Guest Post: Why did humans evolve big penises but small testicles?

by Mark Maslin, UCL

Humans have a much longer and wider penis than the other great apes. Even the largest of gorillas, more than twice as heavy as a human, will have a penis just two and half inches long when erect.

However our testicles are rather small. A chimpanzee’s testes weigh more than a third of its brain while ours weigh in at less than 3%. The relative size of our penis and testes is all down to our mating strategies, and can provide some surprising insights into early human culture.

Primates exhibit all sorts of mating behaviour, including monogamous, polygynous – where males have multiple mates – and multimale-multifemale. One indicator of which behaviour occurs in a species is the size difference between males and females. The greater this sexual dimorphism, the more likely the mating is either polygynous or multi-male to multi-female. This can be shown by observing chimpanzees and gorillas, our closest living relatives.

Male chimpanzees are much larger than females, and they have a multi-male to multi-female mating system. Essentially, male chimps have sex all the time with any female and with any excuse. A female therefore may contain sperm from multiple partners at any one time, which puts the sperm itself – and not just the animals that produce it – into direct competition. For this reason, chimpanzees have evolved huge testicles in order to produce massive amounts of sperm, multiple times a day.

Male gorilla are also much larger than females, but they have a polygynous or harem-style mating system where many females live with a single male. With little or no competition actually inside the uterus, gorillas have had no need for a testicular arms race to facilitate the production of more and more sperm. Their testes, therefore, are relatively small. This is similar to modern humans, whose testes are also of very modest size and produce a relatively small amount of sperm. In fact human sperm count reduces by more than 80% if men ejaculate more than about two times a day.

Chimps have huge testicles for their size.
Steffen Foerster / shutterstock

The human penis is large when compared with those of our closest relatives: chimpanzees, gorillas, and orangutans. However, primatologist Alan Dixson in his wonderfully detailed book, Primate Sexuality, suggests that if we look at all primates, including monkeys, this is just wishful thinking.

Comparative measurements show the human penis is not exceptionally long. The Hamadryas baboon, for instance, a native of the Horn of Africa, has an erect penis that is five and half inches long – slightly shorter than an average human male, but they weigh only a third of our weight.

Some of the complex penises found in multi-male to multi-female mating primates such as chimpanzees (h), brown lemurs (a) or macaques (d, e, f).
Alan F. Dixson, Primate Sexuality

The human penis is in fact extremely dull – it does not have lumps, ridges, flanges, kinks or any other exciting feature that other primates have. In primates, this lack of penis complexity is usually found in monogamous species.

Monogamy mystery

This observation clashes with the fact that men are significantly larger than women. This suggests our evolutionary background involved a significant degree of polygynous, rather than exclusively monogamous, mating. This is supported by anthropological data showing that most modern human populations engage in polygynous marriage. Anthropologists Clellan Ford and Frank Beach in their book Patterns of Sexual Behaviour suggested that 84% of the 185 human cultures they had data on engaged in polygyny.

Primates with simpler penises tend to be monogamous like cotton top tamarins (a) or polygynous like gorillas (g).
Alan F. Dixson, Primate Sexuality

However, even in these societies most people remain monogamous. Polygynous marriages are usually a privilege reserved only for high status or wealthy men. It is worth noting that hunter-gathers around the world practice only monogamy or serial-monogamy which suggests that our ancestors may have used this mating system.

At first sight, however, it would seem sensible for males to reproduce with as many females as possible. Human monogamy has long puzzled anthropologists, and lots of effort has gone in to working out what keeps males hanging around.

Three main theories have been put forward. First is the need for long-term parental care and teaching, as our children take a long time to mature. Second, males need to guard their female from other males. Third, our children are vulnerable for a long time and infanticide could be a risk from other males. So to ensure that children are able to reach maturity the male is likely to stay to protect them, both socially and physically. This may be why males have maintained their larger relative size.

Hamadryas baboons have unusually long penises.
المُصوّر: مُعتز توفيق إغباريّة, CC BY-SA

If we view the evolution of monogamy mating systems in humans through the lens of human society it is clear that it takes a huge amount of social effort to maintain and protect more than one mate at a time. It is only when males have access to additional resources and power that they can protect multiple females, usually by ensuring other males protect them. So monogamy seems to be an adaptation to protect one’s mate and children from other males. This monogamy is reinforced by the high social cost and stress of attempting to do this for multiple partners, and it has become supported by cultural norms.

