When it comes to nurturing their young, mother chimpanzees go the extra mile, according to a new study. Using 10 years of observational data on wild chimpanzees, researchers found that while adults often play, and young chimps play a lot, when food gets scarce, the adults put mutual play aside and focus on survival.
But in the meantime, mother chimps continue to be their offspring's primary playmates, tickling and chasing. That suggests the mother chimps take on an indispensable role fostering their young's physical and social development even when they are under food stress.
The study observations took place in Kibale National Park in Uganda. The study analysis, titled Ecological variation in adult social play reveals a hidden cost of motherhood for wild chimpanzees and published in Current Biology, was led by Kris Sabbi, who graduated from The University of New Mexico with a Ph.D. in Anthropology in 2020 and is currently a college fellow in the Department of Human Evolutionary Biology at Harvard University, and her former doctoral associate Zarin Machanda, an assistant professor of Anthropology and Biology at Tufts University.
Co-authors include UNM Professors of Anthropology Melissa Emery Thompson and Martin Muller, UNM Anthropology graduate student Megan Cole, and Isabelle Monroe, UNM graduate and research lab coordinator at University of Michigan.
This work is an extension of Sabbi's Ph.D. research on chimpanzee development, supervised by Muller and Emery Thompson, co-directors of the long-term Kibale Chimpanzee Project. UNM, in collaboration with Tufts University, curates a long-term record comprising over 100,000 hours of observations of wild chimpanzee behavior.
Kibale is the most primate-dense forest in the world, with 13 species living there, including over 1,000 chimpanzees. Over the decades, teams of researchers took detailed field notes of almost every observable behavior— including climbing, feeding, grooming, calling, aggression, and play.
Research highlights
- Adult chimpanzees played more often with all partners when diet quality was high
- Unlike other dyads, only mother-offspring play persisted when diet quality was low
- Playing with offspring helps balance the costs of grouping with offspring needs
- Like with humans, parent-offspring play may be crucial to development for chimpanzees
Through their previous work, Sabbi and Machanda were familiar with the playfulness of chimpanzees and decided to look deeper into the patterns of play behavior. They expected seasonal variations in food availability would affect adult chimps' time spent playing. For example, when supplies of quality fruits were low, the chimps focused on finding and gathering figs and leaves and put play time aside. Surprisingly, although chimp mothers had the same challenge in finding food, they continued devoting a lot of their time to nurturing their offspring's development through play.
"In the last three data collection seasons, suddenly adults were showing up as players in my study on infant and juvenile development. And they weren't just playing with little ones, they were playing with each other more often than I had seen in previous years. When the pandemic hit, and we needed to pause field data collection, Zarin and I used the opportunity to dig into our long-term records to find out whether that season was just weird, or there was something more going on," Sabbi said.
Social structure in the chimpanzee world may also explain why mother chimps sometimes become the primary play partners for their young. The chimpanzees have a very fluid social system called fission-fusion, which means a group of 60 chimps, for example, may have smaller groups break away for hours or days, which then merge again while other groups break off.
When food becomes scarce, chimp mothers tend to break away into smaller groups or forage solo with their babies.
"But when they're doing that, they are also limiting the ability of their young ones to play with others, and the moms become the primary playmates," said Sabbi. "They're trading off that lower feeding competition in the larger group for more time and energy being spent playing with their little ones."
As humans, we take for granted the fact that we still love to play games as adults and that we invest so much time playing with our children, Emery Thompson said, and over the years of observing chimpanzees, who play fairly frequently with their offspring, and even tickle and goof off with other adults, the researchers also took this behavior for granted.
"We were surprised to learn from our colleagues who study other social species that while play by young animals is near universal, adults play rarely, even with their own offspring," Emery Thompson said. "One hypothesis is that play stops being useful for animals who have completed their cognitive and motor development, but our results suggest an alternative explanation: Animals in nature have too many competing priorities, particularly when resources are scarce. Adults opt for less costly forms of affiliation, like huddling together or grooming…"
"As evolutionary anthropologists, we are motivated to consider whether similar circumstances– prolonged development, since chimpanzees take 15 years to grow up, and periodic separation from other group members – may have made play between adults and children the norm in our own species," she continued. These findings also raise the question of whether play between adults holds any special significance, for example as a way to reinforce a particularly beneficial relationship, or whether our brains evolved to find play rewarding because it is so important for our children."
Types of Play
Play among the chimps often divides depending on their sex.
"It's not uncommon to see male chimps to engage in more aggressive types of play, while females are doing a type of play related to parenting," said Machanda. "You see them practice carrying things—a kind of preparation for future maternal behavior. Males often size each other up, and when they hit their second birthday, play style changes and can get rougher."
Mothers are often the ones that juveniles and older infants come back to.
"If they're playing with somebody and it starts to get a little bit too rough, they'll switch it up and go back to playing with mom, because at the end of the day it's a very safe place," said Sabbi. "If we compare to humans, it's very easy to find lots of evidence in the child psychology literature for how important it is for human mothers and fathers to be playing with their children, especially at really young ages. Moms and dads are important first play partners before kids branch out into their own social networks."
Findings
There are two impactful findings, according to Sabbi.
"First, when researchers think about play, especially in animals, we usually focus on young ones because we know that play is important for learning during development and drops off with age. So, the assumption is that adults don't play because it doesn't really have a function for them. In this study, we found support for an alternative idea: that play does have functions for adults, but they usually can't afford to play because of time or energy constraints.
"However, there was an exception to the rule," she continued. "Even though all the other types of adult play disappeared when diet quality was low, mothers continued playing with their infants… What makes this particularly interesting for chimps is that we generally think of mothers as less social than adult males because they spend so much time on their own, and this is especially true when food availability is low. Our results show that one of the costs of spending time on their own might be that they are left as the only partners for young offspring. It could also be evidence that, unlike many other primates, the importance of parental play is shared between humans and chimpanzees."
