WOOD FROG
Rana sylvatica
 
INTRODUCTION

In Rhode Island, wood frogs are the harbingers of spring as they are typically the first species to arrive at the breeding ponds and begin their loud quacking mating call. Their movement phenology varies annually, depending on climatic conditions; but, in Rhode Island they emigrate to breeding sites in most years during late February to early March, as daily maximum air temperatures starts to reach the low 50s. The en masse migration generally occurs at night during the first heavy downpour, so the first warm rainstorm during late winter or early spring often results in hopping armies of wood frogs headed to the nearest pond.

Compared to other amphibians species of that utilize ephemeral ponds for breeding in southern New England, the wood frog is one of the only species that truly can be considered obligate to vernal pools. Wood frogs successfully breed in ponds with shorter hydroperiods than any other amphibian in this region, with tadpole metamorphosis often complete by mid-July. Therefore, ponds that dry by August still provide perfectly suitable breeding habitat, whereas it takes much longer for the young of most other species to complete metamorphosis. In fact, the state of Massachusetts uses the presence of wood frogs as one of their criteria for certifying a pond as a "vernal pool".

 
IDENTIFICATION

  • The only 'masked' frog in Rhode Island.
  • The most distinctive feature of the wood frog is the dark mask-like pattern extending from the tip of the nose, through the eye and tympanum and tapering off above the base of the forelimb. It is this characteristic that has earned them the nickname 'robber frog'
  • A white line runs below the mask, defining the upper lip.
  • The dorsum of the adult wood frogs varies from light pinkish- tan to dark brown. Females are generally much lighter in color and display a reddish hue during the breeding season.
  • The venter is white, cream, or slightly yellowish in color and immaculate, whereas the similar colored throat and breast may have dark mottling.
  • The underside of the hind legs varies from translucent white to pale yellow (Klemens 1993). Colors usually more brilliant in males.
  • Hind legs are marked with dark bars.
  • Front toes are unwebbed.
  • The dorsal-lateral ridges are pronounced and extend the length of the body (behind the eye to near the vent).

Color variation: male (on top) and female
wood frog during breeding season.

  • Some individuals have dark spotting or dark patches on their sides below the dorso-lateral ridge.
  • Considerable geographic variation occurs in this species, with western and northern populations characterized by a light mid-dorsal stripe extending onto the hind legs (Martof and Humphies 1959; Klemens 1993).
  • On average, females are larger (see measurements below) and lighter in color than males.
  • Breeding males have swollen thumbs with large nuptial pads and thick forearms.
  • The sexes can be distinguished by the shape of the webs between the hind toes, which are convex in males and concave in females (Knox 1999). In addition, the 4th digit on the hind feet of females tends to be much larger than found on males.
  • Voice: croak or quack. Some people, with poor hearing, mistake wood frogs for ducks quacking. Male wood frogs may be heard anytime- day or night- during the breeding season (Wright and Wright 1949). In Rhode Island, it has been our experience that wood frogs calling activity peaks during the middle of the afternoon, when air and water temperatures are over 50°F (see Crouch 1999). Once one wood frog initiates calling, other individuals will readily join in the chorus. In fact, we have found that playing a tape recording of wood frog vocalizations will often start an active chorus.
 
SIZE
AGE / SEX SVL (SNOUT VENT LENGTH) (cm)
SAMPLE SIZE

AVERAGE

RANGE

Std. deviation

ADULT FEMALE

5.4

4.0- 6.5

0.5

696

ADULT MALE

4.6

3.5- 5.9

0.3

1578

JUVENILE

3.2

2.4- 3.9

0.4

61

METAMORPH

2.0

1.1- 3.1

0.3

1061


AGE/ SEX
MASS (g)
SAMPLE SIZE

AVERAGE

RANGE

Std. deviation

ADULT FEMALE

16.6

5.1- 33.4

4.5

696

ADULT MALE

9.5

5.0- 19.4

1.6

1578

JUVENILE

3.1

1.3- 5.4

0.9

61

METAMORPH

0.8

0.2- 3.4

0.3

1061







(University of Rhode Island unpubl. data)
 
RELATIVE ABUNDANCE
Rhode Island
We detected wood frogs in 72% of 119 randomly-selected ponds in the western part of the state ( see Paton and Egan 2001) during the 2000 field season.
Regional
Although disappearing from many urban areas, Rana sylvatica is presently secure in southern New England, occurring in most state parks, forests, and private sanctuaries, as well as in many suburban and rural areas (Klemens 1993).

