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Hyla chrysoscelis Cope, 1880
Cope's Gray Treefrog
George R. Cline1
Cope’s gray
treefrogs (Hyla chrysoscelis) and eastern gray treefrogs (Hyla
versicolor) are members of a cryptic, diploid-tetraploid species complex.
This has resulted in considerable taxonomic confusion, especially in early
reports. As a result of this confusion, many authors have chosen to combine the
accounts and distributions of these species. In this account, I have attempted to
separate, as much as possible, the literature related to H. chrysoscelis
and H. versicolor. For categories where data are lacking or where
there is a question regarding the species identification, data from the sister species
are reported. This approach is at least partially validated by the genetic analysis
of this complex by Ptacek et al. (1994) and the growth and development studies of Ptacek
(1996).
1. Historical versus Current Distribution. Because of the cryptic nature of
Cope’s gray treefrogs and eastern gray treefrogs, discussions of these species are
nearly always intertwined. Cope's gray treefrogs were originally designated as a
subspecies of pine woods treefrogs (Hyla femoralis
chrysoscelis; Cope, 1880). Wright and Wright (1949) listed Cope's gray
treefrogs as subspecies of eastern gray treefrogs (Hyla versicolor
chrysoscelis). The distribution of Cope's gray treefrogs has long been
associated, and usually combined, with that of eastern gray treefrogs. Noble and
Hassler (1936) reported two call types along the East Coast of the United States. A
fast-trilling, harsh call type was found in the southern United States, while a slower
trilling, more mellow sounding call type was found at higher latitudes. The two
call types coexisted in a railroad yard in Baltimore, Maryland. Wright and Wright
(1949), apparently not realizing the importance of the call differences, restricted the
distribution of H. v. chrysoscelis to east-central Texas,
east into southern Arkansas and northwestern Louisiana. Blair (1958a) reported
distinct geographic distributions for two call types of what was then considered
H. versicolor. Recent depictions show the distribution of the
gray treefrog complex to encompass much of the eastern United States, including the
eastern Great Plains (eastern portions of Nebraska, Kansas, Oklahoma, and eastern Texas),
Minnesota south to Louisiana, and east of the Mississippi River to the Atlantic
coastline, excluding northern Maine and peninsular Florida, south of the panhandle
(Conant and Collins, 1998).
Species
identification has long been problematic in this group. Noble and Hassler (1936)
first reported discrete call types along a latitudinal cline. Blair (1958a) plotted
distinct geographic distributions of two call types across the range of what was then
considered H. versicolor. Johnson (1959, 1961, 1963, 1966)
reported genetic incompatibility between the call types, prompting him to designate the
faster trilling call type as H. chrysoscelis and the slower trilling
call type as H. versicolor. The diploid-tetraploid nature of this
complex was first suggested when Wasserman (1970) reported that H.
versicolor was a tetraploid (4N = 48). Later, Bogart and Wasserman (1972)
confirmed the diploid (2N = 24) nature of H. chrysoscelis. Ralin
(1977a) discussed call variation between H. chrysoscelis and
H. versicolor along an east-west transect from central Texas to
Louisiana. Gerhardt (1974a), Ralin (1977a), and Cline (1990) quantified call
differences between eastern and western races of H. chrysoscelis.
Bachmann and Bogart (1975), Cash and Bogart (1978), and Green (1980) demonstrated partial
discrimination between the two species by measuring cell diameter. Ralin and
Rogers (1979) reported that H. versicolor were morphologically and
genetically (Ralin and Selander, 1979) intermediate between eastern and western
populations of H. chrysoscelis. Cline (1990) combined acoustical,
morphological, and genetic analyses in central Oklahoma, Kansas, Missouri, and Arkansas
to show Cope's gray treefrogs range from east-central Texas (excluding the eastern 1/5 of
the state), north into central Oklahoma (excluding the eastern 1/4 of the state), eastern
Kansas, extreme eastern Nebraska, extreme eastern South Dakota, extreme eastern North
Dakota, western and southern Minnesota, Iowa, extreme southwest Wisconsin, most of
Missouri, east in a band across south-central Illinois, much of central Indiana, and much
of western and central Ohio, south through most of Arkansas and Louisiana, Mississippi,
southwestern Tennessee, Alabama, the Florida Panhandle, Georgia, South Carolina, eastern
North Carolina, southeastern Virginia, and southeastern Maryland.
Numerous attempts
have been made to identify and determine the distributions of these cryptic species at
the state level. Hoffman (1946, Virginia), Walker (1946, Ohio), Brown and Brown
(1972, Illinois), Bogart and Jaslow (1979, Michigan), Johnson (1987, Missouri), Matson
(1988, Ohio), and Little (1983, Ohio/West Virginia) used call characteristics.
