In August 2016, scientists from the United States and Mexico assembled at Highlands, North Carolina, a temperate-zone hot spot of salamander biodiversity, to participate in the Special Highlands Conference on Plethodontid Salamander Biology. Hosted by the Highlands Biological Station (HBS), the conference celebrated the 90th year of the HBS, and especially the productive collaboration at Highlands of Drs. Lynne Houck, Steve Arnold, and Rick and Pam Feldhoff. The conference showcased the increasing value of plethodontids as research models, exemplified by eight papers in this special issue of Herpetologica stemming from invited minireviews presented at the 2016 Special Conference. The papers contribute to the dramatic rise in journal articles that use plethodontids to address diverse questions of broad relevance to modern biology. We hope that this special issue of Herpetologica sparks new research on these fascinating salamanders, and we look forward to future conferences on plethodontid salamander biology.

Understanding the role of species interactions as regulatory mechanisms for ecosystem processes presents a challenge to ecologists working in systems with high species diversity and habitat complexity. Recent studies suggest that interactions among intraguild predators, such as terrestrial salamanders and large arthropods, might be important for the regulation of detritivores, fungivores, and perhaps detritus within terrestrial webs. A key prediction is that interactions among predators weaken trophic cascades. Our research examined this prediction by removing predators for 4 yr from unfenced field plots to investigate the effects on litter arthropods, the microbial community, and rates of leaf litter decomposition. We manipulated predator abundance in three treatments (salamander removal, centipede removal, and multiple predator removal) compared to a control in which no predators were removed. Despite difficulties in suppressing centipede numbers, we observed increases in salamanders, millipedes, isopods, slugs, numbers of ant colonies, and gamasid mites in the centipede removal plots. Additionally, several phospholipid fatty acid markers for bacteria were suppressed in plots where salamanders were most abundant. Finally, we detected treatment effects on the rate of litter disappearance from leaf bags in our field plots: those with the most salamanders had the lowest levels of litter decomposition. Overall, we found some evidence for top-down effects of predators in a temperate forest-floor web. Our study is one of few that have employed an unfenced field design and the only study examining the effects of salamanders on forest soil microbes. The results contribute to a growing body of evidence indicating that territorial predators, such as terrestrial salamanders, can be strong regulators of species composition at lower trophic levels in a system that is commonly thought to be regulated primarily through bottom-up effects of organic matter supply.

An examination of courtship in salamanders helps resolve the puzzling problem of long-term evolutionary stasis in behavior. To address the companion issues of stasis and diversification, we summarize and synthesize courtship observations in Rhyacotriton and 13 genera of plethodontids. We use a modular analysis of courtship to identify conservative, as well as variable behavioral elements and sequences. We use time-calibrated, molecular phylogenies to reconstruct the evolutionary history of key elements in courtship at different time levels. In deep time (40–175 mya), we reconstruct the evolution of courtship among plethodontid genera, using Rhyacotriton and Ambystoma as outgroups. On a shorter time scale (10–40 mya), we reconstruct courtship history among species within three genera (Aneides, Plethodon, and Desmognathus). These reconstructions reveal extraordinary stasis for greater than 130 million years (Myr) in the courtship modules that align sexual partners and accomplish sperm transfer. Although some aspects of courtship pheromone delivery predate the origin of plethodontids in the early Cretaceous (66 mya), other aspects of delivery and preliminary courtship have diversified in the last 20–30 Myr. We argue that intricate aspects of sexual communication generate multivariate stabilizing selection that is responsible for evolutionary stasis lasting 100 Myr or longer.

Rapid evolution is a hallmark of proteins involved in reproduction. The protein courtship pheromones in plethodontid salamanders are classic examples of such rapidly evolving reproductive proteins, with male pheromones likely coevolving with female receptors to improve reproductive success. Over the past 66 million years of plethodontid evolution, the structure and composition of the male mental gland has evolved with changes in courtship timing and behavior. More than 20 yr of biochemical and molecular studies have provided insight into how multiple gene families have been duplicated, mutated, and co-opted for pheromone roles. Sequencing and mass spectral proteomic analyses have enabled identification and characterization of multiple pheromone families, some with lineage-specific expression. In this review, we provide a phenotypic tango model to better understand male pheromone and female receptor coevolution that has driven the rapid evolution of multiple diverse pheromone families. To offer support for this phenotypic tango model, we review a combination of behavioral, neurophysiological, and structural studies that inform our understanding of the underlying molecular mechanisms of pheromone signaling.

Plethodontid salamanders have become model organisms for studying many fundamental questions in evolutionary biology, ecology, and behavior. Dozens of studies focused on geographic patterns of phenotypic and genetic variation have been conducted on plethodontids, such that we now have a deep understanding of how many species are in the family and where they occur. This body of work on patterns of speciation has been foundational to exploring the drivers of the major patterns of species richness in the family. Approximately 70% of the world's 696 extant species of salamanders are plethodontids. The distribution of plethodontid species diversity exhibits striking variation globally. Most plethodontids are concentrated into two hotspots of diversity, the Appalachian Highlands and the Mesoamerican Highlands. Moreover, within these montane centers of diversity more species are found in midelevation habitats than at the highest or lowest elevations. The clade has been highly successful in the New World tropics, which harbors more plethodontid species than the temperate zone. Here, I review new insights on the evolutionary and ecological causes of variation in plethodontid species richness over geographic space and evolutionary time. New hypotheses on the phylogenetic relationships of plethodonids and tools from comparative phylogenetics and biodiversity informatics have been critical to this recent progress. Threats to plethodontid diversity arising from global climate change are examined, as is the need to further study and to cross-validate forecasts of species' range dynamics with different modeling approaches and independent data.