Burne (2000) found wood frogs in 77% of 78 randomly-selected ponds he surveyed near Concord, Massachusetts, which was remarkably similar to our results in Rhode Island (72%, see above; Paton and Egan 2001).

 
DISTRIBUTION
Rhode Island
Wood frogs are common on mainland Rhode Island where habitat is suitable (Klemens 1993).
Regional
Wood frogs are common in suitable habitat throughout southern New England.
General
Wood frogs range farther north than any other amphibian in North America, reaching 69° latitude within the Arctic Circle (Martof and Humphries 1959; Klemens 1993). Their northern range limit corresponds with the edge of the northern limit of spruce forests. Wood frogs are found throughout the northeastern United States, the Great Lakes, and throughout Canada into Alaska; their range covers approximately four million square miles (Martof and Humphries 1959). They also range south along the Appalachian Plateau to extreme northern Georgia, and eastern Tennessee and Kentucky, with disjunct populations occur in Alabama, Arkansas, and Missouri (Klemens 1993).
 
HABITAT NON BREEDING

Except for the very brief period spent at breeding ponds, wood frogs are terrestrial. They utilize all types of forests and woodlands, including maritime, deciduous, and coniferous (Klemens 1993); as well as a variety of other types of habitats. Wood frogs were found in shaded boggy areas (Bellis 1962, 1965; Stockwell and Hunter 1989; Roberts and Lewin 1979) in Minnesota. In Michigan, Heatwole (1961) observed wood frogs in hardwood swamps dominated by American elm (Ulmus americanus), red maple (Acer rubrum), and yellow birch (Betula alleghaniensis). In Connecticut, Klemens (1993) found wood frogs in pastures, along abandoned logging roads, and in lawns with forested edge habitat. Wood frogs are uncommon in urban area and other degraded areas (Klemens 1993).

During the coldest months wood frogs hibernate under leaf litter (Pinder et. al. 1992; Windmiller 1996) rotting logs, stumps, rocks, and moss (Wright and Wright 1949; Klemens 1993). Bellis (1965, citing: Blanchard 1933; Heatwole 1961) suspected wood frogs hibernated in mucky, peat substrate in Minnesota.

Wood frogs have perfected the cryogenic freezing process. In the winter, as much as 35-45% of the frogs body may freeze, and turn to ice. Ice crystals form beneath the skin and become interspersed among body's skeletal muscles. During the freeze the frogs breathing, bloodflow, and heartbeat cease. Freezing is made possible by specialized proteins and glucose, which prevent intracellular freezing and dehydration.

 
HABITAT BREEDING
Wood frogs prefer forested vernal pools (semi-permanent and temporary flooded ponds) for breeding. However, observers have noted wood frogs breeding in grassy pasture ponds, ditches, fens, wooded swamps, flooded meadows (Klemens 1993) and, in Maine, abandoned beaver flowages, old gravel pits, cattail swamps, and slow moving streams (Knox 1999). The common characteristic amongst these waterbodies is that they are typically fishless. Wood frogs bred in both closed and open-canopy wetlands (Werner and Glennemeier 1999).
 
HYDROLOGY

Amphibian species richness and productivity in breeding ponds is positively correlated with hydroperiod length in experimental situations (Wilbur 1987; Rowe and Dunson 1995) and in natural population (Pechmann et al. 1989; Semlitsch et al. 1996; Snodgrass et al. 2000a). The importance of hydroperiod length in natural populations was shown by Semlitsch et al. (1996), who monitored a pond in South Carolina for over 16 years that contained standing water an average of 170 days annually (3-391 days). They found that years with short hydroperiods (<100 days) resulted in total reproduction failure, while years with long hydroperiods (>200 days) tended to have the greatest diversity and productivity (Paton and Crouch In press).

