Plethodon cinereus (Green, 1818)
Eastern Red-backed Salamander, Red-backed Salamander, (Northern Redback Salamander)
© 2004 Henk Wallays (1 of 75)
Adult males are slightly smaller than the females, ranging from 58-91 mm in total length and averaging 73 mm. Adult females range from 64-90 mm and average 78 mm. The largest individual on record is 122 mm (Bishop 1943).
The red-backed phase of this species is characterized by a broad, dorsal band running down the midline from the head onto the tail. The color of the stripe varies from light gray or dull yellow to pink, brick-red, and bright red. There are often small flecks of black within the band. The sides are dark gray or black, becoming lighter and mottled toward the belly, which is strongly mottled with white and gray. In contrast, the lead-backed phase lacks the dorsal band and is uniformly dark gray to almost black, with the head and legs usually lighter (Bishop 1943). There is also an erythristic color phase that is mostly red, apparently to mimic juvenile Notophthalmus viridescens (Tilley et al. 1982). Juveniles of the red-backed phase have a well developed dorsal band and the upper sides are strongly pigmented (Bishop 1943)
The body is long and fairly slender, is slightly flattened dorsally, and is well rounded on the sides. The cross section of the tail is nearly circular throughout its length. Regenerating tails are flattened laterally and are usually uniform dark gray. Number of costal grooves ranges from 17 to 20, but there are usually 18 or 19. The gular fold is prominent. The legs are small with short, thick toes. There are four fingers, which in order from longest to shortest are 3-2-4-1. The five toes are slightly webbed, and are 3-4-2-5-1 in order from longest to shortest. The vomerine teeth form two backward-curving lines of 5-7 teeth separated from each other and from the parasphenoid teeth, which are in two imperfectly separated patches. The mouth is fairly large, with the angle of the jaw behind the eye. The small tongue does not fill the floor of the mouth. Males can be identified when in breeding condition by swollen snout, enlarged premaxillary teeth, and proportionally longer legs (Bishop 1943). Black testes can also be seen through the abdominal wall when transiluminated by a strong light (Jaeger et al. 2002a).
As is the case for all members of the genus Plethodon, eggs are laid in terrestrial cavities attended by the female. The larval stage is passed within the egg capsule. The broad, flat, leaf-like gills rise from a common base, are often fully developed at hatching, and then persist for only a few days (Bishop 1943). Embryos average about 19 mm upon hatching and individuals less than 32 mm in snout-vent length are considered to be juveniles (Bishop 1943; Jaeger et al. 2002a). Juveniles have proportionately broad heads, which allows them to forage on a wide range of prey (Maglia 1996). The fingers and toes of the juveniles are well indicated, the inner and outer short (Bishop 1943).
Distribution and Habitat
Country distribution from AmphibiaWeb's database: Canada, United States
U.S. state distribution from AmphibiaWeb's database: Connecticut, Delaware, Illinois, Indiana, Kentucky, Massachusetts, Maryland, Maine, Michigan, Minnesota, North Carolina, New Hampshire, New Jersey, New York, Ohio, Pennsylvania, Rhode Island, Tennessee, Virginia, Vermont, Wisconsin, West Virginia
Canadian province distribution from AmphibiaWeb's database: New Brunswick, Nova Scotia, Ontario, Prince Edward Island, Quebec
Individuals of P. cinereus can be found beneath old logs, bark, moss, leaf mold, and stones in evergreen, mixed, and deciduous forests (Bishop 1943). P. cinereus prefers a moist environment and becomes more abundant and more active upon introduction of seeps (Grover 1998; Grover and Wilbur 2002). It also prefers a higher cover object density, which increases abundance and average body mass by making foraging more effective (Grover 1998).
Life History, Abundance, Activity, and Special Behaviors
P. cinereus is terrestrial and can often be found under cover objects such as logs (Bishop 1943). It also used earthworm burrows as refuges in experimental enclosures, resulting in higher survival rates over winter as well as lower predation risk from the common garter snake (Thamnophis sirtalis) when compared to encosures (Ransom 2010).
