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".

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.
  

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).

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.

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).

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.

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)

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

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.

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).

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.

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.

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.