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Macmillan Childrens Publishing Group

Do Fathers Matter?

What Science Is Telling Us About the Parent We've Overlooked

Paul Raeburn

Scientific American / Farrar, Straus and Giroux



The Roots of Fatherhood: Pygmies, Finches, and Famine

Fathers who are expecting a baby might work with their wives or partners to prepare the nursery, paint the walls, or shop for a crib. Depending upon their budgets, the male parents-to-be might join their wives in assembling an IKEA bookcase, a global twenty-first-century bonding ritual. But while these activities can get men thinking about fatherhood, much of what prepares them to be parents was done long ago. At least three forces are at work. One is natural selection, which has shaped them to be well suited for fatherhood. The second is their own family's genetic inheritance, which is part of what makes each father different from all the others. And the third is diet and toxins and other factors in men's environments. We are now learning not only how these forces shape fathers, but how and why they can sometimes go awry.

Not long ago, on a tranquil summer night in South Florida, I had an experience that made clear how unusual and important human fathers are. I joined scientists observing sea turtles nesting on a beach. We watched a female green turtle dig a deep hole in the sand, midway up the beach, and drop some 150 eggs, each the size of a softball, into the hole. She then buried the eggs with a rhythmic flapping of her rear legs against the sand and scuttled back to the water, leaving her young to hatch, find the ocean and food, and mature with no parental help at all.

While she was laying her eggs, drops of a clear liquid began to fall from her eyes, as it does from many turtle mothers. Legend has it that they are crying for the children they will never know. We were moved as we stood on the beach, watching. But these secretions were not tears—merely a way to shed excess salt that accumulates in the turtle's body. Crocodile mothers also cry sham tears while laying eggs—which gave rise to the phrase "crocodile tears" to describe sorrowful insincerity.

The truth is, the turtle has no regrets. Her artful use of her limbs to dig a hole and conceal her eggs represents the beginning and end of her parenting. The story is the same for many, many other species. The eggs are untended, and the young that hatch are set free to make their own way. Mothers who give birth in this fashion often produce vast numbers of offspring to combat the overwhelming odds against their survival. While the mothers' contributions might be slight, they dwarf the contributions of males, who scarcely play any part in this primal ritual at all, beyond their brief role in conception.

The circumstances are different for mammalian mothers—but not so different for many fathers. Unlike newly hatched sea turtles, who are hungry and on their own from the moment they clamber out of their shells, mammals—the warm-blooded vertebrates that include everything from shrews to human beings—have a ready source of nutrition from their lactating mothers. The arrangement has a price, though. Newborn mammals often take a long time to mature. The mother's nutritional investment prevents her from bearing other offspring for a time. And, as with turtles, in most mammal species she gets little or no help from the father.

But in the 5 to 10 percent of mammal species in which males help, the family arrangements can be strikingly different. Certain monogamous titi and night monkey fathers are among the most devoted parents in the animal kingdom. They set a standard few human fathers could meet. Titi monkey fathers provide food for their offspring and follow mothers around all day, so that whenever the babies are not nursing the fathers can carry them on their backs. By the end of the first week, the mother's contact with her infant during the day is limited to four or five periods of nursing. The father carries his infant 90 percent of the time. Many fathers lose weight carrying their infants around. The baby monkeys, in return, are very attached to their fathers—experiments have shown that the infants tend to be more attached to their fathers than to their mothers. A titi monkey becomes more upset when separated from his father than from his mother. Infants deprived of their fathers squawk more and show a greater elevation of stress hormones than they do when deprived of mothers. And whether it's to be sure that he is the father of her children or just out of affection, a titi monkey father rarely lets his mate get out of his sight.

Human fathers might not show quite the same dedication to their children and spouses, at least in terms of hours spent feeding or carrying the kids. But they are among the most committed mammalian fathers of any species on Earth. There is no example of a human society in which fathers do not help raise the children. Admittedly, some fathers are better at this than others. Some abandon their families for other mates, and some for reasons we can never be quite sure of. But most human males, at the very least, put food on the table. It would be exciting to trace the evolution of fatherhood over the past few million years to find out whether men were always as invested in their children as they are now, or how that contribution might have changed over time. Did our earliest male ancestors put time, energy, and resources into offspring who would be heavily dependent on parental care for years to come? Or did they swiftly resume the search for other willing females, to multiply again and again, increasing the chance that some of their offspring would survive? And if so, when did that change, and why?

