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Life History


Figure 26. Chilhowee Lake at mouth of Abrams Creek. - click to enlarge
Figure 26. Chilhowee Lake at mouth of Abrams Creek.

       Terrestrial Salamanders (Plethodontidae). The life cycle of terrestrial plethodontids takes place in a multidimensional space. Naturalists tend to think of salamanders as surfacedwelling, but surface activity is only a small part of the life cycle of a terrestrial salamander. Most terrestrial species probably do not have a very large home range on the ground surface, including beneath debris and litter. They spend a considerable part of their lives underground, and biologists really know very little about their life history, especially their time spent underground and the depth and range of underground lateral movement. In addition, terrestrial species occasionally become arboreal during the night or under rainy conditions; salamanders often take refuge under loose bark. Salamanders at different life stages may remain nearly entirely underground (tiny juveniles perhaps; adults during egg deposition and mating) or on the surface (adult feeding and territoriality, environmental conditions permitting). It is by no means clear that space is used similarly by different life stages. Thus, detection probabilities may change with life stage within a habitat. The eggs of some terrestrial species have never been seen, and nests have been located only with extreme infrequency. Some plethodontids may be long-lived (5-10 years).

       Semi-Aquatic Salamanders (Ambystomatidae, Plethodontidae, Salamandridae). All attributes that apply to terrestrial salamanders apply to semi-aquatic salamanders in terms of surface and underground habitat use. Semi-aquatic salamanders, however, require water for reproduction. For mole salamanders (Ambystoma) and newts (Notophthalmus), breeding sites are usually standing water (ponds, ditches) free of fishes. For semi-aquatic plethodontids, breeding sites include seeps and streams from little trickle trails to sizeable streams or rivers. Adults (mole salamanders and newts) may migrate synchronously to breeding sites in a quite orderly fashion, although temporally constrained to one or a few nights during the breeding season. Breeding adults and egg masses can be censused, but herpetologists know little about what proportion of a population breeds annually, and from what area they are drawn. Males and females may not stay for equal amounts of time during the breeding season, even when the breeding season is extended.

       Stream-breeding species may live permanently in the streams (Desmognathus marmoratus), streamsides (many other Desmognathus), or at various distances from the stream (D. imitator, Gyrinophilus, Pseudotriton). Distances may range from a few meters to hundreds of meters away, and breeding migrations are not synchronized. Little is known about spatial distribution during terrestrial nonbreeding times. For some species (for example, Hemidactylium scutatum) virtually nothing is known about their lives away from woodland pools and streams/ditches outside of the breeding season. For certain species (D. quadramaculatus) adults can be censused streamside, whereas adults of other species (D. imitator) can be readily found in terrestrial habitats; some species (Pseudotriton) can be found terrestrially as adults usually only by luck, and the adults of a few species (egg-brooding adult female Hemidactylium) are observed only during the breeding season.

       All eggs of semi-aquatic salamanders are deposited in water, and the egg masses of some species (Ambystoma) can be censused easily. All semi-aquatic species have larvae which remain in a larval stage from a few months to as long as 2-3 years. Paedomorphosis (the ability to breed while maintaining a larval appearance) occurs in a few species (Ambystoma talpoideum) under favorable conditions, but no salamanders from the Park are known to be paedomorphic. Larvae metamorphose and presumably take up adult habits, but nothing is known concerning dispersion for most species. Maturation can range from one to many years, depending on species. Individuals of some species (Ambystoma, Notophthalmus, large Desmognathus) may live 10-15 or more years.

       Aquatic salamanders (Cryptobranchidae, Proteidae). Little is known about the life history of most of these species, except for Cryptobranchus. Species within these families are entirely aquatic. The spatial use of habitat is largely unstudied except for Hellbenders, which are known to have home ranges and to guard nesting sites. Fully aquatic species (Cryptobranchus, Necturus) inhabit medium to large streams and rivers in the southern Appalachians. Hellbenders may live 25 or more years. Nothing, however, is known about longevity of the Common Mudpuppy (Proteidae: Necturus), because the larvae are little known and, for the most part, rarely seen.

       Frogs. All of the frogs in the southern Appalachians have a “typical” amphibian life cycle. Adults move to a breeding site, deposit eggs that hatch into larvae (tadpoles), metamorphose to juveniles, disperse, and grow until they are ready to repeat the cycle. For most species, however, many questions about the life cycle remain unanswered (what percentage is breeding in any one year, where do juveniles go, how far do adults disperse). Larval periods may be extremely brief (days in Scaphiopus) to extremely long (years in some Rana). Breeding may be synchronous (spadefoots, many ranids) or extended (Rana catesbeiana). Even when synchronous and explosive (Rana sylvatica), the actual breeding date may extend over a period of months (December to March) as adults wait for the right combination of environmental conditions. Adults (and perhaps juveniles) of many frog species spend most of their lives away from the breeding sites. Individuals have been found hundreds (or even thousands) of meters from the nearest breeding sites. Frogs are often exceptionally hard to locate outside the breeding season, much less to sample them. However, the terrestrial sites are extremely important to survival since individuals spend most of their lives as terrestrial predators.

