Robert G. Jaeger

Microhabitats of a Terrestrial Forest Salamander

The above-ground density of Plethodon cinereus did not vary over 22 sampling days during spring and summer, even during seven-day periods without rain. Surface density was not correlated with amount of rainfall, but microhabitat choice was so correlated: the percentage of salamanders under rocks and logs increased and the percentage in the leaf litter decreased with decreasing rainfall. These data suggest that the P. cinereus population does not move underground (where food is scarce) during short dry periods but instead that it maximizes the amount of time spent on the surface of the ground (where prey are abundant).

Competitive Exclusion as a Factor Influencing the Distributions of Two Species of Terrestrial Salamanders

The salamander Plethodon richmondi shenandoah is restricted to areas of talus on Hawksbill Mountain, Shenandoah National Park, Virginia, while P. cinereus inhabits the soil outside the talus. To test the hypothesis that the former species is excluded from the soil by the latter species, three enclosures were constructed in each of four habitats: (i) in talus consisting only of bare rocks, (ii) in talus where soil has infiltrated between but not under rocks, (iii) in isolated pockets of shallow soil within the rocky confines of the talus, and (iv) in deep soil outside the talus. In each habitat one enclosure contained an isolated sample of shenandoah, another an isolated sample of cinereus, and the third a mixture of the two species. The above testes indicated that: (i) neither species survived the extremely dry conditions of the bard rocks for more than a week, (ii) shenandoah expressed a higher survivorship than did cinereus in this relatively dry habitat, (iii) in the shallow soil shenandoah had lower survivorship in the presence of cinereus than it did in isolation, and (iv) in the deep soil shenandoah had poorer survivorship than cinereus in the mixed species enclosure and poorer survivorship than its isolated control. P. cinereus apparently inhibits the presence of shenandoah in areas of soil, while shenandoah survives better under rocky conditions. However, the talus is a suboptimal habitat for shenandoah, since its survivorship in isolated enclosures is significantly better in the presence of soil. It seems unlikely that salamanders other than cinereus could exclude shenandoah from the soil or that other animals exert a strong influence. Predation, diseases, and parasites also do not appear to exclude shenandoah from areas of deep soil. Competitive exclusion by cinereus has probably restricted the distribution of shenandoah to the suboptimal talus refugium.

Food as a Limited Resource in Competition between Two Species of Terrestrial Salamanders

The terrestrial salamander Plethodon richmondi shenandoah occurs primarily in areas of talus, apparently excluded from surrounding areas of deep soil by Plethodon cinereus. Although the two species are closely related P. r. shenandoah grows to a longer body length and has a broader head. All ages classes of P. r. shenandoah emigrate out of the talus and are found within 3 m of the talus edge, but only adults which are as large as or larger than the largest P. cinereus are able to survive farther than about 3 m from the talus. This distribution is not explained by a tendency for large adults to wander more than juveniles. The large P. r. shenandoah in the soil take prey that are significantly larger than prey taken by conhabiting P. cinerueus, indicating that food may be the limited resource for which competition occurs. Both species are euryphagic and prey upon the same taxa of small animals. Food is periodically limited in availability, but not in abundance. Following a rain, salamanders forage freely and consume a large quantity of prey, based on stomach contents, but prey become increasingly unavailable as the soil and leaf litter dry and the salamanders are restricted to isolated pockets of moisture. Stomachs remain virtually empty until the next rain, at which time they are full again indicating that the abundance of prey is not greatly affected by the intervening dryness. Food may periodically become a limited resource during long dry seasons and as such may affect the survival of the salamanders or their ability to produce eggs. It is not clear whether interspecific competition is through differential exploitation of the limited resource or through interference. Lack of evidence for interspecific aggression may indicate that the former occurs, although it could not be demonstrated experimentally. Contiguous allopatry between the two species seems to result from an interaction of competitive exclusion and differential tolerance to dryness; P. cinereus is presumably a superior competitor for food in the soil and excludes P. r. shenandoah to areas of talus which P. cinereus cannot enter due to its tolerance of drier conditions in the rocks.

Agonistic Behavior of the Red-backed Salamander

The red-backed salamander, Plethodon cinereus, has a small repertoire of visual displays used during territorial contests. All of trunk raised (ATR) off of the substrate and looking toward (LT) the opponent are threat displays while lying flattened (FLAT) on the substrate and looking away (LA) from the opponent serve as submissive displays. Both combatants are almost always in an ATR posture prior to a biting attack, but usually only the winner of the contest maintains ATR and LT postures after the attack; the salamander losing the contest usually remains in FLAT and LA postures. Males are more aggressive toward other males than toward females, but females make no such distinction. Tapping the nasolabial cirri to fecal pellets seems to be a mechanism for monitoring the identity of the opponent. Touching the chin and cloacal areas to the substrate appear to be methods of pheromonal marking.

Salamander territoriality: pheromonal markers as advertisement by males

A laboratory experiment was performed to test the hypothesis that male red-backed salamanders (Plethodon cinereus) use pheromones contained in faecal pellets to identify male-marked territories. Each of 25 males was tested randomly under four conditions: (1) a burrow marked with its own faecal pellet versus one marked with a conspecifics pellet; (2) own-marked versus surrogate-marked burrows (the surrogate being a pellet of wadded paper); (3) conspecific-marked versus surrogate-marked burrows; and (4) a control of two surrogate-marked burrows. Males spent significantly more time in own-marked than in conspecific-marked burrows and significantly more time in surrogate-marked than in conspecific-marked burrows. Males favoured own-marked over surrogate-marked burrows. No position bias was found in the control. Males spent significantly more time nose-tapping (olfactory sampling) to a conspecifics pellet when it was paired with a surrogate but showed no differences in the other three tests. Significantly more time was spent in a submissive posture in front of the conspecific-marked burrow than in front of either their own-marked or the surrogate-marked burrows; no difference was found between own-marked versus surrogate-marked burrows or in the control. Time spent in the threat posture did not differ significantly between burrows in any condition. These data permit the inferences that males of P. cinereus use faecal pellets to mark and identify territories, avoid or display submissively toward burrows marked by conspecific males, and prefer own-marked shelters.

