Reproduction and Mating              

The grey squirrel (Sciurus carolinensis) has two breeding seasons each year, the first in January and February and the second in June and July.  Both sexes remain reproductively active throughout their lives.  Most females do not reproduce until 1.25 years of age.  An enlarged pink vulva is usually visible the day before the onset of estrus and lasts less than eight hours.  The vagina is closed in prepubescent and anestrous females. 

            Male grey squirrels are sexually mature at 10-11 months of age.  Functional testes descend in the scrotum from December to February and May to July, although testes may stay descended without spermatogenesis until October.  Copulation lasts less than thirty seconds.  After ejaculation, a gelatinous white vaginal plug forms, preventing further sperm entry. 

The female generally gives birth in a warm, sheltered nest that she has prepared in a cavity inside a hollow tree.  Sometimes leaf nests are used, especially for a summer litter if a tree den is not available.

Gestation takes 40-44 days and an average of three young are born, although the litter size may range from one to six.  Litters in the summer are usually larger than those of late winter.

Newborns are naked and blind and they weigh from 13-18 grams.  Young are altricial.  They mature quickly, developing hair by four weeks and sight a week later.  By the eighth week the young squirrels are venturing out of the nest for short distances.  Weaning begins around this time and is completed by the tenth week.  Juvenile males are more likely to leave the natal area and disperse than are juvenile females.  Dispersal usually occurs during the fall and young males move between one and sixteen kilometers away from their natal nest.  However dispersal is a high cause of mortality among males, which results in an adult sex ratios of .85 males to 1.6 females.  Adult size and mass are reached at nine months.

Many species maintain access to resources such as food caches or territories outside the period of offspring dependency.  It has been assumed that these resources are secured for the direct benefit of the resource holder and that the amount is sufficient for maintenance and breeding.  It may also be used as a form of anticipatory parental care whereby the resources will be held until they can be relinquished to offspring when the offspring gain independence.  Red squirrels (Tamiasciurus hudsonicus) are from the sciurinae family and therefore comparable to grey squirrels.  Individual red squirrels defend exclusive territories containing a traditional food cache site that is necessary for survival.  Juveniles must establish their own independent territory by winter and some females appear to assist their offspring by giving their food cache to their offspring and relocating to vacant food caches nearby.  Because red squirrels have evolved very advanced forms of both parental care (territory bequethal) and investment for the future (traditional food caches), they appear to be demonstrating the latter hypothesis of the existence of anticipatory parental care.  

Mating Behaviour

 Courtship behaviour begins when a receptive female begins to call continuously from a tree top with characteristic duck-like calls.  Several males soon gather.  Approximately five to ten males begin to follow females up to five days before estrus, although as many as thirty-four males have been documented to follow a single female.  As some males begin to chase the female, other males hear the noise and also join the chase.  The female runs through the trees and leaves scent markings on branches as she travels.  The males pursue her through the trees.  As the chase progresses, the female stops and aggressively chases the males.  The males then back off and begin to act aggressively towards each other.  A dominance hierarchy is formed among the males. For adult males, mating is a frenetic period of intense competition for available females who remain in estrus for as little as four hours.  Males gain access to females through antagonistic displays and fighting.  The potential female mates observe these competitive interactions and will go so far as to drive undesirable males from their nesting territories.  The chase can last for many hours.  On the day of estrus females will mate first with the dominant male.  However she will also mate with several other males throughout the day.  The female always mates in her home range.  Copulation occurs above ground and must be completed as discretely and inconspicuously as possible to avoid interruption by other males.  Multiple paternity is the rule and has been estimated to be 78%.  For males, breeding success is closely correlated to size.  After copulation, the male has no further role in bringing up the young.

Although tree squirrels are dependent on natural tree cavities for nesting sites and winter homes,  when preparing for the birth of its young, the grey squirrel builds a large round nest high up in the trees.  It looks like a large bird’s nest.  Grey squirrels typically use 3 different types of nests: winter dreys, summer dreys, and dens. Dreys are round conspicuous twig and leaf nests built in trees between 25 and 45 cm in diameter. They are waterproof, and made of an outer layer of interwoven twigs with a softer inner lining consisting of moss, bark, leaves, fur, feathers, lichen or other similar material. Summer dreys are less elaborate than winter dreys and may be no more than twig and leaf saucer shaped platforms on exposed branches. Dreys are generally built in the upper 1/3 of the canopy and seldom in isolated trees, which may serve to protect nests from predators.  Tree dens are another type of nest used by grey squirrels. These are holes or cavities in the main trunks of trees which are also lined with soft material. Formation of den cavities requires 8 - 30 years, and are more common in deciduous trees than in coniferous trees. Squirrels often use dens in winter months and dreys in summer months.    

Richardson’s ground squirrels Spermoplilus ricardsonii are from the same subfamily, sciurinae as the grey squirrel and therefore can be compared.  Female ground squirrels tolerate the proximity of closely related female kin and are antagonistic towards all other squirrels, regardless of sex.  Although they tolerate each other’s nearby presence, this tolerance does not extend to sharing the nest in which young are born and reared.  During pregnancy each female establishes her own burrows system with two or three nest chambers in which she rears her offspring.  Thus, for the first month of life infants come in contact only with each other and with the mother.

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