       Although most species of frogs call during the breeding season, some species do not or they have only weak voices that do not carry far. Calling times are variable among species; some call during the day, some call at dusk and during the early evening, and some call only between midnight and early dawn. Some species call only during rains, whereas others will call most evenings of the breeding season. Some frogs breed in winter (even in the mountains of the South), others breed in the spring or summer, whereas others call during an extended breeding season. Calling times and seasons also vary latitudinally and perhaps with elevation.

Areas of Particular Amphibian Species Richness

       Three areas within Great Smoky Mountains National Park are particularly rich in amphibians. Two (Cades Cove, Cane Creek drainage) are lowland sites, whereas the third is the high-elevation spruce-fir forest. The lowland sites are similar in amphibian species composition; they are rich in species because they are the only two sizeable lowland areas within the Park with a large variety of wetlands. As such, they contain most of the frogs and pondbreeding salamanders. Both areas share species affinities with the herpetofauna of the Tennessee Valley, from whence lowland amphibians colonized Cane Creek and Cades Cove (via Abrams Creek). On the other hand, the highelevation amphibians are composed entirely of salamanders, and two species (Plethodon jordani, Desmognathus imitator) are virtually endemic to the Park (D. imitator is found also in the Plott Balsams). Other high-elevation species in the spruce-fir forest (for example, D. ocoee, D. wrighti, P. metcalfi) are found in other restricted regions of the Southern Appalachians. These three areas should be the special focus of amphibian monitoring activities.

Identification

       Most biologists working at Great Smoky Mountains National Park should be able to identify the majority of the amphibians that they observe by using a combination of the color photographs, species descriptions, and identification/life history tables found in this manual and in Dodd (2004). Some individual animals may be impossible to identify with certainty. Larvae, especially small salamander larvae and tadpoles, often cannot be distinguished without microscopic examination. Adult salamanders, especially the duskies (Desmognathus), are notoriously variable with overlapping phenotypic and genotypic characters. Field biologists have found it increasingly difficult to place some individual animals into a species category because of the range of genetic and color variation observed in natural populations. As a result, sometimes an animal must be recorded to genus, species complex, or as “unknown” in field notes.

       One of the best ways to identify salamander and frog larvae, in addition to color and morphology, is to examine their habitats and the times of year they are found. This can most easily be done through a comparative table. Morphological and life history characteristics are listed in tables 1 and 2 to help field biologists identify the species that are being examined. These data can be used in conjunction with the information in Dodd (2004).

SALAMANDERS

       All salamanders in the Great Smoky Mountains have four limbs with four (Necturus, Hemidactylium) or five (all others) toes on each hind foot. They all have tails, lack dry scales covering the body (lizards have dry scales), and have skins that are moist or wet to the touch. The skins of a few species, such as Jordan’s Salamander (Plethodon jordani), are sticky because of glandular secretions, but only the Hellbender and Common Mudpuppy are truly slimy.

       Biologists take two standard measurements with regard to length. The total length (TL) is the length of an animal from the tip of the snout to the tip of the tail. Because some salamanders lose their tails (or parts thereof) to predation, another common measurement recorded is the snout-vent length (SVL). SVL is measured from the tip of the snout to the posterior portion of the vent (the opening of the cloaca, the common receptacle for the digestive, excretory, and reproductive tracts). All scientific measurements are recorded in metric units, usually millimeters.

Figure 27. Body morphology of a salamander larva: (A) pond form; (B) stream form. - click to enlarge Figure 27. Body morphology of a salamander larva: (A) pond form; (B) stream form.

       Salamander larvae sometimes are divided into two general groups, depending on morphology and the type of wetland in which they develop. The pond form (fig. 27A) is stout bodied, with long filamentous gills and a wide dorsal fin which extends well onto the body. Mole salamanders (Ambystoma), for example, have this type of larva. Pond larvae develop in still water, and use the extra surface area of the body and fin as aids in swimming. Stream larvae (Eurycea, Pseudotriton) are slimmer than pond larvae, with more streamlined bodies, shorter gills, and a narrower tail fin that does not extend onto the body (fig. 27B). These larvae usually live in swift flowing water, where extra surface area on the body would be a distinct disadvantage.

       A number of useful characters are available which can be used to identify salamanders to genus or family. A few illustrative examples are provided, but more detailed comparisons are found in Dodd (2004) under the heading “Similar Species”.

