Increasing temperatures are not the only changes we can expect with future climate. The prevalence of infectious disease is also predicted to increase. The persistence of organisms will depend not only on their thermal tolerance but also how well they fight infection. Not all individuals do both these things well.
Males and females differ in many characteristics, from body size to behaviour, and it may be that each sex will also vary in how well they cope with both thermal stress and infection. Yet this question has been neglected up until now. Tess Laidlaw and her colleagues have used a model system to address this gap. In their system they found that females had a higher upper limit of thermal tolerance than males but, when infected with a pathogen, this difference disappeared.
The model system consists of a small aquatic crustacean, Daphnia magna and a common bacterial pathogen, Pastueria ramosa. Using multiple host and pathogen strains, the team exposed Daphnia to one of the pathogens or left them unexposed as controls. They then subjected male and female Daphnia from the different treatments to acute heat stress that is lethal to the animals and recorded the time to immobilisation – known as the knockdown time. Knockdown times measure the capacity of an individual to avoid physical incapacitation during temperature extremes and are a common measure of assessing thermal limits.
They weren’t surprised to find that females were more heat resistant than males. Sexual differences in heat tolerance have been found in other species, and the teams’ findings were consistent with differences in knockdown times for Daphnia collected from a range of latitudes. The greater tolerance of female Daphnia to heat stress is important because females invest more in their offspring. This means that population growth is likely to be more strongly linked to female survival than male survival.
Once infected however, any advantage the females demonstrated in tolerating heat stress was lost. Knockdown times in males and females were now markedly similar. Tess and her colleagues have shown how the introduction of a pathogen can potentially negate any buffer that the higher thermal limits of females provide for a population.