Bribiescas: On men, aging and why sex kills us

DW: Early on in the book, "How Men Age," you almost quip: why do we even still have aging and death? Why haven't we achieved immortality - a thing high on the list of many technologists in Silicon Valley. Aging is part of life for men and women, though, and it's kind of healthy, isn't it?

Richard Bribiescas: Yes, it is. The assumption is that natural selection should select organisms that live forever. In fact the biologist [and Nobel Laureate] Peter Medawar posed that question. The short answer is that evolution doesn't select for longevity, it selects for reproduction. So as long as we have reproducing species, we're going to have aging and death.

But that's what confuses me: we're a sexually reproducing species. This sets us apart from unicellular organisms, like bacteria, which do have a form of immortality. Yet the more we reproduce, the more we - men and women - accelerate the process of aging and death. The time and energy spent on the "reproductive effort" is taxing for our bodies. So why would we do it?

Well, if you don't reproduce, even if you have genes or traits that lengthen your lifespan, those traits will not be passed on and they will die off.

When we talk about natural selection, we're talking about three basic conditions that Darwin laid out way-back-when. One is you need a trait that is variable - whether it is longevity or height or anything you can think of. The other thing is that trait has to be heritable, so it gets passed from one generation to the next. The third condition is there has to be differential reproductive success - there have to be organisms that are more successful at passing on those genes than others. But it's not so much for the good of the species, it's more in terms of your own fertility and survival.

A matter of time and energy

One way you can look at this is that all organisms, including humans, have to allocate two primary resources. One is time and the other is energy. And in many ways longevity is simply another resource, like energy, that we have to allocate in order to pass our genes to the next generation.

The book focuses on aging in men. You write about the effect older men have had on evolution, you write about crude strength versus brainpower… and in terms of the brain, I'm wondering whether there are any health risks to which men are more prone? Aside from things like prostate cancer, for instance, are men more prone to Alzheimer's and other cognitive degeneration?

Men and women die of the same things. It's just that men are more susceptible and more frail when they encounter these illnesses. When it comes to things like Alzheimer's and cognitive degeneration, men are less capable of recovering from those insults. The reasons are unclear, it just seems to be - whether it's cancer, cardiovascular disease, cognitive degeneration - that men are less resilient to those common forms of mortality compared to women.

Men die younger than women

You also make the point that men have a shorter lifespan, even though we suffer from similar things, whether it's prostate cancer or breast cancer. Why do men live shorter lives?

There are a number of hypotheses, and there are probably multiple things that contribute to the shorter lifespan, because you can flip this question and ask, "Do women have extended lifespans?" And again the answer is unclear.

We know that early on testosterone supports the reproductive effort. But it also compromises the male immune system. We know this from humans, and we know this from other organisms, that if you take testosterone out of the system, males become more resilient and can rebound faster from infections.

And then going back to the idea that time and energy are limited, human males burn more energy per day compared to women. Think about the cost of that. If you're spending that much energy to keep body and soul together and support this body mass, that's energy that doesn't go toward your immune system or tissue repair.

Men do 'stupid things'

And there is a fourth reason - and that is behavioral. Young males in particular - between the ages of 15 to 25 - there's a big spike in mortality, because, for whatever reason, there is greater risk tolerance. Males are doing stupid things.

That's what makes this so interesting. It's not just genes and biology - there are environmental factors too. It begs the question, why focus on men at a time when there is so much discussion about sexual identities in society, the changing roles of women - in jobs and as risk-takers - and all that, surely, will have an evolutionary effect.

Sure, absolutely. One thing I make clear in my book is that a lot of this is focused on heterosexual males, and the reason isn't because heterosexual males are special, or that we should have some unique focus on them. But the fact of the matter is, there simply isn't a very robust body of research on homosexual men, or men who have other sexual identities. And that's something that needs to be addressed.

There are also people who undergo sex reassignment… would that have an effect?

The fact that we now have the technology and the ability to do sex reassignment, and we have a growing community of transsexual individuals, is something that's really important to which we need to pay attention. However, on an evolutionary timescale it is a very recent phenomenon.

Even before we entered a world of sex reassignment and transsexual individuals, it was clear in human males that there was a spectrum of sexual identities. The problem is we don't have a robust body of research on those individuals and that's a shame, that needs to be rectified - and we're trying to do that. We recognize there is more than one way to be a male. A lot of that is particular to western, industrialized societies, and the issue of transexual males is a very recent phenomenon, but it definitely deserves more attention.

Richard Gutierrez Bribiescas is Professor of Anthropology and Ecology and Evolutionary Biology at Yale University. He is the author of "How Men Age - What Evolution Reveals about Male Health and Mortality," published by Princeton University Press (2016).