Those are questions we will probably never answer. We're not even sure exactly when, in the course of human evolution, males and females began to forge relationships with one another. But we have some hints, sifted from prehistoric remains examined by archaeologists and paleontologists. They tell us that among australopithecines—the earliest members of the human family, who lived 4 million to 1 million years ago—mates were involved enough for males to have provided food and care for infants and protection from predators. Long-term male-female relationships likely began with the appearance of Homo erectus about 1.5 million years ago. Fathers, mothers, and children slept together, so children could watch and learn from their fathers, and their fathers could protect them. In the Late Pleistocene period, about 120,000 years ago, men hunted for large game and often had multiple wives. They spent a lot of time in camp between hunts, and were often available to their children. As the Late Pleistocene period progressed, more complex technologies and art forms arose, and we know that fathers helped to transmit that culture to their children. Circumstances changed at the end of the Ice Age, about 12,000 years ago. Foraging and rudimentary farming became part of the subsistence pattern, and women contributed more to the diet, gathering vegetables. Monogamy was more common, and fathers, no longer pulled away from camp by the hunt, had more time to care for their children and grow closer to them.

Many authorities think the increasing size of the human brain, which has grown continuously over the past 2 million years, was one reason that fathers became involved with their children. It's unclear why brain size increased, but it might have been to provide the social intelligence needed when human ancestors began living in larger groups. As human brains swelled in size, the bobble-headed infants who threatened to topple over under the weight of all that gray matter had to be born earlier in their development. If they got too far along in their mothers' wombs, their brains would get so large that their skulls wouldn't fit through their mothers' birth canals, a problem that can put a sudden crimp in an otherwise promising evolutionary path.

But being born earlier had a cost—the infants would need more care. Human children take longer than any other animal to reach the point at which they can find enough food on their own to survive. For those of you who might count calories on occasion, think about this: It takes 13 million calories' worth of breast milk, Cheerios, and mashed peas to raise a child to the age of "nutritional independence" at eighteen. Mothers were not going to be able to manage that investment by themselves. They were going to need help.

* * *

To try to fill the gap between what we would like to know about human development and what fossil remains tell us, anthropologists often turn to societies whose circumstances most resemble those of our ancestors—namely, hunter-gatherer groups. Contemporary hunter-gatherer groups are still living much the way our ancestors did for almost all our evolutionary history. Agriculture was invented only about 10,000 years ago, and the industrial age is even more recent, beginning only a couple of centuries ago. Before that, humans were hunters and gatherers, so today's hunter-gatherers should be able to tell us something about fathers before the development of agriculture and industry.

One of the most interesting such groups can be found in the western Congo River basin—not quite the heart of Africa, but close. The landscape there is covered with a sweeping emerald-green canopy, broken by occasional patches of sun-bleached savannah. Gorillas, chimpanzees, red hogs, several varieties of monkeys, squirrels, and small antelopes called duikers wander through the canopy's shadows. Elephants and larger antelopes—the sitatunga—keep to the swampy sections near river valleys. The temperature scarcely varies, and the weather ranges from very rainy to less rainy (the "dry" season).

What might seem to be a tropical paradise is actually a tough place to make a living. Hunters quickly discover that animals are scattered and hard to find. Living off the plants that grow under that green canopy isn't easy either; many of them are inedible. The soil isn't especially good for agriculture. Ecologists refer to the western Congo as "marginal" habitat. But it is not uninhabited. Among those who live here are the Aka pygmies, for whom this is home, a place that has been home to them and to their ancestors for so long that they have learned how to survive and prosper there.

The Aka's skill at foraging and hunting with nets keeps them comfortably fed and content, with time left over. You might even say they live a life of leisure. On hunting trips, men bring their families along to help. Wives are there for more than companionship; they help to chase animals into their husbands' nets. The parents can't pack the kids off to day care, so the kids come, too. The hunts are efficient, the families are almost always together, and the Aka men spend as much time with their children as they can.