During the summer noncourtship season, two-thirds of individuals are found alone, while the other third lives in male-female pairs (Gillette et al. 2000). Breeding takes place from October to December, during which time the pairs remain together (Bishop 1943). In the early spring, groups of 2-7 can be found together under rocks and logs (Jaeger 1979). Insemination takes place in the spring and eggs are laid in June and July (Bishop 1943; Lang and Jaeger 2000). Clutch size ranges from 3-14 eggs, usually from 8-10 (Bishop 1943; Ng and Wilbur 1995). The eggs are suspended by a common pedicel from the roof of the nest cavity, which is usually a well-rotted log (Bishop 1943). The females protect the eggs until they hatch 6-9 weeks later. Brooding females do not actively forage, but will eat opportunistically. This causes them to grow less than non-brooding females (Ng and Jaeger 1995). Females usually breed in alternate years because they normally require two years in order to store enough energy to yolk a clutch of ova and survive brooding. This is due to scarcity of prey (Jaeger et al. 2002a).
P. cinereus, in the red-backed form, may avoid predation by mimicking the red eft (terrestrial) stage of the red-spotted newt, Notophthalmus viridescens, which is toxic; if so, this would be an example of Batesian mimicry since it is assumed that P. cinereus is not toxic (Brodie and Brodie 1980; Cassell and Jones 2005; Robertson 2010). Although Highton (1959) suggested that the most logical explanation for the observed dimorphism of P. cinereus is that the gene for striping (which makes the red-backed phase) is dominant to the gene for the unicolored, nonstriped condition (which makes the lead-backed phase), Highton (1975) observed that the striped morph was dominant in one Virginia locality but recessive in another (in a different county), and suggested that epistatic interaction of two or more loci was responsible for the dimorphism. Fitzpatrick et al. (2009), using model salamanders with and without dorsal stripes, found that the striping polymorphism was maintained due to frequency-dependent selection by ground-foraging wild birds. The two forms appear to differ in various ways: red-striped morphs were found to obtain prey with higher nutritional value than lead-backed morphs (Anthony et al. 2008); one study suggested that red-striped morphs have a different temperature threshold for above-ground activity, as they have higher metabolic rates (Moreno 1989), but another study did not find a consistent difference in metabolic rates (Petruzzi et al. 2006); red-striped forms were found to be less likely to flee from predators and less mobile than the lead-backed forms (Venesky and Anthony 2007); and red-striped morphs had lower stress hormone levels than the lead-phase form, possibly due to differential predation pressure (Davis and Milaonvich 2010).
Amphibians harbor microsymbionts on their skin surfaces, which aid in defense against pathogens. Plethodon cinereus has been shown to harbor different species of bacteria on its skin that serve as part of the innate immune system and protect the salamander against fungal infections. These cutaneous bacteria include Janthinobacterium lividum, which secretes the antifungal metabolite violacein (Brucker et al. 2008a, 2008b), as well as the beneficial bacterium Lysobacter gummosus, which also has antifungal activity (Lauer et al. 2007). The bacterial species J. lividum has been shown to protect the salamander P. cinereus against infection by the chytrid fungal pathogen Batrachochytrium dendrobatidis, when it is present in sufficient numbers on the salamander's skin, and reduces clinical symptoms of the fungal disease chytridiomycosis in the salamanders (Brucker et al. 2008b; Harris et al. 2009b; Becker and Harris 2010). Bioaugmentation with this beneficial skin bacterium (J. lividum) in the laboratory appears to protect not only salamanders (P. cinereus) against chytridiomycosis (Harris et al. 2009b) but also at least one species of frog (Rana muscosa) (Harris et al. 2009a). The strategy of increasing beneficial skin bacteria (bioaugmentation) is now being tried in wild Rana muscosa frogs (Vredenburg pers. comm.), which carry some J. lividum on their skin but have been almost completely extirpated due to chytridiomycosis (Woodhams et al. 2007).
P. cinereus has also been found to sometimes harbor an intracellular bacterium (order Rickettsiales, probably family Anaplasmatacea) within red blood cells. The bacteria live in a membrane-bound vacuole within the erythrocyte that appears as a cytoplasmic violet-colored inclusion following Giemsa staining. Inclusions were generally found in nucleated erythrocytes but occasionally also in enucleated erythrocytes. Davis et al. (2009) found that males were more likely to be infected than females and that infected salamanders were actually larger and had higher body condition scores than uninfected salamanders (even after accounting for gender). It is thought that the parasitic bacteria are likely to be transmitted by trombiculid mites. Trombiculid mites inhabit leaf litter and are the only ectoparasite known for salamanders in the genus Plethodon (Rankin 1937).
