Brazilian researchers have documented thousands of bumblebee catfish (Rhyacoglanis paranensis) climbing a small waterfall on the Aquidauana River in Mato Grosso do Sul, apparently defying gravity as they wriggled vertically over slick rock faces. The spectacle, first filmed by the state’s Environmental Military Police in November 2024, prompted a 20-hour field investigation. Scientists from the Federal University of Mato Grosso do Sul observed the fish forming dense aggregations, stacking atop one another and even slithering upside-down across crevices to make the ascent. Their findings, published on 8 August in the Journal of Fish Biology, suggest the catfish were migrating upstream to spawn. While ‘climbing’ behaviour has been reported in a handful of other species, the researchers say the scale of this event—described as “so massive that specimens were seen above each other”—makes it unprecedented for the genus and underscores the importance of opportunistic field observations in understanding riverine ecology.