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Natural History of Eastern Forest Amphibians
The double life of amphibians (amphi-both; bios- life) reflects an evolutionary life history
that is constrained by a dependence on water for survival and reproduction. Most amphibians
return to ponds, puddles, marshes, and streams for courtship, mating, and egg-laying. The
gelatinous eggs give rise to larvae that are designed for locomotion, foraging, and gas exchange
in an aqueous environment. Amphibians have a tendency toward nocturnal activity when the air
is cool and humid while avoiding warmer drier air and sunlight during the daytime. Rather than
oral drinking, amphibians take up most of their water by skin absorption.
We undervalue amphibians in the ecosystem. This oversight likely stems from their
cryptic nature and nocturnal activities. Surprisingly, salamanders are the most numerous
vertebrate in the forests of eastern North America, hands down. Chipmunks, squirrels, birds, and
deer only seem more numerous because of their size, noisy movements, diurnal activity, and
frequent vocalizations. Breisch and Ducey (Gibbbs, et al. 2007) estimate that the average
hectare of woodland in New York State is home to 4,100 red-backed salamanders. If all of New
York State were uniformly wooded, then approximately 36 billion red-backed salamanders
would be Yankee fans.
The amphibian lifestyle is dependent on ambient environmental conditions. Their body
temperature warms and cools slowly in synchrony with ambient temperatures. Amphibians
display interesting adaptations to survive temperature extremes in their environment. Because
amphibians don’t generate metabolic heat, more of their caloric and organic molecule intake is
directly invested in amphibian biomass. Endothermic birds and mammals regulate their body
temperature and most of the caloric intake is used to generate heat. Thus, amphibians are more
efficient at energy conversion than warm-blooded mammals and birds.
In northern forest, winter presents long periods of sub-freezing temperatures. An
amphibian can’t simply adopt the frozen state of the environment. Water expands upon freezing
and ice crystals form jagged edges. An unprotected frozen cell would have membranes shredded
from the physical transformation from liquid to solid water. Many eastern amphibians strive to
escape freezing conditions. Salamanders burrow deeper into the soil where the ambient
temperature remains safely above freezing. Radio tracking of the green frog in winter, indicates
that they move away from breeding ponds to moist seeps and springs that remain oxygen rich
with flowing water.
Wood frogs and spring peepers are among the exceptional amphibians that freeze solid.
Freezing in these species is not a haphazard circumstance, but an evolutionary adaptation that
where freezing is internally controlled to preserve tissues and cells. Amphibian cryogenics
works quickly. Rapid carbohydrate conversion and flooding the interstitial fluid with glucose
draws waters out of the cells. A higher concentration of glucose in solution likely depresses the
freezing point, but it also forces ice to form smaller crystals in slightly dehydrated cells.
Freezing is tolerated because ice crystals are smaller and the cells have shrunk during the
dehydration. Freezing begins on the frog’s periphery and slowly progresses towards the internal
organs. As the frog thaws in the spring, the preserved cells and tissues slowly hydrate and life is
returned to the amphibian.
The freeze tolerant adaptation of wood frogs allows then to have an expanded range to
the Arctic Circle in northern Canada. In addition, wood frogs and spring peppers emerge early in
the spring and begin to breed before other freeze intolerant species emerge. By remaining close
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to the surface and thawing quickly when temperatures warm above freezing, these frogs can
exploit breeding habitat with reduce pressures from competition and predation.
Amphibians have multiple avenues of exchanging respiratory gases. Aquatic larvae
depend heavily on external gills. The heavy vascularized gills are effective gas exchangers as
the water supports the structure and the gills have a high surface area to volume ratio. Several
aquatic adult salamanders (e.g., mudpuppies) have retained larval features such as external gills.
Amphibians with lungs use positive pressure ventilation to force air into the lungs. Frogs and
lunged salamanders gulp air into the buccal cavity of the mouth and then muscular contractions
force the air into the lungs. In contrast, contraction of the diaphragm in humans increases the
volume of the chest cavity which draws air into our lungs as a form of negative pressure
ventilation.
