The Chimpanzees of Mt. Assirik

25 09 2007

When chimpanzees (Pan troglodytes) appear in documentaries they are often shown inhabiting relatively dense tropical forest, their lives taking place within the green refuge of the forests. As with any other species that is spread over a considerable distance, however, different populations of chimpanzees have different habits, and one of the most remarkable populations are those around Mt. Assirik. Located in the southeastern part of the Parc National du Niokolo-Koba in Senegal, the chimpanzees in this area have to deal with a local ecology that is drier and more open than some of their relatives elsewhere, and their behavioral adaptations to the environment is of great interest to those study human origins.

The Mt. Assirik study area is remarkable in that 55% of the habitat is open grassland, only about 37% being woodland of varying density and only 3% being more dense forest (the remaining area being made up of bamboo forest and isolated trees). Such open spaces allow some of the major Carnivora of Africa to live in close proximity to the chimpanzees; Lions (Panthera leo), Leopards (Panthera pardus), Wild Dogs (Lycaon pictus), and Spotted Hyenas (Crocuta crocuta) are all frequently seen in the area. As if having so many predators at their doorstep were not enough, the Mt. Assirik area seems to have fluctuations of food that aren’t correlated with seasonal changes, and in the dry season water is the most prized of any resource. The apes are not entirely helpless in the face of such pressures, however, and they’ve been behaviorally adapted in some very interesting ways.

Given a choice, the Mt. Assirik chimpanzees prefer to spend their time in the denser areas of forest, but shifting food resources sometimes require them to move across large expanses of open grassland in order to find nourishment. Wandering out onto the open plains alone is so dangerous as to nearly be suicidal, and the apes form large mixed groups when they have to move across the plains. During this time they are at their most vulnerable, especially since they would be unlikely to outrun any of the major predators (especially those that hunt in packs), and they are extremely alert when undertaking such a journey. What is perhaps most striking of all, hearkening back to Raymond Dart’s “Savanna Hypothesis,” is the fact that the chimpanzees sometimes stand up to get a better look at their surroundings, potentially spotting predators before they get too close, although such an observation should not be taken as a sweeping vindication of Dart’s ideas of human evolution.

The presence of just one tree or a few trees spaced far apart doesn’t help the chimpanzees much either; mothers with children and individuals spent much less time in the sparser woodland areas than in the forest, mixed groups seemingly having to issues with the woodlands. Why should this be so? Well, leopards can climb trees (and often do so to stash their kills), as well as lions, and so simply climbing a tree does not equal escape. Lone chimpanzees are far more comfortable in a habitat where they can climb a tree and move through the canopy out of reach of their assailants, something that is not possible in woodlands. The predators may also have another effect on the diet of the chimpanzees; the Mt. Assirik chimps do not seem to eat young ungulates or monkeys, although such behaviors have been made famous where it has been observed (i.e. Gombe). This may be due to some competition, but it may also be due to the restricted forested habitat and the fact that chimpanzees would have to enter the habitat of the carnivores in order to capture young ungulates, predators being likely to quickly learn about any kills that had been made.

Indeed, the Mt. Assirik population is remarkable in that it often moves long distances in order to obtain food as it becomes available, relying on numbers and vigilance to protect itself from predators when it’s habitat only offers a few isolated islands of relief. Although humans did not evolve from modern chimpanzees, this population may give researchers some idea of the behavior patterns of our ancestors when faced with similar constraints when forests became sparser and the plains were filled with predators. Such social behavior is not the only thing that makes the Mt. Assirik chimpanzees stand out, however; they also make use of Baobab trees in a very interesting way.

By now many people are familiar with the ability of chimpanzees to use a piece of wood as a hammer to break a nut placed upon an “anvil” of rock or tree root; such footage has been shown in television programs again and again. Such behavior did not come out of nowhere, however, and the way Mt. Assirik chimpanzees open nuts may represent a stage of tool use that precedes the hammer-and-anvil technology. While it had been disputed for some time whether the Mt. Assirik population used hammers and anvils or just anvils, recent studies have shown that they are cracking open the hard nuts of the tree on branches and not using a hammer. While we might think of an “anvil” as something that can only be used in conjunction with a hammer, mechanically this isn’t necessarily so, and the Mt. Assirik chimpanzees bang the hard nuts they collect on the branches of the tree (therefore staying aloft, not coming down to use stones or the roots of the tree), the tree itself being the anvil.

