For over 40 years, biological anthropologist Robert Martin has studied sex. Martin, the A. Watson Armour III Curator at the Field Museum in Chicago -- one of the world's leading natural history museums -- is fascinated by the evolution of mating. He began by studying mating patterns in primates, completing his PhD on the mating behavior on tree shrews (work which led biologists to remove the animals from the primate category) and has more recently focused on the pregnancy, birthing and nursing lessons women today can learn from our ancestors.
Martin's new book, How We Do It: The Evolution and Future of Human Reproduction, touches on a whole range of issues related to sex and mothering in humans. He explores the context--and consequences--behind everything from declining sperm counts to birth control to IV-fertilization. I spoke with Martin about why he sees the rising rate of cesareans as a cause for concern, the Catholic Church's support of the rhythm birth control method as potentially harmful, and allowing nursing women flexible schedules should be a priority for employers.
There's no hard evidence that humans are adapted for either a monogamous or polygamous system. But what's the most common scenario?
About 85 percent of human societies around the world are polygamous. You have a system of one man with two or more wives. A minority, about ten percent, are monogamous. The predominant condition is polygamy. But what happens in practice is that although a society may allow polygamy, a man must have enough resources to have several wives. In a lot of societies that are polygamous, most men are, in fact, monogamous because they can only afford one wife.
How does social structure affect mothering?
The social system doesn't have too much to do with mothering. Primates all share intensive mothering but you can find all kinds of different social systems. You get pair-living primates, monogamous primates, harem-living (one male with several females) and you get multi-male groups.
What makes humans different from other primates?
In terms of mating systems, I don't see where we're truly unique. One thing is you don't have restriction of mating to a two or three-day period which is typical of mammals generally. But monkeys and apes often mate throughout the cycle. I don't see anything unique about us compared to monkeys and apes.
But human babies, compared with other mammals, are particularly helpless. Is there an evolutionary reason for this?
It's because of our big brains. In most primates, a baby develops in a mother's body until it has half of its brain size. A lot of brain growth occurs in the womb. At birth, human brains are a quarter of adult size. We can't give birth to babies any later because the brain will just fit through the pelvis as it is. It takes about a year for us to get to the position where monkeys and apes are at birth. That makes human babies particularly dependent on care for their first year of life, which has implications for social organization. Special social support is necessary.
So a monkey emerges from the womb in a higher-functioning state?
Do you see specific ways humans should raise children, given the helplessness of infants?
A baby can't move around independently like a rhesus monkey or a chimpanzee. Other primates carry the baby around on her fur, but it can move away and explore. Our babies really can't do that. For the first year of life they require special attention. Social support is needed, whether it's provided by men, women, or other members of the group.
Can you talk about the decline of sperm counts?
The evidence is overwhelming: there is a serious problem with declining sperm counts in industrialized countries. The average human ejaculation is 250 million sperm. Why do we need so many? You only need one for the job. The answer is that if you have below 60 million in an ejaculate, you have declining fertility. This is happening to more and more men. We don't have good data from Africa or South America or areas where people are living in non-industrialized conditions, and it would be interesting to see if this is happening there.
Why is it happening in industrialized countries?
Many factors in how we live now are contributing, but it's definitely environmental. There are chemicals in the environment that behave like steroid hormones. BPA is now used extensively. If you get a plastic container, there's probably BPA in it. It hardens plastic and is used for making parts in vehicles and all kinds of things. It also lines food containers--tin cans have it. I recently thought that water pipes might be lined with this: and they do. The water we drink every day is carried through pipes that may contain BPA. Every single person in the United States has BPA in their blood. It's particularly high in babies, especially premature babies. There are some factories in China that have very few controls and have BPA levels that are 50 times higher than they are in the US. One study shows that sperm counts of people working in factories decline as the amount of BPA increases. That's a pretty direct indication.
What are the implications of the widespread use of birth control? Are there specific kinds that are more "natural" or line up with the natural rhythms of a woman's body?
I have pretty strong feelings about this. The most controversial aspect of my book is the evidence I give that sperm can be stored in the womb for ten days or more. This is a pretty revolutionary idea--in the past, people thought sperm survived for two days. That shoots to pieces the idea of a natural birth control method. If sperm is stored in the womb, it extends a woman's fertility. It ruins the rhythm method. I've been suspicious of the rhythm method for years, and the evidence convinces me that there's no sound basis for it at all.
Isn't the rhythm method the only birth control method condoned by the Catholic Church?