Barry S. Hewlett, an anthropologist at Washington State University Vancouver, began studying the Aka in 1973. He did not initially focus his research on Aka fathers, but that changed when he briefly left his studies of the Aka to take a job as a health coordinator for a child development agency in the United States. There he began to study the psychological literature on child development. And he realized that the Aka represented something unique. The descriptions of Western fathers' role and behavior were completely at odds with what he'd seen in Africa. He went back in 1984 intending to focus on the behavior of Aka fathers, and that research continues. Hewlett now has a house in an Aka village that he visits every year, for weeks or months at a time. He's also had occasion to practice the things he's learned about fatherhood; Hewlett has seven children.

Aka parents, Hewlett quickly discovered, are different from Western parents. Hewlett observed that Aka infants are held almost constantly by someone, usually with skin-to-skin contact, because the Aka usually don't wear shirts. Parents and others "talk to, play with, show affection to, and transmit subsistence skills to their infants during the day," Hewlett writes. Infants "are nursed on demand, and attended to immediately if they fuss or cry." Children as young as a year old, Hewlett reports with some unease, are taught how to use machetes, pointed digging sticks, sharp spears, and miniature axes with sharp blades. It's admirable that children are given responsibility while they are young, and taught to use the tools that their parents use. However, the practice of giving metal axes to one-year-olds might not be one of the features of Aka life that we would choose to emulate.

Despite all this attention and contact, Aka families, unlike many American families, do not let their world revolve around their children. "American parents allow their children to interrupt their conversations with other adults; they ask their children what they want to eat and try to accommodate other desires of the children." That's what Hewlett calls a child-focused family.

Aka society, in contrast, is adult-centered. "Parents seldom stop their activities to pay undivided attention to their children. If an infant fusses or urinates or defecates on a parent who is talking to others or playing the drums, the parent continues his activity while gently rocking the infant or wiping the urine or feces off with a nearby leaf." Aka fathers spend 47 percent of their day holding their infant children or keeping within arm's reach of them. According to Hewlett, infants frequently crawl to their fathers, and fathers pick them up because they intrinsically enjoy infants. The babies are even part of dads' nights out. Hewlett watched men take their kids along when they gathered in the fields to relax and drink palm wine. (Try to imagine an American father slinging his kid on his hip before heading out for a drink with the guys.)

One morning, Hewlett watched a father named Yopo who was in bed with his eight-month-old son, Manda, when Yopo's wife left to fetch water for the camp. Yopo put Manda on his lap, humming to him. Manda reached for a twig on the bed and played with it. Yopo sang as if they were on a net hunt, holding Manda on his chest. Manda cuddled up to Yopo's neck, and Yopo put a leaf on his head. Manda squealed happily. Yopo continued to sing and hold Manda for about an hour, even after his wife returned. On another occasion, a father and a mother saw their fifteen-month-old son have a bowel movement outside, near their hut. The father dropped his work—making string for his net—and cleaned up the boy and the ground, using a handful of leaves. The father then sat down, resuming work on the string. His son walked toward him, put a hand on his leg, and quietly watched him work.

One of Hewlett's many interesting discoveries was that Aka fathers do a lot of their child care in the evenings, when field anthropologists often aren't watching. The usual practice is for visiting scientists to observe when it's convenient, during the day. And they miss what fathers do at night. Too many anthropologists conclude fathers do little child care because they aren't there to see it. We can recognize this in our own families: failure to account for what fathers do at night has bedeviled studies done in industrialized countries, too. "Infants in all cultures wake often during the night, and it is my impression that fathers are often involved in infants' care during this time," writes Hewlett. Since psychological researchers don't generally set up observation in new families' homes, they often miss this. They don't know much about the role of the father because they haven't seen him in action, and so they conclude that he doesn't do much.

Hewlett observed that Aka fathers held their infants about 9 percent of the time during the day, but 20 percent of the time in the evenings. This is not all what we would call quality time—the fathers with children in their arms are often engaged in something else. But the many hours fathers and children spend together leads to unusually intimate relationships, "because the father knows his child exceptionally well," says Hewlett.

