Cœur d’Alene salamander COSEWIC assessment and status report: chapter 6

Biology

Members of the family Plethodontidae comprise 230 of the 380 known salamander species worldwide (Cohn 1994). One distinguishing feature of this family is the fact that all species entirely lack lungs at all developmental stages (Nussbaum et al. 1983), and they respire through their skins or the mucous membranes of their mouths. To facilitate oxygen transfer and to avoid dehydration from moisture loss through the skin, they are restricted to moist environments. Unlike other plethodontids, P. idahoensis lives in the relatively harsh climates of interior British Columbia, northern Idaho and northwestern Montana. These areas are typified by extremes of temperature and humidity. To survive, the species requires the thermal and hygric buffer provided by wet areas such as seeps, waterfalls and streams. It also requires specific over-wintering habitat in the form of moist, deep crevices in rocks into which salamanders can retreat to avoid freezing.

Life cycle and reproduction 

Plethodon idahoensis spends much of its life in subterranean retreats. It is most active on the surface during rainy nights when substrate temperatures are above 4° C (Wilson and Larsen 1988). Previously believed to be above ground only in spring and fall (Nussbaum et al. 1983), the species has been consistently observed on the surface in summer during rainy weather, and at waterfall sites that maintain a moist environment (Ohanjanian 2001b).   

Plethodon idahoensis are relatively long-lived; one individual in captivity reached an age of at least 12 years (A. Wilson, pers. comm.). Sexual maturity occurs in their fourth summer of life (Lynch 1984), and females probably breed in alternate years (Nussbaum et al. 1983). Clutch sizes are small; females in northern Idaho are reported to have, on average, 6.7 ovarian eggs (range 4 -12) (Nussbaum et al. 1983). These factors result in a low reproductive rate. Their elaborate courtship ritual, described by Lynch and Wallace (1987), may take place in spring or fall (Lynch 1984). Early phases of this ritual were observed in October 1995 in southeastern British Columbia (Ohanjanian and Teske 1996).

Females are highly secretive during egg laying and no P. idahoensis nests have been found. It is likely that the female attends the nest and broods the eggs as in other plethodontids (Nussbaum et al. 1983, Truath et al. 2006.). Two nests of the closely related P. vandykei were found under a rock (Noble 1925; cited in Nussbaum et al. 1983) and in a fallen log (Jones 1989).

There is no larval stage. Hatchlings average 18 mm SVL (range 16-21 mm) and emerge from their eggs in fall or spring (Lynch 1984). Several individuals of 19 and 20 mm SVL were observed in May and June 2000 in British Columbia (Ohanjanian 2001a), indicating that hatching occurred in the spring of that year. Preliminary data from recaptures of eleven individuals between 28 May and 16 October revealed a mean growth rate of 0.05 mm per day (SD = 0.01, range = 0.03 – 0.07 mm) (Ohanjanian 2001a). Among P. idahoensis captured between 2000 and 2003 at 13 sites in southeastern British Columbia, 143 out of 391 salamanders (i.e. 37%) were juveniles (Ohanjanian, unpubl. data).

Feeding/predation

No studies of food habits of P. idahoensis have been carried out in British Columbia. At a seepage habitat in Montana P. idahoensis fed primarily on insects, including aquatic larval forms of Hemiptera, Plecoptera, Ephemeroptera, as well as Diptera adults and larvae, Homoptera at various developmental stages, and adult Coleoptera (Wilson and Larsen 1998). Other non-insect invertebrates, including spiders, oligochaetes and gastropods were eaten as well. Similar results were obtained at a falls in Idaho, with aquatic and semi-aquatic insects (primarily springtails, Collembola) predominating (Lindeman 1993).

Predation by the American robin, Turdus migratorius, has been recorded by Wilson and Simon (1985). Other potential predators of adults, young and eggs may include small mammals, such as the pack rat, Neotoma cinerea, garter snakes and large invertebrates.

Physiology

Plethodon idahoensis requires moist micro-environments to facilitate oxygen transfer, and, like other plethodontids, is prone to water loss through evaporation (Spotila 1972, Grover 1998). The species’ rates of dehydration and rehydration, its critical thermal maximum, preferred humidity, and ability to acclimate have not been studied. There are inter-specific differences in these physiological constraints, even within the genus Plethodon, and these differences are reflected in niche partitioning along moisture gradients and in behaviour (Spotila 1972). 

Plethodon idahoensis is active on the surface at temperatures significantly lower than other species of plethodontids, many of which occupy much warmer climates in south-eastern United States or on the west coast (Spotila 1972, Grover 1998, Ovaska 1987). One individual was observed near Kimberley at 3°C (Ohanjanian 2002). Surface movements of P. idahoensis are also significantly reduced when the weather is dry (Wilson and Larsen 1988). This has been shown to be the case with other plethodontids; duration of activity is directly related to substrate moisture level (Keen 1984). In the mountainous regions of south-eastern British Columbia, northwestern Montana and northern Idaho, temperatures are generally low at those times of year when the rains provide adequate moisture for surface activities.

Dispersal/migration

Information on dispersal of P. idahoensis is limited. Plethodon idahoensis has been detected at different locations on the same watercourse (Ohanjanian 2002). This indicates that dispersal must occur along an altitudinal gradient as well as between watercourses. The mechanisms for dispersal may be passive, for example individuals being carried downstream by run-off or flood events, or active, with individuals walking upstream, downstream, or laterally away from one watercourse to another. The physiology of P. idahoensis constrains the timing and distances traveled laterally between watercourses. In the southern parts of its British Columbia range, the intervening forested habitats are arid, thus severely limiting opportunities for dispersal.

The size of home ranges is not known for P. idahoensis. The longest distance recorded in British Columbia was a 52.8 m movement by an individual from its prior location at a waterfall. The same salamander was found back at the waterfall two years later, indicating that it traveled a minimum distance of 103.6 m (Ohanjanian and Beaucher 2002). An annual fall migration of salamanders from the interior of a cave to the outside was observed for three years at a site near Kootenay Lake (Ohanjanian 2001); both adult and juvenile P. idahoensis occupied the cave from the late spring through the summer and fall each year, but moved towards the entrance and then left the cave in late October. The cave was almost 29 m deep, and several individuals moved at least 25 m to reach the entrance. One individual moved > 48 m between September 25 and November 11 1999 (Ohanjanian 2000).

Interspecific interactions

Interspecific displacement and competition between salamander species has been documented in the eastern United States (Grover and Wilbur 2002, Marshall et al. 2004). In British Columbia, the only other salamander species whose range overlaps that of P. idahoensis is the long-toed salamander, Ambystoma macrodactylum. As both are nocturnal and feed at night, there is the potential for competitive interaction between the two species.

Adaptability

Due to the physiological constraints described above, P. idahoensis is not adaptable to alterations in hydrology and the physical structure of its environment; underground refuges with adequate moisture must be available to avoid desiccation. That being said, its use of fractured bedrock retreats (as opposed to the forest floor) is an adaptation that presumably has allowed its persistence in a mountainous region that is not only climatically harsh, but has been subjected historically to recurrent wild fires.

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