Will fertility loss at high temperatures determine species responses to climate change?

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The climate is warming, and this is predicted to result in an increase in extremes of temperatures. Understanding how this will affect the survival and distribution of organisms is vital if we are to prevent massive losses of species, and invasion by harmful pests. The impacts of climate change are often estimated by examining the temperatures that kill animals. However, this may be flawed. In most animals, from beetles to birds to badgers, males typically lose their fertility at a far lower temperature than that required to kill them. If increasing temperatures cause all the males in a population to become sterile, then that population will not survive, even if the temperatures are nowhere near high enough to actually kill any animals. Unfortunately, there has been very little systematic investigation of this, so we do not know whether this possible impact of increasing temperatures on male fertility really is likely to be a threat to nature.We will rectify this situation by examining the impact of ecologically relevant high temperatures on fertility in male Drosophila fruit flies. We focus initially on a model species, D. pseudoobscura, to provide a detailed examination of how temperature impacts on fertility. We will determine the impact of high temperature and extreme temperature shocks on male fertility, and whether cooler night time temperatures can restore fertility. In many insects and reptiles, after mating with a male, females may store sperm for weeks, months or even years. In some cases the females are better at maintaining the sperm at extreme temperatures than the males are, as male insects of many species often die quicker than females in harsh environments. We will examine whether female sperm storage can ameliorate the impacts of temperature on male fertility. Most importantly, this study on D. pseudoobscura will allow us to work out standard techniques to evaluate these impacts of temperature on male fertility generally. With the knowledge gained from this case study, we will then examine how temperature impacts on fertility in a panel of 50 Drosophila species, carefully chosen to cover a range of lifestyles, habitats and temperatures, including tropical species, temperate species, and species that have spread worldwide. Most importantly, all these species are really well known, with excellent data about the climates they live in, and the temperatures they can survive in the laboratory. We will work out the fertility impacts in all 50 species, and then be able to correlate this with the distributions of the species. If the fertility data predicts the climates where the species are found in nature better than the high temperature fatality data other people have already collected, then we will know that male fertility really does impact on where species can survive in nature. We should also begin to be able to predict which groups it is particularly likely to be important for. For example, we might find that species where males mate many times in their lives, in which males typically have large testes and produce huge numbers of sperm, may be better able to remain fertile at extreme temperatures. Species where males typically mate only a few times in their lives may easily be rendered infertile. Alternatively, species restricted to areas where temperatures vary very little (such as rainforests) may be particularly vulnerable to temperature extremes, whereas species that regularly encounter rapidly changing temperatures may remain fertile even in extreme conditions.