So ignoring all the other hydrodynamic inefficiencies associated with sticking a human torso on a fish body, let's just worry about those most distinct of human female features, breasts. The problem with them on mermaids is that no other sea creature has breasts. Sea mammals such as whales and dolphins have teats that calves will suck from when nursing. But, these teats are flush with the skin of the mother and offer no additional resistance when swimming.

Assuming the instantaneous creation of classically shaped merfolk, would evolution select for or against human style breasts on an creature that lives exclusively underwater? If mermaid breasts did remain, how would they be shaped and where would they be located for maximal hydrodynamic efficiency as well as satisfying their role as providing food for merchildren?

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    $\begingroup$ Consider researching existing sea mammals: en.wikipedia.org/wiki/Marine_mammal $\endgroup$
    – DA.
    Mar 9, 2016 at 15:03
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    $\begingroup$ I would like to point out that most primates don't have large breasts unless they're actively using them. So pretty much any animal with constant breasts is pretty strange. $\endgroup$ Mar 9, 2016 at 15:20
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    $\begingroup$ Evolution might minimise large breasts but it would definitely select against long head hair. $\endgroup$
    – Separatrix
    Mar 9, 2016 at 17:07
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    $\begingroup$ Evolution selected for dugongs with breasts (perhaps part of the origin of the mermaid myth). On the other hand, evolution selected for whales and seals withoutsuch obvious mammaries.On the other other hand, nature alse solved the same problem with udders,on land. I think the simple answer is that nature, the blind watchmaker, works with whatever happens to be available, and changes happen randomly rather than because they would be useful -- if the right mutation doesn't happen to occur, no change happens and the critter competes as it is. $\endgroup$
    – keshlam
    Mar 10, 2016 at 0:41
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    $\begingroup$ Some answers seem to assume that the mermaids in question are man-eaters and are interested in trapping or otherwise harming sailors. I'd like to see some clarification from the OP on what kind of mermaid they're asking about (if their primary food source is sailors, that makes baiting food an evolutionary factor). Personally, I think the question is only about swimming efficiency and feeding offspring but clearly this isn't the assumption everyone is making (sometimes it's The Little Mermaid, others it's Harry Potter merfolk etc.). $\endgroup$
    – mechalynx
    Mar 10, 2016 at 2:07

9 Answers 9


A walrus
enter image description here

A hippo
enter image description here

Both of them can swim much faster than a human being, despite the clumsy looking proportions and giant weights.

Next are the winners of different length swimming contests in Dubai 2010 (FINA) as stated on the wikipedia page. I am only posting their names and not their swimsuit images to evade creating controversy here.

Ranomi Kromowidjojo
Camille Muffat
Katie Hoff
Erika Villaécija
Hinkelien Schreuder
Kylie Palmer
Mireia Belmonte

If you google search their images, you would find out they are no less feminine to look at, than other women. In fact some of them are hopelessly gorgeous, both by the face and by the curves.

If having round, bumps on your chest was detrimental to swimming, you would find more flat-chested women champs ... which is not the case. So while mermaids might have minimized breasts or be completely flat-chested due to some other reason, speed in swimming would definitely not be that.

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    $\begingroup$ Excellent +1. This is the kind of answer I like to see on this site. Drawing conclusions from real-life examples is so straightforward and simple yet elegant in figuring out hypothetical questions. That said I do have a couple of comments to make: The walrus and hippo have bodies that are more efficient than ours for swimming (much like sharks) and have limbs that help that along - weight isn't an issue with enough buoyancy. On the swimmers, considering swimming is done for sport in these cases, it's possible that unattractive people are less likely to be athletes (very broadly speaking). $\endgroup$
    – mechalynx
    Mar 10, 2016 at 1:56
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    $\begingroup$ @ivy_lynx Your last hypothetical "unattractive people are less likely to be athletes" seems possible but it might be the other way around "athletes are less likely to be unattractive". A large part of what we consider as being attractive ( in the west) has to do with body shape and body image and toned muscles and small amounts of fat are very much in right now. $\endgroup$
    – DRF
    Mar 10, 2016 at 10:08
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    $\begingroup$ Human female competitive swimmers typically wear one-piece swimsuits which provide a degree of streamlining. Are you suggesting that mer-women should wear swimsuits to gain the same hydrodynamic benefits? $\endgroup$ Mar 10, 2016 at 12:33
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    $\begingroup$ If human women wear one-piece swimsuits, that does not nullify my point. A one-piece swimsuit worn by a flat-chested person would have given them some extra streamlining as compared to a more curvy chest of a feminine looking body. How come many women with normally large breast tissues are able to outperform other women, wearing same quality swimsuits and having more flat chests? In short, wearing one-piece swimsuit might indeed increase the streamlining, but that swimsuit does not give any highly effective advantage to flatter-chested women over bumpier chested women. $\endgroup$ Mar 10, 2016 at 14:21
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    $\begingroup$ @ivy_lynx: weight is an issue, even with buoyancy. Buoyancy simply means how easily you can keep your afloat not how fast you can move it. A hippo would have to use many times more force than a human to push its hundreds of kgs heavy body through water. Same applies to walruses too. I would expect (although I have not researched this topic in detail) that the average density of a human body would be no greater than that of a walrus or a hippo. $\endgroup$ Mar 10, 2016 at 14:24

Evolution would select for human style breasts ...given a few things are true.

