DOI: 10.5061/dryad.9p8cz8wwx
title: Timescapes of fear: Exogenous and endogenous control of moonlight-mediated circalunar foraging rhythms in a weakly electric fish

description: 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.
description: # Timescapes of fear: Exogenous and endogenous control of
 moonlight-mediated circalunar foraging rhythms in a weakly electric fish
 Dataset DOI:
 [10.5061/dryad.9p8cz8wwx](https://doi.org/10.5061/dryad.9p8cz8wwx) ##
 Description of the data and file structure Wild sand knifefish data: 1.
 **wild_monthly_passage_rate.csv** 2. **wild_preemergence_rise.csv**
 Captive sand knifefish data: 1. **captive_monthly_EOD_rate.csv** 2.
 **captive_preemergence_rise.csv** ### Files and variables Dataset keys: 1.
 **wild_monthly_passage_rate.csv**: Data for plotting figure 2a, the wild
 sand knifefish passage rate actogram with field-measured moonlight ground
 illuminance shown as a heatmap. * datetime: Timestamp in datetime format
 (M/D/Y h:mm), at 15-min intervals * average_passage_rate: Number of sand
 knifefish passages per 15 min, averaged across the six EOD loggers
 (continuous) * illuminance: Field measurements of ground illuminance (lux)
 at 15-min intervals (continuous): the light meter continuously integrated
 over 15-s periods, and the 15-min value was calculated as the mean across
 all 15-s periods within each 15-min interval. 2.
 **wild_preemergence_rise.csv:** Data for figure 3a, the pre-emergence rise
 in EOD rate (measured 80-90 min before nightfall) of wild sand knifefish
 on various afternoons across the lunar cycle. Analysed using Model 1
 (ANOVA). Within each sampling day, each row represents a measurement from
 a different individual, to avoid within-day pseudoreplication. Individuals
 were not identifiable across days; therefore, day-to-day resampling of the
 same individuals cannot be excluded. * date: Day of the measurement, for
 example "FM-2_(aug9)" indicates two nights before the full moon
 on August 9, matching the labels in figure 3a (categorical) *
 mean_EOD_rate: Pre-emergence rise EOD rate in Hz (continuous); mean EOD
 rate was calculated for each 15-s recording (recorded at 1-min intervals).
 3. **captive_monthly_EOD_rate.csv:** Data for figure 3b, the captive sand
 knifefish EOD rate actogram with lab-recreated moonlight ground
 illuminance shown as a heatmap. * datetime: Timestamp in datetime format
 (M/D/Y h:mm), at 1-min intervals * average_mean_EOD_rate: Mean EOD rate in
 Hz (continuous), averaged over a 15-s window (recorded at one-min
 interval) and then averaged across all 24 sand knifefish. * illuminance:
 Ground illuminance in the lab at 1-min intervals, in lux (continuous): the
 light meter continuously integrated over 15-s periods, and the 1-min value
 was calculated as the mean across the four 15-s periods within each 1-min
 interval. 4. **captive_preemergence_rise.csv:** Data for figure 3c, the
 pre-emergence rise in EOD rate (mean over the 10-min interval 80–90 min
 before nightfall) of captive sand knifefish on various afternoons across
 the lunar cycle. Analysed using Model 2 (GLMM). * fish_num: Captive sand
 knifefish individual identity number (1 to 24) - date: Day of the
 measurement, in M/D/YY (e.g., 6/4/2021 is June), matching the labels in
 figure 3c (categorical) - mean_EOD_rate: Pre-emergence rise EOD rate in Hz
 (continuous): mean EOD rate was calculated for each 15-s recording
 (recorded at 1-min intervals), and then averaged across the 10-min
 interval 80–90 min before nightfall.