Impacts of parental age and inbreeding on fitness in a wild insect

Parental age and inbreeding have both been shown to have substantial fitness effects in laboratory experiments and in observations of wild animals. These demographic effects are likely to be strongly impacted by habitat fragmentation and warming temperatures, so understanding them is a priority. In insects and other ectotherms, some processes implicated in senescence are dependent on temperature. Anticipated changes in climate may therefore have direct effects on senescence in insects, or indirect effects via parental age. Similarly, although effects of inbreeding are well studied in wild vertebrates, information about how matings between relatives affect fitness in invertebrates comes almost exclusively from laboratory studies. To bridge the divide between field studies of vertebrates and laboratory studies of insects, we conducted an experiment using wild field crickets, Gryllus campestris. We experimentally manipulated the relatedness of parents, their age at reproduction and the temperature they experience as they aged. We then released the offspring of these parents into a natural meadow and used a network of video cameras to monitor their adult behaviour and life history throughout the course of their breeding season. We found no effect of parental age on their offspring. There were effects of inbreeding, but they were restricted to more inbred females mating to fewer males, and more inbred males being slightly smaller than outbred males. Our study suggests that effects that can be detected in laboratory studies may have relatively modest effects on fitness in nature.