Does Size Matter? Evolution Says Yes

In the spirit of Father’s day, let’s talk about male genitalia. This topic may seem strange but male genitals allow us to observe evolution in a way we can relate to.

You may notice a pattern at FTDM; we talk about evolution a lot. Posts on carnivorous sponges, altruism (or not), weird sex, peanut allergies and picking the best plant for your home all mention evolution. Despite the importance of evolutionary principles for understanding biology, observing evolution is difficult. In particular, the time required makes it difficult; evolutionary processes often occur on time scales that are much longer than any typical human life. This fact is also why I love studying microbes. Their ability to grow quickly allows scientists to easily see ‘evolution-in-action’. Although microbes essentially impact all aspects of our daily lives, it is difficult for people to relate to microbes. Out of sight, out of mind if you will.

However, we can all relate to sex.

You have likely heard the phrase, ‘size does (or does not) matter’ in various contexts surrounding male genitals. From huge, gas-guzzling cars, supersized meal options, and ‘Americano’ coffee sizes, you can certainly say that some people prefer things big. However, is there any reason to believe that size does matter in the bedroom?

Evolution says yes.

First, a curious observation: male genital size varies a lot in primates; humans have the largest penises, in terms of length and width, while gorillas have the smallest. However, chimpanzees have us beat for the largest testicle size (they can have it as far as I’m concerned).

The first obvious question is why? Why is there so much variation in male genitalia traits amongst primates?

'Move along, nothing to see here.'    Photo Credit:  Susan E Adams     via  Compfight   cc

'Move along, nothing to see here.' Photo Credit: Susan E Adams via Compfight cc

Girl power and female mate choice

A study published last year in the journal of the Proceedings of the National Academy of Science (PNAS) sought to address this observation. In particular, the researchers asked whether or not female choice could influence the evolution of the male member.

To address this question, the researchers showed women 3D computer-generated models of males, and asked them to rate their attractiveness. The models shown varied in height, shoulder-to-hip ratio, etc. You know, traits people typically weight heavily when considering attractiveness. However, they also added flaccid penis size as a variable.

Why flaccid penises? The key here is that mating decisions by females happen before sex. During earlier stages of human evolution, before the modern use of clothing, the male member would be exposed for all to see, providing females with something else to base their mating decisions on.

As expected, the usual suspects influenced ratings. For example, it is generally accepted that taller men are typically found more attractive and the ratings reflected this pattern. Surprisingly, however, flaccid penis size also correlated with better ratings, as much as aspects like height!

If female choice gave humans larger flaccid penises, why do gorillas see the opposite trend?

In short, female choice is not solely responsible for genital size. Remember, height and body ratios were also very important traits in this study. In addition, social aspects, chemistry, and relationships between individuals can certainly influence attractiveness between humans.

Could social differences between primates explain the variation? It certainly seems like it could.

Another factor that is popular in the field of genital evolution is sperm competition. Although males compete with one another for a woman’s favor, competition can also take place postcopulation, or after the sex has occurred. This kind of competition can proceed two ways: size or numbers. A larger penis has been suggested to actually displace the sperm of another male that has already mated with the same female through physical forces during sex. At the same time, producing more sperm to achieve greater numbers than competitors is also possible. Ultimately, society determines which factors are the most important.

Enough said? Photo Credit:  Meredith Jacobs  via  Compfight   cc

Enough said? Photo Credit: Meredith Jacobs via Compfight cc

Chimpanzee societies tend to be dominated by warring males where dominant males monopolize mating with certain females. To overcome this obstacle, sperm competition favors larger sperm counts to make every opportunity count. Hence, chimpanzees have the largest testicles of the primates. In contrast, silverback gorillas encounter zero sperm competition. Since gorilla society revolves around a single, sexually active male, there was no need to evolve larger penises or testicles negating the influence of both sperm competition AND female choice.

Humans? We likely exhibit a bit of both strategies.

Men with larger testicles tend to be less nurturing fathers

Penis size does not get all the attention. Another study published in PNAS this past March suggests that testicular volume inversely correlated with brain activity in men associated with good parenting behavior.

To examine brain activity, researchers examined how the brain of several fathers reacted to photos of their children. Those fathers with smaller testicles tended to show more brain activity in response. Check out the Discovery News video below for more details:

Is this ridiculous? Possibly.

First off, correlation does not necessarily imply causation. Check out some humorous examples of this here. For example, divorce rates in Maine seem to inversely correlate with the consumption of margarine. Surely, there is no logical reason why the use imitation butter would make for happier marriages. So, don't judge a father by his size!

Large testicles? Too hard to say with any certainty. I am sure we just caught him during a quick phone check. Photo Credit:  Mark Nye,  via  Compfight   cc

Large testicles? Too hard to say with any certainty. I am sure we just caught him during a quick phone check. Photo Credit: Mark Nye, via Compfight cc

Despite the problems, I think the study does properly demonstrate how society could influence the evolution of humans. The paper suggests variation in testicle size may represent two evolutionary strategies: mating more and parenting less, versus mating less and parenting more. Both seem viable; producing more children increases the likelihood you have children that are fit and successful. However, nurturing a child and making sure they succeed could also work.

The mothers of the world certainly deserve serious credit for the amazing things their reproductive system is capable of, but let's not forget about our fathers. This Father's day I hope we can all appreciate how our human sexual prowess got to where it is today while we acknowledge the important Father figures in our lives this coming week.