Climate change affects the relationship between chimps and gorillas in the wild

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Lara M. Southern of Osnabrück University in Germany went on the Gorilla Project podcast to discuss how climate change affected the relationship between chimpanzees and gorillas in the wild. She wrote the famous research paper entitled “Lethal coalitionary attacks of chimpanzees on gorillas in the wild.

“What you usually get with gorillas is you get a scent,” Lara described.

“They really smell very strong. Once you’re a chimp researcher, you can distinguish between this chimp and gorilla scent because it’s very different. We spend our whole days around chimps.”

Lara and the researchers initially thought there were levels of mutual avoidance between chimps and gorillas. Whenever they were in the forest, they could smell gorillas. They could see their feeding tracks and their footprints. So, they always knew they were there. They could also hear them sometimes. But Lara noticed that, to the chimpanzees, there was an indifference to the presence of gorillas. The scientists witnessed nine peaceful encounters between the two species. Lara even remembered seeing them eating figs together in a Ficus tree. Yet these friendly interactions didn’t surprise them because they were in line with research done at other field sites.

“So when these attacks happened,” Lara conceded. “It was something I don’t think I would have ever been expecting on any level.”

On the morning of February 6th, 2019, Lara and the researchers did what they always did- they got up very early to go to the nest site where their chimpanzees had slept the night before. At the nest site, they would begin their “focal follows,” following a different individual chimp each day.

“It’s the most exciting part about field research,” Lara thought. “Because you never know what’s going to happen.”

They set out to find the chimps (about five kilometers from the camp), and little did they know that they would walk thirty-five kilometers that day, one of the biggest “patrols” since the habituation study began.
To defend their territory, explore its boundaries, and ensure everything was in order, the chimpanzees would go on huge walks called “patrols.” They would be very silent, smell everything, listen intently, looking for neighboring communities. The researchers followed them to the very edge of their territory. At the very end of the day, dinner time, everyone was done and ready to go home.

“We were ready to put the chimps to bed and let them go nest,” Lara described.

It was around five o’clock when the forest was already getting dark.

“And that’s when all of the crazy commotions began.”

Listen to the full Gorilla Project podcast episode where Lara Southern tells the story in her own words.

Lethal attacks on gorillas by chimpanzees

Loango National Park in Gabon. The end of the day was when the chimps re-entered their territory after their patrol. Lara Southern and her team of researchers were following the day’s “focal”, an individual chimpanzee named Candy. They entered a dense part of the forest. After a while, they started to lose sight of the chimps. Then, they heard a distinctive scream of another chimpanzee named Freddy. It was immediately after that scream that they heard a bark. Lara’s head did a double take.

“Because they’re always screaming, but this bark really stuck out to me. And after that, it just exploded. A chimpanzee and a gorilla sound nothing alike. And then it was just this crazy cacophony of noise.”

The chimpanzees made alert calls, barks, and roars. The gorillas responded with roaring and barks of their own. The team didn’t have excellent visibility at first.

“We try and really keep our distance because they are completely wild animals, and we also have very high safety protocols,” Lara explained.

“There’s always these negotiations in the forest between when’s the time to leave versus your observation of what kind of data you’re going to get.”

The field guides were more interested in not getting their team injured. They tried to convince Lara that this situation was unsafe and that everyone needed to leave, but as a scientist, she felt they needed to stay and see what would happen. She repeatedly begged the field guides to let them observe for “just five more minutes… five more minutes”.

During this encounter, the chimps attacked a group of five gorillas: one silverback, one infant, and three females.

“This is unprecedented. I’ve never seen this before, I’ve never heard about this before,” Lara remembered thinking.

They tried to find a good vantage point to see what was happening. Lara and her team hid behind a massive log, peeping over the top. They made themselves known while staying far enough away to avoid interfering.

“I could describe it as kind of like a ball of chimps and gorillas, and we could see that the silverback was in the middle.”

Finally, the silverback charged the female adolescent chimpanzee named Gia (pronounced Gee-AH) and sent her flying into the air. Lara was worried when she saw the silverback attack the chimp.

“She was one of my favorite chimpanzees. She was always a big explorer,” Lara describes.

“She didn’t seem like a young female. She was really always with the males.”

The male chimps responded by surrounding the silverback and attacking him. He retreated with his family. The researchers later observed that the chimps captured the gorilla infant before the other gorillas could escape. The chimps passed the baby around for an hour and beat it to death. To this day, no one has found the body of the infant gorilla the chimps abducted.

Listen to the full Gorilla Project podcast episode where Lara Southern tells the story in her own words.

Lethal attacks on gorillas by chimpanzees

Even though Lara and the other scientists were entirely shocked by the first attack, she knew it was important to relay the information to their supervisors stationed in Germany. The most challenging part was to be thorough. Their write-up needed to capture every single detail in the exact way that it happened. Their thoroughness relied heavily on viewing their video footage over and over again.

“One thing I find very helpful is to watch animal behavior from behind the lens,” Lara suggested.