Amphibian skins serves as an important gas exchange organ. The skin serves the purpose
well for several reasons. First, the moist skin is important for facilitating gas exchange from an
aqueous cellular environment to the drier atmosphere. A drier skin would block this exchange.
Second, the skin of amphibians has a greater surface area of veins and arteries than a comparable
mammal skin. The pulmocutaneous artery of the amphibian heart branches into an arch that
carries blood to the skin. Gas exchange through the skin of amphibians accounts for 20-90 % of
the total oxygen uptake and 30 to 100% of the carbon dioxide release. This form of gas
exchange is equally important for terrestrial amphibians and those in fast moving oxygenated
streams. A third feature that is often overlooked is the body size of amphibians. In general
amphibians are smaller than mammals. This feature makes more skin surface area available for
gas exchange relative to the volume of the animal.
The absorptive nature of the moist amphibian skin also makes them more susceptible to
environmental toxins. The green revolution of the 1950’s and 60’s introduced a diverse palate of
fertilizers and pesticides. Although the chemical revolution improved crop yields, they imposed
new pressures on natural ecosystems as through water run-off and persistence. Amphibians have
been very susceptible to these toxins because of their dependence on water for mating and
development as well as their absorptive skin. A common agricultural pesticide, atrazine, was
recently discovered to interrupt endocrine enzymes in the northern leopard frog. Frogs exposed
to low levels (0.1 ppb) have an increased risk of incomplete sexual development and
hermaphroditism.
Other human chemical contaminants such as heavy metal release from mines,
polychlorinated biphenyls (PCBs), nitrogen pollution, and acid rain are known to have
devastating effects on frog development and survival. PCBs from electrical industries are longlived heavy environmental contaminants. PCBs persist in stream, lake, and river sediments for
hundreds of years and are known to bioaccumulate because of their affinity for fats.
Environmental scientists have proposed using frogs as environmental sensors for PCBs because
these molecules are readily absorbed through cutaneous and digestive routes. Degrady and
Holbrook (2006) showed that frogs, toads, and tree frog tadpoles are more prone to PCB
accumulation than adults because their lifestyle and direct interaction with polluted sediments
increases their exposure. While PCBs don’t appear to have toxic effects on amphibians, it is
becoming clearer that they do have significant effects on behavior and endocrine systems in
mammals.
No one knows the effects soaps, cosmetics, lotions, oils, and insect repellents such as
DEET may have on handled amphibians. Naturalists who feel the unnecessary urge to directly
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handle amphibians in the field should take care not to use these products and thoroughly rinse
hands prior to handling the amphibians.
Salamanders have short, stout bodies, four similar sized legs and a long tail equal to or
longer than the body. Lateral surfaces of the thorax have ridged costal grooves of many
salamanders believed to wick water across the body surface. Salamanders are silent but possess
a simple cartilaginous opercular apparatus for detecting sound. Unlike frogs, salamander
communication is chemical, tactile, and visual. The noses of salamanders detect water soluble
chemicals. Plethodontid salamanders such as the red-backed salamander have special nasolabial
folds that are designed for wicking moisture and detecting water-soluble chemicals. Chemical
detection provides information for tracking prey, potential mates, and their home ranges. Special
mucous glands in the skin produce important pheromone secretions to stimulate and attract mates
and induce females to pick-up packages of sperm deposited by males on the forest floor.
Forest salamanders forage upon isopods, ground beetles, and numerous other
macroinvertebrates. These soil detritivores are important carbon and nutrient converters on the
forest floor by consuming dead organic matter. By managing the woodland detritivore
population, salamanders slow the release of carbon dioxide and important nutrients from the
forest. When nitrogen and phosphorus are cycled slowly, more of the nutrients can be
reabsorbed and incorporated into the biomass of the forest. In addition, because salamanders
feed on the forest floor and burrow deeply into the soil, they transport nutrients to plant roots and
fungi where they are readily absorbed. Recently, amphibian biologists have estimated that
salamanders reduce the amount of carbon reentering the atmosphere at 267-476 kg per hectare
per year. Thus, atmospheric carbon balance equations must take the salamander factor into
account.
