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.

Dakotah Tyler, University of California, Los Angeles

Located 163 light-years from Earth, a Jupiter-sized exoplanet named WASP-69b offers astrophysicists a window into the dynamic processes that shape planets across the galaxy. The star it orbits is baking and stripping away the planet's atmosphere, and that escaped atmosphere is being sculpted by the star into a vast, cometlike tail at least 350,000 miles long.

I'm an astrophysicist. My research team published a paper in the Astrophysical Journal describing how and why WASP-69b's tail formed, and what its formation can illuminate about the other types of planets astronomers tend to detect outside of our solar system.

A universe filled with exoplanets

When you look up at the night sky, the stars you see are suns, with distant worlds, known as exoplanets, orbiting them. Over the past 30 years, astronomers have detected over 5,600 exoplanets in our Milky Way galaxy.

It isn't easy to detect a planet light-years away. Planets pale in comparison, in both size and brightness, to the stars that they orbit. But despite these limitations, exoplanet researchers have uncovered an astonishing variety – everything from small rocky worlds barely larger than our own moon to gas giants so colossal that they've been dubbed “super-Jupiters.”

However, the most common exoplanets astronomers detect are larger than Earth, smaller than Neptune, and orbit their stars more closely than Mercury orbits our Sun.

These ultra-common planets tend to fall into one of two distinct groups: super-Earths and sub-Neptunes. Super-Earths have a radius that's up to 50% larger than Earth's radius, while sub-Neptunes typically have a radius that's two to four times larger than Earth's radius.

Between those two radius ranges, there's a gap, known as the “Radius Gap,” in which researchers rarely find planets. And, Neptune-sized planets that complete orbits around their stars in less than four days are exceedingly rare. Researchers call that gap the “Hot Neptune Desert.”

Some underlying astrophysical processes must be preventing these planets from forming – or surviving.

Planet formation

As a star forms, a large disk of dust and gas forms around it. In that disk, planets can form. As young planets gain mass, they can accumulate significant gas atmospheres. But as the star matures, it starts to emit high amounts of energy in the form of ultraviolet and X-ray radiation. This stellar radiation can bake away the atmospheres that the planets have accumulated in a process called photoevaporation.

However, some planets resist this process. More massive planets have stronger gravity, which helps them hold onto their original atmospheres. Additionally, planets that are farther away from their star aren't hit with as much radiation, so their atmospheres erode less.

So, maybe a significant portion of super-Earths are actually the rocky cores of planets that had their atmospheres completely stripped, while sub-Neptunes were massive enough to retain their puffy atmospheres.

As for the Hot Neptune Desert, most Neptune-sized planets simply are not massive enough to completely resist the stripping power of their star if it orbits too closely. In other words, a sub-Neptune orbiting its star in four days or fewer will quickly lose its entire atmosphere. When observed, the atmosphere has already been lost and what remains is a bare rocky core – a super-Earth.

To put this theory to the test, research teams like mine have been collecting observational evidence.

WASP-69b: A unique laboratory

Enter WASP-69b, a unique laboratory for studying photoevaporation. The name “WASP-69b” comes from the way it was discovered. It was the 69th star with a planet, b, found in the Wide Angle Search for Planets survey.

Despite being 10% larger than Jupiter in radius, WASP-69b is actually closer to the mass of much lighter Saturn – it's not very dense, and it has only about 30% the mass of Jupiter. In fact, this planet has about the same density as a piece of cork.

This low density results from its ultra-close 3.8-day orbit around its star. Being so close, the planet receives an immense amount of energy, which causes it to heat up. As gas heats, it expands. Once the gas expands enough, it begins to escape the planet's gravity for good.

When we observed this planet, my colleagues and I detected helium gas escaping WASP-69b rapidly – about 200,000 tons per second. This translates to the mass of the Earth lost every billion years.

Over the star's lifetime, this planet will end up losing a total atmospheric mass equivalent of nearly 15 times the mass of Earth. This sounds like a lot, but WASP-69b is approximately 90 times Earth's mass, so even at this extreme rate, it will only ever lose a small fraction of the total amount of gas from which it is comprised.

The cometlike tail of WASP-69b

Perhaps most striking is the discovery of WASP-69b's extended helium tail, which my team detected trailing behind the planet for at least 350,000 miles (about 563,000 kilometers). Strong stellar winds, which are a constant flow of charged particles emitted from stars, sculpt tails like this. These particle winds ram into the escaping atmosphere and shape it into a cometlike tail behind the planet.

Our study is actually the first to suggest that WASP-69b's tail was so extensive. Past observations of this system suggested the planet had only a modest tail or even no tail at all.

This difference likely comes down to two main factors. For one, each research group used different instruments to make their observations, which could result in varying detection levels. Or, there could be actual variability in the system.

A star like our Sun has a magnetic activity cycle, called the “solar cycle.” The Sun's lasts for 11 years. During peak activity years, the Sun has more flares, sunspots and changes to the solar wind.

To complicate things even more, each cycle is unique – no two solar cycles are the same. Solar scientists are still trying to better understand and predict our Sun's activity. Other stars have their own magnetic cycles, but scientists just don't have enough data to understand them yet.

So the variability observed for WASP-69b may come from the fact that every time it gets observed, the host star is behaving differently. Astronomers will have to continue to observe this planet more in the future to get a better idea of exactly what's going on.

Our direct look at WASP-69b's mass loss tells exoplanet researchers like me more about how planetary evolution works. It gives us real-time evidence for atmospheric escape and supports the theory that hot Neptunes and Radius Gap planets are hard to find because they just aren't massive enough to retain their atmospheres. And once they lose them, all that is left to observe is a rocky super-Earth core.

The WASP-69b study highlights the delicate balance between a planet's composition and its stellar environment, shaping the diverse planetary landscape we observe today. As astronomers continue to probe these distant worlds, each discovery brings us closer to understanding the complex tapestry of our universe.

Dakotah Tyler, Ph.D. Candidate in Astrophysics, University of California, Los Angeles

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