In the United States, quality time for fathers often means playtime. Aka fathers do not often play with their children "because they can communicate their love and concern in other ways … They know subtle means of interacting with their children." The Aka demonstrate the importance of what we might call "quantity" time—simply spending time with children, even if the parents are not always focused on them. Child relationships based on quantity time contribute to emotional security, autonomy, and self-assuredness.

Studies of the Aka and other non-Western societies challenge much of what we think we know about fathers. They show us that fathers can—and will—do more in the right circumstances. It's unlikely, given the pressures of our changing society, that many fathers will have the leisure to spend as much time with their children as Aka fathers do. Still, the Aka give us another view of what fatherhood can be like, and one from which we might be able to learn something about what kind of fathers we'd like to be.

* * *

The Aka give us insight into fatherhood from the time of our prehistoric ancestors, but they don't tell us much about how fatherhood might have changed during the past few decades. As I said at the beginning of this chapter, men are shaped to become fathers not only by evolution but also by their own families and their environments. We are now learning that a family's ill health and exposure to toxins in the environment can adversely affect their future children and even grandchildren.

Most of us know that a woman who becomes pregnant should stick to healthy foods, skip mercury-laden fish, quit smoking, and avoid exposure to paint thinners. All of these things, and more, can affect the health of the fetus. That's easy enough to understand; we're never more intimately connected to our environment than when we are in our mother's womb.

The same sort of reasoning suggests that a father would have little or no impact on the health of the fetus, with which he has no physical connection whatsoever. But that reasoning is faulty: research is showing that a father's environment, his behavior, and even his appearance can have a substantial effect on fetal health—and on the health of his grandchildren.

The first glimmer of this phenomenon came up in the mid-1960s. A pharmacologist named Gladys Friedler was studying the effects of morphine on female rats, and she found that the drug altered the development of their offspring. She then tried injecting males with morphine and mating them with healthy females to see if that exposure would also affect the offspring. The conventional wisdom was that it couldn't; that the morphine might affect the males in a variety of ways, but that it wouldn't affect their sperm. But the conventional wisdom was wrong. The rats' pups were underweight and underdeveloped—solely from the fathers' exposure to morphine before conception. Friedler didn't fully understand what she was witnessing. Neither did anyone else; and nobody believed her. She struggled to get funding for more experiments, and colleagues urged her to abandon the research. But she persisted, and it is only within the past decade that her work has been confirmed.

Researchers have now seen signs of this kind of paternal inheritance in a number of recent studies. Some of the most interesting findings come from what is now an isolated resort community in northern Sweden called Överkalix parish, with mountains lit by the midnight sun in summer and the northern lights in winter.

Swedish researchers were drawn to Överkalix because careful historical records had been kept by town officials during the nineteenth century, when Överkalix was subject to repeated crop failures. Harvest statistics were collected in "Communications from the County Governor in Västerbotten to His Majesty the King," and grain prices were recorded as well. Researchers had information on children born in Överkalix in 1905, as well as data on bountiful harvests and starvation back to the time of the children's grandparents. The idea was to look for any connection between the grandparents' diets and the outcomes of their grandchildren. During bountiful years, the grandparents would have had plenty to eat, and during lean years they would not have had nearly enough. The scientists didn't know what they would find, but the data gave them the opportunity to see whether changes in men's nutrition could have any health consequences for their grandchildren.

They looked at records that would tell them about the diets of grandfathers during their early adolescent years, a period thought to be particularly important for future health. And they found that diet at that stage of life had important consequences. The grandchildren of men who had plenty to eat did not live as long as those whose grandfathers had gone hungry. The grandfathers' hunger was good for grandchildren in other ways, too. The grandchildren of these men were less likely to die of heart disease or diabetes than those whose grandfathers had had plenty to eat as adolescents.

Marcus Pembrey of University College London has reviewed the Överkalix findings and other sources of information to see what else he could learn about men's behavior and diet and their effects on their children and grandchildren. He looked at data on 166 British fathers who said they'd started smoking before the age of eleven and compared their children to those of fathers who started smoking later in life. The sons of the fathers who started smoking early were more likely to be overweight by age nine. There seemed to be a link between fathers and their sons but not between fathers and their daughters.

