Sex differences in the thermal acclimation and tolerance of Daphnia clones along a latitudinal cline

Sex differences in thermal tolerance may significantly impact population responses to climate change, yet most studies examining geographic variation in thermal tolerance focus on a single sex. Here, we investigate how males and females might vary in their capacity to tolerate the heat along a latitudinal cline using the Australian water flea, Daphnia carinata. We measured heat knockdown times in males and females using clones of Daphnia from six populations spanning eastern Australia, raised under two acclimation temperatures (20°C and 25°C). Females consistently showed higher thermal tolerances than males, with greater capacity for thermal acclimation. Critically, we discovered that latitudinal clines in thermal tolerance were sex-specific: females showed a much steeper decline in heat tolerance with increasing latitude compared with males. As a result, sexual dimorphism in thermal tolerance was more than twice as large in northern populations compared with southern populations. These results suggest that sex-specific selection, potentially driven by differential habitat use or the female-biased demography of cyclical parthenogenesis, may be driving divergent thermal adaptation between the sexes. Our findings highlight the importance of considering both sexes when predicting population vulnerability to climate change, as sex-averaged estimates may misrepresent the heat tolerance of populations along environmental gradients.