Tuesday, April 28, 2015

Female chimps are more likely than males to use tools when hunting

The hunted (a galago). Credit: flickr user Robertsphotos1
Jill Pruetz and colleagues recently published a great study about sex differences in tool use in chimpanzees (Pan troglodytes verus). Pruetz works at Fongoli in southeastern Senegal. Fongoli is different than most chimpanzee study sites because it's actually a woodland-savannah. Most chimpanzee study sites, such as Gombe, are characterized by deciduous, woodland, or evergreen forests. Anyways, chimpanzees at Fongoli hunt bushbabies or galgoes (Galago senegalensis ) using sticks they've essentially sharpened into spears. The chimps will break off a branch and frequently use their teeth to sharpen the end of the stick. They're taking an object found in their environment and modifying it to suit their needs. This is the only site where chimpanzees have been observed hunting using tools. Recently and after years of data collection, Pruetz and colleagues discovered some interesting differences in hunting between the two sexes.

Over the course of the study, ninety-nine instances of chimpanzee hunting were recorded, including episodes during which tools were used. Both male and female chimpanzees hunt Galago more than any other vertebrate at Fongoli. After observing and documenting episodes of chimpanzees hunting with the use of tools, females were significantly more likely to use a tool than were males. Pruetz and colleagues (2015) report 170 instances of females using tools compared to 130 instances of males using them when hunting.

Uh oh fellas, does this mean female chimps are smarter than males? No, it doesn't. Let's not jump to any conclusions here.

Pan troglodytes. Photo credit: William Warby
There could be multiple reasons for females hunting with tools more than males. Perhaps females are smarter than males in this population and use tools to aid them when hunting. On the other hand, perhaps females use tools because they are weaker physically than male chimps, thus tool use makes hunting easier for them and compensates for this difference in strength. Both are possible answers and there are likely others. The study showed this difference in males and females is significant, thus it is not random that Pruetz and colleagues observed more females using tools when hunting than males. Pruetz and colleagues hypothesize that males may use tools less often when hunting Galago than females because males usually hunt larger prey, and thus when they flush out Galago individuals, it is opportunistic: they are not using tools to hunt baboons or other large prey they're targeting.

This isn't the only difference between the sexes when it comes to chimpanzee hunting. Males are known to hunt more than females (Fahy et al., 2013; Stanford, 1999). At Fongoli, Pruetz and colleagues also found while Galago made up 75% of females' prey, it accounted for only 47% of males'. Only male chimpanzees at Fongoli hunt patas monkeys (Erythrocebus patas). Successful hunters were more likely to be male than female. However, when both sexes used tools to hunt Galago, there was no difference in success rate between males and females.

As Pruetz and colleagues discuss in this paper, their study has implications for human evolution. Tools likely played a part in how our early human ancestors hunted, thus studying chimpanzees allows us to better understand what hunting may have looked like in early humans.

Food for thought: Can you think of any reasons why females use tools to hunt more frequently than males do?
Why might chimps use tools to hunt at Fongoli and not elsewhere?

Links of potential interest:
Video of Pruetz discussing chimps hunting bushbabies
Nat Geo article on Fongoli chimps
Tool Use, Hunting, and Other Discoveries

J. D. Pruetz , P. Bertolani , K. Boyer Ontl , S. Lindshield , M. Shelley , E. G. Wessling. New evidence on the tool-assisted hunting exhibited by chimpanzees (Pan troglodytes verus) in a savannah habitat at Fongoli, Sénégal. Royal Society Open Science, 15 April 2015
Fahy, G.E., Richards, M., Riedal, J., Hublin, J., Boesch, C. 2013. Stable isotope evidence of meat eating and hunting specialization in adult male chimpanzees. Proc. Natl Acad. Sci. USA 110, 5829-5833 
Stanford, C.B. 1999. The hunting apes:meat eating and the origins of human behavior. Princeton, NJ: Princeton University Press.

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