Olson (1984) used calls and karyotypes to verify H. chrysoscelis in
Minnesota. Jaslow and Vogt (1977, Wisconsin), Chapin and Trauth (1987, Arkansas),
and Hillis et al. (1987, Kansas) used histological techniques. Recent research at
the state level has begun to elucidate the separate distributions of Cope's and eastern
gray treefrogs.
2. Historical versus Current Abundance. Little data comparing historical and
current abundance are available. With the advent of the North American Amphibian
Monitoring Program (NAAMP) and its standardized protocols, this paucity of data should be
resolved. Most of the early reports suggest, at least qualitatively, that members
of the gray treefrog complex were common historically. Personal experience in the
Southwest suggests that both species remain common. They occur densely around
ponds in Alabama, but diffuse "populations" of calling Cope's gray treefrogs
are found in suburban habitats.
3. Life History Features. Ritke et al. (1991a) discussed the life history of a
western Tennessee population of Cope's gray treefrogs.
A. Breeding.
Reproduction is aquatic.
i. Breeding migrations. Calling is probably stimulated by a combination of day
length and temperature. Calling typically begins earlier in the southern portion of
their range. Wright (1932) reports Hyla versicolor (almost
certainly H. chrysoscelis) from the Okefenokee Swamp calling from 10
June–13 August. In northeastern Alabama, calling can begin in late March and
continue through July, but calling is most intense in April–May. In
north-central Oklahoma, calling usually begins in April, with a sharp peak in May to
early June. Calling usually does not extend very far into July in Oklahoma.
Fellers (1997b) reports calling activity in Maryland from April–July or
August.
Cope's gray treefrogs breed from mid March to July (Wright and Wright, 1949). It
seems reasonable to suggest that the peak of the breeding season for both species is late
spring (May–June).
ii. Breeding Habitat. During the breeding season, Cope's gray treefrogs are found
calling near the edges of ponds, ephemeral wetlands, and ditches, and from floating algae
and emergent vegetation (Fellers, 1979b; Godwin and Roble, 1983; Conant and Collins,
1998; personal observations). At dusk, Cope's gray treefrogs begin calling from
high in the trees surrounding a pond. As the evening progresses, individuals move
down the trees (sometimes calling along the way) until they reach lower branches or
shrubs, or they continue until they reach the ground and move to a point usually within
1.5 m of the water’s edge (personal observations). Fellers (1979b) detailed
calling site characteristics. Ptacek (1992) compared calling sites of H.
chrysoscelis and H. versicolor. Godwin and Roble (1983)
suggest that females may mate on the first day they arrive at the breeding pond.
Males and females may mate ≤ 3 times/breeding season (Godwin and Roble, 1983).
Ritke and Lessman (1994) present biopsy data indicating that females develop follicles
throughout the breeding season, and production of later follicles relate to foraging
success. Ritke et al. (1991b) report strong breeding pond philopatry.
B. Eggs.
i. Egg deposition sites. Egg deposition sites are similar for both Cope's gray
treefrogs and eastern gray treefrogs. Eggs are loosely attached to emergent
vegetation at the surface of shallow ponds and pools (permanent or temporary).
These wetlands may be natural or created and can be highly disturbed. Hillis et
al. (1987) reports Cope's gray treefrogs breeding in the rain-filled furrows of
cornfields in Kansas.
ii. Clutch size. Eggs are laid in packets (10 x 12.5 cm, 30–40 eggs/packet)
as a surface film loosely attached to emergent vegetation (Wright, 1932).
C.
Larvae/Metamorphosis. Development is aquatic. Thibaudeaux and Altig (1998)
documented the ontogenic development of the oral apparatus of Cope's gray treefrogs.
i. Length of larval stage. Dickerson (1906) reported a larval period of 3 wk from
eggs to hatching and 4 wk to metamorphosis. Wright (1932) reported 45–65 d
from eggs to metamorphosis. Because developmental rates are generally positively
linked to temperature, one would expect that the average developmental times are shorter
in the South.
ii. Larval requirements. McDiarmid and Altig (1999) have summarized most of what
has been published on tadpoles. The sources cited herein are largely from this
text.
a. Food. No data are available on the food habits of Cope's gray treefrogs or
eastern gray treefrogs. Tadpoles generally feed by filtering food from the water
column or by scraping periphyton from submerged substrates (Hoff et al., 1999).
