It had been a long day in the field and we were all exhausted. I crawled into bed amid a thunderstorm and listened to the loud claps of thunder and the rushing sound of rain falling on the ground outside. I started to doze as the thunder gradually softened and the rain turned to light drizzle. Abruptly, I was jarred awake by the harsh and unmistakable vibrations of an earthquake. We ran outside and waited another 30 seconds or so until it passed. Everything seemed fine. No major damage, but people yelled loudly from the nearby village as monkeys gave loud alarm calls. This earthquake was the second in as many days. The first measured magnitude 5.2. This one felt quite a bit stronger, measuring magnitude 5.7. We had a restless night, with aftershocks as strong as magnitude 5.4 occurring frequently throughout the night. Seismic activity is not uncommon in this area. Western Uganda lies along the Albertine Rift, an active area of tectonic shifts that are slowly separating the Somali Plate in East Africa from the rest of the continent. The earthquakes we’ve experienced in the past week have had their epicenter along the rift in Lake Albert to the west.On our way to the field the next morning, we were all a bit bleary-eyed, but the earthquake was an animated topic of conversation among all. Amid our discussions of how our Ugandan friends responded to the event, I wondered how the chimpanzees in the region fared as well. Ethologists and others have written numerous articles regarding how animal behavior changes preceding and during seismic activity (e.g., Buskirk et al. 1981; Shaw 1977). Most of these accounts have come from captive animals. For example, in 2011 the National Zoo described the responses of various animals to an earthquake of magnitude 5.8 in the Washington, DC area. Orangutans and western lowland gorillas were among the zoo inhabitants to respond to the quake, with members of both species climbing higher in their enclosures and vocalizing.
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What about free-living primates? Snarr (2005) similarly reported that wild mantled howler monkeys (Alouatta palliata) responded to an earthquake by vocalizing and climbing higher in the trees. Fujimoto and Hanamura (2008) provided a rare account of wild chimpanzee responses during a strong earthquake and its aftershocks in the Mahale Mountains, Tanzania. The chimpanzees vocalized loudly (but were not visually observed) during the main quake. During aftershocks, they typically responded by climbing higher or by interrupting their activities. Interestingly, one female was observed to climb down and touch the ground with her hand, as though to explore the vibrations she was feeling from the ground. Few other accounts exist regarding the behavior of wild non-human primates in response to earthquakes, however.
When we arrived in the field, we met with two farmers who gave us their own account of the previous night’s events. They were tending an evening fire at their small hut at the garden’s edge. The garden is situated on a steep hillside in a mountainous rural area. There is a small forest patch near the garden, and the farmers knew chimpanzees were nesting there. When they began to feel the tremors, they also heard the screams of chimpanzees. The vocalizations quickly grew nearer, and soon they saw an estimated 20 chimpanzees in the garden outside their hut. Amid their surprise, they grew concerned that the chimpanzees had run out of the forest to attack them in response to the quake. They banged their machetes on rocks to intimidate the chimps, who went running back into the forest.
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When the farmers awoke in the early morning hours, the chimpanzees called from another patch of forest about 300 meters away. The men were surprised that the chimpanzees seemed to have moved their nests in the middle of the night. Perhaps this shift was related to the strong aftershocks during the night.
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Because we weren’t there, we have no way of confirming whether their account of events is entirely true. However, everything else they told us—the locations of nests, what the chimps had been feeding on, and where they went in the morning—was borne out by the evidence we later found. In short, I have no reason to suspect that their account is anything other than accurate.
What can we make of this account? If chimpanzees did climb down from the trees and leave the forest as the farmers said, this would contrast sharply with Fujimoto and Hanamura’s account of chimpanzee responses to seismic activity. Indeed, most accounts of responses by captive and free-living primates indicate that climbing up is a more likely response. Perhaps, though, there are reasons why climbing down would be more sensible for these chimpanzees. First, their remaining forest is small and degraded, and surviving nesting trees may be much smaller than those seen elsewhere. Smaller trees may move more and provide less stability during seismic events, making them a less secure option than the ground. The frequent felling of trees in this area may also make chimpanzees particularly wary of the possibility of a tree falling during seemingly dangerous situations. In addition, there are no large carnivores like leopards here, as have been reported the Mahale Mountains, so perhaps moving to the ground at night is perceived as less dangerous. Unfortunately, the paucity of observations of such events makes it difficult to observe these responses and clarify their meanings. Nonetheless, it is fascinating to consider how chimpanzees, humans, and our ancestors may have responded to seismic activity over the course of evolutionary history.
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Buskirk, R., Frohlich, C., Latham, G. (1981). Unusual animal behavior before earthquakes: a review of possible sensory mechanisms. Rev Geophys Space Phys, 19, 247 – 270.
Fujimoto, M., Hanamura, S. (2008). Responses of wild chimpanzees (Pan troglodytes schweinfurthii) toward seismic aftershocks in the Mahale Mountains National Park, Tanzania. Primates, 49, 73 – 76.
Shaw, E. (1977). Can animals anticipate earthquakes? Nat Hist, 83, 14 – 20.
Snarr, K. (2005). Seismic activity response as observed in mantled howlers (Aloutta palliata), Cuero y Salado Wildlife Refuge, Honduras. Primates, 46, 281 – 285.
This post was originally published at Scientific American.