The impact of rising global temperatures on survival and reproduction is
putting many species at risk of extinction. In particular, it has
recently been shown that thermal effects on reproduction, especially
limits to male fertility, can underpin species distributions in insects.
However, the physiological factors influencing fertility at high
temperatures are poorly understood. Key factors that affect somatic
thermal tolerance such as hardening, the ability to phenotypically
increase thermal tolerance after a mild heat shock, and the differential
impact of temperature on different life stages, are largely unexplored
for thermal fertility tolerance. Here, we examine the impact of high
temperatures on male fertility in the cosmopolitan fruit fly Drosophila
virilis. We first determined whether temperature stress at either the
pupal or adult life-history stage impacts fertility. We then tested the
capacity for heat-hardening to mitigate heat-induced sterility. We found
that thermal stress reduces fertility in different ways in pupae and
adults. Pupal heat stress delays sexual maturity, whereas males heated
as adults can reproduce initially following heat stress, but lose the
ability to produce offspring. We also found evidence that while
heat-hardening in D. virilis can improve high temperature survival,
there is no significant protective impact of this same hardening
treatment on fertility. These results suggest that males may be unable
to prevent the costs of high temperature stress on fertility through
heat-hardening which limits a species' ability to quickly and
effectively reduce fertility loss in the face of short-term high
temperature events.