Timescapes of fear: Exogenous and endogenous control of moonlight-mediated circalunar foraging rhythms in a weakly electric fish
Data files
Feb 03, 2026 version files 1.41 MB
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captive_monthly_EOD_rate.csv
1.36 MB
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captive_preemergence_rise.csv
4.52 KB
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README.md
3.30 KB
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wild_monthly_passage_rate.csv
38.21 KB
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wild_preemergence_rise.csv
2.60 KB
Abstract
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.
Dataset DOI: 10.5061/dryad.9p8cz8wwx
Description of the data and file structure
Wild sand knifefish data:
- wild_monthly_passage_rate.csv
- wild_preemergence_rise.csv
Captive sand knifefish data:
- captive_monthly_EOD_rate.csv
- captive_preemergence_rise.csv
Files and variables
Dataset keys:
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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.
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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).
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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.
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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)
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date: Day of the measurement, in M/D/YY (e.g., 6/4/2021 is June), matching the labels in figure 3c (categorical)
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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.