© 2009 Mark V. Leppin (1 of 19)
Plethodon dunni Bishop, 1934
R. Bruce Bury1
1. Historical versus Current Distribution. Dunn's salamanders (Plethodon dunni) are distributed from the southwestern corner of Washington State (but not in the Washington Cascade Mountains) southward through western Oregon, including the Cascade Range (Storm and Brodie, 1970a; Leonard et al., 1993), and barely into Del Norte County, California (Bury et al., 1969). In western Oregon, where their range is most extensive, Dunn's salamanders occur from the high tide line of the Pacific Ocean (Ferguson, 1956; Storm and Brodie, 1970a) throughout the Coast Range, and up the west flank of the Cascades to about 1,000 m (Nussbaum et al., 1983; Petranka, 1998). Dunn's salamanders are absent from the low parts of the Willamette Valley (Storm and Brodie, 1970a).
2. Historical versus Current Abundance. Unknown. Unlike many Pacific northwestern species, Dunn's salamanders can be found in forest stands of all ages (Corn and Bury, 1991). However, they are absent or reduced from banks of streams that are in or downstream from clearcut forests in western Oregon (Corn and Bury, 1989a).
3. Life History Features.
A. Breeding. Reproduction is terrestrial.
i. Breeding migrations. Unknown for any species of Plethodon, although movements from forest floor habitats to underground brood sites occur. The breeding season for Dunn's salamanders is not well defined. Gravid females have been found throughout the year (Dumas, 1956). Spermatophores or remnants of spermatophores have been found in the cloacas of mature females in April and October, indicating that courtship may occur over a long time (Nussbaum et al., 1983).
ii. Breeding habitat. Unknown.
i. Egg deposition sites. See "Brood sites" below.
ii. Clutch size. Ranges from 4–15 (average = 9.4), and larger females contain more eggs than smaller females (Nussbaum et al., 1983). Mature ova are 4.5–5.5 mm in diameter (Nussbaum et al., 1983).
C. Direct Development.
i. Brood sites. On 6 July 1952, Dumas (1955) found a Dunn's salamander egg mass located about 2 m from Sugarbowl Creek, in Lincoln County, Oregon. The egg mass was attached by a 3.9 mm stalk to a slab, and located about 37 cm back in a crevice of a shale outcrop on a heavily shaded southwestern-facing slope. Nine eggs were arranged in a grape-like cluster. Eggs ranged from 4.8–5.3 mm in diameter. Dumas brought these into the lab and reared them until the last egg succumbed to a fungus. At this time (after an incubation period of about 70 d), the nearly full-term embryo was 16 mm SVL (in nature, hatchlings as small as 13 mm SVL have been found; Dumas, 1956).
ii. Parental care. As in other members of the Plethodon genus, the egg mass is probably protected by the female.
D. Juvenile Habitat. Unknown.
E. Adult Habitat. Dunn's salamanders are almost always associated with rocks. Along the Coast Range, they occur in sandstone or shale outcrops near seepages, springs, and streams (Dumas, 1956). In the Cascades, they occur in basaltic talus near seepages, springs, and streams. They often are found in wetted rocky areas along with torrent salamanders (Rhyacotriton spp.; Nussbaum et al., 1983; Corn and Bury, 1989a). However, Bury et al. (1991) found that Dunn’s salamanders were mostly along stream banks (about 90% of captures) with fewer (< 10%) in riffles compared to torrent salamanders, with some (about 25%) along stream banks but most (about 50%) in riffles and the rest in pools. In general, torrent salamanders tend to occur in riffles or rocky substrata with fast waters nearby (often with their feet or venters wet), whereas Dunn's salamanders live under rocks just a little less wet and upward on talus slopes (personal observations). In rainy weather, they may be found in or under logs near streams or under surface debris (Corkran and Thoms, 1996).
Dumas (1956) collected animals when ambient temperatures ranged from 4–17 ˚C (see also Storm and Brodie, 1970a). Storm (1955) describes locations where Dunn's salamanders were found as "…under well imbedded rocks, about 4 ft (1.3 m) from the water's edge, along the base of the steep bank of a small stream. The area was well shaded by alders and second-growth hemlock, and many of the rocks were overgrown with moss.” Further, at another site "Two of the salamanders were taken from beneath shaded moss-covered rocks at the edge of a tumultuous mountain brook and 4 were collected among small stones and gravel in a spring seepage of the well shaded east slope…." Dumas (1956, p. 485) provides additional details of this habitat and notes that under high humidity conditions Dunn's salamanders can be found on drier substrates.