First, humans have effectively removed predatory pressure as a cause for human evolution, and with sapience I would say things would be the same for your merfolk.

Without this pressure the selection for reduced breast size/improved aqua-dynamics simply doesn't exist. I don't need to be super fast to get away from predators if I don't really have any predators.

Second, you need to define what the merfolk society values; function or appearance or some combination thereof.

Since it is not specified I will lean towards they act like humans. This means it will vary from time to time. The ideal of beauty will be the primary procreation selection pressure. Which is weird because that means the pressure will vary based on the whims of your male merpeople. In the end I think this means that there is no effective directional pressure because the pressure would not be consistent enough over long periods of time for selection to take place.

Additional considerations:

Breast cancer is something to keep in mind if your mermen prefer a larger bust. It's possible that larger size = higher cancer risk

....so figure out how to do mammograms underwater.

enter image description here

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    $\begingroup$ Mermaids have no predators? What about sharks or orcas? They might not be a threat for us humains, because we don't smell, look or taste like fish, but maybe Mermaids do? $\endgroup$
    – Carlos2W
    Mar 9, 2016 at 16:13
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    $\begingroup$ @Carlos2W They would be as much a threat to merfolk as lions and bears are to modern humans. Sure the threat exists but we humans have effectively removed ourselves from the food chain, my argument is merfolk would be in the same situation. $\endgroup$
    – James
    Mar 9, 2016 at 16:15
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    $\begingroup$ I'd like to point out that sentience and sapience are not the same thing. A dog or cat are sentient. But humans are the only sapient beings on Earth. $\endgroup$
    – AndreiROM
    Mar 9, 2016 at 17:52
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    $\begingroup$ Don't forget sexual appeal . Males will be more likely to mate with them , making it more likely for large breasts to be selected for , like antlers or the feathers on peacocks $\endgroup$
    – user15036
    Mar 9, 2016 at 18:25
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    $\begingroup$ Underwater mammograms are very easy. Just learn to communicate with dolphins; their echolocation can easily see inside mammary tissue. $\endgroup$ Mar 10, 2016 at 6:42

As long as mermen will consider breasts to be sexy, they won't disappear (that's how they came into existence in the first place).

Quoting Wikipedia on sexual selection:

Sexual selection is a mode of natural selection where typically members of one biological sex choose mates of the other sex with whom to mate (intersexual selection) and competition between members of the same sex to sexually reproduce with members of the opposite sex (intrasexual selection). [...]

Sexual selection can lead typically males to extreme efforts to demonstrate their fitness to be chosen by females, producing secondary sexual characteristics, such as ornate bird tails like the peacock plumage, or the antlers of deer, or the manes of lions, caused by a positive feedback mechanism known as a Fisherian runaway, where the passing on of the desire for a trait in one sex is as important as having the trait in the other sex in producing the runaway effect.


The evolution of the enlarged human breast is a bit of an evolutionary mystery, but one prominent theory (I am not making this up) is that human females evolved them so that their chest would look more like a butt (two bumps with a crack in the middle). Most male primates are naturally attracted to the bottoms of their females (which makes sense, since it is near the reproductive organs), but since humans walk on two legs, it made sense for females to have a similarly-shaped sexual trigger that was closer to the male's natural eye level.

Assuming this theory is correct, there is little reason to suppose that merfolk would have the same pressures to evolve breasts, because pre-merfolk (presumably some aquatic mammal) had no legs and therefore no butts to be attracted to.

Therefore, I would presume that they evolved breasts not as sexual selection, but in order to attract human prey. This means that it would make sense for the males to have them as well, unless merfolk are a species where only the female hunts humans.

Since humans are not generally a good primary food source, especially for an aquatic species, it would be unlikely for merfolk to have a specific means for attracting humans... unless humans played an important part in their reproductive cycle. Maybe they lay their eggs in human corpses, like wasps, or need human blood to produce children, like mosquitoes. Though as a survival strategy, this is almost as bad, or perhaps even worse.

It'll take some major handwaving to explain this phenomenon in a logical way.