Steinwascher and Travis (1983) reported that Cope's gray treefrog tadpoles grew faster on
diets with high protein to carbohydrate ratios.
b. Cover. There are no published reports of cover requirements for Cope's gray
treefrog tadpoles. During the day, tadpoles can be seen resting on a variety of
substrates including exposed sediment, leaf litter, and fallen tree limbs. I have
observed hylid tadpoles resting on top of these same substrates at night.
iii. Larval polymorphisms. McCollum and Van Buskirk (1996) reported that reddish
and yellowish tail pigments in Cope's gray treefrogs were induced by the presence of
odonate naiad predators.
iv. Features of metamorphosis. Larval Cope's gray treefrogs are approximately 16
mm TL at metamorphosis (Wright, 1932).
v. Post-metamorphic migrations. Roble (1979) describes post-metamorphic migrations
of eastern gray treefrogs from central Wisconsin. Within a week, juveniles
dispersed from their natal ponds. Juveniles moved an average of 1.58 m/d, with
maximum dispersal distances approaching 125 m (Roble, 1979). Juveniles were active
throughout the day from July–September.
D. Juvenile
Habitat. Roble (1979) reported that juvenile eastern gray treefrogs from Wisconsin
were captured on sedges (Carex sp.) about 1/3 of the time, with false nettle
(Boehmeria cylindrica), reed grass (Phalaris
arundinacea), and swamp white oak (Quercus bicolor) saplings as
preferred habitat. Nearly 3/4 (70.3%) of all captures were below 50 cm above the
forest floor, while > 2% were found above 1.2 m. Roble (1979) surmised that young
frogs did not ascend into trees during their first season.
E. Adult
Habitat. Outside of the breeding season, Cope's gray treefrogs are found on trees
or on mossy or lichen-covered fences, usually above ground (Conant and Collins, 1998),
and will utilize knothole cavities (Ritke and Babb, 1991) and bluebird nesting
boxes (personal observations).
F. Home Range
Size. Little information is known regarding movements outside the breeding
season. Adults are thought to spend the remaining part of the activity season high
in trees where they forage on insects and insect larvae. Short-term movements are
probably limited, but during dry seasons, low relative humidity may drive treefrogs to
seek out high relative humidity microhabitats.
G.
Territories. Little is known about territoriality in these frogs. Gray
treefrogs are known to produce specialized calls (called "turkey roots" by
Wright, 1932) when approached while calling. Fellers (1979a) describes territorial
behavior in treefrogs. Wells and Taigen (1986) note that call duration increases in
high density (mean distance between individuals ~1 m) populations, presumably a response
to intrusion in a territory.
H.
Aestivation/Avoiding Dessication. Not documented.
I. Seasonal
Migrations. The only migrations reported for gray treefrogs are those to the
breeding ponds beginning in March and continuing through June. In July, individuals
may call during periods of high humidity or after rains, but populations tend to be
diffuse. Newly metamorphosed eastern gray treefrogs move ≤ 1.1 km (0.7 mi) from
the breeding ponds (Roble, 1979). Dispersal to winter hibernacula have not been
described, but such movements are probably short and asynchronous.
J. Torpor
(Hibernation). Wright (1932) speculated that gray treefrogs remained active in
Georgia "until November at least." Harlan (1835, in Wright, 1932) relates
collecting a specimen several feet below the surface of the ground from under a root of
an apple tree in winter. Burkholder (1998) discovered hibernacula of Cope's gray
treefrogs near the bases of sugar maple trees (Acer saccharum),
2.5–5.0 cm below the soil surface, as well as within leaf litter of varying depths;
all individuals were found < 1 m from the base of each tree investigated.
Freeze tolerance has
been described for eastern gray treefrogs. Schmid (1982) reported that glycerol
production in eastern gray treefrogs allowed individuals to survive –6 ˚C for
5–7 d. Storey and Storey (1985) reported additional production of glucose as
a cryoprotectant. Gray treefrogs were able to survive –2 ˚C for 5 d and
were also capable of surviving repeated freezing and thawing events. Storey and
Storey (1986) found eastern gray treefrogs could survive moderate freezing temperatures
(–2 to –4 ˚C) for ≤ 2 wk. Cope's gray treefrogs exhibit natural
freeze tolerance via production of elevated levels of glucose in their blood plasma
(Costanzo et al., 1992).
K. Interspecific
Associations/Exclusions. Maxson et al. (1977) reported immunological hybrids
between H. chrysoscelis and H. versicolor in
southeastern Oklahoma. Ralin et al. (1983) reported on electrophoretic hybrids from
Illinois. Gerhardt et al. (1994) cytologically confirmed the natural occurrence of
a triploid hybrid between Cope's gray treefrogs (diploid) and eastern gray treefrogs
(tetraploid).
Petranka (1989b)
reported possible chemical inhibition of southern leopard frog (R.
sphenocephala) tadpoles by Cope's gray treefrog tadpoles. Alford and
Wilbur (1985) and Morin (1987) reported that Cope's gray treefrog tadpoles were smaller
in the presence of other tadpoles, and the order in which species appeared in a pond
influenced the community composition at that pond.
Ralin (1981)
demonstrated that Cope's gray treefrogs and eastern gray treefrogs from Texas had similar
abilities to cope with desiccation in the same habitats. He further noted greater
variability among populations of the same species than he saw between the species.
