2014 | Bakoun, Guinea

Western chimpanzee (Pan troglodytes versus) algae fishing

Fishing involves fish. That’s common sense, right? Not for primatologists. Scientists studying our closest living relatives have a somewhat looser definition of fishing, adopting and adapting it to mean any activity where an animal uses a long probe to reach inaccessible food. This semantic revolution has a long heritage too: Jane Goodall first reported ‘fishing’ for termites (she included those quotation marks) among the Gombe population in 1964.

In the past half century, along with termites, chimpanzees have been reported fishing with tools for ants and for honey, and the quotation marks rarely make an appearance these days. Wild chimpanzees have even been reported ‘fishing’ for crabs in the Nimba Mountains of Guinea, although there were no tools involved—they captured those crabs by hand in shallow streams. If a group of chimpanzees are ever found to actually catch and eat fish (the way that some rehabilitated orangutans in Borneo do), the scientific literature may have trouble coping.

Still, clearing up that confusion is not our task today. Here, we’re actively going to make it worse, as we accompany western chimpanzees out fishing for prey that is not only not fish, it’s not even an animal. It’s algae.

Hunting the great green whale

It’s dry season in the Bakoun Classified Forest, Guinea. As part of a continent-wide research effort known as PanAf, remote cameras are set up across 85 square kilometres of this mountainous landscape, cut by river valleys. Water is critical for the animals living in this mosaic of tropical forest and open savannah, with the rainy season from May to October and a prolonged dry period outside of those months.

In April 2014, the camera network picked up the first evidence of chimpanzees using stick tools to poke into ponds and streams, seemingly to get at freshwater green algae. With re-positioning and added cameras, eventually 11 different sites had a record of similar activity. From the time of the first video, until May 2015, around a third of all the chimpanzees captured by the network (486 videos out of 1473) were filmed fishing around in the water.

The lead author of the report on these fishy primates was Christophe Boesch, a veteran chimpanzee researcher best known for his decades of work in the Taï Forest of the Ivory Coast. Boesch and his colleagues describe ‘algae fishing’ as follows:

All chimpanzees were observed to hold the tool in one hand by its end, dip it into the water until it seemingly rubbed against the bottom substrate of the water source, and then guide and swivel the tool to enroll long fibers of the algae onto the tool's other end. Following this, individuals removed the sticks from the water and proceeded to bring the tool close to their mouths, close to the end where algae had collected, and then pulled the entire length of the algae on the tool sideward through their lips and thereby transferring the accumulated algae from the stick to their mouths.

The algae itself is of the genus Spirogyra, which is sometimes called ‘mermaid's tresses’ or ‘water silk’ because of its long strands. It grows upwards from the sediment at the bottom of relatively calm water, hence the need to scrape or rub those strands up from beneath the surface. In general the chimpanzees avoided getting their own bodies into the water, although ankle-deep wading and submerging the hand holding the tool were common enough on the videos. And, just as for human recreational fishing, the activity was a social one carried out by animals of all ages and sexes.

Here’s a video that accompanied the report, courtesy of ChimpandSee. You’ll see that baboons also use these watery locations, but don’t use tools:

When Boesch and his co-author Anthony Agbor visited one of the riverside fishing spots, they took the opportunity to try the technique for themselves. They found that they could collect 400g of algae in 10 minutes, using a stick tool discarded by chimpanzees at the site. Since chimpanzees had been recorded fishing for more than an hour at the same site, kilograms of algae were potentially available to the animals in a single session. The videos showed that most algae fishing happened towards the end of the dry season, in April and into May, adding significant protein, carbohydrates, fats and minerals to the primate diet at a time of potential food stress.

Getting the scoop

Bakoun isn’t the first site where chimpanzees have been spotted grabbing algae with tools. At Bossou, also in Guinea, a small group of chimpanzees have been seen gathering floating Spirogyra algae on-and-off since August 1995. The Bossou researchers, led by Tetsuro Matsuzawa, termed their observations algae scooping, in which the animal strips the leaves from a fern frond or other stalk and, usually holding the tool between thumb and index finger, uses ‘a gentle swiveling action of the wrist’ to gather in the algae strands.

