Fecundity, a key component of fitness, responds unimodally to temperature in ectotherms. Previous studies have focused on how temperature affects the optimum temperature of fecundity, demonstrating that tropical ectotherms tend to peak in fecundity near the mean environmental temperature while temperate ectotherms tend to have thermal optima far above the mean. A less studied aspect of the thermal response of fecundity is its response breadth, the range of temperatures ectotherms successfully reproduce. In this study, we ask: how does the response breadth of ectotherm fecundity change as a function of latitude and what is its mechanistic basis? We approach this question with two methods: a data analysis describing the relationship between the response breadth of ectotherm fecundity and latitude (in which we consider specific climatic variables such as mean temperature, seasonal variation, and diurnal variation) and an eco-evolutionary model that captures the evolution of the ectotherm reaction norm of fecundity in response to the aforementioned features of the thermal environment.
Results/Conclusions
We report two key findings. Firstly, we find that little or no relationship exists between latitude and the response breadth of ectotherms, contrary to previous studies which predict tropical ectotherms to have narrow thermal niches and temperate ectotherms to have broad thermal niches. Tropical ectotherms have much broader thermal responses than previous studies predict. Secondly, we explain this phenomenon with our eco-evolutionary model making use of a key climatic pattern of latitudinal variation: the magnitude diurnal variation exceeds that of seasonal variation in the tropics while seasonal variation exceeds diurnal variation in temperate regions. This latitudinal asymmetry has a normalizing effect on the response breadth of fecundity. Tropical ectotherms must adapt to diurnal variation with a wider response breadth because diurnal variation largely increases the range of temperatures they experience. Because tropical ectotherms have optima near the mean, diurnal variation facilitates a relatively generalist strategy. In contrast, diurnal variation either has a nominal effect on the fecundity of temperate ectotherms or facilitates further specialization to relatively high environmental temperatures, depending on the specific environmental context. These findings yield novel insights about the evolution of ectotherm life history traits with potential applications to climate warming scenarios.