Geographic divergence in upper thermal limits across insect life stages: does behavior matter?
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Insects with complex life cycles vary in size, mobility, and thermal ecology across life stages. We examine how differences in the capacity for thermoregulatory behavior influence geographic differences in physiological heat tolerance among egg and adult Colias butterflies. Colias adults exhibit differences in morphology (wing melanin and thoracic setal length) along spatial gradients, whereas eggs are morphologically indistinguishable. Here we compare Colias eriphyle eggs and adults from two elevations and Colias meadii from a high elevation. Hatching success and egg development time of C. eriphyle eggs did not differ significantly with the elevation of origin. Egg survival declined in response to heat-shock temperatures above 38–40 °C and egg development time was shortest at intermediate heat-shock temperatures of 33–38 °C. Laboratory experiments with adults showed survival in response to heat shock was significantly greater for Colias from higher than from lower elevation sites. Common-garden experiments at the low-elevation field site showed that C. meadii adults initiated heat-avoidance and over-heating behaviors significantly earlier in the day than C. eriphyle. Our study demonstrates the importance of examining thermal tolerances across life stages. Our findings are inconsistent with the hypothesis that thermoregulatory behavior inhibits the geographic divergence of physiological traits in mobile stages, and suggest that sessile stages may evolve similar heat tolerances in different environments due to microclimatic variability or evolutionary constraints.
KeywordsColias Wing melanin Overheating Heat shock Thermoregulation
We thank the City of Gunnison, the City of Montrose, and Robert and Roxanne Lane of Dayspring Farms for access to collection sites. We also thank the Rocky Mountain Biological Laboratory for access to lab space. We thank Kati Moore for her tireless work with the egg thermal gradient and Edward Shin for the countless hours scoring egg-hatching videos. We thank Kate Augustine, Samuel Shuford, and Autumn Arciero for help with field experiments. We also thank the reviewers and Rob Dunn for useful edits on the manuscript. The research was supported in part by NSF Grants DEB-1120062 to L. B. B. and J. G. K. and IOS-1120500 to J. G. K.
Author contribution statement
H. J. M., L. B. B., and J. G. K. conceived and designed the experiments. H. J. M. and J. K. H. performed the experiments. H. J. M. and J. G. K. analyzed the data. H. J. M. and J. G. K. wrote the manuscript; other authors provided editorial advice.
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