Carl D. Anthony

Social Interactions in Two Sympatric Salamanders: Effectiveness of a Highly Aggressive Strategy

In terrestrial plethodontid salamanders, aggressive behaviour is thought to function in the spacing of territorial residents among contested cover objects on the forest floor. Such behaviour, when exhibited toward heterospecifics, plays an important role in the competitive interactions between species. We compared levels of aggressive behaviour in intra- and interspecific contexts in two species of sympatric salamanders (Plethodon ouachitae and P. albagula) that have similar ecological requirements but differ in adult size. We also tested the effectiveness of such behaviour in holding cover objects (territorial foci) in the laboratory and on the forest floor. We predicted that if one species were more aggressive than the other, then that species would have greater success in obtaining and holding cover objects. In laboratory trials, residents of P. ouachitae (the smaller species) were extremely aggressive in both intra- and interspecific contexts. Individuals of P ouachitae delivered bites at a rate 14 times that of previously studied species of Plethodon and were significantly more likely to escalate to biting when tested as territorial residents (in intra- and interspecific trials) and as intruders (in interspecific trials). Plethodon albagula exhibited a lower level of aggression, similar to other species of Plethodon. In laboratory trials, in which salamanders competed for cover objects of differing quality, residents of P. ouachitae were effective in expelling conspecific intruders, and they were marginally effective at expelling intruding P. albagula. Residents of P. albagula were less effective in expelling conspecific intruders and did not expel intruding P. ouachitae. We conclude that the extreme aggression exhibited by P. ouachitae enabled it to expel intruders from artificial cover objects and to invade cover objects held by larger heterospecific residents. Field data supported intraspecific defence of cover objects by P. ouachitae, but results for P. albagula were inconclusive. These results are consistent with the geographic distributions of these species (P. ouachitae typically outnumbers P. albagula in the Ouachita Mountains) and provide an example of a behavioural mechanism overcoming a size-related disadvantage.

ANTIPREDATOR ADAPTATIONS AND PREDATOR AVOIDANCE BY TWO COLOR MORPHS OF THE EASTERN RED-BACKED SALAMANDER, PLETHODON CINEREUS

Populations of eastern red-backed salamanders, Plethodon cinereus, exhibit color polymorphism, and two color morphs are common: the red-striped morph and the lead-phase morph. A recent hypothesis attributes the maintenance of the polymorphism to selection on behavior and physiology. We conducted a series of laboratory experiments and a field study to determine whether striped and lead-phase salamanders exhibit different antipredator responses and whether predators differentially attacked the color forms. In predation trials, red-striped salamanders spent significantly more time in an “all trunk raised” posture and tended to remain immobile, whereas lead-phase salamanders were significantly more mobile. In field collected museum specimens of individual P. cinereus, we found more lead-phase salamanders with autotomized tails compared to red-striped salamanders, possibly indicating different attack rates on the two forms. The results from this study demonstrate that striped and lead-phase salamanders of P. cinereus respond differently to snake predators and may be attacked differently in the field. These differences may contribute to the maintenance of color polymorphism in this species.

Kinship Influences Cannibalism in the Wolf Spider, Pardosa milvina

Recent studies have called into question the role of Wrights coefficient of relatedness (r) in the interactions among relatives. Kin selection theory predicts a positive relationship between relatedness and frequency of “altruistic” acts, but a number of researchers have reported the opposite relationship. I used a lycosid spider (Pardosa milvina) to test the hypothesis that genetic relatedness would affect the propensity of a cannibalistic species to prey on genetic relatives. I considered lack of predation to be a form of “altruism” where the predator incurs a cost (loss of a meal) that benefits potential prey. Specifically, I questioned whether direct genetic offspring would be avoided as prey items and whether the sex or reproductive condition of a cannibalistic predator would affect the likelihood of predation on conspecific juveniles. As predicted by kin selection theory, spiderling mothers ate significantly fewer of their own offspring than they did of nonkin spiderlings of the same age. Adult virgin female and adult male spiders ate significantly more spiders than females that had recently carried spiderlings. Females with egg sacs consumed significantly fewer spiderlings than did virgin female spiders. These findings support Hamiltons rule and suggest that, in some systems, genetic relatedness plays a strong role in governing altruistic behavior toward relatives.