Compared to all other pond-breeding amphibians in Rhode Island, with the possible exception of marbled salamander (Ambystoma opacum), wood frog has one of the shortest hydroperiod requirements (Paton and Crouch In press). In Rhode Island, wood frogs typically enter breeding ponds in early March and most metamorphs have immigrated by mid-July. Therefore, ponds have to have water for about 145 days for successful reproduction by most populations of wood frogs in the state. Monthly precipitation in Rhode Island is actually relatively constant (3-4" during each month (National Weather Service)). Pond drying, therefore, is not caused by monthly variation in precipitation levels, but rather changes in evapotranspiration rates. Also, during the summer months, as trees need more water, local ground water tables drop rapidly, resulting in many ponds drying (Golet et al. 1993).


 

 
MOVEMENT CHRONOLOGY
Wood frogs are Rhode Island's earliest emerging ranid. They typically emerge in early to mid-March, but movement phenology can be much earlier or later depending on local climatic conditions (Paton and Crouch In press). Both lack of precipitation and snow on the ground can alter movement patterns.

 
REPRODUCTION
Wood frogs emerge from hibernation and begin their annual migration to breeding ponds when melting snow and spring rains saturate the ground. As females enter the water, males actively compete for females by attempting to mount them. If successful, males will remain clasped onto female's back (termed amplexus) until she deposit her eggs. In the lab, we have seen pairs in amplexus for over 24 hours. As the female deposits her eggs, the male fertilizes the eggs by depositing sperm onto the egg mass and into the water. The egg mass is usually attached to woody vegetation near the surface of the water in a relatively shallow section of the pond.

A 3-year study by Crouch and Paton (2000) documented considerable variation in egg-deposition chronology among breeding ponds within the relatively small area that they monitored in southern Rhode Island. This variation probably was due to micro-climate variation among ponds. Ponds with an open canopy (that is no trees covering the pond and presumably with much warmer water temperatures) tended to have much earlier deposition dates than closed-canopy ponds (Crouch and Paton 2000).

Wood frogs in amplexus

Although wood frogs can be explosive breeders, their breeding season may be protracted in adverse weather. In 80% of their breeding ponds where egg chronology was known, most depositions occurred during a 5-8 day period. This supports previous research from other regions of North America that wood frog egg-mass deposition is usually rapid, often requiring less than 1 week (Howard 1980; Seale 1982; Berven 1990). However, Crouch and Paton (2000) documented some ponds that took over 3 weeks to complete deposition. Cold weather can interrupt and extend breeding season. Meeks and Nagel (1973) reported that air and water temperatures of 2.2° C and 5° C, respectively, inhibit egg deposition, whereas air and water temperatures of 11.0° C and 8.8° C increased egg-deposition rates.

 
EGG MASS
The majority of wood frog egg masses are deposited in a large communal aggregation. However there is a tendency for egg masses in ponds with smaller populations (<100 breeding females) to deposit their eggs away from the primary aggregation (Crouch and Paton 2000). Wood frogs are one of only two species that regularly deposits communal egg masses in Rhode Island; the other being northern leopard frog (Rana pipiens). Many theories have been developed to try to explain why wood frogs do deposit eggs in aggregations, whereas most other anurans either deposit solitary egg masses (e.g., American toad) or lay eggs individually (e.g., spring peeper). Waldman (1982) reported that communal egg laying was profitable for the eggs in the center of the cluster. He found that eggs in the interior were insulated from temperature extremes, and were thus warmer than those deposited at the outside of the cluster. The temperature in the center

Communal egg mass (ca. 1200 masses)
can be as much as 1.6° C warmer than the surrounding water (Herreid and Kinney 1967; Hassinger 1970; Waldman 1982); therefore, decreasing development time of larva. Herreid and Kinney (1967) further suggested that egg clustering enhanced overall fertilization and showed experimentally that fertilization rate increased at higher temperatures. Seymour (1995) found that solar heating also promoted interstitial convection currents, which carried oxygenated water to the embryos in the center of the egg mass. Communal egg aggregations may also reduce predation (Herreid and Kinney 1967; Hassinger 1970; Waldman 1982) or reduce other environmental stresses. For example, Karns (1983) found hatching success was higher in larger rafts than smaller, in acidic bog water.

