Hyla squirella Bosc, 1800
Joseph C. Mitchell1
Michael J. Lannoo2
1. Historical versus Current Distribution. Squirrel treefrogs (Hyla
squirella) are found along the Atlantic Coastal Plain from Virginia to the
Florida Keys, and along the Gulf Coastal Plain from south Florida to eastern Texas
(Wright, 1932; Burt, 1938b; Wright and Wright, 1949; Hoffman, 1955; Duellman and
Schwartz, 1958; Hendrickson, 1974; Martof, 1975a; Mount, 1975; Gotte and Ernst, 1987;
Conant and Collins, 1991; Petzing and Phillips, 1998a; Mitchell and Reay, 1999; Dixon,
2000). They occur on numerous barrier islands off the southeastern Atlantic coast
and Florida Gulf Coast (Martof, 1963; Blaney, 1971; Gibbons and Coker, 1978; Braswell,
1988). Squirrel treefrogs have been introduced into the Bahamas (Crombie, 1972).
2. Historical versus Current Abundance. Squirrel treefrogs are one of the most
common species in Florida (Carr, 1940a; Bartlett and Bartlett, 1999a). Deckert
(1914, cited in Wright, 1932) noted squirrel treefrogs in southeastern Georgia are
"…the commonest of the southern tree toads…found everywhere, in corn
fields, sugar cane, about wells and under eaves of stable roofs, barns, outhouses,
etc." Large populations of squirrel treefrogs were observed in the low country
of southern Alabama by Mount (1975), the Mississippi Gulf Coast by Smith and List
(1955), and in eastern North Carolina by Robertson and Tyson (1950) and Palmer and
Whitehead (1961). In Princess Anne County, Virginia, Hoffman (1955) observed
numerous treefrogs that "…came to the bright light of our Coleman
lantern." Werler and McCallion (1951) did not encounter this species in their
surveys of the early 1940s in the same area. Although squirrel treefrogs can be
locally abundant (J.C.M., personal observations), historical and current estimates of
population sizes are lacking.
3. Life History Features.
Reproduction is aquatic.
i. Breeding migrations. Adults migrate from upland sites to breeding pools during
rains (Wright, 1932; Wright and Wright, 1949; J.C.M., personal observations).
ii. Breeding habitat. Woodland or pasture wetlands, flooded roadside ditches,
stock ponds, or other shallow bodies of water (Wright, 1932; Wright and Wright, 1949;
Gosner and Black, 1956; Mount, 1975). Breeding males call while sitting on
shorelines, perched on debris or vegetation, hidden in clumps of grass, and while sitting
in 1–2 cm of water (Carr, 1940a; Wright and Wright, 1949; Gosner and Black, 1956;
Duellman and Schwartz, 1958). Mount (1975) reported that males call from a perch
30–60 cm above the water or from a bank near the water’s edge.
iii. Breeding periods. Seasonal breeding times vary with latitude but usually
coincide with spring and summer rains (Wright, 1932; Wright and Wright, 1949). The
common name of “rain frog” reflects the tendency for this species to call
during the day when rain is approaching, although males produce a raspy, squirrel-like
call dissimilar from the breeding call (Wright and Wright, 1949; Duellman and Schwartz,
1958; Mount, 1975; Garrett and Barker, 1987). Squirrel treefrogs breed from mid
April to mid August in Alabama (Mount, 1975); early April to August in northern Florida
(Carr, 1940a; Einem and Ober, 1956); late March to early August in southern Florida
(Duellman and Schwartz, 1958); May–August in North Carolina (Brandt, 1936a); and
March–October in Texas (Garrett and Barker, 1987). In Florida, “Full
choruses come with July electrical storms” (Carr, 1940a).
Diel activity and
breeding cycles have been understudied. Squirrel treefrogs are nocturnal and
seldom seen abroad during the day. Buchanan (1992) observed treefrogs in Louisiana
leave and re-enter diurnal retreats at dusk and dawn, respectively, and experience peak
periods of activity at 2000–2300 and 0300–0500 h.
i. Egg deposition sites. Wright (1932) stated that eggs are laid in shallow pools
and that "no end of tadpoles must be lost from drying of breeding places,"
indicating the ephemeral nature of the larval habitat. Eggs may be deposited singly
or in clusters on the bottom or attached to vegetation (Wright, 1932; Wright and Wright,
1949; Livezey and Wright, 1947; Brugger, 1984; Bartlett and Bartlett, 1999a). Orton
(1947) noted that eggs "were scattered singly or in pairs on debris in water about
3–6 inches [7.5–15 cm] deep."
ii. Clutch size. Females lay > 950 eggs (Livezey and Wright, 1947). Wright
(1932) cited egg counts of 942 and 972, and Martof et al. (1980) noted 1,000
eggs/clutch. Clutch size from five females in northern Florida averaged 1,059
(Brugger, 1984). Production of > 1 clutch/year has not been reported.
