Kid Question: Why are Daddies Bigger Than Mummies?

by Deb on September 28, 2010

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In general, human men are bigger than women.  They tend to have stronger muscles for the same size and different patterns of body hair.  There are different levels of answers for this.

Growth Patterns

The simplest answer is that boys and girls have different growth patterns.  If you have a look at the WHO growth charts for boys and girls there are a couple of things that stand out.

  • At around 5 years boys and girls have similar height ranges.
  • They stay similar until around 12 years.
  • After 12 years boys start growing noticeably quicker – the teenage growth spurt.
  • Girls stop growing somewhere after 16 years, but boys keep growing until at least 18.

Put those last two together and boys are growing faster than girls for longer, which lets them get bigger.  They also have more testosterone, which is what drives the muscle and hair growth.

Reasons

Differences between males and females are common in the animal Kingdom and even in plants, it’s called sexual dimorphism (literally ‘two forms’).  We are used to seeing mammals and birds which can have dramatic differences, compared with them humans are very similar.  There are two main reasons for these differences – protection and girls.

In some species, like gorillas and lions, the males have the job of protecting the group.  So it makes sense for them to be bigger and stronger so they can fight better.  This is also the reason that many male primates have very large teeth.  We don’t because we started using tools and weapons rather than teeth and claws.

As far as girls are concerned there are two problemsPeacock courting a peahen – getting them and keeping them.  Most of the showier differences are about getting girls, something called sexual selection.  In a very simplified explanation, when some animals pair up females are the ones who choose their mates and they have different taste.  So the males develop to show off, hoping that the females will choose them.  For many birds it is showy feathers, songs, courting dances or beautiful nests that determine whether they get the girl.

Ibex maleMany mammals that live in groups have a different social structure, and it is the dominant male who has the most access to females.  In this case they aren’t selected to show off, but rather to intimidate other males.  They will fight if they have to, but the idea is to look so impressive that you scare the lesser males so they don’t want to challenge you.  This is where many things like horns, tusks and manes come from, they make the males who have them look bigger and scarier.

Interestingly, primates that live in groups show this sort of difference, for example Silverback gorillas.  Primates that live in pairs tend to be similar sizes, like gibbons or tamarins.  Human size differences are a little more than tamarins, but nowhere near other Great Apes.  We seem to have started out like them but at some point in our history we started to pair bond, otherwise known as falling in love, so the dominant males didn’t need to impress each other anymore.  (Well, sort of)

Other Animals

Ironically, mammals and birds are backwards to most other organisms.  We’re just biased because they are the ones we see the most, but in insects and sea creatures (and even some plants) females are almost always bigger than males.  This has a very practical reason – females produce the eggs for the next generation.  Eggs are much bigger and more energy intensive than sperm, so females need to be able to get more energy to put into producing and protecting them.

Ultimately, Daddies are bigger than Mummies because we are animals and work on the same rules as the rest of the animal world.

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{ 6 comments… read them below or add one }

amandab September 28, 2010 at 9:24 am

We have loved watching the “Life” series on ABC1 and have had many discussions about sexual selection which has been very interesting. The “dances” of various birds, and the colour and pattern changing of octopus was very interesting to Princess.

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Slava September 29, 2010 at 12:46 am

That’s an interesting read, but something is unsettling – I remember vividly that around 5th grade girls are much higher than boys. That is not confirmed by WHO charts. (The weird thing actually the tallest girls by that time were the smallest at the end of school.)
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Deb August 27, 2011 at 3:21 pm

I wonder if it’s because the WHO charts are international. I know the baby charts are taken from children all over the world and include very short and very tall populations, maybe it averages out?

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Melbourne Mumma October 4, 2010 at 6:59 pm

I was one of the tallest in year 5! lol I guess, like all species, there are exceptions to the general rule. I like your ”Kid Question” idea, my 4yo is right into questioning everything and loves Scienceworks, so I’ll be back soon to read some more!
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suburp October 6, 2010 at 8:43 pm

while this is probably very valuable factual information (is it a series, i’ll follow that), i kind of dread the questions about size from my son. for the moment he just wants to be ‘big and strong’ like my husband. at some point, he will realize that he’ll never be as TALL as him, because they do not share the same genes.
this mommy is 5’3, the man my son will grow up to call dad…6’6.
oh well.
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Deb October 6, 2010 at 9:03 pm

While it sounds unlikely for you, it happened in my family. My sister is 5’2″, I’m 5’4″ and our little baby brother is 6’5″! We used to regularly be asked if we were adopted or if our parents were married 🙂

There are quite a few posts in the Kid Questions category, hover over the General tab up the top and it’s in a drop down menu. I love it when readers send their kids’ questions in, they’re really fun posts to write.

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