Fossas have a polyandrous mating system.
Most of the details of reproduction in wild populations are from the western dry deciduous forests; determining whether certain of these details are applicable to eastern populations will require further field research.
Mating typically occurs during September and October, although there are reports of its occurring as late as December, and can be highly conspicuous.
In captivity in the Northern Hemisphere, fossas instead mate in the northern spring, from March to July.
Intromission usually occurs in trees on horizontal limbs about 20 m (66 ft) off the ground.
Frequently the same tree is used year after year, with remarkable precision as to the date the season commences.
Trees are often near a water source, and have limbs strong enough and wide enough to support the mating pair, about 20 cm (7.9 in) wide.
Some mating has been reported on the ground as well.

As many as eight males will be at a mating site, staying in close vicinity to the receptive female.
The female seems to choose the male she mates with, and the males compete for the attention of the female with a significant amount of vocalization and antagonistic interactions.
The female may choose to mate with several of the males, and her choice of mate does not seem to have any correlation to the physical appearance of the males.
To stimulate the male to mount her, she gives a series of mewling vocalizations.
The male mounts from behind, resting his body on her slightly off-center, a position requiring delicate balance; if the female were to stand, the male would have significant difficulty continuing.
He places his paws on her shoulders or grasps her around the waist and often licks her neck.
Mating may last for nearly three hours.
This unusually lengthy mating is due to the physical nature of the male's erect penis, which has backwards-pointing spines along most of its length.
Fossa mating includes a copulatory tie, which may be enforced by the male's spiny penis.
The tie is difficult to break if the mating session is interrupted.
Copulation with a single male may be repeated several times, with a total mating time of up to fourteen hours, while the male may remain with the female for up to an hour after the mating.
A single female may occupy the tree for up to a week, mating with multiple males over that time.
Also, other females may take her place, mating with some of the same males as well as others.
This mating strategy, whereby the females monopolize a site and maximize the available number of mates, seems to be unique among carnivores.
Recent research suggests that this system helps the fossa overcome factors which would normally impede mate-finding, such as low population density and lack of den use.

The birthing of the litter of one to six (typically two to four) takes place in a concealed location, such as an underground den, a termite mound, a rock crevice, or in the hollow of a large tree (particularly those of the genus Commiphora).
Contrary to older research, litters are of mixed sexes.
Young are born in December or January, making the gestation period 90 days, with the late mating reports indicating a gestational period of about six to seven weeks.
The newborns are blind and toothless and weigh no more than 100 g (3.5 oz).
The fur is thin and has been described as gray-brown or nearly white.
After about two weeks the cubs' eyes open, they become more active, and their fur darkens to a pearl gray.
The cubs do not take solid food until three months old, and do not leave the den until they are 4.5 months old; they are weaned shortly after that.
After the first year, the juveniles are independent of their mother.
Permanent teeth appear at 18 to 20 months.
Physical maturity is reached by about two years of age, but sexual maturity is not attained for another year or two, and the young may stay with their mother until they are fully mature.
Lifespan in captivity is up to or past 20 years of age, possibly due to the slow juvenile development.
Fossas have a polyandrous mating system. Most of the details of reproduction in wild populations are from the western dry deciduous forests; determining whether certain of these details are applicable to eastern populations will require further field research. Mating typically occurs during September and October, although there are reports of its occurring as late as December, and can be highly conspicuous. In captivity in the Northern Hemisphere, fossas instead mate in the northern spring, from March to July. Intromission usually occurs in trees on horizontal limbs about 20 m (66 ft) off the ground. Frequently the same tree is used year after year, with remarkable precision as to the date the season commences. Trees are often near a water source, and have limbs strong enough and wide enough to support the mating pair, about 20 cm (7.9 in) wide. Some mating has been reported on the ground as well.
As many as eight males will be at a mating site, staying in close vicinity to the receptive female. The female seems to choose the male she mates with, and the males compete for the attention of the female with a significant amount of vocalization and antagonistic interactions. The female may choose to mate with several of the males, and her choice of mate does not seem to have any correlation to the physical appearance of the males. To stimulate the male to mount her, she gives a series of mewling vocalizations. The male mounts from behind, resting his body on her slightly off-center, a position requiring delicate balance; if the female were to stand, the male would have significant difficulty continuing. He places his paws on her shoulders or grasps her around the waist and often licks her neck. Mating may last for nearly three hours. This unusually lengthy mating is due to the physical nature of the male's erect penis, which has backwards-pointing spines along most of its length. Fossa mating includes a copulatory tie, which may be enforced by the male's spiny penis. The tie is difficult to break if the mating session is interrupted. Copulation with a single male may be repeated several times, with a total mating time of up to fourteen hours, while the male may remain with the female for up to an hour after the mating. A single female may occupy the tree for up to a week, mating with multiple males over that time. Also, other females may take her place, mating with some of the same males as well as others. This mating strategy, whereby the females monopolize a site and maximize the available number of mates, seems to be unique among carnivores. Recent research suggests that this system helps the fossa overcome factors which would normally impede mate-finding, such as low population density and lack of den use.
The birthing of the litter of one to six (typically two to four) takes place in a concealed location, such as an underground den, a termite mound, a rock crevice, or in the hollow of a large tree (particularly those of the genus Commiphora). Contrary to older research, litters are of mixed sexes. Young are born in December or January, making the gestation period 90 days, with the late mating reports indicating a gestational period of about six to seven weeks. The newborns are blind and toothless and weigh no more than 100 g (3.5 oz). The fur is thin and has been described as gray-brown or nearly white. After about two weeks the cubs' eyes open, they become more active, and their fur darkens to a pearl gray. The cubs do not take solid food until three months old, and do not leave the den until they are 4.5 months old; they are weaned shortly after that. After the first year, the juveniles are independent of their mother. Permanent teeth appear at 18 to 20 months. Physical maturity is reached by about two years of age, but sexual maturity is not attained for another year or two, and the young may stay with their mother until they are fully mature. Lifespan in captivity is up to or past 20 years of age, possibly due to the slow juvenile development.
Fossa (animal)