More info for the terms: cover, density, litter, natural, parturition, polygamous, torpor, tree
Description: The American marten is a long, slender-bodied weasel about the size of a mink with relatively large rounded ears, short limbs, and a bushy tail. American marten have a roughly triangular head and sharp nose. Their long, silky fur ranges in color from pale yellowish buff to tawny brown to almost black. Their head is usually lighter than the rest of their body, while the tail and legs are darker. American marten usually have a characteristic throat and chest bib ranging in color from pale straw to vivid orange (review by [38]). Sexual dimorphism is pronounced, with males averaging about 15% larger than females in length and as much as 65% larger in body weight (review by [38]). Body length ranges from 1.5 to 2.2 feet (0.5-0.7 m). Adult weight ranges from 1.1 to 3.1 pounds (0.5-1.4 kg) and varies by age and location. Other than size, sexes are similar in appearance (review by [28]).
American marten in southwest Montana.
American marten have limited body-fat reserves, experience high mass-specific heat loss, and have a limited fasting endurance. In winter, individuals may go into shallow torpor daily to reduce heat loss (review by [136]).
Breeding: American marten reach sexual maturity by 1 year of age, but effective breeding may not occur before 2 years of age (review by [136]). In captivity, 15-year-old females bred successfully (reviews by [38,166]). In the wild, 12-year-old females were reproductive [166].
Adult American marten are generally solitary except during the breeding season (review by [38]). They are polygamous, and females may have multiple periods of heat (review by [166]). Females enter estrus in July or August (review by [136]), with courtship lasting about 15 days (review by [38]). Embryonic implantation is delayed until late winter, with active gestation lasting approximately a month. Females give birth in late March or April to a litter ranging from 1 to 5 kits (review by [136]). 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 (review by [28]). In northeastern Oregon, low population reproductive rates were associated with high levels of predation on females prior to weaning kits [20].
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 (review by [28]). American marten females use a variety of structures for natal and maternal denning, including the branches, cavities or broken tops of live trees [18,89,106,147,154,187,188], snags [18,37,76,89,147,154,182], stumps [6], logs [18,58,76,147,187,188], woody debris piles [6], witch's brooms [106], rock piles [18,32,58,147], and red squirrel (Tamiasciurus hudsonicus) nests or middens [18,147]. See Denning for more information on denning structures and habitats associated with denning.
Females prepare a natal den by lining a cavity with grass, moss, and leaves (review by ([165]). In southern Wyoming, females moved kits frequently to new maternal dens once kits were >13 weeks old [82]. In another study in southern Wyoming, the average number of maternal dens per individual was 10.8, ranging from 5 to 24 [147]. In northwestern Maine, females moved kits from tree-cavity natal dens to groundlevel log maternal dens when kits were 7 to 8 weeks old, then moved kits back into large tree dens when they gained coordination at 12 to15 weeks old [187,188]. In southern Wyoming, females did not move kits from aboveground to ground structures between natal and maternal denning; many natal dens were in ground structures [147].
In southern Wyoming, most females spent a majority of their time (>50%) attending dens in both preweaning and weaning periods, with less time spent at dens as kits aged. Females were often away from dens from dusk to midnight [82]. Paternal care has not been documented (review by [28]).
Development and dispersal of young: Weaning occurs at 42 days. Young emerge from dens at about 50 days but may be moved by their mother before this (review by [28]). In northwestern Maine, kits were active but poorly coordinated at 7 to 8 weeks, gaining coordination by 12 to15 weeks [187,188]. Young reach adult body weight around 3 months (review by [136]).
Kits generally stay in the company of their mother through the end of their first summer, and most disperse in the fall (review by [28]). The timing of juvenile dispersal is not consistent throughout American marten's distribution, ranging from early August to October (review by [28]). In south-central Yukon, young-of-the-year dispersed from mid-July to mid-September, coinciding with the onset of female estrus [3]. Observations from Oregon [19] and Yukon [3] suggest that juveniles may disperse in early spring. Of 9 juvenile American marten that dispersed in spring in northeastern Oregon, 3 dispersed a mean of 20.7 miles (33.3 km) (range: 17.4-26.8 miles (28.0-43.2 km)) and established home ranges outside of the study area. Three were killed after dispersing distances ranging from 5.3 to 14.6 miles (8.6-23.6 km), and 3 dispersed a mean of 5.0 miles (8.1 km) (range: 3.7-6.0 miles (6.0-9.6 km)) but returned and established home ranges in the area of their original capture. Spring dispersal ended between June and early August, after which individuals remained in the same area and established a home range [19].
Daily activity patterns: American marten activity patterns vary by region (review by ([165]), though in general, activity is greater in summer than in winter ([18], reviews by [38,136]). American marten may be active as much as 60% of the day in summer but as little as 16% of the day in winter (review by [136]). In north-central Ontario individuals were active about 10 to 16 hours a day in all seasons except late winter, when activity was reduced to about 5 hours a day [169,173]. In south-central Alaska, American marten were more active in autumn (66% active) than in late winter and early spring (43% active) [29]. In northeastern California, more time spent was spent traveling and hunting in summer than in winter, suggesting that reduced winter activity may be related to thermal and food stress or may be the result of larger prey consumption and consequent decrease in time spent foraging [190].