The nocturnal behavior and subterranean habit of salamanders hide them from the casual
forest hiker. At one point in history, salamanders were thought to be pyrophiles. Legendary
stories suggest that salamanders arose from logs that were thrown into fires. Rather than logs
begetting salamanders, it is well-known today that
salamanders are more likely to escape the log habitat
than face a fiery death. Salamanders are fairly easy to
locate in woodlands simply by raising rocks and logs in
the forest. Naturalists should remember to minimize
disturbance in the woodland by carefully replacing
objects and leaving the salamanders in their home range.
Two important salamander families dominate the
eastern woodlands. The mole salamanders
(Amybstomidae) are large robust salamanders known
for the intense nocturnal, spring time migration to
woodland ponds for breeding (Figure Ambystomid
Salamanders). Males begin the migration several days
to weeks before females in early spring as temperatures
warm into the forties and snow begins to melt. It is
likely that mole salamanders utilize scent chemistry to
cue in on their breeding pond. Adults continue to
migrate to the same breeding pond each year throughout
their life.
The mole salamander adults burrow deep
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beneath the forest floor. They utilize both natural cavities and vertical tunnels made by small
mammals to pursue prey and refuge in the three-dimensional forest soil. Earthworms, isopods,
and insects contribute to their diet although other small species of salamander have been taken as
prey. Mole salamander larvae will eat tadpoles, mosquito larvae, and small insects. Larvae are
likely to be preyed upon by other salamanders, frogs, dragonfly larvae, and fish. Chemical
defense seems to work well for adults. Mucous glands along the back and tail release a noxious
milky substance during an attack. The mucous is irritating and distasteful which initiates a quick
release response by the predator. In my experience, mole salamanders are much less likely to
autotomize their tail than other salamanders for defense.
There are several mole salamanders in our woodland fauna. The most awe inspiring is
the spotted salamander (Ambystoma maculatum). This species is large (5-8 inches including
tail), dark-bodied, with large bold yellow spots along the back and tail. Spotted salamanders are
easily seen during the April migration to breeding ponds at night or by excavating woodland
rock piles during the summer and fall. Blue-spotted (Ambystoma laterale) and Jefferson’s
(Ambystoma jeffersonianum) salamanders are also common in spring breeding ponds. These
species are smaller (3-6 inches) and duller in color. The blue-spotted salamander spots are less
regular and more diffuse in shape than the spotted salamander. Jefferson’s salamander is
grayish-brown in color with at most light-flecking of
blue.
The Plethodontidae is the largest family of
salamander (Figure Plethodontid Salamanders). More
than half of the known salamanders are in this family.
The centers of diversity of this family are found in
eastern forests, Central/South America, and the forests
of west coast states. Plethodontids are diverse in
habitat preferences and several important natural
history traits. These salamanders are lungless and
accomplish nearly all gas exchange through the skin.
Plethodontids are well-known for their rapid and
lengthy tongue projection in the capture of insects. The
lack of lungs and breathing apparatus has allowed
natural selection to favor salamanders with a more
elaborate hyoid apparatus for tongue projection
feeding. In the northern hardwood forest, the common plethodontids include the red-backed
salamander (Plethodon cinereus), two-lined salamanders (Eurycea bislineata), dusky salamanders
(Desmognathus sp.), and the northern spring salamander (Gyrinophilus porphyriticus).
The red-backed salamander is the most common species in the northern forest. Redbacked salamanders are more common at drier upland sites in the forest than other species. The
red-backed salamander is entirely terrestrial. The salamanders, mate, lay eggs, and develop on
land. Females attend and aggressively protect the small, pearly eggs in the woodland forest.
Clutch protect is an important adaptation as female reproduction is energetically expensive.
Female red-backs mature and oviposit for the first time at 4-6 years of age. Red-back females
require another 2-3 years to produce a second clutch and they life expectancy is 8-9 years. There
is considerable sexual inequality in reproduction as males mature in 3-5 years and can reproduce
annually thereafter.