“So, when something crazy is happening or when something uncomfortable is happening, if you just film, there’s always this wall between you and what you’re seeing, and you can always remember your place as just an observer. That was something that helped me in both of those instances to document what I was seeing rather than put my own opinions on what I was seeing.”

On December 11th, 2019, Lara and her group of researchers followed the chimpanzee patrol far north of their home range territory.

“Whenever we go up there, everyone gets a little bit grumpy because there are the big swamps. And when there are swamps, it means you have to swim.”

Walking towards the northern swamps, they could see the chimps begin to change their behavior. Once they got outside their territory, they became reticent, displaying similar behavior to the lethal February 6th event. They walked in single file, sniffed the ground, and smelled the eaten fruit near a tree, searching for the presence of rival chimpanzee groups. The chimp named Freddy was in front, near a female named Joy. He stood up bipedal to get a better look at something, reacting to what he saw with alarm barks.

“My initial reaction (because I had already seen them on patrolling behavior) is that they had found a chimpanzee group,” Lara remembered.

The chimps found a family of seven gorillas feeding high up in the trees. They climbed up to attack them, forcing the silverback to retreat, leaving the mother gorillas defenseless. The silverback’s role was to protect his family, but Lara defended the silverback’s decision to flee.

“It was a pretty hard decision in that case. There were seven gorillas against a group of 27 chimpanzees. Chimpanzees are very loud, they’re very vocal, and they cooperate. This is something we talk about in the paper. They’re very cohesive in terms of behavior when they feel like their group is threatened or when they’re going into doing a collective behavior. All of them participate.”

And this occasion was no different. Every chimpanzee joined the attack- even the juveniles were barking at full-grown adult gorillas.

“Which is something that I haven’t really seen before. So, in a sense, the silverback fleeing, I think, was also just a choice of personal safety. I think it was a hard choice to make.”

Several other gorillas had already fled, so only two females and two babies were left in the group. Lara speculated that the silverback might have been attempting to initiate the movement of the remaining members of his family by trying to get them to follow him. The other two females were left high up in the canopy with their babies, allowing the chimps to climb up the tree and trap them. The chimpanzees chased one of the mother gorillas through the trees across the swamp, cornered her, and snatched her baby. Roxy, the older adult female chimp, ripped out the baby gorilla’s intestines and ate them, snacking on its body for hours.

“Roxy played everyone’s least favorite role in the second encounter because she’s the one who ingested the baby gorilla. So, she kept the prey, and she carried it with her, and she fed on it as a piece of prey for most of the afternoon after they caught the gorilla.”

Lethal attacks on gorillas by chimpanzees

The Conclusion: “Wait, but WHY?”

Chimpanzees were mostly frugivores. When they resorted to hunting, they ate things like insects, birds, lizards, and monkeys. No scientist could have imagined that a chimpanzee would ever hunt, kill or eat a gorilla. Lara M. Southern and her fellow researchers noticed these attacks were peculiar for many reasons. They shared characteristics of other chimpanzee hunts: they walked in single-file, made hunting calls, changed their direction silently, scanned the area, and other typical chimpanzee hunting maneuvers. Although it appeared that the chimps targeted the baby gorilla because it was smaller prey, this is where the similarities to other hunting tactics ended. She mentioned this in her research paper titled “Lethal coalitionary attacks of chimpanzees on gorillas in the wild“.

“That’s something we touch upon in the paper. We wanted to emphasize how different this was from hunting behaviors we’ve seen in the past and how much more similar this was to an inter-community encounter with other chimpanzees. They treated the gorillas as if they were neighboring groups intruding on their territory that they were at war with, which we’ve never seen them do with gorillas. It was very different from hunting, and we used the example in the paper where we talk about the noise levels.”

She explained that the chimps made lots of noise before they attacked, but the excitement instantly disappeared. They fell silent as soon as they captured the baby gorilla.

“And when you hunt, it’s completely the opposite,” Lara emphasized.

“There’s this very silent, coordinated hunting strategy, and feeding on the prey is the reward when they catch the prey.”

So here, the researchers saw that it was almost like the killing was the reward- much similar to inter-community encounters.

“There sometimes was cannibalism in inter-community attacks, but it was not so much as this baby as prey but as a symbol or collateral damage within a war.”

She also noticed that the incidents resembled previous attacks by chimpanzees against rival chimps in neighboring territories. For example, adults have killed infants from other communities to eliminate food competition by discouraging female chimps in the competing group from venturing into the disputed territory. However, this attack was the first time anyone witnessed chimpanzees doing the same thing to gorillas.

The researchers noticed that the event was unique due to how the chimpanzees ate the gorilla during the second attack. Chimp hunts typically involved the highest ranking male member possessing and eating the food first. They also tended to involve “food begging” by other lower ranking members and sharing based on rank. In this case, the researchers observed that a female ate the gorilla, only briefly sharing it with another low-ranking chimp. The high-ranking male chimp showed little to no interest in eating it.

