Most animals die once they can no longer have kids, but men and women tend to totally buck this trend, living decades beyond their reproductive years despite drastic changes in their bodies.
A book out in September, How Men Age: What Evolution Reveals About Male Health And Mortality, explores the toll that children take on their parents' bodies, how love handles and paunches can turn "evolutionary lemons into lemonade," and how men may be responsible for humans' relatively long lives.
Here's our conversation with author Richard Gutierrez Bribiescas, a biological anthropologist at Yale University, edited for length and clarity.
Let's start with a simple question. Why does hair go gray?
The cells that actually give your hair color are constantly bombarded by free radicals. And eventually the cells that make the pigment melanin, which colors your hair, go away and your hair turns the neutral color that you would see without these pigments. You see it in other animals, too. Dogs start getting a white muzzle.
On to bigger things. What are some weird things about humans when it comes to aging? Is human aging unique?
Human aging is primarily unique because it's not correlated with the end of reproduction. If you're a female, about a third of your life span is postmenopausal. That's huge and very, very strange.
We also provide care to the elderly. Older individuals who probably would not be able to survive on their own are able to survive well into their 70s and 80s with the help of relatives and friends. That's another thing that's very unique about humans.
Why do we age in the first place? Why don't we just live forever?
Aging and the evolution of sexual reproduction seem to go hand and in hand. An organism that has to reproduce requires energy and resources that would otherwise go to keeping it alive, like repairing or replacing damaged cells.
That reminds me of your quote that "natural selection does not care about your health or if you feel good." What does natural selection care about, if we're going to personify it?
Natural selection favors traits that allow us to reproduce more efficiently. This is the reason why we end up with organisms like mice, which live a couple of years if they're lucky, compared with elephants that can live 70 years.
Longevity is just one trait that will be selected for, but only if it serves to improve reproduction. And if it makes us feel good along the way, that's great. But that's not the goal of natural selection.
Reproduction comes with a huge cost. What's the toll that children can take on females?
It's known pretty well that the more females invest in reproduction, the more their lifespans are compromised. One of my colleagues in Poland, Grazyna Jasienska, did a demographic study on rural women and, looking at church records, showed that their lifespans were shortened by about 18 months for every child that they had. There's a pretty significant inverse relationship between number of children and lifespan, and this has been shown in different populations and other organisms.
My lab's hypothesis has to do with oxidative stress. Every time you take a breath, you support oxidative metabolism, but you also generate toxins in the form of free radicals that cause you to age faster. They cause cellular damage and genetic damage, and it's the reason why you've heard about antioxidants. That's why you eat salmon and blueberries and things like that.
We hypothesized that the more children females have, the more bouts of oxidative stress they have — because every time a woman undergoes a pregnancy, the amount of oxygen that she uses goes up dramatically to support the fetus. So, we looked at biomarkers of oxidative stress in the women in rural Poland. And sure enough, the ones who had more children had higher levels for these biomarkers of oxidative stress.
How about for males?
The reproductive costs of males are a little bit different.
What's interesting is that a lot of the costs are associated with behavioral risks. For example, we know that between the ages of about 15 and 25, in pretty much every population including in nonhuman primates, there's a big increase in male mortality, and usually it's due to risky behavior. You end up seeing males do stupid things to procure mates. That's why it's very expensive to insure an 18-year-old if he has a car.
Risky behavior is one cost. The other cost is energetic.
Like big muscles?
Yeah, absolutely. Skeletal muscle is expensive. A calorie that is burned by skeletal muscle is one calorie that can't be used to repair a damaged kidney cell. So, having big muscles may be great for your social life, but it's not very useful if you have an infection.
Later in life, those muscles go away. If I'm a middle-aged man with love handles and a paunch, are those signs that I've failed?
I call it turning evolutionary lemons into lemonade, because as men get older, it's harder to keep muscle on, their testosterone goes down, and they may develop a little paunch. But it turns out that those traits — say, a little extra fat and lower testosterone — may actually help them reproduce later in life.
It was assumed that when females underwent menopause and stopped reproducing that men did the same. But there was a paper that came out a few years ago by this brilliant biologist at Stanford named Shripad Tuljapurkar. He showed that when you looked at non-Western populations, men sometimes have kids well into their 80s.
It struck me, because it suggested that perhaps the evolution of menopause or longevity might not totally lie with females, but actually with males. If males are reproducing at older ages, they're passing on those longevity genes not only to their sons but also to their daughters.
So, is it a failure or success? I think the ability to actually leverage this paunch in a way that makes you still able to produce children and to increase your fitness — I think you could put that in the win box.
One of your colleagues summarized your early research as "macho makes you sick." Why do men have shorter life spans than women?
Well, it just seems that the physiology of males make them more frail as they get older. And it seems to be universal.
Males and females tend to die of the same things once they get past older ages, like cancer and cardiovascular disease. But it turns out that males simply don't recover as well as females do.
For example, with infections, testosterone suppresses immune function. It's kind of ironic that the hormones and things that make men physically stronger are the same things that actually make males the more frail sex.
Males are the more frail sex. Is that a controversial statement?
No, I think people confuse strength and virility. The cost associated with the way males reproduce results in shorter life spans.
I think a lot of people assume that research going on in North America or Europe is true for all humans, but you say that's not necessarily so. What are some assumptions about aging that non-Western populations have overturned?
You turn on the radio and you hear these commercials for testosterone supplements. It's assumed that as you get older, your testosterone is going to decline and somehow that's bad and you have to fix that. But there's a lot of fuzziness around that assumption. For example, within the United States there's a huge range of variation in testosterone levels at younger and at older ages. They vary by tenfold and we don't know why.
So, what could be "low" or "high" really depends on the individual, and age only accounts for about 15 percent of the variation.
There is sort of a very faint signal to show that testosterone does decline with age in Western males. But it's not universal. If you look at hunter-gatherer groups, their testosterone levels don't change very much throughout their lifetimes. They start out with lower levels as adults and they pretty much maintain that their entire lives. So, the assumption that testosterone is going to change as you age is not necessarily true everywhere.
The other study we did was in Japan, where we looked at testosterone levels. And what we found was really interesting. These were urbanized men living in a suburb of Tokyo. And just like men in the U.S., their testosterone levels were highest when they were in their mid-20s and then they started to drop off. But after about age 40, their testosterone levels stayed constant.
We're trying to figure out why there's this difference compared with what you see in the United States.
So it's very interesting to understand where culture and biology are going to affect each other. And it's going to be reflected in things we may assume to be universal in terms of health. But we have to be cautious about that, because sometimes that's not the case.