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There is considerable variation in the color of the dorsal stripe on red-backed
salamanders. Although many are indeed a brick red, one can find stripes that are grayish,
orange, or even yellowish.
Two-lined salamanders are fairly ubiquitous in streams of deciduous forests. They are
tolerant of the human environment and can be found in shaded urban streams where few other
vertebrates survive. Two-lined salamanders are thin, yellowish with two dorso-lateral brown
stripes. Two-lined salamander adults are adapted for fast moving water, the eggs and larval
development occurs in small stationary puddles alongside streams.
Northern spring salamander is an attractive salmon-brown species in clean, clear, fast
moving streams with rocky bottoms. This species is relatively large muscular plethodontid and
quick moving even in cold waters. Finding this salamander in a stream is a real delight and
brings jaw-dropping amazement to students startled by its size and beauty. Larval development
is slow in this species with at least three years dedicated to growth prior to metamorphosis to
adults.
The eastern or red-spotted newt is a member of the
Salamandridae (Figure Red-spotted Newt). The
Salamnandridae has a rough skin, internal fertilization, and
various degrees of toxicity. Aquatic larvae of newts
transform into a terrestrial juvenile. For the red-spotted
salamander, the red eft or terrestrial juvenile is bright orange
and may live among the forest floor for several years before
returning to the water as an adult. It is unknown how far
juveniles walk during their years on land, but I have found
several efts in forests near the tops of Adirondack mountains.
We don’t know if the newt use seasonal puddles on these
mountains to breed or if the efts have climbed the 1300+
vertical feet from ponds at the base elevation. The
transformation of an eft to an adult is accompanied with a
color change to olive and expansion of the tail and lateral flattening for aquatic mobility. In
addition, the skin is more prone to dessication and less toxic in adults as compared to the red eft.
Both adults and efts assume a defensive posture when provoked by potential predators. The
salamanders will roll onto their back with reverted legs to display the yellow warning coloration
on their belly.
The bright color of the eft and yellow belly of the adult advertise the toxicity of the
salamander. The toxin, tetrodotoxin, is similar in structure to the famous poison of pufferfish.
Several cases of newt poisoning have been reported in humans. The toxin interferes with sodium
channels of neurons and therefore blocks the transmission of nerve electrical impulses. There is
at least one report of a fatal human poisoning following ingestion of rough-skinned newt from
the western United States. Although toxic to humans, the evolution of toxicity in newts is more
likely an adaptation to predation by snakes, birds, and small mammals. The skin of newts is fowl
tasting and will cause vomiting in birds and mammals. Reptiles lack the reverse peristalsis and
suffer the consequences of newt poisoning. Some populations of garter snakes appear to have
evolved resistance to the toxin although with the cost of some impairment to mobility.
Frogs and toads differ from salamanders in several behavioral and anatomical traits.
Anurans have distinct aquatic larvae and terrestrial adults. Anuran larvae are fast moving, live in
ephemeral bodies of water and are voracious vegetarians. Vegetarianism, tail propelled mobility,
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and external gills are three important features of larval survival and development that disappear
during the metamorphoses to adults. Anuran larvae develop rapidly over a period of days to a
few weeks and undergo metamorphosis quickly into a tailless insectivorous adult.
In the spring, frogs and toads aggregate at vernal pools and ponds in the spring to mate.
Mating is synchronized with intense vocalization and territorial behavior. Spring peepers (Hyla
crucifer) and wood frogs (Rana sylvatica) are among the first to emerge and mate in the spring.
The reproductive season has a distinct sequence of mating species. American toads (Bufo
americanus), northern pickerel (Rana palustris) and leopard (Rana pipiens) frogs are next to
breed followed by green (Rana clamitans) and bullfrogs (Rana catesbeaina). Fertilization is
external in frogs. Males grasp females with the forelegs and simultaneously release sperm as
eggs are deposited. Sperm follow a chemical trail to the eggs rather than swimming aimlessly in
the water. Eggs are enclosed in a thick gelatinous coat that buffers the developing embryos
from the aquatic environment.
