactation in rigorously reviewed medical literature, there are numerous reasonably credible reports of men nursing, and the main holdup, if any, for more widespread male lactation is a relatively minor hormonal shortage in men.

It would be far easier for the male hormone balance to change slightly than it would be for women to grow more breasts.


The answer would be yes, they'd have more breasts if they had larger litters. However... one has to consider why larger and more developed animals don't have large litters - they protect and educate the few they do have.

Animals with a high reproductive rate, like rabbits, mice, cats, and dogs, tend to spray out the babies and hope a few make it. Childhood tends to be shorter, with a shorter training period and fewer skills to learn. Cats and dogs have a 6-8 month childhood with the mother starting a new brood within one year of the previous, while human children have an 18 year childhood - and some extend childhood far beyond that time.

Besides, with 6-8 breasts, it would take one all night just to get through foreplay. Might lower the reproductive rate.

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    $\begingroup$ On the other hand, such a long foreplay is a lot of bonding. Might motivate (a larger percentage of) the males to stay around and protect the offspring.... $\endgroup$ – xDaizu Jul 1 '17 at 16:56
  • $\begingroup$ sigh what a problem to have... $\endgroup$ – tj1000 Jul 1 '17 at 17:51

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