The mating system of this species has not been described. However, information on home ranges indicates that the species is probably polygynous. Males have larger home ranges than females, and their home ranges overlap extensively with those of other males and of females. Females, on the other hand, have restricted home ranges, which may overlap those of males, but not those of other females. It is likely then that males have access to many females during breeding, but females are not likely to have access to many males.
Although this is a reasonable assumption, it should be noted that reproductive patterns within the genus Peromyscus as a whole are highly variable. In some species, like Peromyscus polionotus or Peromyscus californicus, mating appears to be strictly monogamous. In other species, like Peromyscus leucopus and Peromyscus maniculatus, breeding can be polygynous or monogamous, depending upon ecological conditons.
Because P. gossypinus occurs in a great variety of habitats and ecological conditions, it is likely that there is some variability in mating systems. Like their sister taxon, P. leucopus, these mice probably form monogamous pairs under some cirumstances.
During estrous, females of this species show little or no consistancy in their patterns of external signals. Swelling and protrusions of the vaginal area are observed inconsistently. It is likely, therefore, that these signals, used by people to determine the reproductive status of the female, are not that important to males of the species.
Males and females probably use a combination of scent cues and behaviors to determine when another animal is ready to mate.
Mating System: monogamous ; polygynous
Peromyscus gossypinus produces at least 4 litters per breeding season. This species averages a 23 day gestation period in non-lactating females and a 30 day gestation period in lactating females. There is a post-partum estrus in this species, which allows a female to rapidly produce addtional litters.
Peromyscus gossypinus females give birth to their young in a nest constructed of primarily cotton. Births usually occur in the early hours of morning. One specimen, observed in captivity by Pournelle, was extremely active on the night prior to parturition, moving around anxiously and frequently stretching her entire body. Once morning arrived, she had quieted down and slept until the first birth.
Litters of 1 to 7 young have been reported, with the average size being 3.7 young. In the sister species of cotton mice, Peromyscus leucopus, variation in litter size with latitude has been reported. Average litter size increases with lattitude, so that litters are larger in northern populations. Because P. gossypinus is also a very widespread species, it is likely that such litter size variation with lattitude also occurs in this species.
All Peromyscus neonates are altricial. They are born hairless, with their eyes closed, incisors underneath the gumline, and pinnae folded. However, cotton mice develop quickly. The ear pinnae are erect by about 4 days of age. By the age of 5 days, these mice are beginning to have a noticable hair cover on their backs. Around day 7 of life the incisors have broken through the gums. The young are fully furred by the time they are 10 days old, and appear to be alert and able to respond to stimuli in their environment. Most baby cotton mice open their eyes for the first time between the ages of 12 and 14 days.
In terms of their body size, neonates average only 2.19 g at birth. They are about 47 mm in length, and their tails are relativle short, measuring only 11 mm. They double in size within the first two weeks of life, and reach 93% of their adult length by the age of 60 days.
The pelage of baby mice is grayish. They undergo their first molt between the ages of 34 and 40 days, with the adult pelage appearing first on their sides, then on the head and face, and finally spreading back over the back and the rump.
Females in captivity reach reproductive maturity between the age of 43 and 73 days. The average age at which the vagina opens up is 43 days, and the average age of first conception is 73 days. It is difficult to say how this is mirrored in wild populations, because nutrition is different in captivity, as are other social and environmental cues which could affect reproductive development.
Male reproductive maturity is harder to determine. A captive male had sperm in his epididymides at about 45 days of age.
Breeding season in this species may vary geographically. In Florida, these mice breed throughout the year, with a peak in breeding activity in the late autumn and early winter. There is a lull in breeding in the middle of the summer. Although the precise reason for this lull is not known, it may be related to temperature, since male reproductive condition appears to be affected by temperature.
In other parts of the range, P. gossypinus has been seen in breeding condition in spring, summer, and autumn. Records from the Great Smokey Mountains do not show animals in breeding condition during the winter months, but it is not clear whether or not animals were examined at this time.
Breeding interval: Females can produce as many as four litters in a breeding season, with litters born approximately every 30 days.
Breeding season: Year-round in Florida
Range number of offspring: 1 to 7.
Average number of offspring: 3.7.
Range gestation period: 22.86 to 23.34 days.
Average weaning age: 3 to 4 weeks.
Average time to independence: 3-4 weeks.
Range age at sexual or reproductive maturity (female): 43 to 73 days.
Range age at sexual or reproductive maturity (male): 45 (low) days.
Key Reproductive Features: iteroparous ; seasonal breeding ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization ; viviparous ; post-partum estrous
Females care for their young in a nest, providing shelter, warmth, and milk to nourish the altricial neonates. It has been shown that in several species of the genus Peromyscus, the male remains with the female to help care for the young, although no data exist for P. gossypinus.
Parental Investment: altricial ; pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female)