Pembrey and his colleagues also looked again at the historical records of harvests in Överkalix to determine which grandparents had good access to nutrition in early adolescence and which did not. They confirmed the increased mortality risk in the grandsons of paternal grandfathers who had good access to food. And they found the same thing in granddaughters whose paternal grandmothers had plenty to eat. The opposite case was also true: grandchildren had lower mortality risk if their paternal grandparents had poor access to food as children.

There's more. We have known that mothers who overeat or are obese during pregnancy increase the chances that their children will be obese. And now we know that a similar thing happens with fathers. The children of obese mothers and fathers are more likely to be obese themselves. This result comes from Margaret J. Morris and her colleagues at the University of New South Wales in Australia. They noticed that overweight children usually had overweight mothers and fathers, and they wondered whether fathers' diets—not just their genes—would affect their children's risk of developing type 2 diabetes.

The researchers fed male rats of normal weight a diet of more than 40 percent fat, which made them obese. Then they mated them with females who had been fed a normal diet. The male pups showed increases in weight and body fat, and tests indicated they had an increased risk of diabetes. The daughters showed a different pattern. Their body fat and weight were normal when they were born, but in adulthood, they developed a diabetes-like condition marked by alterations in the way they handled glucose and insulin. When Morris and her team looked closely at the daughters' genes, they found alterations in the workings of 642 genes related to islet cells—the cells that produce insulin. There was only one explanation for this link: the fathers' high-fat diets had produced alterations in their sperm, which then led to the occurrence of adult-onset disease in their daughters.

These alterations are referred to as epigenetic changes. They do not change the DNA sequence of genes, but they affect whether or not certain genes are expressed—meaning whether they are turned on or off. The findings of the Överkalix and obesity studies reflect such epigenetic changes.

Other studies have shown a similar connection in other ailments. A group at the University of Massachusetts led by Oliver J. Rando found that feeding male mice a diet low in protein substantially altered many genes involved in the metabolism of cholesterol and fats in their offspring. The group offered some interesting speculation about why this might be the case. Perhaps the father's body, detecting that it is in an environment in which protein is in short supply, is altering the genes it passes on to its children to help them adapt to scarcity. "Mechanisms exist that could allow organisms to ‘inform' their progeny about prevailing environmental conditions," the researchers wrote. That is a remarkable and unexpected way for a father to help ensure the survival of his offspring.

Each of these findings led to more research, and the evidence that poor health in fathers can adversely affect their children is mounting. In a more recent finding, Eric J. Nestler of the Mount Sinai School of Medicine in New York and his colleagues exposed adult male mice to chronic stress, and then bred them with normal females. The pups showed physiological and behavioral changes resembling those of depression and anxiety. Lorena Saavedra-Rodríguez and Larry A. Feig at Tufts University School of Medicine in Boston found that female mice passed the effects of stress on to their offspring, but fathers passed those effects on to their offspring and to the next generation as well—another example of the grandfather effect found in the Överkalix studies.

Studies such as these are appearing all the time. The more that researchers look for these changes, the more they find them. In a study presented at the annual meeting of the Society for Neuroscience in November 2013 and later published in Nature, Brian G. Dias and Kerry J. Ressler of Emory University in Atlanta reported that the fear produced by traumatic experiences can be passed on from males to their offspring. They gave male mice small shocks when the mice were exposed to a certain odor, until the mice would show a startle response when exposed to that particular odor, not to others. When the mice were mated, Dias and Ressler found that the offspring showed an increased startle response to the same odor. And this fear was passed on to the next generation, too.

It's important to remember that until the past decade, researchers did not anticipate finding anything like these epigenetic changes in fathers. It's not surprising that the health of mothers would affect their unborn children; a mother and her fetus have a very intimate connection. But the only connection between fathers and the fetus is the single sperm that fertilizes the egg. It carries within a rich, and sometimes harmful, legacy. The question that remains is how these experiences of the fathers manage to change the epigenetic marks on their sperm so that the health risks or fears of the fathers are passed on to their offspring. Researchers have only hypotheses; nobody knows for sure.

