# Understanding the role of ageing in the thermal responses of life history and fitness in Daphnia magna

## TL;DR

This study explores how temperature affects aging and fitness in water fleas, finding that lifespan alone doesn't predict aging effects and that lower fitness in warmer climates may not be due to faster aging.

## Contribution

The study introduces a pace-standardized time scale to compare aging effects across temperatures in ectotherms.

## Key findings

- Lifespan is not a reliable predictor of temperature effects on aging in Daphnia magna.
- Temperature does not consistently alter the shape of aging in survival and reproduction.
- Fitness is positively correlated with the shape of aging within the same temperature.

## Abstract

The current climate warming has raised concerns that increased rates of ageing will lead to reduced fitness and population viability in ectotherms. However, it is currently not known whether temperature effects on the rate of ageing mainly reflect a change in pace of life or changes in the strength of ageing in the life history. Evaluating this question requires that ageing in different temperatures is compared on the same intrinsic pace-standardized time scale. We present results from a laboratory experiment with Daphnia magna, recording complete life histories of 240 individuals from four clonal lines at six temperatures (5°C to 30°C). For each clone and temperature, we calculated measures of pace and shape of ageing in survival and reproduction. Fitness (long-term population growth rate) and mean lifetime reproduction were calculated using matrix population models. Our results highlight three main points: (i) lifespan is not a good predictor of temperature effects on ageing; (ii) there is no strong consistent effect of temperature on shape of ageing; and (iii) within temperature, the shape of ageing was positively correlated with fitness. Hence, lower fitness and population viability of ectotherms in warmer environments may be driven by factors other than increased ageing rates.