Timescapes of fear: Exogenous and endogenous control of moonlight-mediated circalunar foraging rhythms in a weakly electric fish

The timing of activity is fundamental to animal survival. For many nocturnal species, moonlight suppresses activity due to elevated predation risk. However, how animals track moonlight—with its complex nightly and monthly variation—remains poorly understood. We investigated the relative roles of exogenous and endogenous control in the moonlight-mediated foraging rhythm of the nocturnal electric sand knifefish (Gymnorhamphichthys rondoni) inhabiting Amazonian streams. Using continuous field recordings of electric organ discharge (EOD) paired with high-resolution moonlight measurements, together with laboratory experiments under recreated moonlight cycles, we quantified activity across the lunar month. Sand knifefish restricted foraging to moonless periods, closely tracking nightly shifting windows of darkness. Activity was strongly suppressed under moonlight but resumed when dense clouds blocked it, indicating exogenous masking. We also documented a daily “pre-emergence rise” in EOD rate during the afternoon, consistent with anticipation of nighttime emergence. Intriguingly, this afternoon rise—when moonlight was undetectable—was suppressed on nights when moonlight would already be present at nightfall (full moon and several preceding nights), revealing a circalunar rhythm independent of exogenous moonlight cues. Overall, our findings provide evidence for a rare endogenous circalunar rhythm in a vertebrate, acting jointly with exogenous control to shape foraging behavior.