Michigan. American marten are well adapted to snow. On the Kenai Peninsula, individuals navigated through deep snow regardless of depth, with tracks rarely sinking >2 inches (5 cm) into the snowpack. Snowfall patterns may affect distribution, with the presence of American marten linked to deep snow areas. Where deep snow accumulates, American marten prefer cover types that prevent snow from packing hard and have structures near the ground that provide access to subnivean sites. While American marten select habitats with deep snow, they may concentrate activity in patches with relatively shallow snow. Reproduction Breeding American marten reach sexual maturity by 1 year of age, but effective breeding may not occur before 2 years of age. In captivity, 15-year-old females bred successfully. In the wild, 12-year-old females were reproductive. Adult American marten are generally solitary except during the breeding season. They are polygamous, and females may have multiple periods of heat. Females enter estrus in July or August, with courtship lasting about 15 days. Embryonic implantation is delayed until late winter, with active gestation lasting approximately two months. Females give birth in late March or April to a litter ranging from 1 to 5 kits. Annual reproductive output is low according to predictions based on body size. Fecundity varies by age and year and may be related to food abundance. Denning behavior Females use dens to give birth and to shelter kits. Dens are classified as either natal dens, where parturition takes place, or maternal dens, where females move their kits after birth. American marten females use a variety of structures for natal and maternal denning, including the branches, cavities or broken tops of live trees, snags, stumps, logs, woody debris piles, rock piles, and red squirrel (Tamiasciurus hudsonicus) nests or middens. Females prepare a natal den by lining a cavity with grass, moss, and leaves. They frequently move kits to new maternal dens once kits are 7–13 weeks old. Most females spend more than 50% of their time attending dens in both pre-weaning and weaning periods, with less time spent at dens as kits aged. Paternal care has not been documented. Development of young Weaning occurs at 42 days. Young emerge from dens at about 50 days but may be moved by their mother before this. In northwestern Maine, kits were active but poorly coordinated at 7 to 8 weeks, gaining coordination by 12 to 15 weeks. Young reach adult