American marten may be nocturnal or diurnal. Variability in daily activity patterns has been linked to activity of major prey species ([190], review by ([165]), foraging efficiency [29], gender [30], reducing exposure to extreme temperatures ([29,178], review by [165]), season ([76,190], review by [136]), and timber harvest [169]. In northeastern California, activity in the snow-free season (May-December) was diurnal, while winter activity was largely nocturnal [190]. In Grand Teton National Park, American marten activity peaked at midnight and late morning in spring. In summer, activity peaked at midnight, early morning, and mid-afternoon [76]. In south-central Alaska, American marten were nocturnal in autumn, with strong individual variability in diel activity in late winter. Activity occurred throughout the day in late winter and early spring [29]. In western Newfoundland, American marten were more active at night than during the day in winter; this result contrasts with other studies but may be explained by the generally warmer temperatures of the study region [51].
Daily and seasonal movement: Daily distance traveled may vary by age [150], gender, habitat quality [169], season [76], prey availability, traveling conditions, weather, and physiological condition of the individual [110]. Year-round daily movements in Grand Teton National Park ranged from 0 to 2.83 miles (0-4.57 km), averaging 0.6 mile (0.9 km) (n=88) [76]. In Glacier National Park, Montana, year-round daily movements averaged 0.4 mile (0.6 km), ranging from 0.2 to 1.7 miles (0.1-2.8 km) [23]. One American marten in south-central Alaska repeatedly traveled 7 to 9 miles (11-14 km) overnight to move between 2 areas of home range focal activity [29]. Two individuals in southwestern Montana routinely moved >4 miles (7 km) overnight [43]. One individual in central Idaho moved as much as 9 miles (14 km) a day in winter, but movements were largely confined to a 1,280-acre (518 ha) [110] area. Juvenile American marten in east-central Alaska traveled significantly farther each day than adults (x=1.4 miles (2.2 km) vs. 0.9 mile (1.4 km); P=0.001) [150]. In north-central Ontario, daily linear distance traveled was greater for males than females and for adults in logged than in unlogged forest (P<0.0001) [169].
Studies from Wyoming suggest that immigration and emigration are most likely to occur in the fall [39,40,76], with males more likely to move more than females [40]. American marten may also make smaller seasonal movements. Several studies have documented a seasonal shift in home range [6,29,129] (see Home range for more information). Two studies have documented seasonal migration in elevation. In south-central Alaska individuals moved to higher elevations in spring and to lower elevations in autumn, which the author attributed to food availability [29]. At the Kenai National Wildlife Refuge, south-central Alaska, individuals moved to higher elevations during the snow season, likely seeking the increased thermal protection offered by deep snow [148].
Population structure: American marten populations may contain many transient individuals. Of 85 American marten captured in northwestern Montana, 35% were residents (present in study area for >3 months), 55% were transients (present for <1 week), and 9% were temporary residents (present for >1 week but <3 months) [78]. In Wyoming, less than half of the American marten observed in Grand Teton National Park were considered residents and 33% were considered transients. On the Bridger-Teton National Forest, Wyoming, 67% of the population was considered residents, 7% were temporary residents, and 26% were transients [39].
Population age structure depends heavily on whether or not a population is trapped. Age structure of trapped populations responds mostly to the timing and intensity of harvest (review by [28]). Age structure of populations may also fluctuate in response to prey availability (review by [136]). Over a 3-year study in east-central Alaska, age structure of a trapped population was 49% juvenile (<1 year old), 26% yearling (1-2 years old), and 25% adult (≥ 2 years old) [150].
Population density: Compared to other carnivores, American marten population density is low for their body size. One review reports population densities ranging from 0.4 to 2.5 individuals/km² [28]. Population density may vary annually [60,64] or seasonally [3]. It may be influenced by several factors. Low population densities have been associated with low abundance of prey species ([60,150], reviews by [28,136,165]), environmental stress (e.g., weather conditions) [150], logging ([126,128,158], reviews by [28,165]), and trapping pressure (114, review by ([165]).One study from southern Ontario found no detectible relationship between trapping mortality and changes in American marten density, though it did find some evidence of density-dependent population growth [60].
