Skip booklet index and go to page content

Red-legged Frog (Rana Aurora)

Biology

The Red-legged Frog has a biphasic life-cycle typical of aquatic-breeding amphibians in the northern hemisphere: Eggs are laid in water and develop into aquatic larvae, which then metamorphose into juveniles that leave the water. Juvenile frogs forage in terrestrial and riparian habitats for several years before reaching sexual maturity and then return to aquatic habitats to reproduce. Outside the breeding season, adults of the Red-legged Frog are highly terrestrial and can be found far from water. The use of aquatic breeding habitats increases the exposure of the vulnerable, early life history stages to contaminants, as ponds and other water bodies act as sinks for various pollutants. In fragmented landscapes, seasonal migrations of these frogs to and from breeding sites increase their vulnerability to road mortality and to predators. The modification of either their aquatic breeding sites or adjacent terrestrial habitats by human activities and land use practices can be detrimental to local populations. These frogs require sufficient space to allow for seasonal movements are therefore especially vulnerable to habitat fragmentation.

Licht (1969, 1971, 1974) and Calef (1973a, b) studied the reproductive biology and survivorship of the Red-legged Frog in southern British Columbia. These studies remain the most detailed treatments of the species’ biology and natural history in Canada to date. Waye (1999) provided a comprehensive review of the general biology of this species based on the above and other studies reported up to 1997.

 

Reproduction

The Red-legged Frog is an explosive breeder (sensu Wells 1977), and adults congregate at breeding sites for a short period (2 – 4 weeks) in early spring, often immediately after the break-up of ice. Males are vocal but typically call from under water – as a result, breeding choruses are inaudible or only barely detectable to the human ear from above the water’s surface (Licht 1969). The timing of the breeding migration and egg-laying varies both geographically and from year to year depending on air and water temperatures; water temperatures of at least 6 – 7°C appear to be to required for egg-laying to occur, but temperatures frequently drop below this value during embryonic development (Licht 1974, Brown 1975). In southern British Columbia, breeding has been reported from February to April, but it is typically completed by the end of March (Licht 1969, Calef 1973b). While males are capable of breeding multiple times during each breeding season, their mating success appears to be highly variable (Calef 1973a). Adult females reproduce each year (Licht 1974). Sexual maturity by both sexes is attained at three or more years of age (Licht 1974).

As in most aquatic-breeding anurans, fertilization is external. Females lay their eggs in a large (20 – 30 cm diameter) gelatinous cluster, which they often attach to submerged vegetation (Leonard et al. 1993; Figure 1b). The egg masses are typically entirely submerged, about 30 – 90 cm below the surface of the water (Licht 1969). The clutch size is relatively large (up to 1300 eggs; Leonard et al. 1993) and shows a positive correlation with the body size of the female (Licht 1974). The average clutch size in marshes near Vancouver was 680 eggs (range: 243 – 935 eggs; Licht 1974); at another site on the lower mainland (Marion Lake) it was 531+19 eggs (mean+SE; Calef 1973b).

The duration of the incubation and larval period is temperature-dependent and highly variable under natural conditions. Hatching can take place as soon as about nine days from oviposition (under constant temperature of 18.3°C; Storm 1960) but usually takes much longer under the variable temperature regimes encountered under field conditions in the spring (6 – 7 weeks in Oregon, Storm 1960; 35 days in Washington, Brown 1975). In southern British Columbia, hatching typically occurs during the first half of May (Calef 1973b). The duration of the larval period is about 11 – 14 weeks (Calef 1973b). Most tadpoles transform from early July to early August, but the timing of metamorphosis varies both annually and with location (Licht 1969, Calef 1973b); Calef (1973b) found that tadpoles continued to metamorphose until early October at one site. Over-wintering by tadpoles has been documented for the California Red-legged Frog under some situations (Fellers et al. 2001), but there is no evidence of this phenomenon for the Red-legged Frog.

