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On testicle size as an indicator of polygamy (note that this thread is using monogamy to mean any single male system, not strictly mongamy):
The first problem is that it fails to address the fact that humans females have none of the signs of fertility found in gorillas or chimpanzees, and that humans have sex more frequently than either species as result, and far more frequently during infertile periods. That alone could account for the difference.
The next problem is that humans are the only bipedal ape. As a result human females tend to ‘leak’ far more than other apes, resulting in greater sperm loss. Once again, that could account for the difference by itself.
The next issue is that gorillas are not clearly more monogamous than humans. Female gorillas routinely cheat and will almost always try to mate with the next-to-senior bull if they have the opportunity to do so undetected. The observed infidelity rate for gorillas is far higher than anything even suggested for any human population. Much less observed. So gorillas should have much larger testicles than humans, yet they don’t.
The next issue is that humans and are carnivorous and live on high energy, low volume diets. Gorillas are almost exclusively vegetarian and live on low energy, high volume diets. Semen contains a lot of protein, it’s not a cheap resource to produce energy wise. Just based on dietary limitation of resource production we would predict that gorillas would produce less sperm than humans or chimps.
Linked to that is the issue of temperature control. High temperature interferes with sperm production requiring larger testes. Humans and chimps are very active creatures, whereas gorillas are fairly sedentary browsers, as a result gorilla testicles shave less heat build up to worry about. That could also account for a discrepancy between the two groups.
Then we come to the problem that humans have abnormally large penises. The size of the penis and testicles seems to be genetically linked to some degree. Human testicles may be large simply because we have large penises.
Next we have to deal with totally different social structures. Gorilla form strong harems ,with one male ‘exclusively’ servicing numerous females. Any male may only be dominant for a short period of his life, or he ,may never become dominant and never have a chance to mate, yet all males have to be ready to mate immediately if the opportunity presents itself. That produces a huge constraint on gorilla testicles because they can’t afford to permanently carry around energy expensive organs that only get used for a few months of their life, but they can’t afford to have small organs because if they do rise to dominance they may be need them for several years. The solution is to come up with a mid-range organ that works reasonably when required, but doesn’t chew vast amounts of energy when not in use. Chimps in contrast have the opportunity to mate continuously, and as a result can afford to gamble on large organs that constantly use energy because they are in constant use. Humans fall into a mid-range situation where human males may not have an opportunity to mate for many years, particularly while young, but due to air bonding may mate regularly once a mate has been found. We adopt a strategy somewhere between a gorilla and a chimpanzee in producing organs that are moderately expensive to maintain but work quite well. IOW we could just as easily explain the difference because human were intended to be exclusively monogamous with no pre-marital sex.
And finally bonobos and chimps are even more closely linked than humans and chimps. Bonobos are even more promiscuous than chimps, yet they have slightly smaller testicles.
Basically the whole hypothesis compares apples and oranges. By including gorillas which are so different from humans it’s throwing carrots in there as well. It’s a hypothesis that doesn’t have an ounce of evidential support beyond the initial observations that led to its construction. I have never even seen one of the proponents of the theory attempt to back it up by looking at the sperm counts of these animals, rather than It contains numerous assumptions, invalid comparisons and logical flaws.
Tyring to use this ‘hypothesis’ to support the idea that humans aren’t monogamous just doesn’t work.
On size dimorphism as an indicator of polygamy:
The problem with this idea is that humans are a strongly sexually dimorphic species. We don’t just differ in body size we also differ in body shape, secondary sexual characteristics, behaviour/psychology, food sources, interaction with young and so on. IOW a human male isn’t just a scaled up version of the human female, he’s a different creature. That makes it very difficult to draw any inferences regarding the meaning of size dimorphism.
To give you some idea what I mean I will show you four birds. The first is the mute swan, in this species the male and female the same size. The next is the Huia where the male is 10% larger than the female. The next is the hornbill where the male is 15% larger than the female. The final is the red jungle fowl, in this species the male is 20% bigger than the female. Which of these species are monogamous and which are polygamous? Based on the information provided we might guess that the mute swan is almost entirely monogamous while the rest of the species become increasingly less monogamous as we progress down the list. So male hornbills should frequently fight with other males for mating rights, female huias should at least occasionally cheat and jungle fowl should be a harem society.
Of course this is mostly bollocks. In reality the mute swan is approximately monogamous, but they are fairly prone to cheating. Male hornbills aren’t at all aggressive and the species is even more monogamous than the swan. Huias are the most monogamous of all the species and the males and females live together constantly so the opportunity virtually never arises for cheating. The only species we got right was the jungle fowl, which does indeed have a single male harem society. So what went wrong? Simple, we didn’t factor in ecological and reproductive dimorphism in our equation.
Male and female swans are physically identical because they live identical lives even to the extent of incubating the eggs and caring for the young. Whatever evolutionary pressures dictate ideal male size also dictate female size.
Yes male hornbills are significantly larger, but not because they fight off rival males. They are larger because they have a reproductive strategy quite different to most birds. The male seals the nesting female up in a hollow branch using mud until the young have fledged. During that whole time the male is exclusively responsible for feeding himself, his mate and the young. He needs to be bigger because he has more demands placed on him to find food.
Huias are even more extremely dimorphic. They have completely different bills. The male bill is short and robust and good for breaking up wood. The female bill is long and thin and good for probing. Huias survived because the male broke up rotten logs and branches and the female then winkled the grubs out of them. The male needed to be larger not to fight off other males but because he had to perform strength related tasks to get food. The female performed primarily dexterity related tasks that required less physical size and strength. The males and females paired up for life and were never seen individually.
So what does all that tell us about humans? Well humans are a species where the evolution of bipedal gait has forced the production of a helpless infant that requires a lot of care and a debilitating birth process for the female. That leaves a time period where the male is almost the exclusive provider for the pair. That is directly analogous to the situation with hornbills, and just as monogamous male hornbills are forced to become larger to take care of a ‘helpless’ mate the same may well be true of humans. Similarly humans hunt high risk prey animals, and frequently suffer serious injury as a result. Moroever bipedal gait and foetal development means that for several months females can’t engage in such hunts effectively or without risking their own lives and the lives of their offspring. Just as huias developed size dimorphism because males and females had evolved to seek different food types this could be the case for humans.
Note that none of this is evidence for monogamy in humans. After all maybe males did get bigger to fight off rivals. But it is just as plausible that males got bigger to provide for helpless mates and young and because the females couldn’t effectively hunt large game. IOW size dimorphism could be equally used as an argument for exclusive monogamy or against it.