So when living in complex human societies the largest and most important sexual organ is the brain. Somewhere in our evolutionary past how smart and social we are became the major control on our access to sexual partners – not how big or fancy a male’s penis is.



Mark Maslin, Professor of Palaeoclimatology, UCL

This article was originally published on The Conversation. Read the original article.

Guest Post: The psychology of New Year’s resolutions

by Mark Griffiths, Nottingham Trent University

Research has shown that about half of all adults make New Year’s resolutions. However, fewer than 10% manage to keep them for more than a few months.

As a professor of behavioural addiction I know how easy people can fall into bad habits and why on trying to give up those habits it is easy to relapse. Resolutions usually come in the form of lifestyle changes and changing behaviour that has become routine and habitual (even if they are not problematic) can be hard to do.

Happy New Year, 2017!

The most common resolutions are: losing weight, doing more exercise, quitting smoking and saving money.

The main reason that people don’t stick to their resolutions is that they set too many or they’re unrealistic to achieve. They may also be victims of “false hope syndrome”. False hope syndrome is characterised by a person’s unrealistic expectations about the likely speed, amount, ease and consequences of changing their behaviour.

For some people, it takes something radical for them to change their ways. It took a medical diagnosis to make me give up alcohol and caffeine and it took pregnancy for my partner to give up smoking.

To change your day-to-day behaviour you also have to change your thinking. But there are tried and tested ways that can help people stick to their resolutions – here are my personal favourites:

Be realistic. You need to begin by making resolutions that you can keep and that are practical. If you want to reduce your alcohol intake because you tend to drink alcohol every day, don’t immediately go teetotal. Try to cut out alcohol every other day or have a drink once every three days. Also, breaking up the longer-term goal into more manageable short-term goals can be beneficial and more rewarding. The same principle can be applied to exercise or eating more healthily.

Do one thing at a time. One of the easiest routes to failure is to have too many resolutions. If you want to be fitter and healthier, do just one thing at a time. Give up drinking. Give up smoking. Join a gym. Eat more healthily. But don’t do them all at once, just choose one and do your best to stick to it. Once you have got one thing under your control, you can begin a second resolution.

Be SMART. Anyone working in a job that includes setting goals will know that goals should be SMART, that is, specific, measurable, achievable, realistic and time-bound. Resolutions shouldn’t be any different. Cutting down alcohol drinking is an admirable goal, but it’s not SMART. Drinking no more than two units of alcohol every other day for one month is a SMART resolution. Connecting the resolution to a specific goal can also be motivating, for example, dropping a dress size or losing two inches off your waistline in time for the next summer holiday.

Tell someone your resolution. Letting family and friends know that you have a New Year’s resolution that you really want to keep will act as both a safety barrier and a face-saver. If you really want to cut down smoking or drinking, real friends won’t put temptation in your way and can help monitor your behaviour. Don’t be afraid to ask for help and support from those around you.

Change your behaviour with others. Trying to change habits on your own can be difficult. For instance, if you and your partner both smoke, drink and eat unhealthily, it is really hard for one partner to change their behaviour if the other is still engaged in the same old bad habits. By having the same resolution, such as going on a diet, the chances of success will improve.

Don’t limit yourself

Changing your behaviour, or some aspect of it, doesn’t have to be restricted to the start of the New Year. It can be anytime.

Accept lapses as part of the process. It’s inevitable that when trying to give up something (alcohol, cigarettes, junk food) that there will be lapses. You shouldn’t feel guilty about giving in to your cravings but accept that it is part of the learning process. Bad habits can take years to become ingrained and there are no quick fixes in making major lifestyle changes. These may be clichés but we learn by our mistakes and every day is a new day – and you can start each day afresh.

If you think this all sounds like too much hard work and that it’s not worth making resolutions to begin with, bear in mind that people who make New Year’s resolutions are ten times more likely to achieve their goals than those who don’t.



Mark Griffiths, Director of the International Gaming Research Unit and Professor of Behavioural Addiction, Nottingham Trent University

This article was originally published on The Conversation. Read the original article.