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SUNDAY SCIENCE: PLAYING WITH THE KIDS IS IMPORTANT WORK FOR
CHIMPANZEE MOTHERS  
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Zarin Machanda, Kris Sabbi
May 8, 2024
The Conversation
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_ So on this Mother’s Day, consider raising a glass to also
celebrate these amazing – and tired – chimpanzee moms. _

Chimp mothers take on the critical role of playmate with their
young., Kris Sabbi

 

Wild chimpanzees have been studied for more than 60 years
[[link removed]], 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
[[link removed]] 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.

Two moms with babies play with one another on small trees, and two
other young chimpanzees join in.

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
[[link removed]],
as primatologists
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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
[[link removed]] 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
[[link removed]].

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
[[link removed]]. During the months when
these fruits are ripe and plentiful, chimpanzees spend more time
hanging out together in larger groups
[[link removed]].

This sort of energy surplus has been linked to rigorous activities,
such as hunting among monkeys
[[link removed]]. 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.

[Female chimpanzee sits with her infant on a tree branch.]
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Gathering enough food to eat is a critical daily task. Kris Sabbi

When life gets in the way of play

To test this idea, we turned to the long-term records of the Kibale
Chimpanzee Project [[link removed]],
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 [[link removed]] 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
[[link removed]]. Males invest a lot of
time developing friendships
[[link removed]], and, in turn, they
reap the rewards of those bonds: higher dominance rank and more sex
[[link removed]]. 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 [[link removed]].

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
[[link removed]] 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
[[link removed]]. 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
[[link removed]]. 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.

A chimpanzee mom tussles playfully with her young daughter while her
infant nurses.

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 [[link removed]]. 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 [[link removed]], 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.

Chimpanzees and people enjoy some of the same kinds of physical play,
like playing airplane.

Since chimpanzees are one of our closest living relatives
[[link removed]], 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.[The Conversation]

Zarin Machanda
[[link removed]],
Assistant Professor of Anthropology and Biology, _Tufts University
[[link removed]]_ and
Kris Sabbi [[link removed]],
Fellow in Human Evolutionary Biology, _Harvard University
[[link removed]]_

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

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