Geography plays a paramount role in many aspects of speciation, including the amount of morphological and niche variation expected between sister species. Current species distributions, when coupled with phylogenies, offer valuable information on likely modes of geographic speciation. I briefly review past studies of the geography of speciation in neotropical plethodontid salamanders and analyze the spatial distributions and climatic overlap between sister species of this group. Using 66 pairs of sister species distributed from Mexico to Ecuador, I find that vicariant allopatric speciation probably played a dominant role in divergence of the bolitoglossines, but that peripatric and parapatric speciation were likely also important in generating the high diversity of the group. I find no evidence for latitudinal variation in speciation mechanisms within the tropics. Future studies incorporating physiology, spatial modeling, and population genetic estimates of demographic parameters will be critical in determining the importance of these nonvicariant speciation modes in the bolitoglossines, with implications for tropical biodiversity in general.

From the time of Cope's first analysis of plethodontid systematics through Dunn's era of consolidation and integration to the present time, there have been persistent themes. Criteria for recognition of species continue to evolve as methods of data acquisition have progressed and new methods of analysis of data have been developed. Species descriptions continue, often based on new discoveries, even in well-studied areas, but taxon subdivision is more common. What do we, as a community, want our species to be? Understanding of phylogenetic relationships has advanced with voluminous new molecular data; for example, we know that desmognathines are not the sister-group of all other plethodontids and that the two great clades, plethodonines and hemidactyliines, are well supported. As we have added more data, however, resolution within Hemidactyliinae has dramatically improved while, ironically, resolution within Plethodontinae has decayed. Historical biogeographical thinking has changed dramatically; “Out of Appalachia!” has been replaced with “Into Appalachia?” Biogeography of tropical salamanders has advanced, and we recognize Mesoamerica as a major evolutionary arena. The dominant biogeographic mystery is, why are there so few Old World plethodontids, given that they have been there for so long and occur at opposite ends of Eurasia? Abundant challenges persist for students of plethodontids, offering opportunities for discovery and further research.

Plethodontid salamanders exhibit biphasic, larval form paedomorphic, and direct developing life cycles. This diversity of developmental strategies exceeds that of any other family of terrestrial vertebrate. Here we compare patterns of larval development among the three divergent lineages of biphasic plethodontids and other salamanders. We discuss how patterns of life-cycle evolution and larval ecology might have produced a wide array of larval life histories. Compared with many other salamanders, most larval plethodontids have relatively slow growth rates and sometimes exceptionally long larval periods (up to 60 mo). Recent phylogenetic analyses of life-cycle evolution indicate that ancestral plethodontids were likely direct developers. If true, then biphasic and paedomorphic lineages might have been independently derived through different developmental mechanisms. Furthermore, biphasic plethodontids largely colonized stream habitats, which tend to have lower productivity than seasonally ephemeral ponds. Consistent with this, plethodontid larvae grow very slowly, and metamorphic timing does not appear to be strongly affected by growth history. On the basis of this, we speculate that feeding schedules and stress hormones might play a comparatively reduced role in governing the timing of metamorphosis of stream-dwelling salamanders, particularly plethodontids.

Plethodontid salamanders are specialized for a low-energy lifestyle, offering an interesting model for studying vertebrate stress responses. Furthermore, in an increasingly changing environment, it is important to understand how plethodontid salamanders respond to a variety of different stressors. Here, I review findings from my research group on the impact of various stressors on behavior and physiology in three species of plethodontid salamanders. A robust response to putative stressors was a decrease in locomotory activity, which was found after exposure to handling, predator kairomones, and acidic environments. The change in locomotory activity was not related to changes in corticosterone (CORT), a glucocorticoid hormone (GC) considered to be a hallmark of the vertebrate stress response. The uncoupling of CORT from stress-induced behavior might relate to the fact that decreased locomotory activity is not energetically costly, while CORT is a metabolic hormone that functions to mobilize glucose. Although CORT was not related to changes in behavior, experimental elevation of plasma CORT decreased body mass, increased metabolic rate, and suppressed wound healing. In addition, experimental elevation of CORT increased infection abundance, but not disease, when exposed to an amphibian fungal pathogen. My research team has also tested whether individual variation in plasma CORT in free-living animals was a useful biomarker of health and population viability. One study found no relationship between plasma CORT, body condition, and environmental degradation. Another study found no relationship between plasma CORT, body condition, and measures of reproductive effort. Taken together, CORT might not be the primary mediator of responses to putative stressors in plethodontid salamanders, perhaps because of the low-energy lifestyle of plethodontid salamanders. I encourage more studies with a broader range of species and approaches to better understand the stress physiology of plethodontid salamanders.