Temperature variation makes an ectotherm more sensitive to global warming unless thermal evolution occurs.

To assess long-term impacts of global warming on species, there is growing interest in latitudinal intraspecific patterns in thermal adaptation. Yet, while both mean temperatures and daily temperature fluctuations (DTFs) are expected to increase under global warming, latitudinal differences in the effects of DTFs have not been documented. We tested whether low-latitude populations of an ectotherm deal better with greater DTF than high-latitude populations, especially at a high mean temperature close to the optimal temperature for growth where DTF causes exposure to extreme high temperatures. We evaluated the impact of DTFs when assessing the effect of gradual thermal evolution at the high latitude with a space-for-time substitution. We compared effects of both mean temperatures (20 and 24°C) and DTFs (constant = 0°C, low = 5°C and high = 10°C) on growth rates between low-latitude and high-latitude populations of the damselfly Ischnura elegans in a common-garden experiment. DTFs, if anything, reduced growth and were generally stressful as indicated by reductions in body condition, antioxidant defence and metabolic rate, and increases in oxidative damage. Most negative effects of DTFs were only present at a mean of 24°C when too high temperatures were reached during a daily cycle. Notably, while 4°C warming was beneficial in terms of growth rate at both latitudes at a constant temperature regime, this changed in a negative effect at high DTF. Moreover, this modulating effect of the mean temperature by DTF differed between latitudes indicating local thermal adaptation. While 4°C warming at low DTF still caused faster growth in low-latitude larvae, it already slowed growth in high-latitude larvae. This supports the emerging insight that warming would increase growth in high-latitude larvae in the absence of DTF, yet would decrease growth in the more realistic scenarios with DTF. In contrast, a space-for-time substitution approach suggested that under gradual thermal evolution, the evolved high-latitude larvae would no longer suffer a growth reduction in the presence of DTF. Our study provided important proof-of-principle that jointly integrating gradual thermal evolution and the expected increase in DTF generates opposing predictions of effects of global warming on this ectotherm.