Given the basal usage of anvils by the Mt. Assirik chimps and the use of hammers and anvils elsewhere, it becomes possible to hypothesize about the evolution of stone tool use in our own ancestors. The starting point was likely similar to what is exhibited by the Mt. Assirik chimpanzees, banging hard nuts on trees or rocks in order to open them (thus preventing damage to the teeth, if it even would be possible to open the nuts using their jaws). The next step would be adding a hammer, possibly wooden (as seen in some groups today) or possibly stone. At this stage any combination of wood or stone hammers and anvils could be used, but tool use would probably not progress until a population was using stone hammers and stone anvils to open foods. In such a scenario, the apes would sometimes miss their targets and flake off bits of stone, an accident that would shape the tools. When a certain cognitive leap was made, the apes could then move from accidentally flaking their tools to doing it intentionally to truly be making tools rather than making use of naturally occurring bits of wood and stone. The reality of the situation may be forever lost to us, ancient tool use before the knapping of stone became prevalent being notoriously hard to discern, but such a line of behavioral descent is not unreasonable and seems to allow further development merely by chance combinations of naturally occurring resources.

Such a discussion is only a brief sketch based upon what I have only recently learned myself, but I hope that it has been at least somewhat informative. Different populations of chimpanzees show different behaviors and live in differing ecologies, and it would be a mistake to assume what the famous Gombe chimpanzees are doing holds true for all the other populations. Another population that I soon intend to write about spends time in caves, probes trees for bush babies, and may even have the beginnings of a fire culture; others do not show the same exact behaviors, but they have their own cultures and reactions to the local ecology. While we should be careful in analyzing the living populations of chimpanzees and their perceived similarities to humans, it would be foolish to think that they can tell us nothing of our own past, and if very well may be that some of traits (behavioral or otherwise) they now exhibit were present in our own lineage, vignettes of evolutionary history being replayed with different actors in our own time.

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5 responses

27 09 2007
DDeden

Hi Brian,
Interesting conjecture on tools.

I’ve an OT question that relates to dinosaur feathers and hominids.
When homonid researchers in the field find a skull, what is their priority? To pick and brush it off right? Since we know that feather imprints can be retained if the critter was covered in fine mud (rare but not unusual), shouldn’t skull diggers be looking at the inverse of the skull for hair imprints on the dried mudstone? Of course once the skull’s soft tissues decomposed, the surrounding silts collapsing inwards would destroy some of the features, but the dead hair protein would probably leave an imprint, possibly visible through MRI or micro or nano Xray analysis or something. I for one would like to know the precise hair/beard patterns, as I’m tired of paintings, drawings, models of ancient neandertals, erectoids and apiths with short haircuts, since sapiens have hair that grows a yard long when left uncut.

But then again, some people think our ancestors were like chimpanzees on savannas, where long hair is useless. I do wish the bone & stone folks would check on hair, probably the only soft tissue recognizable after 20k years in the hardening mud.

27 09 2007
laelaps

Hi DD; I was just thinking about this today, in fact. In my Topics in African Prehistory course my professor had said that paleobotany at hominid dig sites is very much overlooked and needs more attention.

As for feathered dinosaurs go, the feathers aren’t preserved in just any sort of sedimentary deposit but in lagerstatten deposits, usually anoxic lagoon floors that lack many bacteria that degrade skeletons or ash falls, quickly covering the animals. This deposits are so fine and the burial is so rapid that the minor details of covering can be ascertained, and although I’m not an expert, I don’t know of any hominid lagerstatten sites. There are ash falls, and I wouldn’t be surprised if hominid remains were found in ash falls deposits (there are footprints, after all), but it seems that many of the fossils come from areas that lack this fine preservation. Perhaps a close look might be good, but if I came across a fragmentary skull from a riverbed that did not have a body outline around it I wouldn’t think that any hair would be found.

And as far as the hairy/hairless issue goes, I’m sure you’ve heard before that we’re just as hairy per square inch as apes, just our hairs are often shorter and lighter so it doesn’t seem as prominent. If a fine-preservation specimen were found that was less hairy microscopy would still be able to determine the presence of hairs, perhaps, even if they were reduced in length.

In any case, that is a very good idea, DD, and perhaps one day a more well-preserved specimen will be found. Time will tell.

28 09 2007
DDeden

Hi Brian,
I took a different approach to the use of (hollow) trees and tools use by hominids, and wrote a quick scenario tying together some things and sent it as a post here:
http://tech.groups.yahoo.com/group/paleoanthropology/message/16687

5 10 2007
Apes aren’t the only primates to use tools « Laelaps

[...] populations don’t use tools and some don’t use them in the same way. As I mentioned in my post about Mt. Assirik chimpanzees, the chimpanzees there use the large Baobab tree limbs and trunks as anvils to crack open the fruit [...]

12 03 2012
Yon Fornerod

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