Age-specific Jensen contrasts reveal how ontogenetic responses to thermal variability shape lifetime performance in Daphnia magna
Year: 2026
Type: preprint
Abstract: Temperature shapes life histories through its effects on energy allocation among growth, reproduction and maintenance. The resulting temperature dependence of performance is commonly characterised using thermal performance curves (TPCs), which are often treated as static descriptions that apply uniformly throughout life. Yet the response to thermal variability may change across ontogeny. We recorded growth, reproduction and survival throughout the lifetimes of individual Daphnia magna under four thermal regimes: Constant Low (15°C), Constant Rearing (20°C), Constant High (25°C) and Variable (15–25°C). The mean temperature under Variable closely matched that under Constant Rearing. Using an allometric energy-allocation model, we derived age-specific and lifetime performance measures and quantified their uncertainty with a stratified individual-level bootstrap. Our principal quantity was the Jensen contrast, defined as performance under Variable minus that under Constant Rearing. Age-specific point estimates of the Jensen contrast changed sign across ontogeny. A positive aggregate contrast in full-trajectory growth concealed a negative mid-life contrast; reproductive point estimates were positive during early and mid-life but negative later. Bootstrap support was strong for the positive early-life and negative mid-life growth contrasts, and for the positive early-life reproductive contrast. Under a second-order approximation, this pattern of estimated signs is consistent with ontogenetic changes in effective thermal curvature. The positive early-life reproductive response did not carry through to the lifetime outcome: expected lifetime reproduction was lower under Variable than under Constant Rearing, with strong bootstrap support for the negative lifetime contrast. Because lifetime reproduction is survival-weighted, lower survival under Variable reduced later reproductive contributions; across the estimated trajectories, the accumulated negative late-life contribution outweighed the earlier positive contributions. These results show why forecasts based on mean temperature, a single life stage or an aggregate TPC may misrepresent lifetime performance under variable thermal regimes. Accounting for both ontogeny and temperature variability may therefore improve forecasts of life-history performance in increasingly variable thermal environments.
Language: English
Subfield: Ecology
Field: Environmental Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7214674902