Celebrate World Chimpanzee Day on July 14 with Nat Hab!

I want to open this post by taking you into Kibale National Park in Uganda to experience the sounds you might encounter while chimpanzee trekking on a Great Uganda Gorilla Safari. The video captures the calls of an extended community of chimps gathering in a large fig tree to feed and nest for the night.

This is one of the most thrilling sounds in nature. You can’t help but feel your heart race when you hear the first far-off calls echoing through the equatorial rain forest on a primate safari. Unlike a traditional African safari, you’re not separated from the environment by the shell of a Land Cruiser. Instead, you are fully immersed, walking through the forest with all your senses awakened. And if you know a little about the complexities of chimpanzee behavior, an encounter with these great apes becomes truly fascinating.

Before diving into some interesting facts about chimps, however, it’s important to acknowledge that World Chimpanzee Day—and much of what we have learned about chimpanzee behavior over the last 60 years—would probably not exist without one diminutive woman who has had an outsized impact on science.

How Jane Goodall Changed the Game

Jane Goodall wasn’t the first to study chimpanzees when she set up her camp in Gombe on Lake Tanganyika at the request of the famed paleontologist Louis Leakey, but she changed the way field biology is done. Today, she is universally regarded as a giant in the world of science and conservation. But even this expert had to fight to be taken seriously, and for two main reasons:

First, she had the audacity to do field research as a woman. She was a trailblazer in a field that didn’t want her in the 1960s. In Goodall’s own words, “When I was a little girl, I used to dream as a man, because I wanted to do things that women didn’t do back then such as traveling to Africa, living with wild animals and writing books.” Fortunately for all of us, she saw her gender as an asset rather than an obstacle in the male-dominated field of scientific research.

Second, Goodall did her research differently than anyone else. This initially led traditional scientists to disregard her results. She saw chimpanzees as individuals with personalities and gave them names rather than numbers. She assigned intent and emotion to them. This was unheard of at the time, but today, few people think of chimpanzees as mechanical clusters of instincts. We wouldn’t understand chimpanzee behavior and intelligence in the way we do today if Jane Goodall hadn’t paved the way.

A chimpanzee eating fruit in the rain forests of Uganda.

© Mark Jordahl

Perhaps most significantly, Goodall kept doing her research way longer than anyone else. How can you hope to understand the behaviors and social dynamics of a species that can live for 30 years or more through just a six-month or one-year study? Sixty years of continuous study at Gombe earned Goodall a wealth of insights into chimpanzee behavior, such as the fact that chimps hunt, the ritual sharing of meat, the presence of warfare between chimp communities and, of course, tool use.

Now, let’s learn about some fascinating things researchers have discovered about chimps in the past six decades.

Don’t Tell the Kids—Sugar Builds Big Brains

As their canine teeth show, chimpanzees are equipped for hunting. When the opportunity arises, they will feed on small mammals in the forest, particularly red colobus monkeys, forest antelopes like duikers, and bush babies, which they remove from holes in trees with spears they make. However, meat only makes up about 2% of their diet.

They also eat invertebrates, mushrooms, leaves, and even soil, which is believed to neutralize toxins in their food.

But 60% of their diet is fruit—predominantly figs. Chimps love figs. Figs have a lot of sugar. And large brains really love sugar.

The brain size of chimpanzees is connected to both their diet and behavior. Comparing them to mountain gorillas helps show how different brain sizes can relate to lifestyle.

Chimpanzees and mountain gorillas are both great apes, and they live in very similar environments, sometimes even overlapping as they do in Bwindi Impenetrable National Park in Uganda. Adult chimpanzees weigh, on average, about 100 pounds. Mountain gorillas range from about 250 pounds for an adult female up to 500 pounds or more for a male. Despite these dramatically different body sizes, they have the same size brains.

So, why have two great apes, both living in equatorial rain forests, evolved so differently? (For primate lovers, it’s worth noting that you make your own comparisons by joining a primate safari with Nat Hab, which offers a rare opportunity to spend time with two different great ape species on a single trip!)

Mountain gorillas primarily eat leaves, which don’t provide enough energy to support large brains. Gorillas are also always surrounded by their food, with only minor seasonal variations. They don’t need big brains because they don’t need to figure out where and how to find food. The low nutrition in leaves also accounts for their sedentary lifestyle–they rarely move more than a mile in a day.

Mountain Gorilla in Uganda

Mountain gorilla © Kendra Olson

Chimps, however, have a more varied diet that requires thought and energy to gather across large areas of the forest. They are not constantly surrounded by ripe fruits, nor do they know where they will find them at any given time.

Here in the northern hemisphere, we are used to seasonal cycles with fruits. We know what berries will be ripe in the spring. We know that we can look forward to apples in the fall. Evergreen cones all mature in late summer so that squirrels can stock up for the winter. These regular cycles create a superabundance at one time so that seed or fruit predators can get plenty to eat without devouring them all. They eat some, cache some in their larder, and the rest will take root and grow the next generation of trees or bushes.

In an equatorial rain forest, where the only seasons are the rainy season and the rainier season, fruits ripen randomly throughout the year. In the case of figs, they specifically don’t fruit at the same time as their neighbors to reduce competition. Fig trees have massive canopies that prevent new trees from taking root below, so each tree needs chimps and other primates to feed on their fruits so the seeds will be spread.

This means chimpanzees need the brain power to remember where the best trees are at a given time of year or the energy to scour the forest looking for food. It also means that the conservation of chimpanzees and other primates is critical for overall forest health due to their important role as seed dispersers.