When there are low prey densities, individuals have an indiscriminate diet and normally pursue prey. When there are high prey densities, individuals have a discriminate diet and normally ambush prey (Jaeger and Barnard 1981). Each individual learns through foraging experience which prey types are the most profitable. Gross caloric intake, which depends on size of the prey, and rate at which prey can be digested, which depends on the amount of chitin in the exoskeleton, are both factors that need to be considered (Jaeger and Rubin 1982). Thus, P. cinereus prefers termites to ants, because they are larger and have a softer exoskeleton (Gabor and Jaeger 1995).
In individuals from higher-elevation habitat, stored tail fat relative to body size was found to be greater than in individuals from lower-elevation habitat (Takahashi and Pauley 2010).
These behaviors are often used to establish territoriality. Territories are used by both sexes to defend scarce prey and to avoid desiccation during rainless periods. In addition, they are used by males for courtship (Jaeger et al. 2002a; Lang and Jaeger 2000). Territories are established under cover objects, such as rocks and logs, and can be set within 5 days by placing pheromones on the substrate (Jaeger et al. 2002a). Home ranges for P. cinereus are about 1.15 m in diameter, and may be due to site tenacity, since the range of both adult (max 0.88 m) and juvenile (max 1.22 m) movement between years was roughly equal to the diameter of the home range (Ousterhout and Liebgold 2010). Scent markers are produced by the post-cloacal gland, so marking can be accomplished by touching the cloacal area to the substrate (Jaeger 1984; Simons et al. 1994). Fecal pellets are also used to mark territory (Jaeger et. al. 1986). An intruder can learn characteristics of the resident male, such as size, by sampling airborne odors through gular pumping, or by touching nasolabial cirri to the fecal pellets (Jaeger 1984; Simons et al. 1997).
Females are more attracted to large males, males that have a prey-rich territory, and males that do not have odors from other females (Gillette et al. 2000). Females can discover how prey-rich a male's territory is by squashing his fecal pellets and seeing if it has the residue of light-armored termites or heavy armored ants (Jaeger et al. 1995a). Since prey-rich territories are the more valuable ones, both resident and intruder males are more aggressive when the resident has eaten higher quality food (Gabor and Jaeger 1995). During an invasion of another male's territory, both the intruder and defender assume threat posture about half the time (Jaeger et al. 1982). Both combatants are usually in ATR prior to biting attack, but the defender exhibits the higher rate of biting and successfully defends his territory 74% of the time (Jaeger et al. 1982; Jaeger 1984). Larger individuals are in general better competitors, and are thus more likely to hold the prey-rich territories (Mathis 1990). Since competition is normally harmful, neighboring males exhibit dear enemy recognition, which consists of less aggression and more submissive behavior towards territorial neighbors than toward strangers (Jaeger 1981). Females that are familiar with each other also spend less time in threat displays toward each other (Jaeger and Peterson 2002).
Once a female has selected a male, the two of them form a pair and defend the territory together. In both the courtship and noncourtship seasons, males spend more time in aggression toward invading males than females do, and females spend more time in aggression toward invading females than males do. Thus, pairs can codefend a territory more successfully, but not in a cooperative manner. Their success can be seen in the fact that females spend less time intruding a territory defended by a pair than by a single individual, and that both female and male intruders spend less time on a pair's territory during courtship season than during noncourtship season. Still, the fact that the male and female of a pair cannot cooperate seems to indicate that males are not willing to pass up future polygynous relationships and females are not willing to pass up future polyandrous relationships (Lang and Jaeger 2000).
To some extent, however, the relationship between the members of a pair is monogamous. During the noncourtship season, partners show no preference to associate with each other over novel conspecifics of the opposite sex. Even during the courtship season, they show no preference toward each other over single conspecifics of the opposite sex. At this time, however, they do prefer each other over paired conspecifics of the opposite sex. During the courtship season, the male profits from the presence of the female because it increases his reproductive fitness. As a result he undertakes mate guarding. The female profits from a monogamous male because with no other female in the territory she can obtain more prey for yolking ova (Gillette et al. 2000).
The male takes this monogamous relationship so far as to punish a socially polyandrous female partner, meaning one who has foraged with another male. The male can sense if his partner has associated with another male by detecting the other male's pheromones on her skin. Punishment takes the form of increased used of threat postures and even nipping if it is during the courtship season. Males also stay farther away from female partners that are socially polyandrous during both the courtship and noncourtship seasons, while they spend more time touching socially monogamous female partners. Socially polyandrous females in response show an increase in escape behavior. This sort of sexual coercion on the part of the male is logical, because he should not allow polyandrous females to feed in his territory. This might mean investing his own resources on the offspring of another male (Jaeger et al. 2002a).
