Darby Saxbe, USC Dornsife College of Letters, Arts and Sciences

Parenting makes the heart grow fonder, and the brain grow … smaller? Several studies have revealed that the brain loses volume across the transition to parenthood. But researchers like me are still figuring out what these changes mean for parents.

In a new study that looked at brain change in first-time fathers, my colleagues and I found that brain volume loss was linked with more engagement in parenting but also more sleep problems and mental health symptoms. These results might point to a cost of caregiving, traditionally shouldered by women but increasingly borne by men also.

Brain changes for mom come with new baby

Caring for an infant demands new motivations and skills, so it is no surprise that it might also sculpt the brain. Research in rodents first identified remodeling of both the structure and function of the brain during pregnancy and parenthood. A new body of research is unearthing similar effects in human parents, too.

In a pair of studies, researchers recruited first-time mothers for a brain scan that occurred before they became pregnant and then scanned them again a few months after birth. Gray matter – the layer of brain tissue that contains neuronal cell bodies – shrank in the mothers but not in a comparison group of women who did not become mothers.

Although a shrinking brain sounds bad, researchers theorized that this more streamlined brain could be adaptive, helping process social information more efficiently and therefore facilitating sensitive caregiving. In keeping with this hypothesis, studies have linked maternal brain changes with women's degree of attachment to infants and with their responses to images of their infants. Women who lost more gray matter volume also appeared more bonded with their babies.

New dads' brains change, too

Most studies of the parental brain have focused on women, but emerging evidence suggests that similar brain changes might occur in new fathers, too. My collaborators and I had previously identified brain volume loss in men transitioning to fatherhood, in similar parts of the brain that changed in mothers.

Before you picture the shrunken-head guy from the movie “Beetlejuice,” keep in mind that these changes were subtle. Fathers showed smaller, less statistically significant brain changes than mothers.

Dads vary in how invested they are in caring for the baby, so as a next step, we wanted to know how men's brain changes across the transition to fatherhood map onto their experiences of new parenthood.

To test this question, we looked more closely at 38 men we scanned in California before and after their baby's birth. During pregnancy and again at three, six and 12 months postpartum, we asked the fathers how they were feeling about their infants and how well they were sleeping. We also asked about their symptoms of depression, anxiety or other mental health problems.

As before, we saw significant prenatal-to-postpartum brain differences across the entire cortex, the outermost layer of the brain that carries out many higher-order functions, such as language, memory, problem-solving and decision-making. On average, men in our sample lost about 1% of their gray matter volume across the transition to parenthood.

Consistent with the research on mothers, men's brain volume reductions did indeed seem to track with their parenting. If men told us during pregnancy that they wanted to take more time off from work after the birth, and felt more bonded to their unborn child, they subsequently lost more gray matter volume, especially in the frontal and parietal lobes – parts of the brain involved in executive functioning and sensorimotor processing, respectively.

Greater volume loss also emerged among fathers who told us that they spent more time with their infants at three months postpartum, took more pleasure in interacting with their infants and experienced less parenting stress. Taken together, our results dovetailed with the prior studies of mothers and suggested that more motivated, hands-on fathers lost more gray matter volume across their transition to parenthood.

The plot thickened when we looked at mental health and sleep quality. Men who lost more brain volume also reported greater depression, anxiety, general psychological distress and worse sleep at both six and 12 months after birth. These results held up when we controlled for the same measures during pregnancy.

This finding provides a clue to a possible direction of causality: Rather than prenatal sleep problems or psychological distress predicting greater brain change, we found instead that fathers' gray matter volume loss preceded their postpartum sleep problems and mental health, above the effect of their well-being before birth.

Parenting comes with highs and lows

Importantly, this research is preliminary: We had a small sample of fathers who were willing to participate in our intensive research study. These results need to be replicated in larger and more representative groups of fathers.

Still, as one of the first longitudinal studies of male brain changes across the transition to first-time parenthood, our findings illustrate that perinatal brain changes may reflect both adaptation and vulnerability. The very same changes linked with fathers' greater investment in caregiving also seemed to heighten their risk of sleep trouble and mental health problems.

As any new parent will tell you, caring for an infant is a challenge. Becoming a parent forces a realignment of life priorities and can bring magic and meaning to everyday life. But parenting can also be dull, repetitive, lonely and draining.

Perhaps our findings in fathers point to a cost of caregiving, a burden that has long been familiar to mothers but may be increasingly shared by men as fathers step up their participation in hands-on parenting.

The take-home message here is not that men should stop caring for children. A slew of research suggests that children with involved fathers do better across the board: academically, economically and emotionally. And fathers themselves report that parenthood makes their lives richer and more meaningful.

Instead, results like these support public health priorities that invest in fathers – and parents in general – through policies that reduce stress for new parents in the first months after birth, such as paid leave and workplace efforts to normalize leave-taking among men.