Communication in frogs and toads is primarily vocal in contrast to the chemical
communication of salamanders. Anuran vocalizations are loud and distinctive for each species.
Sound is produced as air is forced out of the lungs by and through vocal cords where hyoid
cartilage vibrates. The sound may be amplified and attenuated through vocal sacs in the throat
region of the frog. The size of the vocal sacs, vocal cords, and size of buccal cavity determine
the loudness and frequency of the vocalization. Large frogs with massive vocal cords and large
vocal sacs produce loud low frequency sounds. The bullfrog “rum” and green frog “gonk” are
examples of loud, low frequency calls, although the bullfrog produces these without vocal sacs.
On the other hand, small frogs such as the spring peeper produce high frequency sounds at a
lower volume. Certainly when the spring peepers calls become a chorus, the sound can be
deafening, but each peeper alone produces a low volume call.
Vocalization is one of the most energetically demanding aspects of being a frog. Male
pickerel frogs are known to lose significant body mass during the breeding and calling season.
As such, the muscles used to contract the lungs are well endowed with fat stores, mitochondria,
and capillaries. These features increase the oxygen delivery for aerobic respiration in the trunk
muscles.
The language of frog communication is complex. Males produce a number of
advertisement and aggressive while on breeding territory. Male bullfrogs are known to
vigorously defend their breeding territory and the frequency and rate of advertisement calls have
a role in this social activity. Large frogs are deemed to be better territory defenders and fighters.
Frogs can assess the size of calling frogs because call frequency is correlated with frog size. In
playback experiments, male bullfrogs alter their call frequency downward to approximate the
recording. Such observations have lead herpetologist to suggest that male frogs may alter call
responses to falsely advertise their size and fighting ability.
It is clear that frogs perceive and respond to a variety of calls in nature. The significance
of these interactions is not well-known and a subject of intense research. Pickerel frogs vocalize
both in air and underwater. The discovery of underwater vocalizations suggests that we have
only uncovered the tip of the iceberg in anuran language.
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In eastern North America, frog and toad
diversity is greatest in the southeast, along the Gulf
Coast, and the southeastern Atlantic Coast where
nearly 40 species are found. Of the 14 anurans
occurring in New York, New England, and southern
Canada, eight are relatively common and
widespread. These species are spring peeper, wood
frog, green frog, gray tree frog, northern leopard
frog, pickerel frog, bullfrog, and American toad
(Figure Common Anurans).
Toads differ from frogs by possessing a
granular, dry, warty skin; conspicuous parotid
glands above the shoulders; cranial crests behind the
eyes, and shortened legs. The warty appearance of
the skin is due to the presence of numerous glands
that secrete bufotoxin. The toxin is distasteful and
repels many mammals from predating toads. Nevertheless, some reptiles are unaffected by the
toxin, and predators like raccoons have learned to eat toads from the belly and leave the
distasteful dorsal skin alone. Toads are most active at dusk and in the early evening hours
especially on rainy nights. Toads move slowly and bounce more than leap along the ground.
The American toad vocalizes a distinctive trill that frequently lasts 20 seconds or more.
Wood frogs are light brown with black eye masks. They are common in forests across
New England and Canada, the Midwest and southern Appalachian mountains. They are early
breeders and adults are freeze tolerant. Egg masses warm in cold northern waters by focusing
light upon the dark embryo. Calling wood frogs produce a curious, hoarse-sounding “wuckwuck” in short series.
Great color variation exists within populations of green frogs from dark melanic forms to
bronze, and bright green. Green frogs have two dorso-lateral skin ridges that extend from the
eye along the body. These lines distinguish the green frog from bullfrogs quickly. The hind legs
are long and thin with dark bands crossing the upper and lower leg sections. The sound of the
green frog, a nasal “glunk”, has been described as a string pluck on a banjo or a stretched rubber
band. Green frogs can produce high frequency alarm calls when caught and roughly handles.
