Squirrels, like many other mammals, engage in a process of courtship and mating that involves female mate choice. Squirrels are polygynandrous, meaning that both males and females may mate with several partners. Once mated, the female takes on sole responsibility for parental care of the offspring, and in some species, the female will rear two litters per year.
The Mating Chase
When a female squirrel is approaching estrus (going into heat), male squirrels gather near her territory and wait for her to become receptive. When one of the males approaches and tries to mate with the female before she is ready, she will violently defend her territory against her pursuer, resulting in a temporary stalemate. Once she is ready, the female squirrel will run off and engage the males in a mating chase within her territory. Typically, the dominant male will find the female first and can mate with her, but not always. The female is in estrus for only a matter of hours, and the act of copulation may take anywhere from one to 25 minutes, according to an observational study published in the Journal of Mammalogy 1982 by Rolf Koford, a unit leader of the Iowa Cooperative Fish and Wildlife Research Unit.
Male - Male Competition
Male squirrels have adapted several strategies to help facilitate reproductive success. If a female has given chase and then stops to mate, another male may attack the mating male viciously, occasionally injuring the female in the process. A strategy used by younger, less dominant males has been documented by Koford as well as John Koprowski from the University of Kansas in his 1992 article in Journal of Mammalogy. These males may sit out the chase and wait within the female's territory, thus avoiding the risk of injury associated with chasing a female while there is a dominant male nearby. According to Koford's study, the dominance hierarchy of the males may vary geographically within the female's territory, so that the male that is dominant in one part of the territory may not be the same male that is dominant in other parts of the territory.
One reproductive strategy employed by some squirrel species, such as the Idaho ground squirrel, is mate guarding, whereby the dominant male will stay in close proximity to the female and fend off any other males that try to approach her. Typically a display of physical dominance is enough to keep competing males from attempting to reach the female, but the Formosan squirrel has adopted a different approach. Formosan squirrels emitted a call after mating that was identical to that species' anti-predator call -- which caused other squirrels to leave the area or become immobile to avoid detection -- according to research published inn 1995 by Noriko Tamura, a biologist with Tokyo Metropolitan University, in the journal Behavioral Ecology and Sociobiology. These post-copulation anti-predator mimic calls sometimes lasted up to 17 minutes.
Sperm competition can take many forms in animal societies where the female mates with multiple males, and may include the development of more, smaller sperm cells, the forceful removal of copulatory plugs by subsequent males, mate guarding and other strategies to ensure insemination. Kaprowski documented that female tree squirrels sometimes manually remove the plug and either discard or consume it, thus enabling insemination by subsequent suitors. Tactics like mate guarding and copulatory plugs suggest that the late male to mate with the female may have the reproductive advantage; however, female removal of copulatory plugs suggests that perhaps female squirrels have a different reproductive strategy that may include mixed paternity litters.