Yes, it's still the church's position today, which I find very worrying. A guy named John Rock, a gynecologist in Boston, was a devout Roman Catholic, went to Mass every morning, and he was working on human fertility--primarily interested in helping women have babies. He got involved in the early work on the Pill, wanting to use it to kick-start women's fertility. He saw it as natural, since the pill mimics a situation where the woman is pregnant. That's how it prevents a new pregnancy starting. Rock campaigned heavily to have the Church accept the pill as a natural form of birth control. But the Pope decided against it, and that has been the position ever since. But the rhythm method, which I regard as totally unreliable, does have the approval of the Roman Catholic Church. And it gets worse: If a woman becomes pregnant using the rhythm method, the chances are that a fresh egg was fertilized by an aging sperm, or that an aging egg was fertilized by a fresh sperm, because you've deliberately been avoiding the time of ovulation. That will lead to increased likelihood of miscarriage and fetal deformity.
More and more couples are waiting longer to have children. Do you see this as a problem? Do you see humans adapting to have births later, especially as our life span increases?
Women are born with a certain number of starter cells for eggs, which gradually decline. Evolution has acted to stop production of eggs at menopause: it's not accidental. Lots of evidence points to the optimal time for a woman having a baby to be between 20-25. Now many women delay that because of careers. We do know that it increases the risk of miscarriage and birth defects. But whether, over time, we will adapt to this, I don't know. We don't know enough about it.
And doesn't the age of male sperm matter as well?
It's typical -- people study a lot of things in women and quite often haven't gotten around to studying it in men. It's long been known that the probability of having a baby with Down Syndrome increases with a woman's age. But a man's age contributes, too. And sperm counts decline as men get older.
But one thing that hasn't been studied is the fact that the increasing risk of reproduction with age may be related to copulation. As people get older, it happens less and less frequently. So the probability of having an aging sperm fertilize an egg, or an aging egg being fertilized, increases. This could be the connection between age and Down Syndrome -- the decline in frequency of copulation.
What do you think of assisted reproduction, like IVF?
When you deposit semen directly into the womb, it bypasses the filter of the neck of the womb, which produces mucus, and the sperm have to swim up. There's a selective process -- we know that sperm are filtered, and only healthy sperm get through that barrier. If you inject sperm directly, you bypass the filter. There's also intracytoplasmic sperm injection, injecting sperm directly into an egg. There's no selection, it's just a sperm picked at random. You haven't had the filtering effect of the neck of the womb, or any other filter that happens before the sperm gets to the egg. It comes back to the question of why we need so many sperm: there's a filtering that helps the sperm reach the egg in top condition.
In your book, you discuss the rise of C-sections. Why should we be worried?
It's really quite frightening. There's an epidemic: around one in three women in the US now gives birth by C-section. Twenty years ago, the World Health Organization said C-sections shouldn't be above 15 percent, which I thought was a high figure anyway. They gave that as a target, and we're more than double that. The highest figures are in Asia--in China, it's around 45 percent and growing. But apparently, there are propitious birthdays--quite often the C-sections are fixed so the baby is born on that specific day.
There's a huge downside to this. One effect of C-sections is removing the selective effect. If we get into a situation where 50 percent of women are having C-sections, are we going to remove the selection on overly large heads? What we're doing is stopping natural selection. And generally, cesareans take place too early. The baby is born prematurely, and premature babies have all kinds of problems. And with every operation, there is a risk. Women should be advised that this is a pretty dangerous option.
You write that women are "biologically adapted for frequent suckling in response to the infant's needs," rather than feeding on a schedule. Can you talk about this?
Suckling on demand means that a baby decides. Primates are a good example--usually, babies are held by the mother all day and the baby can move to the teat when it wants. You can tell from milk composition: if you get high fat and high protein but low sugar, it's associated with suckling on schedule. Suckling on demand is associated with low fat and low protein but high sugar. This is what you find in all primate milk. If we look at human milk, it's exactly like other primates. Our milk has been adapted for suckling on demand.
Okay, but what about women who work and can't suckle on demand?
The more important question is the natural period of lactation for women, which is about three years. This is not exclusive breast-feeding--after the first year it can be mixed with supplementary feeding. That is what women's bodies are adapted for. I want to emphasize that women don't need to go back and do this. What we need is find out what the baby is biologically expecting from the mother and find ways of meeting it. If bottle-feeding is used, that formula must have all the necessary constituents.
My message is not that women have to suffer: by all means, use artificial milk. Just make sure the composition of that artificial milk is right. The last thing I want to do is make women feel guilty. I also think women need to maximize physical contact with the baby. If she can't do that while at work she should make up for that some other time. There's a strong message here for employers: If they're employing women with babies, move heaven and earth to provide them with the right conditions so that they can suckle the babies appropriately.
This interview is edited for clarity and length.