Can a mans testicles betray his cheating habits?

Many people wonder if humans are monogamous by nature or by nurture. Do we choose one partner because society (and often religion) tell us to, or is it in our DNA?

Most people will say that it’s a societal issue. To a point, they are probably right. However, there appears to be evidence that humans are a lot more monogamous by nature than would be anticipated. In the same way monogamy can be nurtured by societal pressures, men can also be deceived by society into thinking multiple female partners make them more of a “man” or more successful.

Untangling biology from society is extremely difficult, and often the answer is a combination of the two. So what do testicles have to do with anything?

Looking to our cousins
Mistakenly, people often point to chimpanzee behaviours as an explanation of our own. The logic being that we evolved from chimpanzees and therefore they represent a more ancestral human population. This logic is flawed on the basis that humans did not evolve from chimpanzees, but rather, both humans and chimpanzees evolved from a common ancestor. From then on, we have both been on our own evolutionary path.

For example, it is likely that a human/chimpanzee ancestor neither walked on two legs nor “knuckle-walked”in the way chimpanzees do, but probably somewhere in the middle . Our ancestor is thought to have had a generalised arboreal behaviour (i.e. was good at climbing and living in trees) and chimpanzees evolved to walk on the ground like they do, while we evolved bipedalism (walking on two legs). Gorillas too, would have evolved to “knuckle-walk”.

Testicles
Ok, the reason anyone is reading this in the first place is probably to find out how to check their boyfriends/husbands testicles and see if they are cheating. Unfortunately for many, that’s now how it works. Instead, it appears that a lot can be told about the history of human sexual behaviour as a whole, by looking at our testicles, as a species.

Sexual strategies
A key characteristic that humans lack compared to chimpanzees and other promiscuous primate species is sperm competition. The theory is that if many males are mating with each female when she enters her ovulation period, then natural selection will favour those with higher quality sperm, leading to a competition of sperm between males. And it is said that testicular size may have something to do with this.

Indeed gorillas (160 kg) and orangutans (50–100 kg) are the heaviest of the primates but have a breeding system where one male mates with a number of females. Sperm competition therefore is absent, and as expected, their testicles weigh around 30 g and 35 g respectively. Compare this with chimpanzees (30–60 kg), where males fight with each other to mate with as many females as possible and whose testicles weigh 120 g. The idea is that larger testicles produce more sperm, and therefore chimpanzees have to have enough sperm to out-compete the sperm of other males who have mated with the same female. Indeed, a correlation between breeding systems in primates and testicular size has been shown.

But what if gorilla sperm is just way more efficient than chimpanzee sperm? Well, this question too was answered and it was shown that not only is testes size related to the rates of sperm production (i.e. bigger testicles produce more sperm), but that primates that mate with multiple females also have higher quality sperm that is more motile and more numerous. Testicle size is also associated with sperm reserve: basically the bigger the testicles, the more sperm reserve can be held for “lucky” periods or breeding seasons.

Slightly more graphic, but equally as important, is the ability of the sperm of various species to coagulate (harden) and form a vaginal plug which blocks penetration of competing sperm. Macaques, chimpanzees and other sperm-competing primates show this quality, while thankfully, humans do not.

Where do humans fit on this scale?
Humans have bigger testicles than gorillas, but smaller than chimpanzees. This would suggest that in our evolutionary past, we are somewhere between monogamous and polygamous. Human sperm quality and reserves however are among the lowest of the primates. And of course, humans don’t have sperm that coagulates to form a plug.

Obviously an entire sexual strategy can’t be assumed based on one biological structure. There is other evidence for a largely monogamous lifestyle of early humans, such as a lack of large canine teeth in humans (which are indicators of aggression) and surprisingly, the fact that humans walk on two legs. The logic here is that walking on two legs allows food transport over long distances. Additionally, it is likely that in an early hominid ecological context, females adopted the strategy of exchanging copulation for food (as seen in chimpanzees and bonobos for example). This would create a situation where a pair-bonded male and female worked together, with the female and their offspring enjoying the benefits of reduced predation risk (by staying in a safe place) while the male gathered food and brought it back. This and other reasons (such as concealed ovulation where a female hides her ovulation period thereby assuring the male she is not mating with other males and her offspring are his, thereby making in his interest to aid in childbearing) are indicators that early humans were largely monogamous. These topics are quite complex and beyond the scope of testicular size, so I will save writing about those topics for my next blog entry and will link it here.

So what do a mans testicles say about his cheating habits?
On an individual basis, not a lot. The research on testicular size does not mean that a man with larger testicles is more likely to cheat, while a man with smaller ones is more faithful. There are too many variables when it comes down to individual testicular size. However, taken in context, it does seem to imply that early humans adopted a more monogamous lifestyle. Despite this, the fact that our testicles and sperm quality are not as low as say, gorillas, indicate a certain level of infidelity in our past. There it appears that humans are quite sexually liberal naturally. In the past, various societal pressures have forced humans to form single monogamous lifestyles, which often haven’t worked out. People have responded by saying we’re not programmed to live like that. It seems the answer is somewhere in the middle. Overall though, it is wrong to say humans are programmed to spread their seed in the way chimpanzees do for example, because although they are our closest cousins genetically, our behaviours have changed dramatically since we split from the last common ancestor (thankfully).