The wood mouse is a murid rodent native to Europe and northwestern Africa. It is closely related to the yellow-necked mouse but differs in that it has no band of yellow fur around the neck, has slightly smaller ears, and is usually slightly smaller overall: around 90 mm (3.54 in) in length and 23 g in weight. It is found across most of Europe and is a very common and widespread species, is commensal with people and is sometimes considered a pest. Other common names are long-tailed field mouse, field mouse, common field mouse, and European wood mouse. This species is a known potential carrier of the Dobrava sequence of hantavirus which affects humans and may pose serious risks to human health.

Sperm competition is the competitive process between spermatozoa of two or more different males to fertilize the same egg during sexual reproduction. Competition can occur when females have multiple potential mating partners. Greater choice and variety of mates increases a female's chance to produce more viable offspring. However, multiple mates for a female means each individual male has decreased chances of producing offspring. Sperm competition is an evolutionary pressure on males, and has led to the development of adaptations to increase male's chance of reproductive success. Sperm competition results in a sexual conflict between males and females. Males have evolved several defensive tactics including: mate-guarding, mating plugs, and releasing toxic seminal substances to reduce female re-mating tendencies to cope with sperm competition. Offensive tactics of sperm competition involve direct interference by one male on the reproductive success of another male, for instance by mate guarding or by physically removing another male's sperm prior to mating with a female. For an example, see Gryllus bimaculatus.