Another interesting behavior among P. cinereus is the association between juveniles and adults. Juveniles normally inhabit the leaf litter between cover objects. They are attracted by the pheromones of adults and when the leaf litter dries out and foraging becomes difficult, they enter the adults' territories. Males are less aggressive toward juveniles than toward adult males and both male and female adults are more tolerant of juveniles with which they have cohabited previously. This type of behavior seems to be some sort of kin-selection. When it rains, the juveniles return to the leaf litter (Jaeger et. al. 1995b).
A final fact about P. cinereus behavior is that they seem to exhibit a certain degree of homing ability. The average daily movement of individuals is only 0.43 m/day, yet when they are displaced 30 m, 90% of them return to their territories. This return is usually along a fairly straight path and is almost immediate. When displacement increases to 90 m, only 25% of individuals return to their territories (Kleeberger and Werner 1982).
Trends and Threats
The major threat is clearcutting, which has reduced salamander populations in the southern Appalachians by almost 9%, or more than one-quarter of a billion salamanders (Alford and Richards 1999). Logging exposes terrestrial salamanders to altered microclimates, increased soil compaction and desiccation, and reduced habitat complexity.
Another threat is presented by invasive species of earthworms, which decrease forest leaf litter and thus habitat for the small arthropods that serve as prey items for salamanders. Maerz et al. (2009) conducted a mark-recapture study of woodland salamander abundance at ten sites in central New York and northeastern Pennsylvania, examining whether earthworm or plant invasions were associated with decreased salamander abundance. At these sites, P. cinereus constituted 80-99% of the salamanders captured. Salamander abundance was found to decline exponentially with decreasing leaf litter volume and was significantly associated with non-native earthworm abundance but not invasive plants. Earthworm invasions can be major drivers of change in temperate forests.
It has also been suggested that salamanders in the vicinity of military installations might be at risk from high copper contamination (due to its use in bullet casings, shot, and explosives), based on toxicity studies (Bazar et al. 2008). Mercury accumulation might also pose a threat; salamanders from a contaminated site on the South River in Virginia had elevated mercury concentrations in their tissues, at much higher levels (14-fold higher) than those shown to negatively impact development and metamorphic success in the frog Rana sphenocephala. However, P. cinereus had much lower levels than the sympatric species Eurycea bislineata, probably due to life history (direct development in a terrestrial environment for P. cinereus vs. an aquatic larval stage and riverine association plus aquatic prey in the adult stage for E. bislineata) (Bergeron et al. 2010).
Although some previous studies have shown that P. cinereusis sensitive to increased habitat acidity, Moore and Wyman (2010) reported that 87% of juveniles and 83% of adults were found under coverboards on a highly acidic forest floor (pH less than or equal to 3.8) in a northern hardwood forest of Québec, Canada.
Possible reasons for amphibian decline
General habitat alteration and loss
This species was featured as News of the Week on 6 June 2016:
Eastern North American woodlands have been invaded by Asian earthworms, potent ecosystem engineers. They accelerate leaf litter decomposition and nutrient release, consume detritus, and alter edaphic properties, all potentially significant to co-occurring salamanders. An experimental lab and field study (Ziemba et al. 2016) examined the impact of invading earthworms on Eastern Red-backed Salamander, Plethodon cinereus, in Ohio. Salamanders use lower quality microhabitat and consume fewer prey in the presence of earthworms, and behave aggressively toward earthworms. Earthworms and salamanders share cover objects less often than random expectations. Earthworm abundance was negatively associated with abundance of some salamander classes. Loss of cover and physical exclusion of salamanders likely hinders salamander performance, Thus, a scenario following earthworm invasion is reduced recruitment and abundance. While P. cinereus is widespread and abundant and is not under immediate threat, related species with restricted geographic ranges might well face great threat from future earthworm invasion of their isolated ecosystems (Written by David B. Wake).
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Originally submitted by: Christopher Searcy, Kellie Whittaker (first posted 2003-01-09)
Trends and threats by: Michelle S. Koo (updated 2021-03-17)
Edited by: Kellie Whittaker, Ann T. Chang (2021-03-17)
Species Account Citation: AmphibiaWeb 2021 Plethodon cinereus: Eastern Red-backed Salamander <https://amphibiaweb.org/species/4126> University of California, Berkeley, CA, USA. Accessed Oct 5, 2022.
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Citation: AmphibiaWeb. 2022. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 5 Oct 2022.
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