Wood frog eggs are deposited near the edge of the pond on the water's surface where water temperature is highest (Wright and Wright 1949). They are usually attached to submerged branches and herbaceous vegetation. Overhanging branches of red maple trees, (Acer rubrum), flooded shrubs (e.g., buttonbush [Cephalanthus occidentalis]), and herbaceous plants such as Beggar's Tick (Bidens spp.) were common plants utilized during 2000 in Rhode Island.

A wood frog egg mass consists of clear jelly capsules surrounding individual bi-colored embryos; the embryos are black above and white below. When initially deposited, an egg mass is approximately 7 cm in diameter (about the size of golf ball) (Duellman and Trueb 1986), but begins to expand as it absorbs water and eventually reaches the size of soft ball. Documented clutch sizes vary: Wright and Wright (1949) reported an average of 2000-3000 eggs per clutch, which is considerably higher than reported by Possardt (1974) or Duellman and Trueb (1986). Possardt reported 1020 as average in Massachusetts; and Duellman and Trueb (1986) found an egg mass contained about 1750 eggs. Eggs hatch in 10 to 30 days (Degraaf and Rudis 1983; Oliver 1955), depending on the temperature of the water; although in Rhode Island it probably takes at least 20 days in most years (P. Paton pers. obs).

Wood frog egg masses can easily be distinguished from those of the spotted salamander (Ambystoma maculatum) by the lack of an outer gelatinous sheath. Pickerel frog (Rana palustris) egg masses are similar in shape and design but contain brown embryos; and furthermore, are deposited much later in the season than wood frogs.

Wood frog egg mass
 
LARVAE

Newly hatched wood frog tadpoles are dark brown to black. As they grow, their dorsum lightens to a mottled olive brown (Altig 1970) and their venter color changes to a distinguishing pinkish-bronzy iridescence (Knox 1999). The tail fins of wood frog tadpoles are translucent and sometimes mottled with gold and blackish flecks. The tail fin is rounded dorsally, tapering to a fine point (Altig 1970); the tail musculature is bicolored.

Wood frog tadpoles grow at varying rates depending on water temperature, tadpole density, and available food resources (Wilbur 1977; Riha and Berven 1991). Minton (1972) reported that tadpoles develop in 6 to 15 weeks. In Rhode Island, we have found that egg typically hatch in mid-April and metamorphosis is complete by mid-June to mid-July (Paton and Crouch In press). Therefore, in Rhode Island it generally takes at least 8 weeks for metamorphs to emerge from ponds after hatching. Previous research has shown that the timing of pond drying can also affect the length of the larval period, with protracted development in ponds that have longer hydroperiods (Wilbur 1987; Semlitsch 1987; Semlitsch and Wilbur 1988; Skelly 1996).

Identification of wood frog tadpoles is usually relatively easy because wood frogs are the earliest anuran to breed in ponds in Rhode Island; therefore, they often are the only tadpoles encountered in ponds during April and early May - particularly in ponds with short hydroperiods. In ponds with longer hydroperiods, green frog or bullfrog tadpoles could also be detected in March or April, but their tadpoles tend to be much larger, greener in color, and exhibit dark spots on the dorsum and tail.

Recently hatched tadpoles initially feed on their remaining yolk and on the remains of the egg capsules (Raithel, unpubl. data). As they develop, young tadpoles feed on algae and various microorganisms scraped from aquatic vegetation with their specialized rasping mouth parts. As they grow older and larger, decaying plants and some animal matter are also consumed (Knox 1999). Petranka et al. (1994) demonstrated that wood frog tadpoles will readily prey upon the eggs and smaller tadpoles of American toads (Bufo americanus) and the eggs and unhatched larva of spotted salamander (Ambystoma maculatum) (Petranka et al 1998).