Darby Saxbe, Professor of Psychology, USC Dornsife College of Letters, Arts and Sciences

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Zarin Machanda, Tufts University and Kris Sabbi, Harvard University

Wild chimpanzees have been studied for more than 60 years, but they continue to delight and surprise observers, as we found during the summer of 2017 in Kibale National Park in Uganda.

We were observing young chimpanzees' play to better understand how they grow up. For most group-living animals, play is an integral component of development. Beyond just having fun, social play allows them to practice critical physical and social skills they will need later in life.

But that summer, we realized that it wasn't just the young ones playing. Adults were joining in on play more often than we had seen before, especially with each other. Watching fully grown female chimpanzees tickling each other and laughing surprised even the most veteran researchers of our project.

What made this so unusual was not that adult chimpanzees were playing at all, but that they were doing it so frequently. A behavior that we typically might see once every week or two became something that we saw every day and sometimes lasted for hours.

So what had changed that summer? For us, as primatologists, this is where the fun began.

Why would adults play in the first place?

Scientists tend to think the main reason play declines with age is that individuals essentially grow out of it as they master motor and social skills and shift toward more adult behaviors. By this logic, adults only rarely play because they no longer need to. The situation is different for domesticated species like dogs because the process of domestication itself preserves juvenile behaviors like playfulness into adulthood.

Neither of these reasons would explain why our adult chimpanzees were shoving babies out of the way to play with each other that summer. Instead of asking why adults would play, we had to ask what might, in other circumstances, stop them from playing. And for this, we had to go back to the basics of primatology and consider the effects of food on behavior.

The summer of 2017 was notable because there was an unusually high seasonal peak of a lipstick-red fruit called Uvariopsis, a favorite and calorie-rich chimpanzee food. During the months when these fruits are ripe and plentiful, chimpanzees spend more time hanging out together in larger groups.

This sort of energy surplus has been linked to rigorous activities, such as hunting among monkeys. We wondered whether fruit abundance might be linked to social play as well. Perhaps adult play is constrained because grown chimpanzees just don't usually have the extra time and energy to devote to it.

When life gets in the way of play

To test this idea, we turned to the long-term records of the Kibale Chimpanzee Project, extracting nearly 4,000 observations of adult play over 10 years.

Whether tussling with a young chimpanzee or playing chase with another adult, the frequency of adult play was strongly correlated with the amount of ripe fruit in the diet in any given month. When the forest was full of high-quality food, adult chimpanzees played a lot.

But when their prized fruits dwindled, their playful sides all but disappeared – that is, except for mothers.

A surprising sex difference

Among chimpanzees, males are much more social than females. Males invest a lot of time developing friendships, and, in turn, they reap the rewards of those bonds: higher dominance rank and more sex. For females, the high energetic costs of pregnancy and lactation mean socializing comes at the cost of sharing food that they need for themselves and their offspring.

We expected that play, as a social behavior, would follow other social patterns, with males playing more and being able to afford to play even when food abundance was low. To our surprise, we found the opposite. Females played more, especially during months with less fruit – because mothers kept playing with their babies even when all other chimpanzees had stopped.

A hidden cost of motherhood

Chimpanzees live in multimale, multifemale societies that exhibit what researchers call fission-fusion. In other words, the whole social group is rarely, if ever, all together. Instead, chimpanzees break up into temporary subgroups called parties that individuals move among throughout the day.

When food is scarce, parties tend to be smaller, and mothers are often alone with just their young. This strategy reduces feeding competition with group mates. But it also leaves mothers as the only social partners for their offspring. Mothers' time and energy that might be devoted toward other daily tasks, such as feeding and rest, go toward play instead.

Not only did our study reveal this previously unknown cost of motherhood, but it also highlighted how important play must be for these young chimpanzees that their mothers accept this cost.

You might be curious about how chimpanzee fathers fit in here. Chimpanzees mate promiscuously, so males do not know which offspring are theirs. Mothers are left to bear the costs of parenthood on their own.

An ape connection

Child development researchers know that play, and especially play with parents, is critically important for human social development. In fact, caregivers of young children might be reading this in between bouts of play with their little ones right now.

Since chimpanzees are one of our closest living relatives, these kinds of behavioral similarities between our species are not uncommon.

But not all primate parents reckon with costly play. In fact, there are almost no records of monkey mothers playing with their babies at all. Most other primate species, such as baboons and capuchins, don't go their separate ways during the day, so babies can play with each other and moms can catch a break.

Whether maternal play is a product of fission-fusion grouping or the developmental needs of offspring still needs to be tested directly. But the responsibility to play with your little ones certainly resonates with many human parents who experienced a sudden shift to become their children's main play partners when COVID-19 interrupted normal activities.

So on this Mother's Day, consider raising a glass to also celebrate these amazing – and tired – chimpanzee moms.

Zarin Machanda, Assistant Professor of Anthropology and Biology, Tufts University and Kris Sabbi, Fellow in Human Evolutionary Biology, Harvard University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Cara Wall-Scheffler, University of Washington

Think about taking a walk: where you need to go, how fast you need to move to get there, and whether you need to bring something along to carry the results of your errand.

