How Big are Camera Shy Chimps? Measuring Body Size in the Forest

Young Explorer Aaron Sandel studies the largest community of wild chimpanzees ever observed. Trekking through the forests of Kibale National Park, Uganda, he is investigating different aspects of behavior and morphology, with a focus on development, dominance, and play.


How big are chimpanzees? If you are a red colobus monkey, or me—sitting amongst 15 males screaming as they charge around fighting over a chance to mate with Bartoli, an estrus female—the answer is, big enough. A more accurate answer is difficult. The chimpanzees at Gombe, where Jane Goodall did her research, used to be lured onto scales with bananas. Today researchers do not provision chimpanzees, so getting an estimate of weight or body size is difficult. The solution: lasers.

Adult males corner Bartoli in a tree, fighting over a chance to mate with her (Photo by Aaron Sandel)
Adult males corner Bartoli in a tree, fighting over a chance to mate with her (Photo by Aaron Sandel)

Take two laser pointers, make them parallel at a set distance apart (e.g. 5 cm), then shine the two dots on your target and take a photo. On the computer, you can determine how many pixels are between the laser dots in the photo, which correspond to the known distance of 5 cm. Then you measure how many pixels are between, for example, the shoulder and rump, and then determine the distance in centimeters. The distance between the dots acts as a scale, enabling measurements of body and limb lengths. First used in primates by Jessica Rothman and colleagues (2009) and then applied to chimpanzees by Nick Brazeau and a team from Harvard last summer, this method allows non-invasive measurements of body size in wild animals.

Here I train an undergraduate student how to take photos with the lasers by acting like a chimp; don’t let them see the lasers! (Photo by Caleb Vogt)

While the lasers do provide an excellent solution to an important problem, there are some challenges that arise.

(1) Trying to precisely calibrate the lasers in the field is difficult. Both laser pointers have to be exactly parallel; that way, the two dots are always the same distance apart no matter how far you are from the chimp. Ngogo Camp is not exactly up to the standards of the Applied Physics department at the University of Michigan, but luckily there are a handful of relatively flat surfaces. Using a clamp borrowed from a village carpenter, I am able to keep the lasers in working order.

Calibrating the lasers in our make-shift lab (Photo by Caleb Vogt)
Calibrating the lasers in our make-shift lab (Photo by Caleb Vogt)

(2) Many female chimpanzees are camera shy. My goal this summer is to take photos of mothers and infants to look at variation in body size and understand the factors that influence growth. But the mothers are not the most cooperative bunch. Luckily Ngogo is the largest known community of chimpanzees, so I’m sure that among the 30 infants, there will be some who want to be stars.

Camera shy infant chimpanzee (Photo by Aaron Sandel)
Camera shy infant chimpanzee (Photo by Aaron Sandel)


NEXT: Searching for Chimps & Other Misadventures in an Ugandan Forest

Read the entire blog series


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Meet the Author
I am a PhD student in Anthropology at the University of Michigan. I write from Ngogo, in the center of Kibale National Park in Uganda, home to 200 chimpanzees, the subjects of my dissertation. A National Geographic Young Explorers grant in 2013 got me through the angst of finalizing a dissertation project. Now, I study angst, or at least the closest thing to it: friendship and the transition to adulthood in male chimpanzees. When I’m not focused on following a chimpanzee, my forest daydreams include opening a vegan burrito restaurant in a rural Ugandan village and making a documentary about an old fig tree.