 
METAMORPHS

Stebbins (1951) found that metamorphosis occurs on average 67 days after hatching. The majority of the metamorphs exit the breeding ponds by mid-July in Rhode Island (Paton and Crouch In press), usually during rainy nights, congregating under leaves and other debris while making their way to the upland (Knox 1999). Synchronized breeding and rapid emergence by metamorphs may be an adaptation to minimize tadpole predation and cannibalism (Petranka and Thomas 1995). Unfortunately, most of them (70% or more) succumb to predation and other natural hazards before reaching adulthood (Stebbins 1951).

Within a few weeks, recently emerged metamorphs acquire adult characteristics in terms of coloration and markings.

 
JUVENILES

The term juvenile refers to individuals too young to breed. Juvenile wood frogs are similar in appearance to that of the adult, differing only in overall size and distinctiveness of identifying characteristics. Heatwole (1961) thought that individuals encountered mid-summer less than 3.4 cm SVL were juveniles from the previous breeding season. Age to Maturity: Collins (1975) and McLaren (1965) found that male wood frogs mature in 1-2 years, whereas females mature in 2 years. Another study by Berven (1981) found that male and female wood frogs in northern Maryland mature in 3-4 years.

It has been our experience in Rhode Island that juveniles are most evident from mid-April through mid-June and in early September (Paton and Crouch In press). Interestingly in the spring, juveniles tend to arrive at breeding ponds just after the last of the adult males have immigrated from ponds. However, just before metamorphs emerged from ponds, juvenile capture rates decline. There is another small pulse of juveniles that are captured near breeding ponds in the early fall (see movement phenology above). Presumably, the pre-breeding aged frogs are prospecting across the landscape, searching for breeding sites once they mature enough to initiate breeding. Previous researchers have documented young wood frogs dispersing over 2 km from their natal pond to a new breeding pond (Berven and Grudzien. 1990). Therefore, this exploratory phase by juvenile wood frogs is probably critical.

 
FOOD
Adults
Adult wood frogs feed on a variety of invertebrate including flies, beetles, spiders, earthworms, moth larva, slugs, snails, and annelids (Klemens 1993; Knox 1999).
Larvae
Newly hatched larva feed on their egg masses and associated algae, as wood frog tadpoles are "microphagous filter-feeders" (Petranka et al. 1998) with a largely herbivorous diet (Nyman 1985).

Larger tadpoles use their specialized mouth parts to feed on algae and various microorganisms scraped from aquatic vegetation, decaying plants and some animal matter (Knox 1999). Petranka (1994) found that larger tadpoles will feed on the tadpoles of American Toads.

 
PREDATION
Adults
A variety of animals feed on wood frogs. In breeding ponds, adults fall prey northern water snakes (Nerodia s. sipedon), snapping turtles (Chelydra s. serpentina), herons, mink, and raccoons (Knox 1999). On land, predators include: black racer snakes (Coluber constrictor), ribbon snakes (Thamnophis sauritus) garter snakes (Thamnophis sirtalis) (Klemens 1993), skunks, foxes, coyotes, and some larger birds (Knox 1999).
Larvae
Tadpoles often succumb to cannibalism, especially to their larger 'relatives'. Other predation accounts cited by Knox (1999) are: tadpoles are eaten by predacious diving beetles (Dytiscidae spp.) ( Young 1967), Ambystoma salamanders (Caldwell et al. 1980), Blandings (Emydoidea blandingii), and wood turtles (Clemmys insculpta) (McCollough unpubl. data). Knox (1999) also noted that tadpoles high and dry in ponds that dried untimely are consumed by wading, shore, and land birds.
 
CONSERVATION CONCERNS
Although relatively common in Rhode Island and New England, there is evidence that their populations are negatively impacted by suburban sprawl (Paton and Egan 2001). We found that wood frogs population sizes tended to be much larger in breeding ponds in relatively unfragmented forested stands in the western part of the state compared to urban areas, such as Providence. At breeding ponds near busy roads, we have seen large numbers of wood frogs killed while trying to cross the road. As suburbanization increases in Rhode Island, more and more breeding ponds could become isolated from other breeding ponds. This could eventually to lead to extirpations of many populations at some breeding ponds, unless steps are taken to ensure protection of all vital habitats including breeding ponds, the adjacent upland forested habitat, and forested dispersal corridors between breeding ponds.
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