Fossa pups develop slowly and don't leave the den for about four to five months.

Fossas use scent to communicate and keep track of each other, scent marking rocks, trees, or even just the ground with scent glands on the chest and under the base of the tail. As solitary animals, their home ranges rarely overlap.

The only time they seem to vocalize is during breeding: the female mews to attract males; males howl and yowl when competing for a female. A fossa may roar to intimidate a fellow fossa or in defense. Fossa pups make a purring sound when nursing or near their mother.

Fossas are ready to start their first family when they are about four years old, and breeding season is between September and December. When it is almost time to give birth, in December through March, the mother makes a den in a place like an old termite mound, underground den, a rock crevice, or the hollow of a tree. There are two to six white-haired pups in a litter, and they are born toothless and with their eyes closed. The mother raises them without help.

Fossa pups are very dependent on their mother for the first few weeks. Their eyes begin to open and their fur begins to darken when about two to three weeks old. The pups develop slowly and don't leave the den for about four to five months; they are dependent on their mother for another eight months. To get the mother's attention, the pups make a high-pitched noise called mewling. Fossa youngsters continue to grow until about two years of age. They then find a space of their own and usually only meet up with other fossas during the fall breeding season.