Abstract
Species increasingly face environmental extremes. Morphological responses to changes in average environmental conditions are well documented, but responses to environmental extremes remain poorly understood. We used museum specimens to investigate relationships between a thermoregulatory morphological trait, bird bill surface area (SA) and a measure of short-term relative temperature extremity (RTE), which quantifies the degree that temperature maxima or minima diverge from the 5-year norm. Using a widespread, generalist species, Junco hyemalis, we found that SA exhibited different patterns of association with RTE depending on the overall temperature regime and on precipitation. While thermoregulatory function predicts larger SA at higher RTE, we found this only when the RTE existed in an environmental context that opposed it: atypically cold minimum temperature in a warm climate, or atypically warm maximum temperature in a cool climate. When environmental context amplified the RTE, we found a negative relationship between SA and RTE. We also found that the strength of associations between SA and RTE increased with precipitation. Our results suggest that trait responses to environmental variation may qualitatively differ depending on the overall environmental context, and that environmental change that extremifies already-extreme environments may produce responses that cannot be predicted from observations in less-extreme contexts.
Highlights
As climate change is predicted to increase the extremity and variability of environmental conditions [1,2], understanding the royalsocietypublishing.org/journal/rsos R
The avian bill plays a substantial role in thermoregulation [17]
We predicted that bill surface area would be positively associated with relative maximum and minimum temperature [24], and that relative Tmin (a) 0.4
Summary
As climate change is predicted to increase the extremity and variability of environmental conditions [1,2], understanding the royalsocietypublishing.org/journal/rsos R. Effects of environmental extremes on phenotypes is a priority [3]. Understanding responses to environmental extremes, how they differ from linear extensions of responses to less-extreme conditions, will be crucial in predicting and understanding the changes to come [2,3,11]. Characterizing the evolutionary effects of environmental extremes is challenging due to the rarity and inherent unpredictability of extreme events [3]. Natural history collections with deep sampling over time can resolve this difficulty by capturing historical responses to past environmental conditions, including extreme conditions [12]. The avian bill is well-suited to such examination of historical responses due to its preservation in museum specimens [13] and its strong ties to fitness (e.g. [14,15])
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