temperature drops below -7 to -4 degrees C. Both males and females are at greater risk of injury during breeding season. Males extend their range in search of mates which puts them at greater risk of injury from motor vehicles and predators due to unknown territory. Females carrying young are slower moving and have to forage earlier in the evening and later into the night, also increasing their risk of injury from motor vehicles and predation. Reproduction The breeding season for the Virginia opossum can begin as early as December and continue through October with most young born between February and June. A female opossum may have one to three litters per year. During the mating season, the male attracts the female by making clicking sounds with his mouth. The female's estrus cycle is 28 days and lasts 36 hours. Gestation lasts 11–13 days and the average litter size is 8–9 infants, although over 20 infants may be born. Opossums have a very high mortality rate of their young; only one in ten offspring survive to reproductive adulthood. Newborns are the size of a honeybee. Once delivered through the median vagina or central birth canal, newborn opossums climb up into the female opossum's pouch and latch onto one of her 13 teats. The young remain latched for two months and in the pouch for months. The young then climb onto the mother's back, where she carries them for the remainder of their time together. It is during this time that the young learn survival skills. They leave their mother after about four or five months. Like all female marsupials, the female's reproductive system is bifid, with two lateral vaginae, uteri, and ovaries. The male's penis is also bifid, with two heads, and as is common in New World marsupials, the sperm pair up in the testes and only separate as they come close to the egg. Lifespan Compared to other mammals, including most other marsupials except dasyuromorphians, opossums have unusually short lifespans for their size and metabolic rate. The Virginia opossum has a maximal lifespan in the wild of only about two years. Even in captivity, opossums live only about four years. The rapid senescence of opossums is thought to reflect the fact that they have few defenses against predators; given that they would have little prospect of living very long regardless, they are not under selective pressure to develop