Plethodon cinereus (Green, 1818)

Common Name and Size

Plethodon cinereus, commonly called the red-backed salamander, is a small terrestrial salamander that reaches a total length of 5.7–10.0 cm (2.2–3.9 in) including the tail.

Habitat and Abundance

It typically inhabits forested areas, where it lives under rocks, logs, bark, and other debris, and is one of the most abundant salamander species across its entire range.

Skin Permeability

Like all amphibians, red-backed salamanders have permeable skin.

Respiratory Trait

As members of the family Plethodontidae, they lack lungs, an ancestral trait for this group, so they rely entirely on cutaneous respiration for gas exchange.

Climatic Constraints

Because their permeable skin is prone to desiccation and must stay moist to enable cutaneous respiration, most of the species’ ecology and behavior is constrained by climatic and microclimatic variables, especially dryness and temperature.

Epibiotic Skin Bacteria

The skin of red-backed salamanders hosts the epibiotic bacterium Lysobacter gummosus, which produces the chemical 2,4-diacetylphloroglucinol that inhibits the growth of certain pathogenic fungi.

Soil pH Influence on Distribution

Red-backed salamanders live in close contact with forest floor soil, and soil pH strongly influences their population density and distribution.

Soil pH Preference

While acid deposition accelerates soil acidification in maturing deciduous forests, which can limit the distribution of amphibians and populations of related species, P. cinereus prefers to occupy more neutral soils over acidic soils.

Lower pH Occurrence Threshold

The species is rarely found in soils with a pH of 3.7, and occurs more frequently in soils with a pH of approximately 3.8 or higher.

Juvenile pH Tolerance

Juvenile P. cinereus have never been recorded in soils with a pH lower than 3.7, and laboratory studies have supported these observations.

pH Related Mortality

A pH between 2.5 and 3 causes acute mortality in P. cinereus, while a pH between 3 and 4 causes chronic mortality.

Sublethal Low pH Effects

Low soil pH also reduces the species’ growth and respiration.

Low pH Predation Risk

Slower growth and delayed metamorphosis make juvenile P. cinereus more vulnerable to predators, which creates major risks for population survival.

Moisture and Temperature Effects

Moisture and temperature are other factors that affect the species’ population density and distribution.

Dry Period Behavior

During prolonged dry periods, individual salamanders move deeper into the soil; during short dry periods, they retreat under logs or rocks.

Temperature Avoidance

They avoid very warm areas, and retreat underground when temperatures drop to 4–5 degrees Celsius.

Preferred Temperature Range

Their preferred optimum temperature range is 10 to 15 degrees Celsius.

Competition Effects on Distribution

Both intraspecific and interspecific competition also shape the species’ distribution.

Microhabitat Use During Inclement Conditions

When surface conditions are too dry or hot to be hospitable, red-backed salamanders stay within moist microhabitats — under rocks, woody debris, or within the soil — for extended periods to maintain hydration.

Surface Activity Purpose

They only come to the surface for short periods to travel, forage, or reproduce.

Surface Activity Limitation

The length of time they can remain active on the surface is directly limited by their rate of cutaneous water loss to the environment, which is affected by environmental factors including altitude, forest canopy cover, and the amount of recent precipitation.

Seasonal Distribution Patterns

The spatial distribution of Plethodon cinereus follows seasonal patterns.

Spring Grouping Behavior

Compared to other seasons, in spring red-backed salamanders are more likely to gather in groups of 2 to 7 individuals under cover objects such as rocks and wood, while overall population density on the forest floor remains constant.

Spring Grouping Cause

This grouping pattern occurs because territorial spatial disputes begin in spring.

Seasonal Density Study Results

A study conducted on Blackrock Mountain, Virginia found that the mean number of salamanders per 100*100 m quadrat ranged from 1.6 to 3 in spring, compared to 0.8 to 1.8 in summer.

Summer Spread Cause

The increased spread of individual salamanders across the landscape from spring to summer is attributed to intraspecific interference competition.

Cover Object Use in Rainy Season

Cover objects on the ground provide good moist refuges for P. cinereus during the rainy season.

Foraging Range Constraint

Because red-backed salamanders cannot forage underground, they are restricted to areas under and around these cover objects.

Spacing Pattern Driver

Observed spacing patterns are driven by aggression and territoriality when resources such as food and shelter are limited.

Color Morph Physiological Differences

This species has two distinct color morphs with different standard metabolic rates; lead-phase P. cinereus prefer warmer habitats and microclimates.

Color Morph Selection Evidence

This provides strong evidence that the geographic distribution of color morphs in the species is shaped by selection on physiological traits.

Homing Behavior

Like many species in the genus Plethodon, Plethodon cinereus exhibits homing behavior.

Homing Targets

Females return to their nests, and both non-attending females and males return to their established home ranges.

Homing Benefits

Homing ensures females maintain necessary contact with their eggs to support egg survival, and allows non-attending females and males to travel outside their home range to escape predation pressure, search for food or cover, and return if they do not find a more favorable microhabitat.

Male Territorial Aggression

Male red-backed salamanders actively defend their territories from intrusion by other males, and are less aggressive towards invading females and juveniles.

Competitor Interaction Patterns

Red-backed salamanders typically show aggressive behavior towards both conspecifics and heterospecifics that they identify as potential competitors, and they interact positively with both conspecifics and other potential competitors such as centipedes.

Threat Posture Trigger

A visual stimulus is required to trigger the species’ threat posture; if only chemical cues are present, red-backed salamanders do not show obvious aggressive behavior towards conspecifics or heterospecifics.

Chemical Cue Response Study

In experimental tests where male P. cinereus were individually exposed to four substrate chemical cue treatments (control, self, conspecific, or centipede (Scolopocryptops sexspinosus)) to test their behavioral response to conspecific and heterospecific cues, males increased the time they spent in aggressive postures when paired with centipedes, but did not show increased aggression when paired with conspecifics.

Residency and Aggression

An individual’s residency status also affects its level of aggression, but regardless of residency status, red-backed salamanders will attack centipedes.

Reproductive Territory Structure

For reproduction, male and female P. cinereus usually establish separate feeding and/or mating territories under rocks and logs.

Social Monogamy

However, some red-backed salamanders are thought to practice social monogamy, and may maintain co-defended territories throughout their active periods.

Breeding Season

Breeding takes place in June and July.

Egg Production

Females produce 4 to 17 eggs per year.

Egg Development Time

Eggs hatch 6 to 8 weeks after being laid.

Neonate Dispersal

Little is known about neonate dispersal, but both neonates and juveniles are thought to be philopatric.

Sperm Storage

Like many species in the genus Plethodon, female red-backed salamanders can store sperm as spermatophores, and have been documented storing sperm for up to eight months before the June and July oviposition period.

Post Oviposition Sperm Retention

No sperm or spermatophores are retained after oviposition is complete.