Bats Biodiversity Climate change Conservation planning

Little brown and Yuma myotis maternity colonies are negatively impacted by extreme weather

Rensel, L. J, Hodges, K. & Lausen, C.

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Key points

• Urban bats often rely on bat boxes and buildings because natural roosting sites such as large trees and rock crevices can be scarce in cities. These artificial roosts expose bats to different conditions than found in natural roosts.
• Bats need extra energy to stay warm during cold, wet conditions, which can affect their health and ability to reproduce.
• The study found that a colder, wetter spring led to poorer outcomes for bats: fewer females were pregnant or nursing, and bats weighed about 8% less on average.
• Extreme heat can also be dangerous. During a 2018 heat wave, temperatures inside some bat boxes exceeded 44°C, causing a mass mortality event in a colony that had no nearby alternative roosts.
• Bat conservation efforts should aim to provide a variety of roost types close together, including warmer roosts for spring breeding and cooler refuges for hot summer weather, especially as the climate warms.

Abstract

Bats in urban spaces can lack access to natural roosts — like snags, large trees, and rock crevices — so some species use artificial roosts like bat boxes and buildings. Artificial roosts often differ from natural roosts in their thermal and hygric properties, thus exposing bats to different conditions than found in natural roosts. Extreme weather events, such as excessive rain or heat waves, may cause energetic stress in bats using artificial structures due to overheating or increased torpor requirements. The timing of such weather events, especially for pregnant or nursing females, can negatively affect reproduction and body condition of bats in maternity colonies using artificial structures. We examined whether early season temperature and precipitation influenced bat body mass and reproductive condition in maternity colonies of Yuma myotis (Myotis yumanensis) and little brown myotis (Myotis lucifugus) using artificial roosts in western North America. We captured bats at three maternity colonies in 2018 and 2019 near Vancouver, British Columbia, Canada. We collected roost and ambient weather data using temperature loggers and nearby weather stations. Spring 2019 was colder and wetter than in 2018, and 2018 had several extreme heat events. We predicted that cold, wet spring conditions would reduce body condition and reproduction in adult female bats due to increased energetic needs for thermoregulation. As predicted, a lower proportion of bats were identified as reproductive in 2019 than in 2018. Similarly, bats weighed on average 0.47 g less (ca. 8%) in 2019 than in 2018. A major heat wave in 2018 also affected the colonies differently. Bats with high fidelity to an array of bat boxes experienced a mass mortality event in July 2018 when internal roost temperatures exceeded 44°C; the two sites where bats had nearby alternative roosts in buildings did not experience mortality during this heat wave. These results highlight that early season conditions affect bat body condition and reproduction. Bats need a range of roosting habitats to align their roosting locations with weather, energy needs, and reproductive stage. Specifically, artificial roosts that are warm in spring promote gestation and save bats energy, whereas roosts that are cool are important refuges from overheating in summer. Having an array of roost types in close proximity may thus be valuable for maintaining bat colonies in a warming climate.

Recommended citation

Rensel, L. J, Hodges, K. & Lausen, C. (2026). Little brown and Yuma myotis maternity colonies are negatively impacted by extreme weather. Acta Chiropterologica. https://doi.org/10.3161/15081109ACC2026.28.2.017