Edge effects and intraguild predation in native and introduced centipedes: evidence from the field and from laboratory microcosms

Human alteration of habitat has increased the proportion of forest edge in areas of previously continuous forest. This edge habitat facilitates invasion of exotic species into remaining fragments. The ability of native species to resist invasion varies and may depend on intrinsic variables such as dispersal and reproductive rates as well as external factors such as rate of habitat change and the density of populations of introduced species in edge habitat. We examined the distributional and competitive relationships of two members of the class Chilopoda, Scolopocryptops sexspinosus, a centipede native to the eastern US, and Lithobius forficatus, an exotic centipede introduced from Europe. We found that L. forficatus was most abundant in edge habitat and S. sexspinosus was most abundant in the interior habitat at our field sites. Although L. forficatus was present in habitat interiors at 11 of 12 sites, there was no correlation between fragment size and numbers of L. forficatus in interior habitat. The native centipede was rarely found occupying fragment edges. We used laboratory microcosms to examine potential competitive interactions and to indirectly assess prey preferences of the two species. In microcosms both species consumed similar prey, but the native centipede, S. sexspinosus, acted as an intraguild predator on the introduced centipede. Native centipedes were competitively superior in both intraspecific and interspecific pairings. Our results suggest that intraguild predation may aid native centipedes in resisting invasion of introduced centipedes from edge habitat.

Differential Parasitism by Sex on Plethodontid Salamanders and Histological Evidence for Structural Damage to the Nasolabial Groove

-In the Ouachita Mountains of Oklahoma and Arkansas, the trombiculid mite, Hannemania dunni, infests five endemic salamander species (Plethodon ouachitae, P caddoensis, P fourchensis, P kiamichi and Desmognathus brimleyorum). Plethodon albagula, which is widely distributed throughout much of Arkansas and southern Missouri, is rarely infested. In a field survey of 322 salamanders, 80.2% of P ouachitae, 26.3% of P caddoensis and 0% of P albagula were infested with chiggers (larval mites). Males of P ouachitae and P caddoensis had significantly more chiggers than females, and site of infestation differed significantly between species. Histological examination of the nasolabial groove (a chemosensory structure) indicates that chigger attachment in the snout area can occlude the groove. Loss of chemosensory function in plethodontids has been shown to impair foraging ability and may adversely affect a salamanders ability to find mates and/or defend a territory. Host tissue response and positioning of the chigger in the dermis was similar to that reported in other amphibian hosts.

Do Behavioral Interactions between Salamanders and Beetles Indicate a Guild Relationship

Abstract Guilds consist of groups of species that use mutual resources or collections of overlapping resources. Guilds are often comprised of members of the same family or genus, but they can span larger taxonomic categories. Predacious arthropods, such as carabid beetles, centipedes and spiders, and small carnivorous vertebrates, such as woodland salamanders, overlap extensively in microhabitat and prey types. These taxa, though unrelated, may form important guilds of forest floor predators. We examined the behavioral interactions between a carabid ground beetle (Platynus tenuicollis) and a small woodland salamander (the red-backed salamander, Plethodon cinereus) in laboratory encounters. These species occur syntopically and feed on similar prey. Individuals of P. cinereus are territorial and defend cover objects against conspecific and congeneric intruders. Because both species require moist conditions, it seems probable that they would compete for resources, such as cover objects and prey, as the forest floor and leaf litter dries. We posed two general hypotheses: (1) salamanders will display territorial behavior toward intruding salamanders and beetles and (2) beetles will display territorial behavior toward intruding beetles and salamanders. Residents of each species were paired with control, intra- and interspecific intruders. In both species, residency status affected behavior—residents behaving more aggressively than intruders. Resident salamanders behaved similarly toward intruding salamanders and beetles, but no biting was observed. There were no significant differences in aggressive behavior of resident beetles among treatments. When physical attacks by beetles were observed, they were brief and often escalated to biting and chasing. Platynus tenuicollis bit intruders of both species in a significant number of trials; this often resulted in the production of adhesive secretions by P. cinereus that successfully immobilized P. tenuicollis. Antipredator behaviors observed in P. cinereus, and biting by P. tenuicollis of P. cinereus, suggest that intraguild predation may be as important as interspecific resource competition for these species of forest floor predators.