i. Length of larval stage. Time from deposition to hatching is 24–43 hr
following ovulation (Wright, 1932; Beck, 1997). Tadpoles metamorphose after
40–50 d (Wright and Wright, 1949; Garrett and Barker, 1987) at 11–13 mm
(Wright, 1932; Martof et al., 1980). Beck (1997) reported experimental larval
periods of 54–77 d at 22 ˚C with low to constant feeding rates and body mass
at metamorphosis from 0.25–0.52 g.
ii. Larval requirements.
a. Food. Tadpoles are suspension feeders that eat organic and inorganic food
particles they scrape from rock, plant, and log substrates.
b. Cover. Tadpoles use high habitat complexity to escape predation by dragonfly
naiads (Anax junius) and giant water bugs (Lethocerus
americanus; Babbitt and Tanner, 1997). Some ephemeral pools, however,
lack cover (J.C.M., personal observation), and tadpoles probably experience variable
rates of mortality.
iii. Larval polymorphisms. Not known to occur.
iv. Features of metamorphosis. Metamorphosis appears to occur over extended
periods. Wright (1932) reported a 2-wk duration.
v. Post-metamorphic migrations. Recently metamorphosed animals emerge from
breeding pools and migrate to upland feeding sites and shelters. Goin and Goin
(1957) observed numerous squirrel treefrog juveniles entering their yard in northern
Florida from October–December, but none of the migrations were en masse. Pool
emergence and post-metamorphic migrations have not been quantified.
Habitat. Juveniles occupy habitats similar to adults, although Neill (1951a) noted
that many squirrel treefrogs found in bromeliad plants are recently metamorphosed
animals. Goin and Goin (1957) found that newly arriving juveniles initially used
palmettos for cover, but subsequently moved to more permanent shelters including oaks,
holly trees, and magnolias. They noted that suitability of holly leaves as
overwintering sites reflects the structure of the leaves, as they are leathery and tend
to curl toward the underside. Juvenile squirrel treefrogs frequently select a
position between two leaves that lie one over the other. Juveniles also remained
under inverted tin cans atop stakes in their yard during the winter. Site shifts
occurred from natural to unnatural cover from autumn to spring.
Habitat. Squirrel treefrogs show little discrimination in their selection of major
habitat types (Carr, 1940a). They occur in and around buildings, in gardens,
grasslands, weed or brush tangles, bottomland hardwoods, riparian zones, open woodlands,
pinelands, trees, vines, cypress stands, longleaf pine/turkey oak/wiregrass associations,
and longleaf pine/slash pine flatwoods—in almost any place associated with
moisture, food, and cover (Wright, 1932; Carr, 1940a; Wright and Wright, 1949; Anderson
et al., 1952; Funderburg, 1955; Goin and Goin, 1957; Goin, 1958; Duellman and Schwartz,
1958; McComb and Noble, 1981; Enge and Marion, 1986; Garrett and Barker, 1987; Conant
and Collins, 1991; Dodd, 1992; Lamb et al., 1998). Squirrel treefrogs exhibit some
preference for open woodlands, such as mature pine and mixed hammock forests and open
woody areas (Wright, 1932; Carr, 1940a; Wright and Wright, 1949; Delzell, 1979).
They occur in trash piles (Ashton and Ashton, 1988) and bromeliad plants (Neill, 1951a),
and often select narrow spaces in human structures (Goin and Goin, 1957). Squirrel
treefrogs are known for their ability to change colors to match their backgrounds
(Wright, 1932; Wright and Wright, 1949).
squirrel treefrogs occupy coastal estuarine and harsh barrier island habitats.
Engles (1952) found them on, in, and under logs surrounded by meters of bare, dry sand in
the wrack zone, and in all vegetated habitats on the Outer Banks of North Carolina.
Neill (1958a) noted that this treefrog breeds in rainwater pools affected by saltwater
spray in the Florida Keys. Webb (1965) determined that the salinity of a pool with
squirrel treefrog tadpoles on Bogue Bank (a barrier island off North Carolina) was 47% of
full-strength sea water.
F. Home Range
Size. Home ranges have not been reported, but Neill (1957b) noted that squirrel
treefrogs apparently establish temporary home ranges, which they occupy for days to
weeks. Adults in northern Florida returned to the same resting places day after
day, even after retreating to other sites during inclement weather (Goin and Goin, 1957).
Territories. Analyses of territorial behavior have not been published.
Aestivation/Avoiding Dessication. Squirrel treefrogs will often aestivate
communally. Carr (1940a) wrote, "Several are found in the same hollow tree or
in the axils of the same royal or coco-palm petiole when numerous available and
apparently identical retreats are unoccupied." Of the eight species of hylid
frogs studied by Farrell and MacMahon (1969), squirrel treefrogs had the lowest water
content, and females were composed of less water than males. Squirrel treefrogs
will tolerate an average of 34% of total weight loss in desiccating conditions before
succumbing—about average for the species studied by Farrell and MacMahon
Migrations. Migrations are tied to breeding and metamorphic events; most are
apparently tied to periods of rain.