  • $\begingroup$ The basic theory is that human females (and almost no other animals) have large breasts (mostly non-functional fat) because human males have brains. I.e. humans can reason, and since people instinctively want to have many healthy children, they (incorrectly) assume that large breasts produce more milk, and so tend to use that as a factor when choosing their mates. So females with a genetic predisposition to excess breast fat tend to produce more children, which in the long run tends to remove the small-breast genes from the pool. $\endgroup$ Aug 10, 2019 at 1:51
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    $\begingroup$ @RayButterworth "Because humans can reason?" That theory would only make sense if the attraction was operating on a conscious level. I guarantee that the majority of men are not actively thinking "I am attracted to that woman with large breasts because she can produce a lot of milk for my children". It is possible that this stems from males instinctively selecting females that could produce more milk and this would lead to runaway sexual selection resulting in ornamental fat deposits, but this has nothing to do with thinking. $\endgroup$ Aug 12, 2019 at 5:50

I see two possible directions:

The first, they keep their parts where they are because they grew them in the first place to attract sailors and fishermen to their fate ;). Though it is possible they only show up (down) when the mermaid 'relaxes' some muscles in order for them to 'stick out' either for feeding young or attracting strange humans, kind of like my belly when I sit (though much less attractive). Otherwise they would be 'flat' against the body, like when a woman lays on her back, and stretches. Though really busty mermaids are likely to be pretty rare.

The other is more along the lines of other animals. Well let's take the manatee as our model. They would likely be large round animals since that shape helps mammals in the water, all of the examples I can think of are, Whales, manatees, seals, sea lions, walruses. Dolphins are the least round, and are the fastest/sleekest.

The best I could imagine would be a line of teats like a pig, where they form a line or trow and could be more of a rudder than anything else. enter image description here


I’ll attempt another, not well documented approach, here.

  • Mermaids activities include most notably luring sea farers with their charms and singing voices in order to wreck ships. This is usually described as a group behaviour.
  • While little is known about what happen once the ships go down, I think it’s safe to assume it is beneficial to mermaids. Collecting riches makes little sense as they’d have no way to spend it. It only seems logical to assume they eat the sailors they manage to drown. I actually heard that a Captain’s kidney is considered a delicacy under the sea1.
  • Swimming speed thus brings little benefits to their hunting method.
  • This making-noise-to-attract-prey attitude pretty much seems a top-of-the-food-chain behavior. Fleeing a regular danger doesn’t seem a vital selection criterium either.

Now, consider that most sailors throughout history were mainly men. Men with little regular contacts with women.

It would thus my humble opinion that, from an evolutionary perspective, the size of individual mermaids would matter little but, as a collective, a larger variety of chests would increase the probability of attracting more members of a ship crew, thus increasing the probability of a shipwreck.

  1. Where nobody beat you, fry you and eat you in fricassee if you’re a fish or a mermaid princess.
  2. I must admit that the size of my sample makes statistical results contestable.

While I am not a mermaid, I am a female who free dives (diving under the water with no air supply only lasting on one breath) in the ocean. I am naturally large chested (size US 36H for reference) and often I am swimming in a one piece swimsuit or a wet-suit.

  1. With my chest secured I think the only thing I notice is being more buoyant (due to extra fatty tissue) then smaller chested or male free divers. So while my brother can dive easily with no weights, I use a weight belt to help me dive easier.

    • Thinking about mermaids, I would think of this to be a negative effect for a creature that would be swimming and diving all the time, who probably doesn't want/need the extra buoyancy. Tho this may depend on what type of buoyancy system the mermaid has if any at all.
  2. With my chest not secured by a swimsuit we run into some different issues. One time practicing alone in a private pool I wanted to try swimming and diving without my top on to test the difference. First off while floating in the water breasts are buoyant and float as if gravity is no longer weighing them down. This is a positive at first because large breasts are heavy and cause back pain/problems, so at least mermaids wouldn't have to deal with that. But when I started swimming/diving under water I encountered an issue, large breasts move and pull when swimming creating a similar feeling as jumping without a bra on. This is a very uncomfortable experience, and is why I put on 3 sports bras when going to the gym. This would probably cause some kind of damage to the tissues after a while.

    • So if mermaids do have larger breasts they would probably be tightly bound to prevent this awful experience. Out of what material you would make this binding, maybe fish/seal skin or maybe some type of sturdy kelp. I'm just gonna say here that seaweed, shells, and sea stars/starfish are not viable options for securing breasts in general.

So with that field research data out of the way, I would conclude that mermaids would probably have smaller breasts, flatter breast shape, and/or that the milk producing lobes and ducts would be more spread out, maybe even located in the chest cavity. The stress caused to large breasts while swimming and the added buoyancy would probably in the end outweigh any benefits of either attracting mates or prey (especially if they do not function to produce milk for young). And with creating bras/binders out of marine materials being so difficult, having a large rack is just too difficult.


