The red-backed salamander (Plethodon cinereus) is a small, hardy woodland salamander species in the family Plethodontidae. It is also known as the redback salamander, eastern red-backed salamander,Integrated Taxonomic Information System [Internet] 2012. [updated 2012 Sept; cited 2012 Nov 26] Available from: www.itis.gov or the northern red-backed salamander to distinguish it from the southern red-backed salamander (Plethodon serratus). The species inhabits wooded slopes in eastern North America, west to Missouri, south to North Carolina, and north from southern Quebec and the Maritime provinces in Canada to Minnesota. It is one of 56 species in the genus Plethodon. Red-backed salamanders are notable for their color polymorphism and primarily display two color morph varieties ("red-backed" and "lead-backed"), which differ in physiology and anti-predator behavior.
Description and ecology
The red-backed salamander is a small terrestrial salamander, in total length (including tail), which usually lives in forested areas under rocks, logs, bark, and other debris.Conant R, Collins JT (1998). A Field Guide to Reptiles and Amphibians of Eastern and Central North America, Third Edition. Boston: Houghton Mifflin. 616 pp. . It is one of the most numerous salamanders throughout its range.
As with all amphibians, the red-backed salamander has permeable skin. They also lack lungs, a condition which is an ancestral trait of the Plethodontidae. Red-backed salamanders are thus entirely reliant on cutaneous respiration for gas exchange. Permeable skin is susceptible to desiccation and must be kept moist in order to facilitate cutaneous respiration; as a result, much of the ecology and behavior of the red-backed salamander is restricted by climatic and microclimatic variables, particularly dryness and temperature.
The skin of red-backed salamanders was found to contain Lysobacter gummosus, an epibiotic bacterium that produces the chemical 2,4-diacetylphloroglucinol and inhibits the growth of certain pathogenic fungi.
Polymorphism Plethodon cinereus exhibits color polymorphism, the common ones being the red-striped morph and the lead-phase. The "red-backed" or "red-stripe" variety has a red dorsal stripe that tapers towards the tail, and the darker variety, known as the "lead-backed" (or simply "lead") phase, lacks most or all of the red pigmentation. The red-backed phase is not always red, but may actually be various other colors (e.g., yellow-backed, orange-backed, white-backed, or a rare erythristic morph in which the body is completely red). Both morphs have speckled black and white bellies. Additional color anomalies of this species also exist, including iridistic, albino, leucistic, amelanistic, and melanistic anomalies. These color morphs are rarer than the red-backed, lead-backed, and erythristic morphs, but still have been reported with consistency among varying populations of this species.
Evolution of color polymorphism
Color polymorphism is thought to be an adaptive strategy in a heterogeneous environment, so the maintenance of polymorphism is derived from behavioral and physiological choices. The color polymorphism of the red-striped morph Plethodon cinereus and the lead-phase Plethodon cinereus show different anti-predator responses in behavior, and predators attack differently based on the color form. Compared to red-striped morph P. cinereus which prefers an "all trunk raised" posture and tends to stay still, the lead-phase P. cinereus is significantly more mobile. Moreover, lead-phase P. cinereus has the ability to automatically cut off the tail, indicating that the two forms also differ in the frequency of being attacked.
As evidence that polymorphism is an adaptation to the environment, P. cinereus color morph frequencies are correlated with climatic variables, suggesting habitat temperature and, more broadly, climate to be potential sources of selective pressure on P. cinereus polymorphism. The red-backed form is found with greater frequency in colder regions at more northerly latitudes and easterly longitudes throughout its range, whereas the opposite is true of the lead-backed form. Additionally, lead-backed morphs withdraw from surface activity earlier in the autumn than red-backed morphs, presumably to avoid cooling temperatures. Standard metabolic rate has also been found to differ between the morphs at certain temperatures, with significantly lower metabolic rates being displayed by the lead-backed form at 15 °C; in the same study, lead-backed individuals were also more active on the ground surface at this temperature. These findings suggest that the lead-backed color variant is less tolerant of cool temperatures than the red-backed color variant, and that the two color forms differ physiologically and behaviorally at certain temperatures.
An alternate explanation for the uneven geographic distribution of the red-backed and lead-backed P. cinereus color morphs involves phenotypic plasticity responding to developmental temperature. Although the genetic origins of the P. cinereus polymorphic condition are not fully understood, initial studies indicate that color morph dominance is likely subject to epistasis, and that multiple loci may interact to determine an individual's morph condition. However, more recent research indicates that a plastic response to thermal conditions during development also contributes to color morph determination; in one study, P. cinereus eggs incubated at a higher temperature hatched a greater proportion of lead-backed morphs than eggs incubated at a lower temperature. Temperature-dependent color morph determination may therefore also potentially influence the spatial distribution of P. cinereus color morphs.