A chimpanzee feeding on fig fruits in the Kibale nature reserve, Uganda.

A chimpanzee feeds on a fig, aka brain food!

Chimpanzees Have Complex Social Dynamics

Chimps live in what’s called a fission-fusion society. They are part of one large community of 50 to 190 individuals with a single alpha male, but they spend most of their time in smaller groups to travel, feed and mate.

Sub-groups change daily based on a series of decisions, so social dynamics like status must be constantly negotiated. Coalitions of lower-ranked males plot against higher-ranked males while away from them. Sub-groups need to know how to regroup with the parent group later.

Think about what this means for an alpha chimp, where most community life happens outside of his view, compared to mountain gorillas, where the family of 8 to 20 spend all of their time in close proximity. The silverback can relax; he knows everything that’s happening. But alpha chimps’ cortisols, or stress hormones, are always elevated, because you never know what’s happening behind your back.

A chimpanzee family in Uganda.

Looking For Love in ALL the Places

In many mammal species, the largest and strongest alpha male controls breeding and sires most offspring in the group. For mountain gorillas, the dominant silverback is the father of at least 85% of the young. In chimpanzees, the alpha is only the father to a third to a half, depending on the size of the community. As community size increases, paternity of the alpha drops.

This is not a surprise when you think about chimps being spread out across the forest in smaller groups. No alpha could control mating in that situation, no matter how big or intimidating he is. Sometimes a lower-ranked male will mate with a female while traveling with their subgroup. They may even go “on safari” and slip off together for a few days or weeks. Some females will mate with many males while they are in estrus. One female at Gombe, Jane Goodall’s site, was observed mating 65 times with 18 different males in 11 hours.

Clearly, these females are not only mating with the strongest or fittest males. With so many potential fathers, sperm competition becomes a more important factor than physical size or aggressiveness. Male chimps have large testicles instead of large bodies in the hopes that their sperm will be more successful than the others. Chimp testicles weigh 4 times as much as gorilla testicles.

For females, mate selection is not important. They often don’t have a choice, so they cast a wide net and hope the strongest sperm will succeed. Their main concern is to live in a food-rich environment so they can raise healthy young.

Chimps Form Gangs

Coalitions of male chimps regularly patrol the boundary of their territory and make incursions into neighboring territories. These can be violent, as was first recorded by Jane Goodall in the 1970s. Later research has supported her findings that chimps wage war on nearby communities, slowly picking off the males from a neighboring community until it is weak enough for them to take over entirely. Sometimes, if they already live in an area with a reliable food source, they might wage a preemptive strike to protect their territory.

There is a lot of intention that goes into these patrols, from choosing members to deciding where to go. Often the chimpanzee coordinating the patrol will select male relatives that he trusts. Sometimes they choose chimps who have a history of being part of successful raids. Sometimes a chimp is selected because they are seen as a potential ally who will support a bid for power against an alpha male later. These are all sophisticated decisions requiring the highly developed brain discussed above.

Once the patrol reaches the edge of their territory, they get quiet and alert while they assess the situation. Whether or not a physical confrontation happens depends entirely on numbers. If the patrol finds a lone male or a small group, they are very likely to attack. If they come across another patrol that has as many, or more, than their own, they will retreat.

Chimpanzee patrol

Chimpanzees Are Political Animals

Unlike in many other species, it is not the largest or most aggressive male chimpanzee that becomes dominant—it’s the best coalition-builder. A weaker male who builds a strong coalition can dominate a stronger male, and they are very generous with meat-sharing while they are working their way to the top.

A good alpha also supports the community. Showing empathy and providing comfort in stressful situations helps him maintain his position. In medium and low ranks within the community, females exhibit the most comforting behavior. In the higher ranks, it is the males who do the most consoling, and the alpha is involved in more comforting interactions than any others.

It’s not easy to be the alpha in a fission-fusion community. Most of the community’s activities take place out of your sight, and there are always younger males plotting against you. You need to reassert your status every time the troop re-gathers. Your cortisol levels are always elevated, and you are always at risk of injury from fights. The average tenure as alpha is only four years.

So why bother to try to climb the ranks in a society where all males have a chance to mate and where alphas live under constant stress?

One reason seems to be the promise of a longer lifespan. Let’s bring it back to human politics. The first eight U.S. presidents had an average lifespan of 80 years. And this was in the 1700s and 1800s, when the average life expectancy for white men in the United States was just 38 years!

For chimps in Gombe, the average lifespan for males who don’t become alphas is 25.5 years. For alphas, it’s 33.4 years. Alphas generally lose dominance at about the same age other males die, then live another eight years. Researchers have not yet found an explanation for this, but there must be something to it.

Power is a mysterious elixir.

Chimpanzees Just Might Have Your Back

I want to end this post with a story I heard from a researcher studying chimpanzees in Budongo Forest in Uganda. He had been working with a team habituating a family of chimps, getting them comfortable with human presence so they could be studied. The team had spent weeks with them, and the chimps had stopped being afraid or aggressive towards their human observers.

One day, 15 of the chimps dropped out of a tree and charged the researchers. They were screaming, baring their teeth and trying to scare them away. The researchers didn’t know what was happening until they saw three of the chimps gather in one spot, looking at the ground not far from where they had been standing. There was a massive python coiled there. Far from attacking, the chimps had been protecting their new friends.

Chimpanzees are complex and fascinating animals that we are only beginning to understand. Celebrate World Chimpanzee Day by learning more about them and making plans to see them in the wild.