Thinking as a Hobby


Home
Get Email Updates
LINKS
JournalScan
Email Me

Admin Password

Remember Me

3022479 Curiosities served
Share on Facebook

Orangutan and Chimps with Spears
Previous Entry :: Next Entry

Read/Post Comments (2)

There are some photos that have just been posted on-line showing an orangutan holding a fisherman's spear that he found and apparently thrusting it into the water.

Links are here and here.

You've got to be careful with how you interpret such things. I've been taking a class on the evolution of primate cognition this semester, and it's an easy trap to confer all sorts of subjective emotional and mental states onto non-human primates that may or may not be there. The best route is to be conservative, and try to explain the phenomenon with the simplest explanation that you have evidence for.

In this case, we have no idea of the orangutan has any idea about what it's doing. The individual could just be mimicking a human fisherman as a form of play, without even realizing the causal link between spearing the water and catching a fish. There may not even be any fish present when the orangutan thrusts it into the water!

Some evidence that might support the "fishing" hypothesis would be data on when the orangutan last ate, coupled with some feeding habit information on orangutans. If you could establish that the behavior took place at a time when the orangutan hadn't recently eaten, that would be some support. Actually documenting that a fish was present at the time when the behavior occurred would go a long way to supporting the hypothesis as well. But just based on what we have, you can't call this "fishing" or "hunting" behavior. It's an orangutan sticking a spear in water.

I previously wrote about a study suggesting that chimpanzees use "spears" to hunt. Mary Roach wrote a piece on the research for National Geographic magazine, which is on-line here.

Roach describes the "hunting" behavior this way:


Things like sharpening sticks to spear bush babies. It is a different kind of hunting than the organized colobus monkey raids documented at other sites. A chimp who comes across a dead, hollow tree limb—promising real estate for day-sleeping bush babies—will sometimes break off a branch from a nearby tree, remove the leaves and the flimsy ends, and then use its teeth to whittle one end to a point. This tool is then stabbed into an opening in the tree limb until the animal inside is out of commission. Whereupon it is eaten, head first, Pruetz says, "like a Popsicle."


What she fails to mention, and what you would see if you read the actual paper, is that there was only one recorded incident of a chimpanzee using such a tool in a tree hollow and then retrieving an inert bushbaby from the hollow and eating it. One instance.

Tools were seen being constructed 26 times, and were seen being stuck in tree hollows 22 times as of the writing of the published paper, but there was only one instance of any food source being removed from a probed tree hollow.

Roach makes it sound like there's a male-dominated conspiracy against Pruetz for her heretical findings:


The media ruckus spurred by Pruetz's report of spear-wielding chimps made her absence as a speaker at last year's Mind of the Chimpanzee conference perplexing. She was in the audience but wasn't invited to present a paper. On top of that, Pruetz's post-doc adviser, Cambridge University primatologist William McGrew, made a passing reference to the Fongoli hunting behaviors but did not credit her with the work. He credited her co-author and former student Paco Bertolani, now a student of McGrew's. Bertolani witnessed the first—of now 40—observed instances of the behavior, but scientific etiquette would call for the principal investigator to be mentioned. McGrew apologized afterward. Some primatologists took Pruetz to task for overstating the bush-baby-spearing behavior. When your prey is smaller than your hand, are you really hunting? Male primatologists tend to make the distinction along gender lines: The traditional view has been that chimpanzee hunting—along with aggression and murder—is the domain of the male. "Small mammals that females and juveniles obtain are 'gathered,' " Pruetz says, "while males 'hunt.' "


This is really an instance of horribly sloppy science writing. Roach says that as of this her writing there have been 40 observed instances of "the behavior", without really saying what the behavior is. The implication is that the behavior was clearly hunting. The issue is not the size of the prey, or the chauvinism of her colleagues, but the fact that she's extrapolating all sorts of intentionality from one observed instance.

Here's a highly relevant passage from Pruetz and Bertolani's paper Savanna Chimpanzees, Pan troglodytes verus, Hunt with Tools (no hedging or ambiguity in that title, eh?):


Chimpanzees forcibly "jabbed" tools into hollow trunks or branches multiple times and smelled and/or licked them upon extraction. In only two of the 22 cases was the tool use playful (in the case of an infant male) or exploratory in nature ("investigatory probe"). In all other cases, chimpanzees were judged to use such force in inserting the tool that prey within the cavity could have been injured. In all observed cases, chimpanzees used on hand in a "power grip" to jab the tool downward multiple times into the cavity. In the single instance in which a chimpanzee was observed to extract a bushbaby, it was unknown whether the prey was alive or dead after the use of the tool, but it made no attempts to escape, nor did it utter any vocalization.


Several things to note here. First, it was also unknown whether or not the bushbaby was dead or alive before the chimp even used the tool. They say that in only 2 of 22 cases was the use of the tool "playful" or "investigatory", but as far as I know there is no photo or video of any chimpanzee engaged in the behavior she describes.

There is some video footage that accompanies the research that shows the aftermath of such an episode, but none showing the actual "hunting" behavior at question. The only photo of anything resembling this behavior is one that accompanies the National Geographic story. Here's the photo gallery. Check out the fourth photo.

Does that look like the behavior that Pruetz describes? Does that look like a power grip to you? Or is this one of those rare cases of investigatory behavior?

Now, she may be right about the hypothesis that chimps are hunting. But the evidence is weak right now. Some actual photographic evidence of a chimpanzee carrying out the power grip-jamming behavior she describes would bolster her claims. As would additional instances of chimpanzees pulling bushbabies out of tree hollows following such behavior.

But one possibility we discussed in class was that this behavior is an extension of the termite fishing behavior chimps engage in, stripping a stick and using it to fish for termites. With the evidence that's presented in her paper, I'm willing to call the tools "probes", but not go further than that without additional evidence.

And I think Roach does a disservice to science when she invokes some sort of good ol' boy conspiracy for explaining the skeptical reaction of the scientific community to the claims that these chimps are hunting with spears. The simple fact is that Pruetz is overstating the case.



Read/Post Comments (2)

Previous Entry :: Next Entry

Back to Top

Powered by JournalScape © 2001-2010 JournalScape.com. All rights reserved.
All content rights reserved by the author.
custsupport@journalscape.com