Diet Red-backed salamanders are mostly insectivorous, but prey on a wide assortment of other small invertebrates including isopods, millipedes, centipedes, pseudoscorpions, harvestmen, spiders, and gastropods.
The two primary P. cinereus color morphs also differ in diet. The prevalence of certain prey taxa and the overall diversity and quality of prey items have been observed to differ seasonally between the two morphs in the spring and autumn when surface activity is greatest. The diets of striped and unstriped P. cinereus differ the most in the spring and fall seasons. The striped salamanders have a red-colored dorsal band that runs from the head/neck to their tail, and the unstriped ones lack this red stripe and are instead totally black. These salamanders are at the surface the most during these seasons. Contrasting diets during the fall and spring are due to differences in two types of prey consumed during this time. In the fall, the striped salamanders eat more entomobryomorph Collembola, the largest species of elongated springtails, as opposed to in the spring when they eat more oribatid mites.
Some studies have suggested that the unstriped morph has adapted to be better suited for drier and warmer conditions explaining the differences in diets. Unstriped morphs are less aggressive and less likely to hold territories because they are more well suited to find a territory that these striped salamanders are less adapted to withstand. The unstriped salamanders can forage in drier leaf litter, so they do not need to protect their territory to the extent that striped morphs do. The less pressure the unstriped salamander feels to hold territory does change what type of access of prey it has access to compared to the striped salamander during the drier months. Striped salamanders defend territories underneath objects such as rocks and logs when the conditions are dry. During these dry conditions, arthropods are forced to hide in these same moist areas that the striped salamanders claim as their territory since some arthropods will desiccate in dry periods. These arthropods then become the (red-backed) striped salamander's prey while the unstriped salamanders miss this opportunity. This allows striped salamanders to feed on springtails, mites, ants, and other small invertebrates.
There are some disputes on which morph has the more diverse diet. In one study, the autumn diet of red-backed morphs was more diverse and of higher quality, and found to be dominated by mites, springtails, and ants, whereas the most important prey for lead-backed morphs were ants, mites, and isopods. A later study notes that this was because the earlier study only compared diets during the fall season, while the later study compared their diets throughout all of the seasons. The later study concludes that the unstriped morph has a broader diet and encounters prey the striped morph does not. They link this back to unstriped salamanders being able to roam more freely between drier territories during this time.
Distribution Distribution of P. cinereus are in close contact with the soil on the forest floor. As deciduous forests mature, acid deposition can accelerate the acidification of soils. Acidic conditions can limit the distribution of amphibians and the numbers of sibling species, while the pH value of soil has a strong effect on the density and distribution of P. cinereus. When choosing between acidic and neutral soils, P. cinereus prefers to occupy more neutral soils. P. cinereus is rarely found in soils with a pH value of 3.7, and relatively more to be found in soils with a pH value of about 3.8 or higher. Juvenile P. cinereus have never been found in soils with a pH value lower than 3.7. Similar conclusions have also been supported in the laboratory. P. cinereus prefers to occupy substrates near neutral pH. A pH value between 2.5 and 3 results in acute mortality, while a pH value between 3 and 4 results in chronic mortality. Low pH will reduce their growth and respiration. Slowed growth and delayed metamorphosis make juvenile P. cinereus more vulnerable to predators and has serious consequences for population survival.
Several other factors, such as moisture and temperature, can affect the population density or dispersion of Botrytis as well. During prolonged dry periods, individuals move down into the soil, while during short dry periods they retreat under logs or rocks. They will avoid very warm areas, and when the temperature drops to 4–5 Celsius degrees, they will retreat to the ground as well. The optimum temperature is 10 to 15 Celsius degrees. Moreover, intraspecific and interspecific competition also affected the distribution of P. cinerea. Individuals confine themselves to moist microhabitats (beneath rocks, woody debris, etc. as well as beneath the soil) for long periods of time in order to maintain hydration when surface conditions are inhospitably dry or hot, and are only active on the surface to travel, forage, or reproduce for short periods. The duration of surface activity is directly limited by the rate of cutaneous water loss to the environment, which is influenced by environmental variables such as altitude, forest canopy cover, and the amount of recent precipitation.