On Oct. 9, 2004, a group of researchers studying gorillas in northern Congo happened upon a never-before-seen event. In a swampy forest clearing, a female gorilla yanked a roughly 3-foot-long branch from a dead tree and waded into a deep pool of water. Keeping the stick in front of her and her upper body above water, the gorilla slowly advanced about 30 feet into the pool as she tested the water’s depth.
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