I'm currently supervising a graduate student who's writing a review of the literature on tool use in wild chimpanzees. This has become a most enjoyable interaction: it's a topic I've been interested in for quite a while now, so the supervision role is an excuse to extend my own knowledge, and it's great helping the student to enhance their own skills in relation to academic research and writing.
Anyway, a couple of days ago I came across a new paper (Boesch et al., 2016) on an intriguing aspect of chimpanzee behaviour, and my student and I had a stimulating discussion about it at our regular weekly meeting this morning. (There's a general summary of the findings and the project which generated them here.) I'd previously heard of (& shared with her) what appeared to be an isolated incident of 'fishing' by an orangoutan, but this new paper documents wild common chimpanzees, Pan troglodytes, using a new technique to obtain freshwater algae. (Of interest in the orangoutan example were the claims that the image of the animal in action were faked, claims discussed here and dismissed as false.)
It seems that it's unusual for primates to eat aquatic plants, although they may eat fish and invertebrates when available. Both bonobos (Pan paniscus) and gorillas eat plants growing in swampy areas. Common chimpanzees do the same, but have also been reported eating algae - something that's really unusual in animals apart from marine species. And it's highly unusual in chimps too:
despite decades of chimpanzee research, there are only a few observations of algae harvesting, suggesting that this behavior is indeed rare
and in most of these observations the chimps used their hands, rather than tools, to scoop algae from the water. It's possible, of course, that the local ecology of other well-studied chimpanzee groups just don't favour consumption of aquatic algae. But this behaviour could also be due to cultural evolution in a few small social groups.
So, Boesch and his team set up a research station at Bakoun, in Guinea (not far south of the equator), as part of a continent-wide attempt to
contribute to a fuller understanding of the extent of chimpanzee behavioral variation and flexibility
in order to help get a handle on the actual level of behavioural diversity in wild chimps, and to answer questions around the relative effects of ecological diversity and cultural evolution on differences in behaviour shown by different groups of animals.
The chimps in the study area at Bakoun hadn't been studied before, and to minimise the potential impacts of interaction with humans, all observations were made using 'remote video camera traps', triggered to begin recording on detecting movement. These cameras were set up at sites where there was other evidence of chimp activity, such as remains of tools. Obviously they captured much more than chimpanzee activity, but of the 1,473 video clips that showed chimps, 486 (from 11 different sites), showed the animals 'fishing' for algae (Spirogyra sp.). Most of these events happened during the dry season, when water levels were lower, peaking in the 'hot dry' season when chimps returned repeatedly to the same sites over several days.
The chimpanzees were observed to fish for algae at sites where the algae occurred in large accumulations at the bottom of the river bed.We rarely observed free floating, surface algae being targeted... [and we] observed all age and sex classes perform and succeed in fishing for algae from deep ponds or river shores.
Interestingly, the researchers found that every single animal used a tool to collect algae, even those only 2 or 3 years old - and they tended to use the same hand each time they fished. They fished by holding one end of a long stick, reaching it down to the bottom of the water, and then twirling the stick so that strings of algae were wound onto it. They then withdrew the stick and pulled the algae off with their lips. And, when algae fishing, the chimps usually avoided getting wet as much as possible.
To see how successful this was as a food-gathering strategy, two of the research team used a discarded chimp tool - they managed to collect 400g of Spirogyra in just 10 minutes. Since individual chimps were seen fishing for an hour at a time, algae fishing could make quite a contribution to their seasonal diet:
chimpanzees may be fulfilling substantial dietary requirements [for protein, carbohydrates, and lipids, plus antioxidants and minerals] by ingesting large amounts of Spirogyra algae during the dry season
And just what were these tools? Mostly woody branches, modified by stripping off smaller branches and fraying one of both ends; some of these branches were up to 4m long, allowing access to algae that was otherwise unreachable in deeper parts of the river. In around 20% of events chimps arrived at their fishing sites already prepared ie bringing tools with them.
As I commented to my student, research like that described by Boesch and his colleagues goes well beyond simply documenting the activities of our close cousins. This is because, while it's likely our own hominin ancestors used a variety of plant-based tools, these aren't the sort of thing that's likely to be found by palaeontologists, and so
research on primates can illuminate the potential repertoire of tool use behaviors that may reasonably be assumed to have been present in our last common ancestor (Boesch et al. 2016).
we suggest that in Bakoun, tool use permits a more efficient access to a rarely available but highly preferred resource, such as algae, that permits chimpanzees to flourish in an environment otherwise more limited in food and water. It is therefore probable that our last common ancestor would have similarly made and used tools to also engage in rudimentary fishing, to collect and consume rich aquatic fauna, and perhaps flora too (ibid.).
This [research] demonstrates the flexibility in [chimpanzee] technical skills and how this helps them to obtain access to valuable resources in a drier habitat and new context. Such technological skills have been suggested to be present in our human ancestors when they invaded drier, savanna habitat during the course of human evolution (ibid.).
C.Boesch, A.K.Kalan, A.Agbor, M.Arandjelovic, P.Dieguez, V.Lapeyre, and H.S.Kuhl (2016) Chimpanzees routinely fish for algae with tools during the dry season in Bakoun, Guinea. American Journal of Primatology 78(12), published on-line 3 November 2016. DOI: 10.1002/ajp.22613