Carole Hooven, Ph.D.

We're nursing.

We're producing the milk with our own bodies.

We have to grow the baby in a relatively energy-restricted environment.

The burden for female mammals

The energetic and time burden for female mammals is enormous to produce each offspring.

And if you don't have the right egg or the right sperm, you should care about where you're getting the sperm, then you've lost a huge chunk of your potential reproductive output.

Men don't lose a big chunk.

That just doesn't happen to them.

And this is the sex difference in parental investment that shapes, that's why eggs and sperm matter in terms of our bodies and our behaviors, because we have to do very different things and live in different ways to maximize our reproduction.

OK, I want to come back to what you said about mini puberty and the differences in hormones.

So I do think it's the differences in the increase in testosterone that males have that explain why they're more likely to have rough and tumble play, more energy.

It starts within a month after birth, but then peaks around three months and I think then goes down until something like six months.

And it appears that it has important effects on brain development and on lengthening the penis.

Yes.

There's a lower postnatal peak, but the mini-puberty in boys appears to also be associated with activity levels in the boys and even growth trajectories.

So that's interesting.

Yes, yes.

So in terms of the activity levels, it could be that that postnatal time, that the play in boys has something to do with differences in activity levels, differences in novelty seeking, different temperament, less fear.

also.

But if you think about it from an evolutionary point of view, in male mammals that have to compete for status and operate in a dominance hierarchy, there's a lot of male mammals have dominance hierarchies which tend to function to reduce aggression overall, because instead of duking it out every time there's a fertile female or a delicious piece of fruit in a tree, you just signal