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.

Mental glands and their associated delivery behaviors during courtship are unique to the plethodontid salamanders. Because previous interpretations of the evolution of these features were conducted using older phylogenetic hypotheses, we reanalyzed these traits with newer courtship descriptions and contemporary phylogenetic methods. Using Bayesian ancestral state reconstruction methods that have been developed since the first phylogenetic analyses were conducted in the mid-1990s, we reconstructed mental gland and courtship behavior evolution on a Bayesian phylogeny of the nuclear gene Rag1. The most probable ancestral condition for plethodontids was resolved as presence of a mental gland. Loss of a mental gland occurred in each subfamily and was recovered as the most probable ancestral condition for the Spelerpinae. In contrast, parsimony reconstruction recovered the presence of a mental gland in the ancestor to Spelerpinae with multiple secondary losses. We hypothesize that that absence of a mental gland is possibly ancestral in some clades (i.e., Spelerpinae) and secondary in others (e.g., paedomorphic Eurycea). The most probable ancestral form of the mental gland is likely to be the large pad-type distributed extensively in Plethodontinae and Bolitoglossinae. Desmognathans have the most unique mental glands, occurring in an anterior protrusion or bifurcated form (in Desmognathus wrighti). Fan-shaped mental glands evolved independently in Eurycea and Oedipina. Small pads arose independently in Bolitoglossinae, Plethodontinae, and Spelerpinae. Head-rubbing behavior for mental gland delivery mode was recovered as the most probable and parsimonious ancestral state for the Plethodontidae, with independent losses of this behavior in Plethodontinae and Spelerpinae. Because head-rubbing was observed in outgroups, we hypothesize that head-rubbing behavior predated mental gland evolution. Pulling, snapping, slapping, and biting behaviors evolved independently in the Plethodontinae and Spelerpinae and are not homologous with head-rubbing. All hypotheses of mental gland and courtship evolution invoke homoplasy.

The southeastern US has an incredibly diverse salamander fauna that contains ∼17% of the world's species. Furthermore, there is more variation in salamander life-cycle there than elsewhere—the diversity includes metamorphosing, paedomorphic, and directdeveloping species. The most diverse family in the world and in the southeastern US are the salamanders in the family Plethodontidae. I tested the hypothesis that variation in larvalgrowth history causes variation in metamorphic timing in Eurycea wilderae (Blue Ridge Two-lined Salamander), a member of 1 of the 3 lineages of plethodontid salamanders that metamorphose. Larval Blue Ridge Two-lined Salamanders were grown at 2 food levels and at 2 temperatures. Larvae that were fed more food grew faster, but effects on earlier metamorphic timing were minimal (3.84%, 13 d, P = 0.071). In contrast, larvae grown at a high temperature metamorphosed an average of 32.98% (55 d) earlier that those grown at a low temperature. In many amphibian species, the timin...

We examined how variation in food availability and temperature influences patterns of energy allocation to growth, storage, metamorphosis, and reproduction in larval Ambystoma maculatum and A. tigrinum. In each species, an allocation vector consisting of metamorphic size, duration of larval period, fat body mass, and gonad mass was influenced by both food and temperature. Larvae grew rapidly at high food levels and high temperature, and delayed metamorphosis when grown at low temperature. High food levels resulted in larger fat bodies and decreased time to metamorphosis in A. tigrinum, and larger gonads in both species. The allocation vector collectively representing growth rate, storage, reproduction and metamorphosis differed between species: in A. maculatum, only the correlation between metamorphic size and fat body mass was significant, whereas all four variables were influenced in A. tigrinum. An interaction of food and temperature on the allocation vector was only detected for A. tigrinum. Our results indicate that interaction among allocation variables is stronger in some species than others and that these associations might be important in understanding life history variation between these species.

We tested the hypothesis that salamanders growing at different rates would have allocation patterns that differ among male and female metamorphic and larval salamanders.We raised individual axolotls, Ambystoma mexicanum, on four food regimes: constant high growth (throughout the experiment), constant low growth (restricted throughout the experiment), high growth switched to low growth (ad libitum switched after 140 d to restricted), and low growth switched to high growth (restricted switched after 140 d to ad libitum). Because axolotls are obligate paedomorphs, we exposed half of the salamanders to thyroid hormone to induce metamorphosis. We assayed growth and dissected and weighed gonads and fat bodies. Salamanders that were switched from restricted to ad libitum food regime delayed metamorphosis. In all treatment groups, females had larger gonads than males and males had larger fat bodies than females. The association between storage and reproduction differed between larvae and metamorphs and depended on sex.

We collected larvae of the Patch-nosed Salamander, Urspelerpes brucei, and analyzed size-frequency distributions throughout a 12-mo cycle to determine the length of the larval period and size at metamorphosis. The hatchling-size class entered the population across a wide time window, from early winter to late summer, and at a minimal size of 6-mm snout–girdle length (SGL). Examination of both pooled total sample and the samples across six seasonal periods strongly show that the typical larval period is approximately 24 mo, with metamorphosis occurring during late summer to early fall at 18–19 mm SGL. Because oviposition and hatching are extended across several months, it is possible that some animals metamorphose up to 30 mo after hatching. Larval growth in Urspelerpes is 0.5 mm/mo, which is comparable—and even slower—to the slow larval growth seen in all other stream-dwelling plethodontid larvae. Our samples of recently metamorphosed animals and mature adults indicate that males are sexually mature at metamorphosis. This is more difficult to know for females, and we suggest that females may be a mixed group that contains individuals that mature at metamorphosis and individuals that delay maturation for an additional 12 mo. There is no difference in either sex between body size at metamorphosis and at sexual maturation. As in other species in the Spelerpini, the timing of metamorphosis and the timing of sexual maturation appear to be able to evolve independently, with the remarkable result that the early sexual maturation in Urspelerpes results in a tiny adult size despite having a multiyear larval period.