* * *

Other researchers have looked at the threats to fathers from toxins and pollution, to see whether exposure to these substances can produce changes in their offspring. Can toxins alter the operation of fathers' genes the way stress, diet, and anxiety can? The first studies to address this question were done by Michael K. Skinner, a biochemist at Washington State University. He began by exposing lab rats to a fungicide called vinclozolin, used in vineyards and on fruits and vegetables. He wouldn't have been surprised to find that exposure to the chemical harmed the rats. But he found much more than that. The fungicide switched on genes in the rats that normally were switched off, and vice versa—and these changes in the operation of the genes were passed on to their offspring. Researchers have known for a long time that chemicals in the environment can alter the operation of genes. But they thought that the genes in sperm and eggs were scrubbed clean of these changes before being passed along at conception. Skinner found that this was not the case. It was quite the opposite—the alterations had become permanent. The flipped switches were passed on to the next generation.

If exposures to the environment could alter the workings of genes that were once thought to be protected from outside influences, then it made sense to see whether men's exposures to potentially toxic substances at work could produce harmful alterations in the operation of their genes. Tania A. Desrosiers and colleagues at the University of North Carolina did an epidemiological study in which they looked at large populations of male workers to see whether some jobs were associated with health problems in the men's children. The hypothesis proved to be correct. Certain occupations of fathers were associated with a greater risk of birth defects in their kids. The riskier occupations included petroleum or gas worker, chemical worker, printer, computer scientist, hairdresser, and motor vehicle operator. Certain jobs were associated with particular birth defects: cataracts and glaucoma were linked to photographers, while digestive abnormalities were linked to landscapers. Epidemiological studies such as this always require confirmation in the lab and clinic, so we can't yet be sure that this finding is correct. But it's an important warning sign.

* * *

These studies represent one unexpected way that fathers and even grandfathers can affect the health of their descendants. But there are other ways to look for connections between fathers and children's health. One way is to see whether any other paternal attributes contribute to health outcomes in children. Some researchers are trying to find out whether a man's looks have any consequences for his children. And they are finding provocative answers in, of all animals, zebra finches.

These birds, native to Australia, are about four inches long. The males have orange cheeks, striped gray-and-white throats, and red beaks. They might seem an unlikely species in which to pursue questions concerning men's attractiveness. How, for example, would an investigator distinguish a particularly handsome finch from his plainer counterparts? Yet finches have taught us something interesting about fatherhood: the handsomeness of a male makes a difference to his children.

I heard this story from James P. Curley of Columbia University, an authority on the genetics of fatherhood. He doesn't work on finches. He works on mice, which are also quite useful in the study of male genetics. But when I went to see him, he told me about the finches and walked me down the hall from his lab to a small room where some of his colleagues kept a noisy population of chattering zebra finches. Genetic tests of these small birds have shown that males can make important contributions to their offspring through an indirect route: by altering mothers' behavior. Male finches can help their offspring's chances of survival by making females become more adept at caring for their young.

The scientists who work with the finches looked at the question of whether the attractiveness of a male affects a female's parenting behavior. The experiment was prompted by the curious sexual preferences of female finches, which have demonstrated that they prefer males wearing a red leg ring over males without one. The females show little interest in males with green leg bands. This discovery saved the researchers from having to figure out which finches are the best-looking. It turns out to be a question of choosing the proper accessories.

It's impossible to know for certain why the females prefer males adorned with red rings, but female zebra finches find males with big red cheek patches extremely attractive, Curley said, and the red rings could somehow be mimicking the cheek patches. Even without a firm explanation, this was a phenomenon the researchers could use to their advantage. They put red leg bands on half a group of male finches and green leg bands on the other half. Then they compared the offspring of the attractive males to those of the homely green-banded males.

The offspring of the attractive red-banded males were found to have distinct advantages. They begged for food more often than the others and were rewarded: mothers gave them more food. Females laid eggs containing more growth hormones when the eggs had been fertilized by the attractive males. You might guess that the attractive fathers simply had better genes, but that wasn't the case. Somehow, making the male finches more attractive encouraged mothers to devote more resources to the offspring. The attractive males didn't have better genes than their green-ringed competitors, although the females might have been tricked into thinking that they did.