       Desmognathus: All dusky salamanders have a light line which extends from the back of the eye to the angle of the jaw. The duskies also have well-developed muscles on the sides of their heads. They need these muscles to raise the upper jaw in order to open their mouths, since the lower jaw is fused to the skull.

       Gyrinophilus versus Pseudotriton: Although these colorful salamanders are superficially similar in appearance, Spring Salamanders (Gyrinophilus) have a canthus rostralis, a large white line bordered by black lines, that runs from in front of each eye to the nostrils. Salamanders of the genus Pseudotriton do not have this line. Spring Salamanders use the canthus rostralis as a “gunsight” to zero in on prey.

       Plethodontidae versus all other salamander families: All lungless salamanders have a nasolabial groove that extends from each nostril to the upper jaw. The nasolabial groove transmits chemicals to the salamander from the substrate; no other salamander family has this groove.

FROGS

Figure 28. Body morphology of a tadpole. - click to enlarge Figure 28. Body morphology of a tadpole.

       Like most salamanders, frogs have four legs with four toes on the front limbs and five toes on the rear limbs. The hind limbs are much larger than the front limbs, and are used to propel the body when walking, hopping, or jumping. Frogs are measured in TL, that is, from the tip of the snout to the end of the body between the hind limbs (that is, at the end of the urostyle). Of course, there are many other measurements which could be made, such as the length of the various sections of the hind limb, but these data generally are not important in amphibian monitoring-programs except in studies of fluctuating asymmetry (Alford and others, 1999, but see McCoy and Harris, 2003).

       Tadpoles are morphologically complex. As with salamander larvae, there are two general tadpole body types, the pond type and the stream type. Pond-type tadpoles have deeper bodies and higher tail fins than do stream-type tadpoles. Structures important in the identification of tadpoles are labeled on figure 28. The oral disk consists of the mouth parts; the narial aperture is the opening to the nostrils; the spiracle is the opening from the gills (water is taken in through the mouth, passes over the gills, and is expelled via the spiracle); the anus is the opening from the digestive tract. The total length (TL) consists of the body length (BL) and tail length (TAL). Sometimes additional morphological measurements are taken, such as the maximum width of the tail musculature (TMH) or the maximum tail depth (MTH). The location and size of these characters, or their ratios in relation to one another, are useful in identifying what otherwise appears to be just another drab, olive-green, or black tadpole.

Figure 29. Oral disc (mouthparts) of a tadpole. - click to enlarge
Figure 29. Oral disc (mouthparts) of a tadpole.

       The tadpoles of different species of frogs often appear identical to one another, but the structure of their mouthparts readily separate them. Biologists may need to examine mouthparts to determine which species is in hand. For this reason, a diagram has been included of tadpole mouthparts is provided in figure 29. The nomenclature follows Altig and McDiarmid (1999). The location, number, and degree of separation among labial teeth and papillae are important characters for identifying tadpoles. Examining tadpole oral disks (sometimes incorrectly termed teeth) also gives researchers an opportunity to check the health of the tadpole. For example, the horny jaw sheaths drop out when the tadpole is exposed to certain toxic compounds and to the dangerous disease, chytridiomycosis. However, tadpoles should not be held too long before examination or preservation, since some tadpoles may shed denticles in the laboratory.

       As with salamanders, there are certain useful defining characteristics that help to identify certain superficially similar animals. Some of these are listed below (also see “Similar Species” in Dodd, 2004).

       Bufonidae versus Pelobatidae: Frogs in both of these families are terrestrial. However, the true toads (Bufo) are dry-skinned and “warty”, and have prominent cranial crests and parotoid glands. The spadefoot toads (Scaphiopus) are smooth-skinned, lack cranial crests and parotoids, and have a sharp digging spade on their hind feet.

       Hylidae versus other frog families: all hylid frogs (Acris, Pseudacris, Hyla) in the Great Smokies have slightly to completely expanded toepads, but only in the treefrogs (Hyla) are these greatly expanded for climbing; the other hylids are mostly ground-dwelling (however, note that Spring Peepers, Pseudacris crucifer, often call from the trees from late fall to early spring before descending to breeding ponds).

       Rana palustris versus R. pipiens: these very similar frogs are both green and spotted. In R. palustris, the spots are squarish, paired and of nearly equal size, whereas in R. pipiens they are smaller, rounded, and more randomly scattered on the frog’s back.

Additional Information

       Information on the etymology, identification of adults, larvae, and eggs, similar species and how to differentiate them, taxonomic problems, distribution both within the Park and elsewhere in North America, life history, abundance and status, and remarks on interesting aspects of the biology of the species are found in Dodd (2004). Data on 44 amphibians are presented, including information on species no longer thought present (for example, Aneides aeneus) or which were reported historically from the Park, but whose actual occurrence may be doubtful (Acris crepitans). Distribution maps, color photographs of amphibians from the Park, and original color illustrations accompany each account.

Monitoring programs

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