Territorial Behavior and Ecological Divergence in a Polymorphic Salamander

Polymorphic species provide an opportunity to examine the process of sympatric divergence as it occurs. The Eastern Red-backed Salamander, Plethodon cinereus, is a polymorphic species that has served as a model organism in behavioral and ecological studies. Recent work suggests that the two most common color phenotypes (striped and unstriped) exhibit weak assortative mating and are diverging along a number of niche dimensions including temperature optima, diet, and response to predators. Males and females of P. cinereus are territorial and this behavior is thought to function in the context of prey and mate acquisition. Striped males have been shown to gain access to larger, and presumably more fecund, females. We posited that this pattern emerges through differential territorial behavior between the two phenotypes. We predicted that striped and unstriped salamanders would differ in their use of cover objects in the field, and in their aggressive responses to intruders in the laboratory. We examined salamander cover use and movement by placing artificial cover objects (ACOs) on the forest floor and monitoring them for 3.5 years. We compared residency time, number of recaptures, and number of ACOs occupied between the two phenotypes. The proportion of striped salamanders that were territorial residents was significantly greater than the proportion of unstriped salamanders that were territorial residents. Striped salamanders also exhibited significantly longer territorial residency, were recaptured more often, and were more often found under multiple, adjacent cover objects than unstriped salamanders. In the laboratory, we examined territorial behavior of the two morphs. As residents, striped salamanders were more aggressive and less submissive than were unstriped residents. When compared to intruders, resident salamanders of both morphs behaved more aggressively, but significant differences between resident and intruder behavior were only detected for the striped phenotype. These differences in aggression and cover object use may help to explain how striped males gain access to larger females and may be important in the interpretation of sympatric niche divergence and assortative mating by color in this species.

Interactions among Forest-floor Guild Members in Structurally Simple Microhabitats

Abstract Intraguild predation in structurally complex habitats is thought to weaken trophic cascades and increase food web stability. However, many predators commonly found in leaf litter become restricted to simple microhabitat beneath rocks and logs during periods between rains. It is within this structurally simple microhabitat that some predators defend rich prey resources and are likely to interact strongly as the surrounding forest becomes too dry to forage broadly in space. We conducted a 4-y press experiment where we removed focal predators from unfenced field plots. To evaluate the effects of predators on one another we removed either salamanders or centipedes from beneath artificially placed cover objects and compared abundances of these and other intraguild predators to those in non-removal controls. We predicted that salamanders and centipedes would have strong negative effects on each other and on carabid beetles and spiders. We removed a total of 1288 salamanders and 1056 centipedes over 98 sampling dates. In salamander removal plots spider abundance increased by 34%, and carabid beetles decreased by 15% relative to the control. In centipede removal plots salamanders increased by 18% and carabid beetles increased by 29%, but spider abundance decreased by 15%. Interaction strengths were strongest in the drier summer months when territorial predators were confined in spatially fixed microhabitats. It is during these periods that predators may strongly regulate the abundances of guild members. In territorial species that defend areas beneath natural cover, the effect of intraguild predators may be an important mechanism that regulates distribution and abundance of forest floor predators.

Assortative Mating in a Polymorphic Salamander

Reproductive isolation due to divergent selection is thought to be one of the mechanisms that promote speciation in sympatry. A key element of reproductive isolation is assortative mating. We examined a polymorphic population of Eastern Red-backed Salamanders (Plethodon cinereus) for evidence of reproductive isolation through assortative mating. Our study population was made up of two common color morphs, striped and unstriped. In the field, we turned over natural cover objects to find male–female pairs of P. cinereus during peak mating season. We recorded sex, color morphology, and snout–vent length of 112 pairs of salamanders. Estimates of sexual isolation indicated weak assortative pairing in the field with more same-color pairs than expected by chance. Striped females paired with striped males were significantly larger in size than those paired with unstriped males. Intermorph pairs were observed and such pairings, if successful, would interfere with the potential for divergence. Laboratory experiments were conducted to assess the ability of females of each color morph to distinguish between the two phenotypes through fecal odors and male scent. Additionally, mating trials were conducted to examine assortative mating in the laboratory. We found no evidence that scent or natural diet cues (fecal odors) of males contributed to assortative pairing, but females of both phenotypes were more likely to be associated with striped males during mating trials. Our study provides additional evidence that striped males of P. cinereus may be more attractive to females and this may contribute to positive assortative mating in the field. Territoriality and diet may be important factors that influence this pattern.

Recognition of Conspecific Odors by Plethodon caddoensis and P. ouachitae

Recognition of conspecific odors plays a crucial role in intraspecific territoriality of plethodontid salamanders, but it is not known how widespread odor recognition is in the family. I tested the ability of Plethodon caddoensis and P. ouachitae to distinguish their own odors from those of conspecifics. I also examined the responses of these species to fecal pellets of conspecifics, a known source of territorial markers in other groups of plethodontid salamanders. In experiment I, salamanders were allowed to associate either with substrates that had been previously marked by conspecifics or with substrates that they themselves had marked previously. During the initial 60 min of testing, salamanders of each species spent significantly more time on substrates that had been marked by conspecifics. In experiment II, salamanders were presented with fecal pellets from conspecifics and with soil pseudopellets. Salamanders of each species showed increased rates of olfactory behaviors (nose tapping and time spent in attendance of pellets) toward fecal pellets of conspecifics. Results from these two experiments indicate that P. ouachitae and P. caddoensis do distinguish between their own odors and those of conspecifics and that fecal pellets are a likely source of intraspecific odors.