Are you going on this walk with someone else? Does walking with a friend change your preparation? If you're walking with a child, do you remember to bring an extra sweater or a snack? You probably did – because people intuitively vary their plan depending on their current needs and situations.

In my research as an anthropologist, I've focused on the evolution of human walking and running because I love the flexibility people bring to these behaviors. Humans in all kinds of environments across space and time vary how far they go, when they go and what they go for – whether food, water or friends – based on a multitude of factors, including season, daylight, rituals and family.

Anthropologists split their studies of human activity into two broad categories: what people need to do – including eat, keep their kids alive and so on – and what solutions they come up with to accomplish these needs.

How people keep their children alive is a key issue in my research because it has a direct impact on whether a population survives. It turns out that kids stay alive if they're with adults. To this end, it is a human universal that women carry heavy loads every day, including kids and their food. This needs-based behavior seems to have been an important part of our evolutionary history and explains quite a few aspects of human physiology and female morphology, such as women's lower center of mass.

The solutions to other key problems, like specifically which food women will be carrying, vary across time and space. I suggest that these variations are as integral to explaining human biology and culture as the needs themselves.

Impacts of uncommon activities

Evolutionary scientists often focus on how beneficial heritable traits get passed on to offspring when they provide a survival advantage. Eventually a trait can become more common in a population when it provides a useful solution.

For example, researchers have made big claims about how influential persistence hunting via endurance running has been on the way the human body evolved. This theory suggests that taking down prey by running them to exhaustion has led to humans' own abilities to run long distances – by increasing humans' ability to sweat, strengthening our head support and making sure our lower limbs are light and elastic.

But persistence hunting occurs in fewer than 2% of the recorded instances of hunting in one major ethnographic database, making it an extremely rare solution to the need to find food. Could such a rare and unusual form of locomotion have had a strong enough impact to select for the suite of adaptive traits that make humans such excellent endurance athletes today?

Maybe persistence hunting is actually a fallback strategy, providing a solution only at key moments when survivorship is on the edge. Or maybe these capabilities are just side effects of the loaded walking done every day. I think a better argument is that the ability to predict how to move between common and uncommon strategies has been the driver of human endurance capacity.

Everyday life's influence on evolution

Hunting itself, especially of large mammals, is hardly ubiquitous, despite how frequently it is discussed. For example, anthropologists tend to generalize that people who lived in the Arctic even up to a hundred years ago consumed only animal meat hunted by men. But actually, the original ethnographic work reveals a far more nuanced picture.

Women and children were actively involved in hunting, and it was a strongly seasonal activity. Coastal fishing, berry picking and the use of plant materials were all vital to Arctic people's day-to-day sustenance. Small family groups used canoes for coastal foraging for part of the year.

During other seasons, the whole community participated in hunting large mammals by herding them into dangerous situations where they were more easily killed. Sometimes family groups were together, and sometimes large communities were together. Sometimes women hunted with rifles, and sometimes children ran after caribou.

The dynamic nature of daily life means that the relatively uncommon activity of hunting large terrestrial vertebrates is unlikely to be the main behavior that helps humans solve the key problems of food, water and keeping children alive.

Anthropologist Rebecca Bliege Bird has investigated how predictable food is throughout the day and the year. She's noted that for most communities, big game is rarely caught, especially when a person is hunting alone. Even among the Hadza in Tanzania, generally considered a big-game hunting community, a hunter acquires 0.03 prey per day on average – essentially 11 animals a year for that person.

Bird and others clearly argue that the planning and flexible coordination done by females is the crucial aspect of how humans survive on a daily basis. It's the daily efforts of females that allow people to be spontaneous a few times a year to accomplish high-risk activities such as hunting – persistence or otherwise. Therefore it is female flexibility that allows communities to survive between the rare big-game opportunities.

Changing roles and contributions

Some anthropologists argue that in some parts of the world, behavior varies more for cultural reasons, like what tools you make, than for environmental ones, such as how much daylight there is during winter. The importance of culture means that the solutions vary more than the needs.

One of the aspects of culture that varies is the role assigned to specific genders. Varying gender roles are related to the distribution of labor and when people take on certain solution-based tasks. In most cultures, these roles change across a female's life span. In American culture, this would be like a grandparent going back to college to hone a childhood passion in order to take on a new job to send their grandchildren to college.

In many places, females go from youth when they might carry their siblings and firewood, to early parenthood where they might go hunting with a baby on their back, to older parenthood where they might carry water on their head, a baby on their back and tools in their hands, to postmenopausal periods when they might carry giant loads of mangoes and firewood to and from camp.

Even though always load carrying, our capacity to plan and change our behavior for diverse environments is part of what drives Homo sapiens' success, which means that the behavior of females across their different life stages has been a major driver of this capability.

Cara Wall-Scheffler, Professor and Chair of Biology at Seattle Pacific University and Affiliate Assistant Professor of Anthropology, University of Washington

This article is republished from The Conversation under a Creative Commons license. Read the original article.