Evolution of bums

[Bird on a Fire]

Most animals don't really have boobs or bums.

Boobs are quite easy to explain scientifically - they are sexy.

But what about bums? Are they a hangover from ancestral primate mating habits? In which case, is the only reason I (a male) have one (and a very nice one at that) the same reason I have nipples?

I'm basically defining a bum as the sticky-out bit of muscle and fat (your proportions may vary) above the legs. Some ideas:
- convenient place to store important muscles for legwork
- convenient place to store fat that won't get in the way of anything important
- more comfortable for sitting down
- protects the bumhole

But, to be honest, none seem particularly satisfactory.

In my head, if I contort an endurance-running animal like a horse or wolf or ostrich into an upright, human-like posture, I don't think they'd have human-like bums (but maybe if I could find better pictures of shaved/plucked ones it would help).

No comment on the second one.

There's no way in hell comfy sitting is an evolutionary pressure.

In which case, what on earth were our distant ancestors doing with their bumholes that necessitated evolving an entire bum to protect it? I'm getting serious evolutionary jealousy here - call me Fomo sapiens.

[Bird on a Fire]

(I'm not even drunk or owt - think I've been spending a wee bit too much time alone in recent weeks. These are the thoughts that keep me up at night)

[Woodchopper]

I think that the muscle mass is probably explained by the whole walking on two legs thing. Other animals like horses or wolves walk on four legs so aren’t really comparable.

But you raise a good question about why did we evolve fat deposits there. I don’t have an answer.

[Woodchopper]

Most animals don't really have boobs or bums.

Boobs are quite easy to explain scientifically - they are sexy.

Only because some people think they are. I’d take some convincing that the modern western obsession with breasts isn’t wholly cultural.

I remember talking to an anthropologist who mentioned that one thing she had to get used to on fieldwork was that the people she was studying didn’t consider breasts to be anything other than a utilitarian organ for feeding children. So she saw them all the time.

[shpalman]

Most animals don't really have boobs or bums.

Boobs are quite easy to explain scientifically - they are sexy.

Only because some people think they are.

Sexiness is a bit like fashion, in that people are attracted to certain traits because people are attracted to certain trait. Or rather, the current generation seeks attractive mating possibilities to have offspring which are attractive to the next generation. It doesn't matter what the trait is as long as it's genetically heritable. It may have had a basis in something "useful" but it doesn't need to anymore.

I’d take some convincing that the modern western obsession with breasts isn’t wholly cultural.

One piece of evidence is that women have enlarged breasts (compared to men) even when they don't have children to utilitarianly feed with them at that specific time.

I remember talking to an anthropologist who mentioned that one thing she had to get used to on fieldwork was that the people she was studying didn’t consider breasts to be anything other than a utilitarian organ for feeding children. So she saw them all the time.

[bob sterman]

In my head, if I contort an endurance-running animal like a horse or wolf or ostrich into an upright, human-like posture, I don't think they'd have human-like bums (but maybe if I could find better pictures of shaved/plucked ones it would help).

Yup - flat as a pancake...

bojack.jpg (4.6 KiB) Viewed 1392 times

[nekomatic]

I’m sure Desmond Morris said that boobs (or specifically, cleavages) were sexy because they look like the tops of bums, which were what our ape ancestors used to enjoy looking at on approach, as it were.

By some odd coincidence it was around the time I read this that I started becoming the teeniest bit sceptical about some of the claims of evolutionary psychologists.

Have you ever actually seen a horse though? They totally have bums.

[Chris Preston]

Most animals don't really have boobs or bums.

Boobs are quite easy to explain scientifically - they are sexy.

Only because some people think they are. I’d take some convincing that the modern western obsession with breasts isn’t wholly cultural.

I remember talking to an anthropologist who mentioned that one thing she had to get used to on fieldwork was that the people she was studying didn’t consider breasts to be anything other than a utilitarian organ for feeding children. So she saw them all the time.

Just because they are utilitarian, doesn't mean they are not involved in sexual selection. It does appear that certain body shapes have arisen through sexual selection in some societies, but they are not common across cultures and are mostly utilitarian. There are a couple of factors arguing against intense sexual selection in humans that I can think of. One is fashions in body shapes are subject to too much change and the second is that humans are not that picky who they mate with and even those low in the current pecking order for sexiness seem to find a mate and have children.

