Lethal attacks on gorillas by chimpanzees (part 4 of 4)

In part 3 of the story, Lara M. Southern described what it was like to witness both instances of lethal attacks on gorillas by chimpanzees. These events were unprecedented in the history of primatology, forcing scientists to study what was happening to cause it. Part 4 of our story asks: “why did the chimps do what they did?”

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.

Did climate change cause the lethal attacks on gorillas by chimpanzees?

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.