Caveat emptor: rank transform methods and interaction

When distributional assumptions for analysis of variance are suspect, and nonparametric methods are unavailable, ecologists frequently employ rank transformation (RT) methods. The technique replaces observations by their ranks, which are then analysed using standard parametric tests. RT methods are widely recommended in statistics texts and in manuals for packages like SAS and IMSL. They are robust and powerful for the analysis of additive factorial designs. Recently, however, RT methods have been found to be grossly inappropriate for use with non-additive models. This severe limitation remains largely unreported outside of the theoretical statistics literature. Our goal is to explain this shortcoming of RT methods.

Plant Climbing by Salamanders: Periodic Availability of Plant-Dwelling Prey

Terrestrial salamanders (Plethodon cinereus) leave the forest floor during rainy and foggy nights and climb plants where they forage on plantdwelling homopterans and hemipterans. Salamanders that climb plants at night ingest a significantly larger volume of food than do conspecifics that forage on the forest floor at night. Plants are not climbed on dry nights when desiccation of the salamanders occurs rapidly. Plant-dwelling insects may provide an important but periodic food resource for the salamanders.

Resource quality affects the agonistic behaviour of territorial salamanders

Abstract Previous studies suggest that territories of red-backed salamanders, Plethodon cinereus, function as exclusive feeding areas, because prey are limited in availability during rainless periods. To test this idea, males were paired in laboratory chambers under three feeding conditions: (1) termites, a high-quality food resource, (2) ants, a low-quality resource and (3) ants plus termites. Termites are a superior food because they pass through the digestive tract significantly faster and yield significantly higher digestion efficiencies than do ants. Both territorial residents and intruders showed significantly more aggressive behaviour when higher-quality food had previously been fed to the resident than when lower-quality food had been fed to the resident. These data provide evidence that P. cinereus defends feeding territories and that the quality of a territory may be a function of the value of available food resources.

COMPETITION LEADS TO AN EXTINCTION-PRONE SPECIES OF SALAMANDER: INTERSPECIFIC TERRITORIALITY IN A METAPOPULATION

The Shenandoah salamander, Plethodon shenandoah, is a federally endangered species that is restricted to Pleistocene-age talus on three mountains in Shenandoah National Park, Virginia, USA. Each population is surrounded by the red-backed salamander, P. cinereus, which defends intraspecific territories on the forest floor. On Hawksbill Mountain, P. shenandoah consists of a metapopulation, with a large “source” subpopulation on the top of the mountain and at least five small “sink” subpopulations below. Here we report that P. shenandoah is constrained in escaping from the talus into deep soil, and thus in emigrating from source to sink subpopulations, by interspecific territoriality from P. cinereus. We performed competitive release experiments during two summers by removing P. cinereus from under experimental rocks while leaving territorial residents in place under control rocks. Plethodon shenandoah responded by invading experimental rocks significantly more frequently than control rocks in both years. Quadrat analyses indicated that only a small number of individuals escaped from the talus at any given time. Behavioral experiments revealed no significant differences in propensity to bite by the two species, indicating that more subtle tactics are employed by P. cinereus in excluding P. shenandoah from territorial sites. One sink subpopulation of P. shenandoah became extinct in 1970 as a consequence of a severe drought, which affected areas of talus more drastically than areas of deep soil containing P. cinereus. Our data now suggest that interspecific territoriality by P. cinereus can contribute to the extinction-prone status of P. shenandoah by inhibiting movements from source to more distant sink subpopulations.

The influence of tail autotomy on agonistic behaviour in a territorial salamander

Assessment of potentially asymmetrical characters (such as fighting ability and resident advantage) is often important in determining the outcome of agonistic interactions. Loss of body parts, a predator defence mechanism used by many animals, may lead to a reduction in fighting ability and may be easily assessed by competitors. We investigated the influence of tail loss on the expression of agonistic behaviour in the territorial red-backed salamander, Plethodon cinereus. Residents and intruders were matched for body size, and pairs were tested in all combinations of tailed or tailless residents with tailed or tailless intruders. Neither residents nor intruders altered their behaviour based on their own tail condition, but they did alter their behaviour based on the tail condition of their opponents. Intruders showed more aggression or less submission towards tailless residents than towards tailed residents. When contests were between residents and intruders of the same tail condition (both tailed or tailless), intruders were more aggressive towards residents when both were tailless than when both were tailed, indicating that tail loss does not directly hamper aggressive displays. In contests where the asymmetry between residents and intruders was small (based on tail condition and residency status), intruders showed more aggression and less submission than in contests where the asymmetry was large. Residents did not differ in their behaviour for most comparisons. Thus, for intruders, the tail condition of residents is an important determinant of agonistic behaviour displayed in territorial contests. For residents, factors other than tail condition (such as resource value) may be more likely to influence their behaviour. Copyright 1998 The Association for the Study of Animal Behaviour. Copyright 1998 The Association for the Study of Animal Behaviour.