Effects of Northern Latitudes on Bat Nightly Activity Patterns in Western Canada

North American insectivorous bats provide critical ecosystem services, contributing to natural pest control, agricultural productivity, and human health. These nocturnal mammals typically exhibit variable nightly activity patterns that align with prey availability, though activity timing and structure are also influenced by ambient light conditions. At higher latitudes, extended twilight periods result in delayed sunsets and early sunrises, while day-night transitions occur more rapidly at lower latitudes. Previous research on bat responses to light in northern regions has largely focused on single locations, typically examining how emergence timing shifts across seasons or in response to changing light conditions. This landscape-scale study examines bat populations across a broad latitudinal gradient (49-61°N) in the northwestern portion of their ranges in Western Canada. We aimed to determine how latitude shapes bat activity patterns by examining differences in activity curve shapes, modality, and peak timing across this region, where natural light regimes vary significantly during summer months. We hypothesized that (1) bat activity curves would be more similar at latitudes closer together than those farther apart, (2) bats at higher latitudes would exhibit unimodal activity patterns due to shorter nights while lower-latitude bats would show bimodal patterns, and (3) peak activity timing would shift with latitude, with northern bats showing earlier peaks due to higher light tolerance and southern bats showing later peaks. We acoustically sampled 127 sites across Alberta and Yukon during May–August of 2021–2022, deploying autonomous recording units to capture bat echolocation calls. We used Wasserstein distance calculations to quantify dissimilarity between latitudinal activity curves, Hartigan's dip test and linear regression to assess changes in activity pattern modality with latitude, and modeled peak activity timing using standardized nightly activity data and kernel density estimates. Results did not support our initial hypotheses. Wasserstein distances between latitudinal bands showed no systematic relationship to geographic distance, with some adjacent latitudes showing high dissimilarity while widely separated bands exhibited low dissimilarity. Only 24% of our sites exhibited statistically significant multimodal activity patterns, with multimodality predominantly occurring at latitudes below 56°N. However, unimodal patterns were observed across all latitudes, and latitude was not found to be significant in explaining multimodality patterns. Peak activity timing revealed a complex cubic relationship with latitude when all sites were included, but when analysis was restricted to Alberta sites to control for longitudinal confounding, peak activity occurred progressively later at higher latitudes, contrary to our prediction of earlier peaks. These findings suggest that while factors such as landscape characteristics, species composition, and proximity to roosts are influential in shaping bat activity patterns, latitude, and by extension atmospheric light levels, still demonstrates a measurable impact on the timing of peak activity. The later peak activity at higher latitudes indicates that despite shorter nights, bats delay activity until ambient light levels fall sufficiently, prioritizing darkness over extended foraging time. This research contributes to understanding how bats adapt their temporal activity patterns across large geographic scales and provides baseline knowledge for investigating environmental drivers of bat behavior. As climate change drives range expansions into northern latitudes and artificial light pollution increases globally, understanding natural variation in bat responses to light conditions becomes increasingly important for developing geographically targeted conservation strategies and predicting species responses to environmental change.

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