[Tessa K]

I’m sure Desmond Morris said that boobs (or specifically, cleavages) were sexy because they look like the tops of bums, which were what our ape ancestors used to enjoy looking at on approach, as it were.

By some odd coincidence it was around the time I read this that I started becoming the teeniest bit sceptical about some of the claims of evolutionary psychologists.

Have you ever actually seen a horse though? They totally have bums.

Yes, he said that women evolved boobs to encourage men to have face to face sex. So many things wrong with this. Boobs only look like bums in a push-up bra. Why did women evolve to need face to face sex and then have to evolve boobs to get men to do it that way round? Look at bonobos, our close ancestors, they do it in all manner of positions so why would we need to do it face to face to reproduce when it's not anatomically necessary? Maybe Morris had a very dull missionary position sex life. And other reasons.

Bum muscles are useful for climbing, jumping, walking, running. See this article on the now extinct giant kangaroo:

Like walking primates, their pelvises fanned out at the rear. “They had big bums, and much more room for these large gluteal muscles than today’s kangaroos,” says Janis. These muscles would have supported each leg during walking.

https://www.newscientist.com/article/mg ... ey-walked/

Ot his:

The gluteus maximus is the largest muscle in the human body. It is large and powerful because it has the job of keeping the trunk of the body in an erect posture. It is the chief antigravity muscle that aids in walking up stairs.

https://www.loc.gov/everyday-mysteries/ ... e%20heart.

As the largest muscle it might make sense to put fat over it to keep it warm. Maybe?

On sexual selection in humans: although preferences do change across time and cultures, there are some constants. Markers of fertility, youth (peak fertility time) and health/fitness are desirable, for example.

[WFJ]

Have you ever actually seen a horse though? They totally have bums.

I think a horse's "bum" is actually its hamstrings, rather than its arse. The relative protrusion and pertness of our bums is needed to keep the weight of our upper bodies upright.

A weak arse combined with a massively heavy upper body results in this:
Spoiler:

which no one wants and would likely result in lower back issues.

[nekomatic]

IIRC the proposed sequence of evolutionary development went upright posture -> vagina moves towards the front -> face to face sex becomes more reliable for successful insemination, in which case it would make some sort of sense for traits to evolve that made it more attractive. But no, breasts really don’t look much like bums unless you squish them in (evolutionarily) modern underwear.

[Bird on a Fire]

Bum muscles are useful for climbing, jumping, walking, running. See this article on the now extinct giant kangaroo:

Like walking primates, their pelvises fanned out at the rear. “They had big bums, and much more room for these large gluteal muscles than today’s kangaroos,” says Janis. These muscles would have supported each leg during walking.

https://www.newscientist.com/article/mg ... ey-walked/

Ot his:

The gluteus maximus is the largest muscle in the human body. It is large and powerful because it has the job of keeping the trunk of the body in an erect posture. It is the chief antigravity muscle that aids in walking up stairs.

https://www.loc.gov/everyday-mysteries/ ... e%20heart.

As the largest muscle it might make sense to put fat over it to keep it warm. Maybe?

Excellent! Thank you Tessa - I think you've answered my question. Bums are for upright walking, rather than sitting!

No pictures of giant kangaroo bums, even in the linked paper. Possibly for the best.

[Bird on a Fire]

The problem with a purely utilitarian explanation for boobs is that all mammals feed their offspring by lactating, but only humans have boobs. If they were useful for feeding young you'd expect all mammals to have them, unless they can be successfully related to some idiosyncrasy of human physiology or behaviour.

Whereas they'd count as the first two of

Markers of fertility, youth (peak fertility time) and health/fitness are desirable, for example.

for sure, not sure about general health/fitness.

[Bird on a Fire]

Most animals don't really have boobs or bums.

Boobs are quite easy to explain scientifically - they are sexy.

Only because some people think they are. I’d take some convincing that the modern western obsession with breasts isn’t wholly cultural.

I remember talking to an anthropologist who mentioned that one thing she had to get used to on fieldwork was that the people she was studying didn’t consider breasts to be anything other than a utilitarian organ for feeding children. So she saw them all the time.

Yes, in Guinea Bissau last year it wasn't unusual to see women topless and breastfeeding, but also wearing hijabs, in a nice illustration of African syncretism.

