Natural levels of polyandry: differential sperm storage and temporal changes in sperm competition intensity in wild yellow dung flies

1. Polyandry is common in insects. Nevertheless, the evolutionary causes and consequences of this phenomenon remain contentious, in part because of a lack of information about natural mating rates and the fact that most post-copulatory processes are hidden from view within female reproductive tracts. 2. We captured wild female yellow dung flies (Scathophaga stercoraria) over the whole spring season and genotyped the sperm from their spermathecae to obtain information on sperm transfer, sperm storage and natural levels of polyandry for this model species of post-copulatory sexual selection research. 3. On average, females stored sperm from a minimum of 2-47 males (based on the most conservative estimate). Incorporating knowledge of population allele frequencies yielded a slightly higher estimate of 3-33 mates per female. 4. Sperm storage and therefore sperm competition intensity showed high temporal variation. The proportion of multiply mated females (i.e. females with sperm from ≥ 2 males within their sperm stores) and the absolute number of ejaculates detected within females increased strongly over the spring season before sharply decreasing as midsummer approached. 5. Interestingly, we detected a positive relationship between the number of stored ejaculates and females' wing injuries, suggesting that mating not only causes measurable cumulative damage to wild females but also provides a potential mechanism by which males may be able to assess the intensity of sperm competition within a female. 6. Our study found no evidence for intraejaculate sperm sorting, but importantly, the number of ejaculates in storage differed amongst the three sperm storage organs (spermathecae) of female yellow dung flies. Different sperm mixtures across the spermathecae could enable females to bias paternity towards certain males if females can selectively use sperm from a certain spermatheca at the time of fertilization.