Heatwaves are increasing in frequency and intensity due to climate change, pushing animals beyond physiological limits. While most studies focus on survival limits, sublethal effects on fertility tend to occur below lethal thresholds, and consequently can be as important for population viability. Typically, male fertility is more heat-sensitive than female fertility, yet direct comparisons are limited. Here, we measured the effect of experimental heatwaves on tsetse flies, Glossina pallidipes, disease vectors and unusual live-bearing insects of sub-Saharan Africa. We exposed males or females to a 3-day heatwave peaking at 36, 38 or 40°C for 2 h, and a 25°C control, monitoring mortality and reproduction over six weeks. For a heatwave peaking at 40°C, mortality was 100%, while a 38°C peak resulted in only 8% acute mortality. Females exposed to the 38°C heatwave experienced a one-week delay in producing offspring, whereas no such delay occurred in males. Over six weeks, heatwaves resulted in equivalent fertility loss in both sexes. Combined with mortality, this lead to a 10% population decline over six weeks compared to the control. Furthermore, parental heatwave exposure gave rise to a female-biased offspring sex ratio. Ultimately, thermal limits of both survival and fertility should be considered when assessing climate change vulnerability.

For many terrestrial species, habitat associations and range size are dependent on physiological limits, which in turn may influence large-scale patterns of species diversity. The temperature range experienced by individuals is considered to shape the breadth of the thermal niche, with species occupying temporally and/or geographically stable climates tolerating a narrow temperature range. High-elevation environments experience large temperature fluctuations, with frequent periods below 0 °C, but Grylloblatta (Grylloblattodea: Grylloblattidae) occupy climatically stable microhabitats within this region. Here we test critical thermal limits and supercooling points for five Grylloblatta populations from across a large geographic area, to examine whether the stable microhabitats of this group are associated with a narrow thermal niche and assess their capacity to tolerate cold conditions. Thermal limits are highly conserved in Grylloblatta, despite substantial genetic divergence among populations spanning 1500 m elevation and being separated by over 500 km. Further, Grylloblatta show exceptionally narrow thermal limits compared to other insect taxa with little capacity to improve cold tolerance via plasticity. In contrast, upper thermal limits were significantly depressed by cold acclimation. Grylloblatta maintain coordinated movement until they freeze, and they die upon freezing. Convergence of the critical thermal minima, supercooling point and lower lethal limits point to adaptation to a cold but, importantly, constant thermal environment. These physiological data provide an explanation for the high endemism and patchy distribution of Grylloblatta, which relies on subterranean retreats to accommodate narrow thermal limits. These retreats are currently buffered from temperature fluctuations by snow cover, and a declining snowpack thus places Grylloblatta at risk of exposure to temperatures beyond its tolerance capacity.