Spears or probes?

Last year, in a paper published in the journal Current Biology, Jill Pruetz and Paco Bertolani reported on their observation of ten different chimpanzees thrusting wooden "spears" into holes in trees 22 times over the course of more than a year, presumably to stun bush babies that sleep in the hollows during the day. The report has been somewhat controversial, especially since chimpanzees often shove sticks and twigs into holes, but the observations are receiving some new attention in a National Geographic article about the Fongoli population in Senegal.

Before considering what the Current Biology paper says, though, I couldn't help but notice that the National Geographic piece tried to play up the controversy surrounding the research at Fongoli. The hypothesis that the Fongoli chimpanzees are actually hunting has been met with some skepticism, and reporter Mary Roach digs into this angle to suggest that male primatologists have treated Pruetz unfairly because 1) she's a woman, 2) the chimpanzees that use the hunting tools most often are females, and 3) only humans are "allowed" to use tools to hunt, so obviously chimpanzees can't.

There is some truth to the claim that Pruetz has not been treated fairly; her post-doc adviser William McGrew overlooked her contributions while still citing her work in connection with her co-author Bertolani. (Roach also notes that Pruetz was not asked to speak at a recent conference about chimpanzees, but that could be due to any number of reasons.) Consequently, Roach paints Western anthropology as a boy's club of stodgy old men who won't have any woman telling them what's what. Like any other scientific discipline, there are indeed gender and minority equality issues in anthropology, but I think that Roach's assertion that Pruetz's research has been ignored because the chimpanzees exhibiting the jabbing behavior are female is off the mark. (Strangely enough, the article makes some passing, superficial references to the "Savanna hypothesis" of human origins, the classic example of the "Man the Hunter" narrative.)

Pruetz certainly should be given due credit for her work and should be invited to speak about what she and Bertolani observed. Between the paper I'm about to discuss, another short communication about chimpanzees using caves, and the fact that the Fongoli chimpanzees represent a population on the margins of the forest makes the work Pruetz is conducing especially interesting, and I hope we hear more about what's going on at Fongoli in the months and years to come. That said, scientists are not obliged to agree with the findings of a new paper just because it has been published. In order to understand why some researchers are somewhat skeptical that the Fongoli chimpanzees are hunting with tools, though, we need to look at what the Current Biology paper says.

The paper by Pruetz and Bertolani is short, but it contains some significant information. Out of 22 observed "attempts" at spearing bush babies there was only one confirmed success (13 of the instances occurring between June 13 and July 13, 2006). Such raw numbers provide the background for what's happening at Fongoli, but it's the preparation of the tools used that is more interesting. According to the paper, the chimpanzees that used the probes locate an appropriate branch, break it off, trim the leaves/side branches, trim off one or both ends of the branch/strip the bark off, trim the tip of the tool, and then use it. Not all the chimpanzees trimmed the tip or fully stripped the branches, but there is some amount of preparation involved in the activity rather than just picking up any stick that happens to be around and using it until it breaks.

While the term "spear" is often used to describe these tools, though, they're more like bludgeons or even probes. In the instance in which a bush baby was killed, the chimpanzee broke off part of the branch and then extracted the immobile bush baby (which also did not make any sound), so if the chimpanzee struck the small primate it likely stunned it or killed it with a blow rather than running it through. Many of the chimpanzees that used the tools also smelled/licked the tools after forcibly inserting them into the holes, perhaps to tell if they struck anything or if there was anything worthwhile inside the hollow. The problematic part of the behavior, though, is that it's difficult to tell whether it's purposeful (i.e. the chimpanzees are making a tool with an intent to injure, capture, and consume bush babies) or they're creating general-use probes to try and see what food there might be in the hollow branches.

The makeup of the individuals that were exhibiting this behavior should also be noted. The chimpanzees reported to have made or used hunting tools were one adult female, one adult male, three adolescent females, two adolescent males, one juvenile female, one juvenile male, and one infant male. In the paper Preutz noted that a few of the probing attempts were likely play, and I doubt that the infant male was truly involved with hunting smaller primates. Out of all these individuals it was the adolescent females that engaged in the jabbing behavior most often (particularly the females marked as TM and NI in the study, with a total of five attempts each), although the reason for this is unclear. Were they trying to independently gain access to a high quality food (meat) that other chimpanzees wouldn't share? Was one simply curious, poking a stick into a hole, and ended up catching a bush baby (a behavior that some other members picked up on)? How regular is the behavior, and does it occur only under certain circumstances? These are among the questions that remain to be answered.

Perhaps part of the reason why this research is so controversial is because other researchers have not been able to see the jabbing behavior. Some supplemetnal material has been made available for viewing, but the actual "hunting" behaviors are not seen, making the force used by the chimpanzees in their attempts more difficult to visualize. Based upon the available material, a few chimpanzees fashioned probes out of sticks and shoved them into holes in trees, and in one case the hollow branch was broken off, the bush baby inside being extracted and eaten. The evidence is tempting, but more information is needed before we can fully understand what these chimpanzees are doing. Are they just probing around in the trees, or are they actually hunting small prey with tools? The preparation of the sticks for this particular purpose would suggest that it is intentional behavior, but that conclusion has yet to be supported by a greater amount of evidence.

I don't think it's a bad thing, then, to remain somewhat tentative about what the research presented by Pruetz and Bertolani reflects. More information is needed (hopefully including some video of the actual jabbing behavior), and while I don't know when the information will be presented I'm sure it will eventually make its way into the literature. The hypothesis that the Fongoli chimpanzees are intentionally trying to stunt bush babies with prepared tools is exciting, but more research needs to be done before it can be confirmed.

Roach does not mention this in her article, though, painting a false view of the scientific process where publication in a peer-reviewed journal is the establishment of a fact, not the beginnings of a more intensive peer-review process that involves many researchers over a number of years. Speaking for myself, I didn't truly grasp this concept until I started reading scientific papers and seeing all the debates that are actively waged in the pages of journals; just because you can get something published doesn't mean your research is beyond criticism or re-analysis, nor should it be. Science requires an active exchange of ideas, and while debates are not always friendly, they are an essential part of the process.

References;

Pruetz, J.D.; Bertolani, P. (2007) "Savanna Chimpanzees, Pan troglodytes verus, Hunt with Tools." Current Biology, Vol. 17 (5), pp. 412-417

Sayers, K.; Lovejoy, C.O. (2008) "The Chimpanzee Has No Clothes: A Critical Examination of Pan troglodytes in Models of Human Evolution." Current Anthropology, Vol. 49 (1), pp. 87-114