Only humans have large breasts outside of child-rearing phases (coincidentally, other than some equine species, humans are also the only mammals to exhibit overt menstruation). As such, the function of human breasts is not strictly for feeding children.

It's been theorized, rather controversially, that humans have breasts because cleavage looks like a butt, and because humans tend to interact sexually and socially by facing each other, they needed a new sexual cue other than the female posterior.

Personally I think a more plausible theory (though not a mutually exclusive one) is that large breasts signify both maturity and capability of motherhood from a biological standpoint. Humans are very weird creatures in the animal kingdom and we exhibit a lot of strange traits not found among other primates or mammals in general. Humans show a relatively high amount of neoteny (keeping childish traits in adulthood), so we need some secondary sexual characteristics to signify that we are of breeding age. For this same reason, humans show an unexpectedly high amount of sexual dimorphism as well.

For women this is breasts, buttocks, and hips. For men this is muscles, facial hair, and broad shoulders/chest. For women, their traits signal that they can become capable mothers able to raise children into adulthood successfully, and thus pass on the genes of their partners (strong hips suggest safe childbirth, large breasts suggest capability to breastfeed babies, etc...). For men, their traits signal an ability to protect and provide, and thus ensure they can help their partners pass on their genes. Each gender is complementary in their dimorphic traits and what they signify. This is somewhat supported by scientific study that shows a specific waist-to-hip ratio in women is invariably considered "attractive" across cultures on the planet. The same is true of muscular, athletically fit men (there appears to be an especial aversion in heterosexual women to men showing a feminine waist-to-hip ratio, perhaps because it may signify low testosterone or other unfit characteristics). We are more or less genetically programmed on what to find appealing.

So to answer your question, any similar drives in merfolk would select for breasts. Do your merfolk interact socially/sexually facing each other? Do they breastfeed their young? Do they show neoteny? Do they have a selection pressure to show strong sexual dimorphism? If the answers to these questions are yes, then they are likely to select for large breasts.

The hydrodynamic aspect is essentially irrelevant. Breasts are mostly water, and so behave (mostly) neutrally buoyant, despite jokes about "flotation devices." They aren't just big blobs of fat; they exhibit complex structure of ligaments, milk ducts, and other tissues. They're specialized organs.

Humans are stamina-based pursuit predators. We actually have some of the best stamina in the human kingdom, and actually have the best among mammals. Large breasts obviously are not conducive to running for long distances. Even so, humans still have them, so it is reasonable to assume that the selection pressures for them outweigh their disadvantages to our ecological niche. Similarly, I would not expect their hydrodynamic properties to being a limiting factor in whether or not your merfolk would evolve them.

It will also depend a little bit on their evolutionary ancestors, but since they're merfolk, it seems reasonable to assume they evolved from humans. As such, it's likely they would have had breasts before they took to the water. The fact that they're descended from humans means they would be starting with human preferences regarding sexual selection, and it's unlikely they would lose their breasts completely without a strong pressure away from them.

For this to happen, there would have to be a very strong disadvantage to having breasts, one so strong that those whose genes make them prefer large breasts would die out over time, as well as those who have them being unable to pass on their genes to their descendants, presumably because they died before reproducing. The only thing that I can think of that might cause this in an intelligent species is going to be some kind of super predator that only fast merfolk can escape from, and even in that case breasts would have to provide a strong disadvantage for swimming speed.

However, merfolk are assumed to be at least as intelligent as their land dwelling human cousins, so they aren't simply subject to natural selection. Humans invent artifacts to allow us to survive in our environment, one of which is clothing. In this case, we're subject to cultural selection as well. It's likely that even if large breasts were a disadvantage to swimming fast and thus escaping predators, that the merfolk would create clothing to offset this disadvantage (i.e. things that bind their breasts and make them more hydrodynamic). Assume one tribe of merfolk realize this and begin binding their chests. Because cultural changes can propagate far faster than the slow chaos of evolution, the idea of chest binding would spread to other tribes long before evolutionary pressures caused slow mermaids to be eaten and mermen who prefer large breasts to die out.


Another prominent theory about breasts is that large breasts indicate that the female is able to collect sufficient food for herself and her children. In this it serves a similar purpose to peacock tails although in a different sex. I.e. large breasts are a deliberately wasteful use of calories because young females who can deliberately waste calories in this way are more likely to be able to raise healthy, well-fed children.

I find this hypothesis more convincing than a resemblance to buttocks. Because in my opinion, the fatty tissue in breasts hangs differently than the muscle in buttocks. They look most alike when the breasts are in a bra or other binding that creates cleavage, but they would have evolved prior to the creation of such clothing.

This would be just as true of mermaids as of human women.

This would also tend to counteract the evolutionary downsides, like more difficult swimming. Because a female who can survive even with a handicap is again more likely to be able to raise children who survive to adulthood. Because children are also a handicap when running/swimming.


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