The observation was made at Mbeli Bai, a swampy clearing in Nouabalé-Ndoki National Park where researchers have watched habituated gorillas from a platform for decades. A female entered a pool, found the water deepening, retreated, took up a detached branch and used it to test the depth as she advanced. A second female was recorded using a trunk as a prop and then as a makeshift bridge over swampy ground.
That this took so long to see is itself informative. Gorillas eat abundant leaves and stems that require strength rather than extraction, so there is little payoff in probing tools. Captive gorillas, by contrast, readily use sticks and containers, which suggests the capacity was never in doubt.
What remains unclear is how widespread the behaviour is. Wading sticks have been reported only rarely and mainly from bai-dwelling groups, so the rate across ordinary forest life is essentially unmeasured.