pretending for the moment that squirrels were like that, it would indicate that squirrels engage in "sperm competition" like chimpanzees do.

When females, by choice, force or instinct, tend to have the sperm of more than one male in them at the same time there is selection pressure for producing more and more sperm... to swamp the other guy's contribution numerically.

Chimps have unusually large balls. Gorillas, being mostly monogamous in harem arrangements, have no competition and thus have small testicles despite having many mates.

The big balls of dogs and cats are what we would expect in a species where females go into a "heat" period of being sexually indiscriminate.

our front yard.

Why most women had those pointy things (the bras in the 60's created an odd pointy look...at least I thought it looked odd.)

Link: http://en.wikipedia.org/wiki/The_Naked_Ape

Don't remember the ear lobe thing, but a highly entertaining book on this whole subject.
See also the last part of Lucy (about finding that fossil), where some anthropologist went on a long tear about how human evolution was driven by sex - he was mostly trying to explain the expressiveness of our facial expressions, and how upright walking and our large brain could all be explained by this. Another interesting read.

feels good.

bright red buttocks as a sexual signal. I'm happy with our breasts, penes, and beards.

at least by primate standards.

But I don't think the purpose was to intimidate other males.

Humans, unlike other primates, have a tendency to pair-bond. This trait effectively doubled a parent's ability to care for and protect their offspring. Anything that enhanced the pair-bond tended to be reinforced via natural selection, and sexual satisfaction was a part of that.

It is seldom discussed, but this is one of the more drastic differences between us and the rest of the ape family. That, and the brain thing of course.

I will admit that I love breasts, not large ones just those that fit the form of the owner. Breasts have a purpose as do penis's but the primary function is not the only function.

Breasts might be considered decorative as an enticement but really? Who want's to look at a penis and how does it enhance your appearance?

Because if the issue was just about intimidating other males, the scrotum would be tiny, like a gorilla, and it's not.

You probably read about the theory that the reason for the shape of the male penis might be to displace semen, I assume. The idea is that the glans forms a "scoop" that would displace other another male's sperm. This action has been observed in other species - so it's an adaptation that already has some success in other species.

Chimpanzee males have penises of different shape and a different strategy to ensure their sperm is favored among female chimpanzees who have sex with various males. Their sperm secretions are different. Their sperm forms a "plug" to seal off the cervix of a female after sex. The shape of the chimpanzee penis is thought to serve to get rid of this plug. A female chimpanzee mates with just about every male when she is in estrus.

One primatologist thinks the human penis is shaped the way it is to scoop out vaginal secretions - that oxytocin idea of encouraging pair bonds by sharing more than just a three second turn at just any female.

Most every primatologist these days would find a reason to consider that the vagina influenced the shape of the penis because the goal is fertilization for both parties.

You focus too much on male competition.

Our other closest primate relative doesn't have the same social structure as the chimpanzee, and they are more closely related to one another than to humans (they diverged from one another later) - and the three - common chimps, bonobos and humans, all diverged from gorillas even before - and all three have social structures that are different from gorillas.

This is another reason to doubt the male competition idea as the reason for the length of the human penis. When females have no choice, the penis is smallest compared to body size and dimorphism is far more prominent.

Human dimorphism is, again, more like pan than gorillas. Lucy is small - but there's no indication that a male austropith. was significantly larger than a female, at least the last time I heard anything about this subject.

Humans are not entirely monogamous.

This is evident in DNA research and in comparison to other species who either are entirely promiscuous or entirely monogamous, or in species with males and harems.

Humans have also used technology to change our social structures in a variety of ways. Humans have examples of cultures in which males control females, all property is inherited through sons, etc. Humans also have examples of cultures in which a female maintains control of her own property through her family line and the brother, rather than the man with whom a female has children, has the male "authority" in a family. In these cultures, women have more than one partner and do not lose standing in society by doing so because her family is where her resources are located - and in these cultures, children trace their ancestry through the mother.

So, we have examples of social structures in humans that exist in two different primate species - but neither of them absolutely tells us about our social structures before technology.

Since we're the only primates in which estrus is not advertised, it's pretty obvious there's some female agency going on there. The elevation of hormones that inclines a female toward sex also influence secondary sexual features, like breasts. So it's very likely that female interest in sex (and the hormonal soup that would create this) is the reason for breasts that remain enlarged even when they are not lactating.

Other primates don't and thus can't explain that for humans. However, it's obvious that there's a benefit to human females to be interested in sex even when they are not ovulating or else it would not have lasted as long as it has. This is an investment of energy in the female's body to maintain hormonal cocktails that are also part of the ovulation and lactation cycles.

The only other example we have of females that are interested in sex when they are not in estrus are bonobos, and they use sex to create social cohesion and to ease conflicts, rather than whaling on one another all the time. They're not homo or hetero - they're literally pan sexual.

Female bonobos have power within their communities.

...just to say - your assumption about male display being for other males isn't deserved based upon comparisons with other social structures and to female anatomy.

http://link.springer.com/content/pdf/10.1007%2Fs10508-009-9567-6

The present form of the human penis, reached over several million years of evolution, was orchestrated by the vagina of the human female. The multifunctional vagina serves as a birth canal, a component of the structural support for the internal pelvic organs, and as a coital organ. In this last function, it has had an evolutionary effect in shaping male sexual anatomy. Hominid fossils so far discovered show that the female pelvis evolved to accommodate the increasing size of the brain of the human infant. Larger cranial capacity necessitated a larger birth canal and, as a result of sexual selection, an increase in the size of the penis. Compared to that of the other great apes, the human penis is considerably larger, and evidence suggests that its unique configuration may also be a result of vaginal influence.

It has been proposed that the shape of the glans with its distinctive corona facilitates the scooping out of previously deposited semen, enabling the subsequent deposition of other genetic material. This is not a likely occurrence. The healthy vagina has a low pH to protect it from invading organisms. This intense acidity will kill all sperm not promptly reaching the safety of the cervical mucus. I suggest the evolutionary function of the glans and its corona is not to scoop out semen but vaginal secretions, which are then absorbed by the encompassing foreskin. The mucous membrane of the foreskin adjacent to the glans is highly absorbent, as evidenced by the increased susceptibility of the uncircumcised male to HIV. It has been suggested that there may be a glans protecting feature of the foreskin, but it is entirely possible that its primary function is to absorb behavior modifying chemical messengers from the female. Numerous studies have shown the vaginal mucosa to be secretory. In the primate rear-approach coital position, the corona rubs against the anatomically posterior wall of the vagina, the most dependent surface when the woman is standing, and therefore the place more likely to collect vaginal secretions. The deepest end of the posterior vagina is the cul de sac, which commonly contains a small amount of fluid. An analytical study of the vaginal secretions of an ovulating female for neuropeptides, such as pitocin and vasopressin, known to enhance pair bonding and protective behavior in the male, could be revealing. Sexual intercourse may be a medium of communication between humans older than language itself.