Home range: Home range size of the American marten is extremely variable, with differences attributable to sex [6,19,26,29,129,132,156,187,188], year [66], geographic area (review by [28]), prey availability ([19,66,80,150], review by [28,165]), cover type, quality or availability ([19,80,126,156,178], review by [28,165]), habitat fragmentation [80], reproductive status [90], resident status [23], predation [19], and population density (18,116, review by ([165]). Home range size does not appear to be related to body size for either sex [156]. Home range size ranged from 0.04 mile² (0.1 km²) in Maine to 6.1 miles² (15.7 km²) in Minnesota for males, and 0.04 mile² (0.1 km²) in Maine to 3.0 miles² (7.7 km²) in Wisconsin for females (review by ([165]). For a review of American marten home range size and variability throughout its range as of 1989, see Buskirk and Lyman [26]. For more recent home range information, see the following sources: Alaska [150], Idaho [176], Maine [129], Michigan [168], Montana [43], Oregon [19], Wisconsin [186], Wyoming [120], British Columbia [6,106,132], Labrador [156], Newfoundland [66], Quebec [64]. Home range estimates are difficult to compare between studies because of different techniques used to obtain locations and/or to calculate areas (review by ([165]).
Males generally exhibit larger home ranges than females [6,19,26,29,129,132,156,187,188], which some authors suggest is due to more specific habitat requirements of females (e.g., denning or prey requirements) that limit their ability to shift home range [129]. However, studies in east-central Alaska [150] and southeastern Quebec [64] did not find male home range to be larger than female. In both studies, 2 females exhibited unusually large home ranges; in one study both individuals were juvenile [150], and in the other study, much of the home range consisted of logged forest [64]. Males and females in northeastern California appeared to have approximately equal home range size [152].
Home range is generally larger in logged than unlogged areas [61,80,126,135,158,172], though all studies supporting this assertion are from New England or eastern Canada. In northern Maine, regenerating clearcuts (3-18 years old) comprised 16% to 50% of the home range of the adults studied [162]. In north-central Ontario, distances between core areas of individual home ranges were greater in logged (<5 to >30 years) than unlogged forest (P<0.001) [173]. In northeastern British Columbia, removal of immature forest cover of 17% of the study area resulted in home range shifts at the individual level but no detectable impact at the population level, though 5 American marten dispersed out of the treated area and 1 died [132]. In southeastern Quebec, most predictive models included an element of human or natural disturbance to explain increases in home range size; home ranges tended to be larger as road density increased or the landscape contained a higher proportion of unlogged stand with a light outbreak of eastern spruce budworm (Choristoneura fumiferana) [64].
In Wyoming, home range size varied with no apparent pattern relative to age, season, or year, including years with timber harvesting [120]. Similarly, home range sizes did not differ when comparing undisturbed to clearcut (100% removal) and selectively cut (40% removal)) habitat in Wyoming, though individuals may have shifted their home range in response to these disturbances [40].
Home ranges are indicated by scent-marking. American marten male pelts often show signs of scarring on the head and shoulders, suggesting intrasexual aggression that may be related to home range maintenance (review by ([165]). Home range overlap is generally minimal or nonexistent between adult males [3,23,29,30,40,76,186] but may occur between males and females [3,23,29,30,40,111], adult males and juveniles [29,148], and between females [30,40,178]. In northeastern Wisconsin a few individual male home ranges overlapped extensively (88% overlap) in winter [186]. In Grand Teton National Park, male home range overlap was small or nonexistent except in the fall [76]. On Vancouver Island, British Columbia, overlap within and between the sexes generally occurred at the periphery of home ranges [6].
Individual American marten tend to exhibit high fidelity to an established home range [19,29,120,129], though observations in Grand Teton National Park suggested that home range boundaries frequently shift [76]. In north-central Maine males tended to show more seasonal and year-round fidelity to home range than females, with some females exhibiting high home range fidelity while others abandoned or shifted home ranges seasonally [129]. In north-central Maine adult males shifted or expanded their home range when bordering males died [129]. In south-central Alaska, one male shifted home range completely, but most others showed small seasonal shifts in concentration areas within an established home range [29]. Seasonal shifts in home range were observed in Alaska [29] and Vancouver Island, British Columbia [6], but not at a different site in Alaska [148].
Observations from Alaska [29], California [113,152], Idaho [110], and Vancouver Island, British Columbia [6], suggest that American marten may concentrate activity within small parts of their home range. In Alaska [148] and on Vancouver Island [6], core use areas shifted seasonally. In northern California, individuals would occupy small areas of their home range for a few weeks, then completely shift activity to a new area [113]. In central Idaho, daily winter movements generally do not extend beyond a 1 mile² (260 ha²) area, though throughout the winter an overall area 12 to 15 miles² (3,100-3,900 ha²) was used [110].
Several authors have reported that home range boundaries appear to coincide with topographical or geographical features. In northeastern California, movements and home range boundaries were influenced by cover, topography (forest-meadow edges, open ridgetop, lakeshores), and other American marten [152]. In south-central Alaska, home range boundaries included creeks and a major river [29]. In an area burned 8 years previously in interior Alaska, home range boundaries coincided with transition areas between riparian and nonriparian habitats [178]. In northwestern Montana, home range boundaries appeared to coincide with the edge of large open meadows and burned areas; the authors suggested that open areas represent a "psychological rather than physical barrier" [78].