How Climate Change Affected the Relationship between Chimpanzees and Gorillas in the Wild

The question remained: what caused the chimpanzees to suddenly view gorillas as rivals, attack them, and eat their baby? Lara Southern’s research cites Emma R. Bush’s paper titled “Long-term collapse in fruit availability threatens Central African forest megafauna.” The study used a rare long-term dataset of tree reproduction and a photographic database of forest elephants in Lopé National Park in Gabon. Did this long-term phenological data reveal the reason behind the high food competition between gorillas and chimpanzees? The paper-based its findings on one of the largest phenological databases in the world, indicating that the weather was getting hotter and dryer. The lowest average daily temperature in Lopé was increasing by approximately 0.25 degrees per decade.

“And that’s HUGE,” Lara stressed. “If you’re just thinking about in twenty years to come, we’re talking about a degree level of already extreme heat and extremely high temperatures for these species to cope with.”

The problem was that the trees in Lopé relied on the temperature to fall below a certain threshold to know when to begin flowering and producing fruit. Therefore, tree reproduction was cued less often due to the rising temperatures. The scientists discovered that trees in Lopé were producing fruit less often, and the probability of finding fruit was declining significantly over time. They witnessed a massive drop between 2000 and 2003. For example, September was the time for fruit scarcity in Lopé. It changed to between October and March, which was previously the historical time for fruit abundance. The average rate of finding fruit in trees went from approximately 1 in 10 in 1987 to 1 in 50 by 2018. In addition, it showed an 81% decline in fruiting between 1986 and 2018 and an 11% decline in the body mass of forest elephants from 2008 to 2018. The researchers witnessed that the decrease in fruit availability harmed the megafauna (specifically elephants, gorillas, and chimpanzees) residing in Lopé National Park.

“And I think you would see the exact same thing in Loango (National Park in Gabon) if we had that same comparable data set which hopefully we’ll be able to do soon.”

Lara explained in detail how these findings affected chimpanzees.

“They constantly know which fruit resources to go back and visit each year at a very specific time.”
Lara believed that this talent may not have been innate.

“It’s obvious that they learn, and it’s obvious that there are certain members of the group who teach others that there’s this rhythm and flow. We see this in all natural ecosystems. Nature has a problem and then a solution to everything. And it is things like climate change that really complicate these natural systems.”

“So chimpanzees, just imagine, for year after year, they’ve been going to these same sites, and all of a sudden, there’s these slight gradual changes that will have huge payoffs in their daily lives because it will affect their energy levels, their body mass, how far they’re able to travel, how tired they are, and how able they are to protect themselves against other groups. There are so many knockdown effects (that we couldn’t even quantify at this stage) that could be really harmful to not only these populations but populations everywhere.”

She went on to explain how this situation deteriorated the relationship between chimps and gorillas.

“So, chimpanzees and gorillas have a high dietary overlap,” Lara explained.

“They’re both great ape species, and they both share the same food types. Generally, we see on many other sites that there’s a level of niche partitioning. So, they’ve evolved over time to eat different species to limit these levels of competition. But what we think happens at Loango [National Park in Gabon] is that, at certain times of the year, where there are certain key [fruit] species that are favorite species of both gorillas and chimpanzees, you might get these levels of very high competition. And in this competitive atmosphere that’s already there, if you throw in a lower production season for all of those fruit species, you will get even higher levels of competition. So, we think that this might be happening, especially for certain years. This might be more pronounced. There’s a lot less ripe fruit available if it’s a bad fruiting season. And if this continually keeps happening, of course, sources of fruit become more clumped or even more dispersed and harder to find, and you’re attracting two great ape species to a limited resource. So, this creates a base for any form of conflict which you wouldn’t have if you had a larger abundance of fruits.”

There were countless researchers around the world collecting climate change information. Still, the data from the Emma R. Bush paper was a cumulative effort that took phenological data on such a precise level for decades. Then, finally, scientists began to see these critical patterns. Loango National Park researchers also had a phenology route to check the fruiting species. It was something that they monitored.

“But it’s so hard to know long-term seasonal trends in a normal variation because you really do need these long-term data sets. So what we did with the (Emma R.) Bush paper is we’ve taken a lot from that. Because I think there you can really see the drastic figures that they’re reporting: the number of things that are getting dryer, big problems with the rainy season. There are so many fruiting species that have those very specific requirements for temperature changes in order to fruit. Loango National Park, as well as Lopé, is a big stronghold for elephants, gorillas, and chimpanzees. So, it’s probably the exact same thing that’s happening. We just don’t have the data to support it. But we’re really working on getting there.”

Lara Southern and the researchers continued to study the encounters between gorillas and chimpanzees, their root causes, and piece together the data.

“We’re trying to fill in the gaps of what we couldn’t fill in in the first paper. This was really exciting research, and we wanted to get it out there and say, ‘Look! This is happening.’ There are so many angles to look at it from and why it’s interesting. It’s interesting from a species perspective just because we haven’t ever heard of this reported across any other field site where there are both chimpanzees and gorillas. They do coexist in other places, and this hasn’t happened yet. So, it seems that there must be something going on. And what you need is long-term data, so it’s an ongoing process. What I can say is that there do continue to be encounters.”

Listen to the full Gorilla Project podcast episode where Lara Southern tells the story in her own words.