Supplementary material from "Chronic and acute thermal stressors have non-additive effects on fertility"

Climate change is driving both higher mean temperatures and a greater likelihood of heatwaves, which are becoming longer and more intense. Previous work has looked at these two types of thermal stressors in isolation, focusing either on the effects of small, long-term increases in temperature or large, short-term increases in temperature. Yet, a fundamental gap in our understanding is the combined effect of chronic and acute thermal stressors and, in particular, its impact on vital processes such as reproduction. Here, we investigated the independent and interactive effects of higher constant temperatures and short-term heatwave events on reproductive success and offspring fitness in an insect study system (Nicrophorus vespilloides). We found a substantial reduction in key fitness traits (fecundity, hatching success, offspring size) after exposure to both a heatwave and higher constant temperatures, but not after exposure to only one of these thermal stressors. This indicates that the effects of chronic and acute thermal stressors are amplified when they act in combination, as is very likely to occur in natural populations. Our findings therefore suggest that by not considering the potential multiplicative effects of different types of thermal stressors, we may be underestimating the effects of climate change on animal fertility.