While Dumas (1956) notes that Dunn's salamanders "are sluggish salamanders with rather low mobility," Petranka (1998) characterizes them as active and agile and rapidly fleeing when disturbed.
F. Home Range Size. Varies with the season (see "Seasonal Migrations" below).
G. Territories. Dumas (1956) found Dunn's salamanders under 113 cover objects. Of these, only four objects had two animals together, and one concealed three salamanders. This suggests that either individuals avoid each other or territories are established and defended. However, this needs further study.
H. Aestivation/Avoiding Dessication. Ray (1958) points out that Dunn's salamanders generally live in wetter situations than most Plethodon, and that the single individual he tested for desiccation tolerance showed a low resistance to drying and demonstrated no protective behaviors.
I. Seasonal Migrations. Coastal populations may be active year-round in wet weather, but in many areas surface activity is restricted by cold temperatures and dry conditions (Nussbaum et al., 1983; Petranka, 1998). During the winter, some populations remain associated with talus slopes but move to rocky streamsides with warmer spring temperatures. Under dry summer conditions, individuals move deep within talus, into deep cracks in rocks, or beneath stones and rocks in cool habitats along streams. With autumn rains, individuals return to the surface. Surface activity is greatest in the spring and autumn (Dumas, 1956; Petranka, 1998).
J. Torpor (Hibernation). Dunn's salamanders tolerate cold temperatures well and are found at slightly higher altitudes than congeneric western red-backed salamanders (P. vehiculum; Dumas, 1956).
K. Interspecific Associations/Exclusions. Co-occur with Del Norte salamanders (P. elongatus), Larch Mountain salamanders (P. larselli), Van Dyke's salamanders (P. vandykei), and western red-backed salamanders (P. vehiculum; Dumas, 1956; Storm and Brodie, 1970a). Dumas (1956) examined sympatric Dunn's salamanders and western red-backed salamanders and found that Dunn's salamanders prefer wetter sites and feed on a more diverse assemblage of prey items than western red-backed salamanders. Ovaska and Davis (1992) found that Dunn's salamanders and western red-backed salamanders can distinguish between each other's fecal pellets, but neither species avoids the other’s burrow. Dumas (1956) suggested that Dunn's salamanders exclude western red-backed salamanders from preferred cover objects.
Other amphibians associated with Dunn's salamanders include the more aquatic California giant salamanders (Dicamptodon ensatus), Olympic torrent salamanders (Rhyacotriton olympicus), tailed frogs (Ascaphus truei), and the more terrestrial clouded salamanders (Aneides ferreus), rough-skinned newts (Taricha granulosa), and ensatinas (Ensatina eschscholtzii).
L. Age/Size at Reproductive Maturity. Animals become sexually mature at about 50–55 mm SVL, at an age of between 2–4 yr (Brodie, 1970; Nussbaum et al., 1983). Dumas (1956) concluded that for the population he studied there were more males than females.
M. Longevity. Unknown.
N. Feeding Behavior. Individuals forage for invertebrates on the forest floor on wet nights (Dumas, 1956). Prey include annelids, snails, centipedes, millipedes, isopods, scorpions, pseudoscorpions, mites, phalangids, and insects, including collembolans, coleopterans, dipterans, and hymenopterans (Dumas, 1956; Altig and Brodie, 1971). In the laboratory, adults can survive for > 5 mo without food (Dumas, 1956).
O. Predators. Birds, such as dippers (Cinclus mexicanus) and Stellar's jays (Cyanocitta stelleri), and northwestern garter snakes (Thamnophis ordinoides) are reported to take Dunn's salamanders (Nussbaum et al., 1983). Potential predators include beetles (Scaphinotus sp.), shrewmoles (Neurotrichus gibbsi), and other shrews (Sorex trowbridgei and S. pacificus). While cannibalism is rare, Stebbins (1951) observed a female ingesting some newly laid eggs. Dissection of 21 adult Dunn's salamanders from the Oregon Coast Range revealed that two adults had cannibalized juveniles (Riesecrer et al., 1996).
P. Anti-Predator Mechanisms. Dunn’s salamanders may remain motionless or rapidly flee when disturbed (Brodie, 1977).
Q. Diseases. Dumas (1956) has described a fungus attacking the eggs, and sometimes adults, in captivity.
R. Parasites. Unknown.
4. Conservation. This is a fairly widespread species, but it appears to require wet rocky substrate and often is most abundant along streamside habitat. There is no clear evidence to indicate declines in populations, but they are absent or reduced in clearcut forests in western Oregon.
1R. Bruce Bury
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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