Abstract
When a change in the environment occurs, organisms can maintain an optimal phenotypic state via plastic, reversible changes to their phenotypes. These adjustments, when occurring within a generation, are described as the process of acclimation. While acclimation has been studied for more than half a century, global environmental change has stimulated renewed interest in quantifying variation in the rate and capacity with which this process occurs, particularly among ectothermic organisms. Yet, despite the likely ecological importance of acclimation capacity and rate, how these traits change throughout life among members of the same species is largely unstudied. Here we investigate these relationships by measuring the acute heat tolerance of the clonally reproducing zooplankter Daphnia magna of different size/age and acclimation status. The heat tolerance of individuals completely acclimated to relatively warm (28°C) or cool (17°C) temperatures diverged during development, indicating that older, larger individuals had a greater capacity to increase heat tolerance. However, when cool acclimated individuals were briefly exposed to the warm temperature (i.e. were ‘heat-hardened'), it was younger, smaller animals with less capacity to acclimate that were able to do so more rapidly because they obtained or came closer to obtaining complete acclimation of heat tolerance. Our results illustrate that within a species, individuals can differ substantially in how rapidly and by how much they can respond to environmental change. We urge greater investigation of the intraspecific relationship between acclimation and development along with further consideration of the factors that might contribute to these enigmatic patterns of phenotypic variation.
Highlights
Organisms can remodel their phenotypes within their own lifetimes to counteract potentially negative fitness effects of environmental change
We employed linear mixed effect modelling to evaluate the relationship between Timm and body size among individuals that were in the process of acclimating to a change in temperature relative to individuals who had developed under constant temperature (i.e. 28°C heat-hardened versus 17°C control individuals and 17°C cold-hardened versus 28°C control individuals)
We evaluated the relative support of four models, two of which considered the influence of body size and hardening treatment on heat tolerance as either interactive or additive, and two testing these same relationships but with treatment duration considered as an additional covariate
Summary
Organisms can remodel their phenotypes within their own lifetimes to counteract potentially negative fitness effects of environmental change. To investigate the relationship between size/age and acclimation in D. magna, we measured the size scaling of heat tolerance among clonal replicates of a single genotype exposed to four different temperature treatments. We employed linear mixed effect modelling to evaluate the relationship between Timm and body size (both variables transformed by taking natural logarithms) among individuals that were in the process of acclimating to a change in temperature relative to individuals who had developed under constant temperature (i.e. 28°C heat-hardened versus 17°C control individuals and 17°C cold-hardened versus 28°C control individuals). Body size effects on the rate of acclimation were estimated using the best-fitting model to predict Timm values for each of the hardened and control groups over the range of body sizes for which a hardening response was observed (1.1–3.3 mm) Across this size range, predicted Timm values (back-transformed to arithmetic scale) were used to calculate the acclimation shown by hardened individuals as a proportion of the total amount of acclimation possible (figure 1b). All statistical analyses were conducted in R version 3.5.2 [29]
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