Curley called the findings so unexpected as to seem ridiculous. How could a colored leg band have such an important effect on mothers' behavior? He decided to see whether he could replicate the experiment with his mice, comparing males raised in isolation to "enriched males" raised in a more natural environment. Then he mated each one with a female. The females who mated with the enriched males devoted more resources to their offspring and engaged in more thorough maternal behavior. It was similar to what was going on with the finches—females invested more in their offspring when they had a more desirable mate.

Encouraged, Curley did another test, this one with stressed and normal males. Females who mated with normal males nursed and licked their offspring more often, and their pups exhibited less anxiety than the offspring of the stressed males. It was yet another demonstration of the same effect: making the males more desirable turned the females into better mothers. And that was good for their pups.

Continuing along these lines, Curley looked at whether a male's anxiety could affect his pups in the same way that stress did. To produce high-anxiety males, he took males out of their cages and dropped them into unfamiliar enclosures. Those who were the least willing to explore their new surroundings were the mice with the highest anxiety. He bred these males with females and found that the daughters of the high-anxiety fathers exhibited similar symptoms. The pups were raised solely by their mothers. The researchers concluded that marks on the fathers' sperm were being passed on to affect daughters' behavior, independent of any change in mothers' behavior. (These marks are referred to as epigenetic changes, because they change the operation of genes—whether they are turned on or off—without actually changing the DNA.) And the sons did not inherit their fathers' anxiety. This, too, parallels other findings. The nutritional status of the Överkalix grandfathers affected only their sons, not their daughters. It's clear that some of these effects apply only to sons and others only to daughters. The inability to explain this is a sign of how much more researchers need to find out about these odd generational effects.

Curley and his colleagues are now exploring a gene called Peg3 that likewise has different effects on sons and daughters. The name stands for "paternally expressed genes": in this family of genes only the father's copy is expressed in his offspring, while the copy from mothers is silenced. "That means what your father passes on to you is of massive significance," Curley said. He is studying the gene in mice, but a form of Peg3 occurs in humans, too. So anything he discovers in his mice is likely to be true in us as well. To help me understand what the gene does, Curley started with a short lecture on mouse sex. Virgin male mice, he explained, begin with a trial-and-error mating strategy: they pursue any female they can, whether or not the female is in estrus, ready to reproduce. The males usually manage to mate, and then their troubles are over. Once they've mated, they develop the ability to detect, by smell, which females are in estrus. Curley wanted to know how Peg3 might be involved in this behavior, and so he used a lab trick to inactivate, or "knock out," the Peg3 gene in some of his mice. The knockout mice were unable to detect when females were in estrus, even after they'd mated. They continued to try mating with females who were not ready to reproduce. After a while, the directionless males gave up. So now Curley knew that Peg3 is essential for the development of proper behavior regarding sex and mating in males.

Curley then looked at females. There he discovered that knocking out Peg3 had a very different effect. Knocking out Peg3 in female mice doesn't affect mating, the way it does in males. Instead, females whose Peg3 gene is knocked out become poor caretakers. They don't eat as much as they should early in their pregnancies. After birth, they are supposed to lick their pups, nurse them, eat the placenta (a source of nourishment), and build a nest. Females whose Peg3 has been knocked out do those things much less frequently than normal mice.

To sum all this up, Peg3 affects how well a father's male pups will mate and how well his female pups will care for their offspring. The mating ability of his sons and the nurturing qualities of his daughters will both affect the health of his grandpups. Once again, we have an effect that extends from males not only to their children but also to their grandchildren. It's reasonable to expect that the human version of Peg3 has similar effects in human males and their offspring. That's not the same as proving the connection, but it gives Curley and others confidence to look for a similar phenomenon in humans.

We can't put people in cages, tag them with colorful jewelry, and allow them to mate. Unlike finches, who had no say in which leg band they got, human males might object to being made unattractive for the sake of an experiment. Nor would they appreciate being manipulated in a way that could harm their children. But mice, finches, and humans are enough alike that's what true in them is often true in us. Even though these findings haven't been confirmed in humans, it might be wise for men who are about to become fathers to think about their health and about what they should be eating even before their wives or partners become pregnant. This would be good advice for fathers even if it doesn't have any beneficial effects on their children. And it would be an even better idea if it does.

Copyright © 2014 by Paul Raeburn