(Hibernation). Carr (1940a) writes: "They are gregarious hibernators, thirty
or forty sometimes congregating under a loose slab of bark." Squirrel
treefrogs hibernate beneath bark of pine stumps and logs and decaying bark of trees
(Allen, 1932; Neill, 1948b). Adults may be seen on warm days in northern Florida
during the winter; the lowest air temperature at which a juvenile squirrel treefrog was
active measured 6.7 ˚C (Goin and Goin, 1957; Goin, 1958). They also noted
that upon the approach of a cold front, frogs sought more secure shelters, such as under
leaf mold and rotten bark.
Associations/Exclusions. Wright and Wright (1949) noted that squirrel treefrogs
breed in association with eastern spadefoot toads (Scaphiopus
holbrookii) and gopher frogs (Rana capito). In New
Orleans, squirrel treefrogs called with Gulf Coast toads (Bufo
nebulifer), Fowler's toads (Bufo fowleri), and eastern
narrow-mouth toads (Gastrophryne carolinensis; Volpe, 1956). One
of us (J.C.M.) observed breeding in a shallow wetland in eastern North Carolina along
with southern toads (Bufo terrestris), barking treefrogs (Hyla
gratiosa), eastern spadefoot toads, and southern leopard frogs (Rana
sphenocephala), and in Virginia with Fowler's toads, Cope's gray treefrogs
(Hyla chrysoscelis), green treefrogs (Hyla cinerea),
eastern narrow-mouthed toads, and eastern spadefoot toads.
Goin and Goin (1953)
and Goin (1958) noted that where squirrel treefrogs co-occur with green treefrogs
(Hyla cinerea) in Alachua County, Florida, the latter prefer low
shrubbery and more permanent breeding sites and the former prefer the canopy and
ephemeral breeding sites. Squirrel treefrogs were found trapped in a ditch dug to
accommodate a pipeline along with seven species of salamanders, eight other species of
hylids, five species of ranids, five species of bufonids, and eastern narrow-mouthed
toads (Gastrophryne carolinensis; Anderson et al., 1952). Volpe
(1956) found amplexed pairs of squirrel treefrogs and eastern narrow-mouthed toads (two
pairs of male squirrel treefrogs with female eastern narrow-mouthed toads, three pairs of
male eastern narrow-mouthed toads with female squirrel treefrogs). None of the
pairs produced viable eggs; the sperm did not penetrate the eggs.
L. Age/Size at
Reproductive Maturity. Wright (1932) noted 13 mm body length as size at
metamorphosis and estimated that juveniles grow to 19–23 mm during their first year
and that 2-yr-olds were > 26 mm in size. Adult body length is 22–41
mm (Wright and Wright, 1949; Martof, 1975a; Martof et al., 1980; Conant and Collins,
1991) and maximum size will reach 45 mm (Mount, 1975). Males and females are
similar in size (Wright and Wright, 1949; Duellman and Schwartz, 1958).
A wild-caught adult lived 8.5 yr in captivity (Snider and Bowler, 1992).
Behavior. Squirrel treefrogs are aggressive predators that feed on insects and
other invertebrates (Wright, 1932; Garrett and Barker, 1987). Carr (1940a) noted
that they are "Often present in enormous numbers along lake-shores when chironomids
(Diptera) are emerging; they also collect around lamp posts and lighted windows at
night. I once saw nine young in a circle around a pile of newly deposited cow-dung,
awaiting and devouring the midges attracted thereto." Duellman and Schwartz
(1958) examined the stomachs of 20 individuals and found that 9 were empty, 2 contained
only plant debris, and 4 contained beetles (Coleoptera); other stomachs contained
crustacean (crayfish?) remains, a spider (Arachnida), a cricket (Orthoptera), and an ant
(Hymenoptera). Brugger (1984) found that two groups of arthropods (Arachnida,
Isoptera) and three orders of insects (Coleoptera, Orthoptera, Hymenoptera) dominated the
diet of frogs from northern Florida. Goin and Goin (1957) noted that differences in
habitats selected by adults and juveniles may result in dietary differences.
Duellman and Schwartz (1958) observed ribbon snakes (Thamnophis
sauritus) feeding on squirrel treefrogs. Other predators undoubtedly
include small mammals, birds, other frogs, other snakes, and aquatic invertebrates.
Dragonfly naiads and water bugs (Lethocerus) eat tadpoles (Babbitt and Tanner,
Mechanisms. Their small size, propensity for seeking small hiding places, and
ability to change colors to match their background probably aid in avoiding visual
4. Conservation. Pague and Mitchell (1987) noted that some populations in
urbanized areas of southeastern Virginia had become extirpated. However, Neill
(1950a) and Delis et al. (1996) found squirrel treefrogs relatively abundant in urban
areas of Augusta, Georgia, and Tampa, Florida, respectively. Although squirrel
treefrogs are well known to cross roads at night in rains (e.g., Mount, 1975), the effect
of road mortality on size and structure of urban populations is unknown.
1Joseph C. Mitchell
Department of Biology
University of Richmond
Richmond, Virginia 23173
2Michael J. Lannoo
Muncie Center for Medical Education
Indiana University School of Medicine
Ball State University
Muncie, Indiana 47306
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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