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.