Of all the chimpanzee tool-use activities seen at Bossou, algae scooping is the least commonly seen by scientists. That may be because the periodic field seasons there do not overlap with the time of greatest algae eating, with the ponds drying up. Or it may be that the fad for algae scooping is a recent one at Bossou.

Boesch and his team compared what was known about the tools made and used at Bossou with those they found at Bakoun. The recorded Bakoun chimpanzees had a fairly even split between arriving at the fishing hole with a ready-made tool, using a tool left behind by another fisher-ape, or making a new tool themselves. Their tools were around 70-75 cm in average length, similar to the height of a sitting chimpanzee, while the Bossou tools were shorter, averaging around 55 cm.

A shared feature of the algae tools at both sites is the occasional presence of hooks. Neither research team has shown that these are deliberately made by the animals, but they occur when the base of stripped leaves are left behind on the fishing probe, allowing the probe to catch on the floating algae strands. At both sites hooked tools were shorter, but the Bakoun ones were still longer than the alternative ‘smooth’ probes used at Bossou, so it’s not clear whether this difference has ecological or psychological relevance.

What is clear is that much more information is needed from understudied chimpanzee groups like those at Bakoun. The PanAf program is working hard to redress that imbalance, but it is up against forces of destruction that take us ever closer to a world in which chimpanzee populations die out before we even know of their existence, let alone how carefully they flick their wrists while capturing algae strands at the end of a long dry season.

A parallel example comes from what we might consider the original chimpanzee ‘fish’: termites. In 2020 Boesch and a large number of other chimpanzee field researchers published data showing that there are fine but clear differences in how each group across Africa selects termite fishing tools, and then modifies and uses them. Those differences are enough to identify which community a given animal belongs to, based only on a description of how they gather termites. If human groups did the same, the distinctions would be called culture (and this recent research was in fact published in the journal Nature Human Behaviour, despite emphatically not describing human behaviour).

What we don’t know, and may never know without remedial action, is whether algae fishing has the same kinds of ‘chimpanzee etiquette’. But you can help—just head over to ChimpandSee to join more than 10,000 other volunteers using the PanAf cameras to identify animals and discover new behaviour. At a time when field research all over the world is restricted, it’s a direct way to help out our primate cousins.

Sources: Goodall, J. (1964) Tool-use and aimed throwing in a community of free-living chimpanzees. Nature 201: 1264-1266. || Brewer, S. & W. McGrew (1990) Chimpanzee use of a tool-set to get honey. Folia Primatologica 54: 100-104. || Koops, K. et al. (2019) Crab-fishing by chimpanzees in the Nimba Mountains, Guinea. Journal of Human Evolution 133: 230-241. || Russon, A. et al. (2014) Orangutan fish eating, primate aquatic fauna eating, and their implications for the origins of ancestral hominin fish eating. Journal of Human Evolution 77: 50-63. || Boesch, C. et al. (2017) Chimpanzees routinely fish for algae with tools during the dry season in Bakoun, Guinea. American Journal of Primatology 79: e22613. || Matsuzawa, T. (2019) Chimpanzees foraging on aquatic foods: algae scooping in Bossou. Primates 60: 317–319. || Humle, T. et al. (2011) Algae Scooping Remains a Puzzle. In: T. Matsuzawa et al. (eds) The Chimpanzees of Bossou and Nimba, pp. 117-122. || Boesch, C. et al. (2020) Chimpanzee ethnography reveals unexpected cultural diversity. Nature Human Behaviour 4: 910–916.

Main image credit: Boesch et al. (2017) || Second image credit: Matsuzawa et al. (2019) || Third image credit: Humle et al. (2011) || Video credit: ChimpandSee, YouTube: https://www.youtube.com/watch?v=qEk_sNYAyCo