L. Age/Size at
Reproductive Maturity. Cope's gray treefrogs range in size from 32–60 mm
(Wright and Wright, 1949; Conant and Collins, 1998). Wright (1932) suggests that
gray treefrogs from the Okefenokee Swamp begin breeding at 2 yr of age.
M. Longevity.
There are no published reports on longevity of Cope's gray treefrogs.
N. Feeding
Behavior. General reports list insects as the prey of gray treefrogs (Holbrook,
1842, in Wright, 1932). Dickerson (1906) and Ritke and Babb (1991) found grey
treefrogs to be "sit-and-wait" predators, consuming caterpillars, beetles,
flies, wood roaches (Parcoblatta sp., Ischnoptera
deropeltiformis), and camel crickets (Ceuthophilus sp.).
O. Predators.
Numerous potential predators exist for hylids, ranging from invertebrates through
vertebrates. Dickerson (1906) indicated that diving beetles preyed upon Cope's gray
treefrog tadpoles. Resetarits (1998) reported that odonate naiads and larval
dytiscids (diving beetles) both preyed on Cope's gray treefrog tadpoles, but larval
dytiscids were major egg predators as well. Dragonfly naiads were also used in
experimental studies of tadpole response to predators (McCollum and Van Buskirk,
1996). A wide variety of fish could prey upon all life stages of Cope's gray
treefrogs. Petranka et al. (1987) noted the ability of Cope's gray treefrog
tadpoles to detect chemicals associated with fish predators (green sunfish,
Lepomis cyanellus). Smith et al. (1999) reported that bluegill
sunfish (Lepomis macrochirus) substantially reduced eastern gray
treefrog tadpole abundance in field experiments. Potential amphibian predators
include salamanders and salamander larvae (chiefly newts [Notophthalmus sp.] and
ambystomatids [Ambystoma sp.]) and some adult frogs (i.e., American bullfrogs
[Rana catesbeiana]). Skelly (1992) used tiger salamanders
(Ambystoma tigrinum) in field experiments of antipredator costs.
Numerous turtles and snakes represent potential predators on the various stages of the
life cycle of frogs. Wading birds, especially herons (Ardeidae), prey upon tadpoles
and frogs of many species. Additionally, raccoons (Procyon lotor)
and skunks (Mephitis mephitis) are potential mammalian predators.
P. Anti-Predator
Mechanisms. Cope's gray treefrogs produce mucous secretions that are foul tasting
and cause burning sensation and inflammation in mucous membranes of eyes (personal
observations). While these secretions have antipredator functions, it is possible
that they also function as antimicrobial agents. Cline (1986) reported death feigning
(thanatosis) in Cope's gray treefrogs from northeastern Oklahoma.
Tadpoles may not use
chemical defense compounds. Kats et al. (1988) categorized Cope's gray treefrogs
tadpoles as "palatable" in their predation studies. McCollum and Van
Buskirk (1996) reported that predators (odonate naiads) induced production of reddish or
yellowish tail pigments in Cope's gray treefrogs. Subsequently, two papers
(Semlitsch, 1990; Figiel and Semlitsch, 1991) noted that substantial tail damage (>
75%) must occur before tadpoles suffer increased mortality (using odonate and crayfish
predators). Thus, it appears that tadpoles distribute these pigments in the tail to
misdirect predator attacks. Several studies have also reported decreased tadpole
activity (Fauth, 1990) and a shift in habitat usage (Petranka et al., 1987; Kats et al.,
1988) in the presence of predators. Resetarits and Wilbur (1989) concluded that
Cope's gray treefrog tadpoles were capable of detecting chemical odors of potential
predators in water conditioned by these predators.
Q. Diseases.
So far, no known reports of hylid frog declines have been related to diseases such as
those caused by chytrid fungi or ranaviruses.
R. Parasites.
A variety of parasite hosts have been suggested for gray treefrogs. Armstrong et
al. (1997) studied reproduction of a monogenean flatworm parasite in Cope's gray
treefrogs. Delvinquier and Dresser (1996) reported an opalinid (Sarcomastigophora)
from eastern gray treefrogs. Hausfater et al. (1990) chose eastern gray treefrogs
as a model organism for studying the effect of parasitism on mate choice, in part because
males harbored a “wide range of helminth parasites."
4. Conservation. Cope's gray treefrogs are moderately tolerant to the
pollutants tested so far, and they are tolerant to human habitat disturbance.
Confusion between
this species and eastern gray treefrogs continues to confound conservation efforts.
Cope's gray treefrogs are listed as Endangered in New Jersey, where a permit issued by
the New Jersey Division of Fish, Game, and Wildlife is required for all activities
involving this species.
1George R. Cline
Biology Department
Jacksonville State University
700 Pelham Road North
Jacksonville, Alabama 36265-1602
gcline@jsu.jsu.edu
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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