Comments & suggestions  should be directed to Visit my website Reproduction The age of puberty in mice varies according to the strain of mouse, nutritional status, and environmental influences, but in general occurs between 28 and 49 days of age.  Signs of puberty in the female mouse include opening of the vagina and the presence of cornified epithelial cells in a vaginal smear.  Fertility in female mice is greatest between 75 and 300 days of age. Mice are polyestrous and breed year round.  The onset of receptive heat generally occurs between 10 p.m. and 1 a.m.  If females are maintained on a constant light-dark cycle they will ovulate once every 4 - 5 days (with some variability) at approximately 3 - 5 hours after the start of the dark period.  Ovulation is spontaneous and usually occurs 8 - 11 hours after the onset of estrus.  Estrus generally lasts 14 hours.  Ovulation does not always occur in every estrous cycle.  The current stage within the estrous cycle can be determined by examining a vaginal smear.  A postpartum estrus occurs 14 - 28 hours following parturition, at which time the female may be re-bred to maximize production.  If a timed pregnancy is required, breeding the female at the postpartum estrus should be avoided because delayed implantation (4 - 10 days) is common. Mating is detected by the presence of a milky white to yellow vaginal plug due to secretions from the male's coagulation glands.  This plug normally persists 16 to 24 hours and can last up to 48 hours.  If timed pregnancies are required by the investigator, mice should be checked for the presence of a vaginal plug at least every morning and, if possible, twice daily because some females will mate during the day.  Hidden, deep copulation plugs may be found in some mice bred during the postpanum estrus or routinely in some stocks of mice.  A magnifying glass and a small probe may be useful in detecting vaginal plugs. When the cervix and vagina are stimulated by breeding, prolactin is released from the anterior pituitary which in turn causes the corpus luteum to secrete higher levels of progesterone for about 13 days.  If fertilization has occurred, the placenta will take over the production of progesterone. If fertilization has not occurred, females will still appear to be pregnant for this period of 13 or so days; this phenomenon is referred to as pseudopregnancy.  Grouping of females may also induce pseudopregnancy. Gender determination is possible by observation of the anogenital distance, which is 1.5 to 2 times greater in males than in females.  Average gestation in mice is 19 - 21 days.  Implantation of the embryos usually occurs on the fifth day postbreeding.  Litter size varies from 1 - 13 pups, with first litters typically smaller than subsequent litters.  Although female mice do not commonly mutilate or cannibalize their pups, whelping animals or those which have recently whelped should be undisturbed for at least 2 days postpartum.  Mouse pups are altricial at birth; that is, they are born blind, deaf, and naked. Newborns are often referred to as "pinkies".  The "milk spot", which consists of the stomach filled with milk, can be seen through the pup's thin skin and can be used to determine whether nursing has occurred.  Fine hair covers the pup by 10 days of age and their ears are open at this time.  By day 12 the eyes are open as well.  Lactation lasts for approximately 3 weeks; pups are generally weaned at 21 days of age, at which time they weigh 10 - 12 grams.  Commonly used mating systems include pair mating, trios (one male and 2 females), and harems (one male and multiple females).  Males left continuously with females are more likely to re-breed the females at the postpanum estrus, thereby decreasing time between litters.  Within harem systems, females with litters near the same age tend to pool the pups and share in their care.  This does, however, make identification of pups and their dams more difficult.  If identification of individual pups is required for research purposes, pregnant females should be isolated in separate cages near the time of parturition. Pheromones Pheromones are chemical substances produced by one animal which provide olfactory stimuli and communication to another animal.  Pheromones play important roles in the behavior and reproduction of mice.  Mice secrete two types of pheromones, known as signaling and priming pheromones.  Signaling pheromones include a fear signal, male and female sex attractants, and aggression inhibitors.  Preputial gland pheromones of male mice provide female attractant stimuli.  Urine from dominant male mice contains both aversion and aggression promoting pheromones.  Application of urine from dominant males discourages investigation of the area by subordinate animals and incites aggression in other dominant males. Priming pheromones include an estrus inducer, an estrus inhibitor, and adrenocortical activators. These pheromones can affect the estrous cycle of female mice; therefore, an understanding of these pheromonal effects is crucial for successful management of mouse breeding colonies.  Three effects have been well characterized in mice; the Bruce effect, the Lee-Boot effect, and the Whitten effect. The Bruce effect, or strange-male pregnancy block, occurs when a recently mated female is housed with or near a strange male.  Implantation is inhibited in 30% of females and pregnancy is blocked when the strange male is introduced within 24 hours of mating.  Affected females return to estrus in 4 - 5 days.  The maximum effect occurs when the strange male is of a different strain than the breeding male.  Direct contact with the male is not required for the block to occur.  Males castrated prior to puberty cannot induce this effect.  The Lee-Boot effect is induced by housing female mice in groups of 4 or more.  A higher incidence of pseudopregnancy is observed in these groups of female mice than in singly housed females in the absence of matings.  This suggests that female mice produce a pheromone which influences the estrous cycle.  Anestrus (cessation of cycling) can occur if female mice are housed in groups of 30 or more. The Whitten effect is seen when female mice are paired with male mice for breeding after an extended time of housing with other females only.  Most of these females will mate on the third night after being introduced to the male.  For comparison, mating is normally evenly distributed over the first 4 nights after pairing females with males when the females were previously caged individually.  This effect can be taken advantage of to orchestrate timed pregnancies. The current stage within the estrous cycle can be determined by examining a vaginal smear. (A) Metestrous / Diestrous - smear consists mostly of leukocytes with a few cornified epithelial cells. (B) Proestrous - leukocytes and rounded, nucleated epithelia Gender determination.  The anogenital distance is 1.5 to 2 times greater in the male (left) than in the female (right).