These alarm calls are surprisingly loud and sound like a baby crying. Green frogs are somewhat
tolerant of human activity and can be found near agricultural ponds, artificial ponds on golf
courses, wet ditches, and even the occasional landscape yard ponds.
Bullfrogs are similar in appearance to green frogs. Both species are large and greenish,
but bullfrogs lack the dorsolateral skin ridges and dark lines crossing the hind legs. Bullfrogs
have a ridge of skin that circumvents the dorsal border of the eye and travels laterally behind the
tympana. The tympana of male bull and green frogs are larger than the eyes. Rather than to
enhance hearing, these large tympana may act as additional surface membranes to produce the
loud resonating songs of these species. Although I will probably avoid partaking in a feast of
frog legs, I hear that bullfrogs are the preferred species for their size. Apparently they taste like
chicken which is another reason to eat chicken and not frogs. Bullfrogs are also the favored
entry at the former frog pull competition at the Preble Hotel in Preble, NY.
Spring peepers become active early in the spring as day time temperatures inch above 40°
F. They can even be heard “peeping” during warm humid spells in the winter months. Spring
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peepers are one of the best hidden frogs in our area. They have the ability to change skin color
rapidly to tan, gray, and light green. The frogs are small bodied with delicate front feet with toes
ending in small suction disks. Peepers sense vibrations or shadows and quickly become quiet as
humans or other potential predators approach. Once found, peepers are unmistakable with an
irregularly-shaped X on the back. Spring peepers spend the winter frozen in leaf litter of
woodlands. Like the wood frog, peepers undergo a controlled freezing that protects cells and
tissues from ice rupture. In the spring, peepers migrate short distances to open puddles and
ponds. Vocalizations are important in attracting females and in defending small breeding
territories.
Pickerel and northern leopard frogs are the two most challenge species to distinguish.
Leopard frogs bear rounded spots that have a thin, light-colored boarder. Pickerel frogs have
irregular-shaped spots and yellow coloration between the hind legs and belly. Both species are
very common and hibernate beneath the ice in pond sediments. They emerge and begin to breed
a few weeks after spring peepers and wood frogs. Pickerel frogs have a creaking vocalization
that sounds like a rusty spring stretching on an old wooden screen door. Leopard frogs produce a
dull, snore-like sound that has the tempo of a slow rocking motion.
References
Bee, M.A. and A. C. Bowling. 2002. Socially mediated pitch alteration by territorial male
bullfrogs, Rana catesbeiana. Journal of Herpetology 36:140-143.
Davic, R.D., and H.H. Welsh. 2004. On the ecological roles of salamanders. Annual Review of
Ecology, Evolution, and Systematics. 35:405-434.
DeGarady, C.J. and R. S. Halbrook. 2006. Using anurans as bioindicators of PCB contaminated
streams. Journal of Herpetology 40: 127-130.
Gibbs, J.P., A.R. Breisch, P.K. Ducey, G. Johnson, J. Behler, R, Bothner. 2007. The
Amphibians and Reptiles of New York State: Identification, Natural History, and Conservation.
Oxford University Press.
Given, M.F. Vocalizations and reproductive behavior of male pickerel frogs, Rana palustris.
Journal of Herpetology 39:223-233.
Leclair, M.H., M. Levasseur, and R. Leclair. 2008. Activity and reproductive cycles in northern
populations of red-backed salamander, Plethodon cinereus. Journal of Herpetology 42:31-38.
Kohn, N.R., R. G. Jaeger, and J. Franchebois. 2005. Effects of intruder number and sex on
territorial behavior of female red-backed salamanders (Plethodon cinereus: Plethodontidae).
Journal of Herpetology 39: 645-648.
Lamoureux, V. J.C. Maerz, and D. M. Madison. 2002. Premigratory autumn foraging forays in
green frog, Rana clamitans. Journal of Herpetology 36: 245-254.
Lamoureux, V. and D. M. Madison. 1999. Overwintering habitats of radio-implanted green
frogs, Rana clamitans. Journal of Herpetology 33: 430-435.