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Unlike humans who go through a menstrual cycle that includes a monthly ovulation period, squirrels go through a period called "estrus." Estrus is a reproductive cycle that animals like dogs and cats experience, during which their bodies ovulate and they become highly receptive to mating. Like humans, this ovulation period is when squirrels become impregnated, but the similarities end there.
What Is Estrus
Once or twice a year, a squirrel experiences an estrus period. While a human's menstrual cycle gives her several days a month during which she is ovulating and likelier to become pregnant, a squirrel's estrus cycle limits this to one or two days per year. Estrus is the result of her body preparing itself to receive a male and become impregnated -- her uterine tissues thicken and her vulva swells, and she will accept a male's advances rather than rebuffing him.
Finding a Mate
A squirrel approaching estrus doesn't have to do much -- or anything -- to attract a mate. Her body does that work for her, as her swelling vulva is a visual indicator to potential suitors that she is ready to mate, or will be soon. In the days leading up to her estrus period, she also emits pheromones that attract males. Males can detect these pheromones from more than 500 yards away, and may follow a female around for five days in hope of mating with her.
Window of Opportunity
The female's actual estrus period is short-lived, and may last only about eight hours to one day. In some species, like the red squirrel, she may mate with one or several males, and may mate with the same male several times within this single day. In other species, like the eastern gray squirrel, only one male gets the opportunity, as her vaginal opening seals itself off after copulating to prevent other suitors from mating with her.
Gestation and Reproduction
Though the gestation period varies between species, it is typically between 36 and 44 days, at which point she gives birth to a litter of young. Despite this relatively short gestation period, a squirrel does not give birth more than once or twice a year, as she only mates during her estrus period. Unlike menstrual cycles that give humans a regular opportunity to become pregnant, the estrus period gives her a limited time frame for reproducing every year.
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