I'm no anthropologist, but if cultures can gain attraction to body parts, can they not also lose them?

There's also context-dependence. Seeing someone breastfeeding isn't necessarily an instant turn-on in the way somebody taking their top off in your bedroom might be.

[Woodchopper]

On sexual selection in humans: although preferences do change across time and cultures, there are some constants. Markers of fertility, youth (peak fertility time) and health/fitness are desirable, for example.

Certainly, but in practice the actual markers tend to be defined by culture.

For example, in some times and places women having lots of bodyfat is or was considered to be a marker for health and fertility, and being thin a sign of being sickly. And in others being thin is a sign of health and fitness and bodyfat a sign of ill health.

[Woodchopper]

I'm no anthropologist, but if cultures can gain attraction to body parts, can they not also lose them?

Of course, but for me that tends to undermine the suggestion that boobs being seen as sexy is due to evolution.

[Opti]

Since I fractured my hip 3 months ago, I have lost considerable mass from my right buttock. It's proving fairly difficult to rebuild the muscle/fat there.
Walking up stairs/steps is considerably harder than it used to be and finding a comfortable sitting position that isn't more like laying down is very annoying.
Don't know what that adds to the discussion.
I'm trying hard to keep things moving and at least I can walk about 400m a day unaided but if I overdo things I'm in pretty bad pain for about 3 days.
Still, it's better than when I was mostly immobile ... when my Gluteus Maximus would cramp up regularly. Holy f.ck! you really don't want your biggest muscle to cramp up, believe me.

[Woodchopper]

Since I fractured my hip 3 months ago, I have lost considerable mass from my right buttock. It's proving fairly difficult to rebuild the muscle/fat there.
Walking up stairs/steps is considerably harder than it used to be and finding a comfortable sitting position that isn't more like laying down is very annoying.
Don't know what that adds to the discussion.
I'm trying hard to keep things moving and at least I can walk about 400m a day unaided but if I overdo things I'm in pretty bad pain for about 3 days.
Still, it's better than when I was mostly immobile ... when my Gluteus Maximus would cramp up regularly. Holy f.ck! you really don't want your biggest muscle to cramp up, believe me.

Ouch!

Glad to hear you are getting better.

[Boustrophedon]

Have you ever actually seen a horse though? They totally have bums.

So does our cat, Mimi, and she wiggles it when she walks.

[nekomatic]

For example, in some times and places women having lots of bodyfat is or was considered to be a marker for health and fertility, and being thin a sign of being sickly. And in others being thin is a sign of health and fitness and bodyfat a sign of ill health.

And if being strong is good for survival you would expect a healthy display of musculature to be considered attractive in both sexes. Meanwhile the Victorians were driven wild by the mere thought of ankles.

[Bird on a Fire]

Human culture is certainly capable of overriding evolutionary drives and base instincts (thank goodness). But I'm not aware of many examples where cultural/behavioural practises have shaped human evolution. The obvious example is cooking, which led to a drastic reduction of digestive apparatus and subsequently we got these big brains.

[Tessa K]

On sexual selection in humans: although preferences do change across time and cultures, there are some constants. Markers of fertility, youth (peak fertility time) and health/fitness are desirable, for example.

Certainly, but in practice the actual markers tend to be defined by culture.

For example, in some times and places women having lots of bodyfat is or was considered to be a marker for health and fertility, and being thin a sign of being sickly. And in others being thin is a sign of health and fitness and bodyfat a sign of ill health.

Health definitely but more in terms of things like markers of being able to fight off infection or having survived infection. Like the peacock's tail. Smallpox scars should have been attractive as they were a sign of a survivor.

Being fat is a good sign in cultures where resources are scarce as the body has something to fall back on. Being too thin can reduce fertility. Being thin in some modern cultures is/was considered a sign of having HIV/AIDS or in an earlier generation, TB. So lean and fit yes, skinny, no. Lean and fit in a man might indicate a good hunter, for example.

Isn't it only in modern times in Western culture that fatness has become associated with ill health because a) we understand more about the downsides of obesity, b) there are no shortages for the majority and c) fashion - none of which happened long enough ago to influence evolution.

Anyway, back to bums: the average shape of bums varies with ethnicity - as do calf shapes. I don't know enough about sport to say how this would affect the ability to walk or run upright.

I do know that even when I'm really hot my bum is still cool.