Red-backed salamander

The red-backed salamander ("Plethodon cinereus") is a small, hardy woodland salamander species in the family Plethodontidae. 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 also known as the redback salamander, eastern red-backed salamander, or the northern red-backed salamander to distinguish it from the southern red-backed salamander ("Plethodon serratus"). 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. It is one of the most numerous salamanders throughout its range. 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. On one occasion an individual was found to have cannibalized a "P. cinereus" embryo. Predators of the red-backed salamander are many and varied, and include small mammals, snakes, ground-foraging birds, and larger salamanders.
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. 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. Hydric limitations are thus a major constraint on red-backed salamander growth and reproductive success as well as the genetic interconnectedness of red-backed salamander populations.
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.
The red-backed salamander exhibits color polymorphism and two color variations are common: 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.
"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.
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. 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.
Behavior.
Antipredator behavior of "P. cinereus" was found to differ between the two color phases; the lead-backed phase has a tendency to run away from predators, whereas the red-backed phase often stays immobile and possibly exhibits aposematic coloration. Stress levels of each color phase were estimated by determining the ratio of neutrophil to lymphocyte cells in the blood, and the results suggest stress levels are higher in the lead-backed phase than in the red-backed phase. This may be a consequence of a higher predation risk experienced in the wild by the lead-backed phase, and may also mean lead-phase salamanders could be more vulnerable in captivity settings.
"Plethodon cinereus", like many plethodon species, exhibit homing behavior, with homing of females to their nests, as well as non-attending females and males to a home range. This allows for essential contact between a female and her eggs in order to ensure their survival, as well as for non-attending females and males to explore beyond the home range when under predation pressures, or searching for food or cover, and return to their home range if a more favorable microhabitat is not found.
Reproduction and biomass.
Males and females of "P. cinereus" typically establish separate feeding and/or mating territories underneath rocks and logs. However, some red-backed salamanders are thought to engage in social monogamy, and may maintain co-defended territories throughout their active periods. Breeding occurs in June and July. Females produce from four to 17 eggs in a year. The eggs hatch in 6 to 8 weeks. Not much is known about the dispersal of neonates, although neonates and juveniles are thought to be philopatric.The species largely consumes invertebrates and other detritus dwellers. In some areas with good habitat, these salamanders are so numerous, their population densities may surpass 1,000 individuals per acre (2,471 individuals per hectare). Pokagon State Park in Indiana is one such place.
As in many "Plethodon" species, female red-backed salamanders have the ability to store sperm as spermatophore, and have been evidenced in doing so up to eight months prior to the oviposition period in June and July. Sperm or spermatophores are not retained following the oviposition period.