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 variations-and how endogenous and exogenous processes interact to control this behaviour remain poorly understood. We investigated these mechanisms in Gymnorhamphichthys rondoni, an electric fish inhabiting shallow, clear-water Amazonian streams. We quantified activity across the lunar cycle using field recordings of electric organ discharges (EODs) from autonomous loggers, paired with high-resolution moonlight measurements, and through laboratory experiments conducted under recreated moonlight cycles. G. rondoni restricted foraging to moonless periods, closely tracking shifting nightly windows of darkness associated with changes in moonrise and moonset, and the local effects of canopy blocking and cloud cover-indicating strong exogenous masking. We also documented a daily 'pre-emergence rise' in EOD rate, beginning approximately 4-5 h before nightfall, consistent with anticipation of emergence. This rise was selectively suppressed on nights when moonlight was already present at nightfall ('moonlit-onset nights'), when emergence would be risky. This suppression was already evident on the first moonlit-onset night, following several entirely moonless nights. These observations provide rare evidence for an endogenous component to a non-reproductive circalunar rhythm in a vertebrate.