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.

“Trade-offs in cold-resistance at the northern range edge of the common woodland ant Aphaenogaster picea (Formicidae)”

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Andrew D. Nguyen, Megan Brown, Jordan Zitnay, Sara Helms Cahan, Nicholas J. Gotelli, Amy Arnett, and Aaron M. Ellison (Dec 2019)

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Geographic variation in low temperatures at poleward range margins of terrestrial species often mirrors population variation in cold resistance, suggesting that range boundaries may be set by evolutionary constraints on cold physiology. The northeastern woodland ant Aphaenogaster picea occurs up to approximately 45°N in central Maine. We combined presence-absence surveys with regression-tree analysis to characterize its northern range limit and assayed two measures of cold resistance operating on different timescales to determine whether and how marginal populations adapt to environmental extremes. The range boundary of A. picea was predicted primarily by temperature, but low winter temperatures did not emerge as the primary correlate of species occurrence. Low summer temperatures and high seasonal variability predicted absence above the boundary, whereas high mean annual temperature (MAT) predicted presence in southern Maine. In contrast, assays of cold resistance across multiple sites were consistent with the hypothesis of local cold adaptation at the range edge: among populations, there was a 4-minute reduction in Chill-Coma Recovery Time across a 2-degree reduction in MAT. Baseline resistance and capacity for additional plastic cold hardening shifted in opposite directions, with hardening capacity approaching zero at the coldest sites. This trade-off between baseline resistance and cold hardening capacity suggests that populations at range edges may adapt to colder temperatures through genetic assimilation of plastic responses, potentially constraining further adaptation and range expansion.