American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Climate change and thermoregulatory consequences of activity time in mammals”

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Timothy C. Bonebrake, Enrico Rezende, and Francisco Bozinovic (July 2020)

Climate change and thermoregulatory 🧥 consequences of activity time 🌙☀️ in mammals 🐭

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Will nocturnal or diurnal mammals be more vulnerable to global warming? Recent studies have shown that mammal species are shifting their activity patterns in response to human disturbance and also that such shifts could conceivably be one way for mammals to mitigate future warming impacts, e.g. shifts from hot diurnal hours to cooler nocturnal hours. However, to date there is little understanding of the global implications of such shifts or how different traits might filter such climatic variation.

Supported by a Universitas 21 fellowship, Timothy Bonebrake (The University of Hong Kong) worked on this question in Santiago, Chile, with Francisco Bozinovic and Enrico Rezende at the Pontificia Universidad Católica de Chile. To address this issue, the team developed a modeling framework for evaluating the consequences of nocturnal and diurnal activity for climate change implications. Using this model, the team found that, broadly, nocturnal mammals may see increased habitable thermal space with future warming while diurnal species will likely see decreased habitable space. Importantly, traits such as thermal conductance and body size will determine how individual species are affected by warming. Nocturnal small mammals tend to be smaller and have lower thermal conductance than diurnal species – features that will likely have key implications for climate change vulnerability.

These results are particularly important in a world of rapidly changing climates (global warming) and light (artificial night light). With multiple drivers of anthropogenic change globally, diurnal and nocturnal mammals will face unique challenges. An understanding of underlying physiological patterns that structure environmental change responses will hopefully aid in the conservation of these diverse species.


Activity times structure the thermal environments experienced by organisms. In mammals, species shift from being nocturnal to diurnal and vice versa but the thermal consequences of variable activity patterns remain largely unexplored. Here we used theoretical thermoregulatory polygons bounded by estimates of basal metabolic rates (BMR), maximum metabolic rates (MMR) and thermal conductance (C) in small mammals to explore the metabolic consequences of exposure to global scale day-time and night-time temperatures. Model predictions indicated higher metabolic scope for activity for nocturnal species at low latitudes and that reduced minimum C and larger body size increased the geographic range in which nocturnality was advantageous. Consistent with predictions, within rodents, nocturnal species have low C. However, nocturnal mammals tend to be smaller than diurnal species likely reflecting the importance of additional factors driving body size. Projections of warming impacts on small mammals suggest that diurnal species could lose habitable space globally. Conversely, warming could lift cool temperature constraints on nocturnal species and increase habitable space, suggesting that a shift towards nocturnal niches might be favored in a warming world. Taken together these findings demonstrate the importance of energetic considerations for endotherms in managing global change impacts on nocturnal and diurnal species.

Cambio climático y las consecuencias termorregulatorias del tiempo de actividad en mamíferos

Los periodos de actividad estructuran los ambientes térmicos que experimentan los organismos. En mamíferos, las especies pasan de ser nocturnas a diurnas y viceversa, sin embargo, las consecuencias térmicas de los patrones de actividad variables permanecen inexploradas. Aquí, utilizamos polígonos de termorregulación teóricos delimitados por estimaciones de las tasas metabólicas basales (BMR), las tasas metabólicas máximas (MMR) y la conductancia térmica (C) en pequeños mamíferos para explorar las consecuencias metabólicas de la exposición a diferentes temperaturas diurnas y nocturnas a escala mundial. Las predicciones de los modelos indican una mayor expansividad metabólica en las especies nocturnas de latitudes bajas, y que una reducción en C y un mayor tamaño corporal aumentan el rango geográfico en el cual la nocturnidad es ventajosa. De acuerdo con las predicciones, dentro de los roedores, las especies nocturnas tienen una C baja. Sin embargo, los mamíferos nocturnos tienden a ser más pequeños que las especies diurnas, lo que probablemente refleja la importancia de factores adicionales que determinan el tamaño corporal. Las proyecciones de los impactos del calentamiento global en los mamíferos pequeños sugieren que las especies diurnas podrían perder espacio habitable a nivel mundial. Por el contrario, el calentamiento global podría relajar las restricciones debidas a temperaturas frías sobre las especies nocturnas y aumentar el espacio habitable, lo que sugiere que un cambio hacia nichos nocturnos podría verse favorecido en un planeta en proceso de calentamiento. En conjunto, estos hallazgos demuestran la importancia de las consideraciones energéticas para comprender los impactos del cambio global en las especies nocturnas y diurnas de endotermos.