Tell me how small you are and I will tell you how hot you are

Pincebourde, Sylvain; Dillon, Michael; Woods, Art

View of our study site, a meadow at the University of Wyoming - National Park Service (UW-NPS) research station, near Jackson Lake in Grand Teton National Park (USA). As is common in the region, arrowleaf balsamroot (Balsamorhiza sagittata; Asterales: Asteraceae) and other forbs are interspersed with sagebrush in open meadows surrounded by mixed conifer forests. Photo: Sylvain Pincebourde.
View of our study site, a meadow at the University of Wyoming – National Park Service (UW-NPS) research station, near Jackson Lake in Grand Teton National Park (USA). As is common in the region, arrowleaf balsamroot (Balsamorhiza sagittata; Asterales: Asteraceae) and other forbs are interspersed with sagebrush in open meadows surrounded by mixed conifer forests. Photo: Sylvain Pincebourde.

When it comes to climate, size matters. Large animals, including humans, feel hot or cold based primarily on air temperature and the strength of incoming sunlight. For these large animals, the temperature of nearby objects is largely irrelevant. Most animals, however, are tiny, and the surfaces they live on, including the soil and rocks, stems and leaves, and even other animals, can strongly dictate their body temperatures. For example, temperatures of plant leaves bathed in sun can quickly rise to more than 10°C above surrounding air temperatures, rapidly heating the small animals like insects that live on leaf surfaces.

At what size do animals escape the micro-climates of surfaces, such that their body temperatures decouple from temperatures of the surfaces they live on?  To answer this question, we studied insects living on arrowleaf balsamroot, a common and abundant plant in the northern Rocky Mountains, USA. Using both clay models and naturally occurring insects, we found that temperatures of animals whose bodies extended less than 1 mm above the plant surface (e.g., lace bugs) stayed within 2°C of leaf surface temperature. Animals with body heights of 3 mm or greater (e.g., green stink bugs), by contrast, had body temperature that differed more from nearby surface temperatures. These results have three key implications. First, 3 mm of height appears to be a transition size between thermal regimes, perhaps reflecting the typical thicknesses of air layers (the ‘boundary layers’) that adhere to all terrestrial objects. Second, the diversity of temperatures available to the smallest animals depends on the temperature diversity of their local surfaces.

In our study, leaf surface temperatures varied by >10°C within a single plant, providing an enormous range of possible body temperatures available to insects simply by moving a few cm or less. Third, understanding the effects of climate change on the smallest species will depend on understanding how climate change affects spatial and temporal patterns of variation in surface temperatures.

Read the article in full here.

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