Reproductive biology

Tree squirrels have a polygamous mating system in which either sex can have more than one mate. Most knowledge comes from studies in North America and Europe where mating behavior among the different species of tree squirrel is very similar. Females are in estrus for less than one day and males compete for access to the females. During "mating chases" in which a female is often followed by several males, she will try and avoid pursuing males and mate in sheltered locations with reduced risk of attack and injury. Generally, there is a hierarchy among the males pursuing the female with the dominant males closest to the female. Approaches of males towards the female are cautious with withdrawal movements, vocalizations, and flicking actions of the tail to check whether the female will accept the male.

The onset of estrus is not synchronous in females within a population and depends on body condition. Males do not contribute to parental care. Litter size and reproductive suc-

An eastern fox squirrel (Sciurus niger) leaps upward from a log. (Photo by Nick Bergkessel/Photo Researchers, Inc. Reproduced by permission.)
Abert's squirrel (Sciurus aberti) eating on a branch. (Photo by C. K. Lorenz/Photo Researchers, Inc. Reproduced by permission.)

cess are linked to habitat quality and food availability with some females succeeding in raising more than one litter a year.

Squirrels nest in tree cavities (dens), nests (dreys), or in some cases holes in the ground. Dreys are elaborate circular structures made up of a coarse outer layer of leaves or needles and sticks and a woven inner layer of grass, moss, bark, feathers and other similar materials. Communal nesting, particularly during periods of adverse weather occurs in a number of species.

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