 

Survival

The Red-legged Frog exhibits a Type III survivorship curve, which occurs when juvenile mortality is extremely high. Annual survivorship of those individuals that survive the critical early period then increases greatly. For this species the greatest mortality occurs during the tadpole stage, whereas embryonic mortality and that of metamorphosed individuals is relatively low (Calef 1973b, Licht 1974). Licht (1974) reported survival rates of over 90% for embryos from oviposition to hatching and less than 1% for tadpoles from hatching to metamorphosis in marshes near Vancouver. Calef (1973b) reported 5% survival through the tadpole stage at another lower mainland site (Marion Lake); small tadpoles were particularly vulnerable, and most mortality occurred within the first 3 – 4 weeks from hatching. In Licht’s (1974) study, the overall survival rate of metamorphosed juveniles to the end of the growing season was 4.8% from the egg stage and 5.3% from the tadpole stage. Survival of the these recruits to the population over their first year as frogs was estimated to be relatively high at 52%, based on mark-recapture data over one year; the annual survival of adults was also high at 69%. The high embryonic survival of the Red-legged Frog contrasted with that of the syntopic Oregon Spotted Frog (R. pretiosa), in which most early mortality occurred in the egg-stage, often due to stranding of the eggs laid in shallow water. Survivorship of other life history stages was similar for the two species (Licht 1974).

While fungal infections and desiccation due to fluctuating water levels contribute to embryonic mortality, predation is thought to be the main source of mortality of tadpoles of the Red-legged Frog (Calef 1973b, Licht 1974). Experiments in field enclosures where numbers of predators (Rough-skinned Newt Taricha granulosa) were manipulated emphasized the importance of predation as a mortality factor for tadpoles (Calef 1973b).

Little is known of the demography of the Red-legged Frog. Adult males greatly outnumber females at breeding sites, but outside the breeding season the sex ratio appears to be even (Calef 1973a). Adults live for multiple years, but their longevity under field conditions is unknown; a lifespan up to 15 years has been reported in captivity (McTaggart Cowan 1941). Populations of many aquatic-breeding anurans fluctuate widely from year to year (Pechmann and Wilbur 1994), and this species is probably no exception. Waye (1999) pointed out that populations of the Red-legged Frog are likely to withstand 1 – 2 years of low recruitment through the survival of adults for multiple years.

 

Predators and Parasites

Predators of tadpoles of the Red-legged Frog include predatory fish such as the introduced Rainbow Trout (Salmo gairdneri), salamanders such as the Rough-skinned Newt and Northwestern Salamander (Ambystoma gracile), and various invertebrates such as dragonfly larvae (family Odonata) and the Giant Water Bug (Lethocerus americanus) (Calef 1973b). Leaches prey on anuran tadpoles and eggs (Licht 1974), and the Rough-skinned Newt has been observed to feed on eggs of the California Red-legged Frog (Rathbun 1998). The introduced Bullfrog is a predator of both larvae and adults. Various other vertebrate and invertebrate predators that include metamorphosed frogs and tadpoles in their diets are often present in aquatic habitats occupied by this species, including the Raccoon (Procyon lotor), Great Blue Heron (Ardea herodias), Belted Kingfisher (Megaceryle alcyon), and the Common Garter Snake (Thamnophis sirtalis) (Licht 1974).

The Red-legged Frog is a host for various parasites and disease-causing organisms. Infection by the parasitic yeast Candida humicola alters the behaviour of tadpoles of this species and increases their susceptibility to predation (Lefcort and Blaustein 1995). In western United States, infections by the parasitic trematode Ribeiroia ondatrae are linked to limb abnormalities in several amphibian species (Johnson et al. 2002). Johnston et al. (2002) found a relatively high frequency of limb malformations (mean of 10.8%) in metamorphs of the Red-legged Frog, which were associated with Ribeiroia infection. The parasite was found at 5 of the 11 sites sampled where this frog was present. The Red-legged Frog is susceptible to infections by iridoviruses, a group of pathogens that infect invertebrates and ectothermic vertebrates. An iridovirus identical to that in sympatric fish species has been isolated from the Red-legged Frog, suggesting that fish (native and introduced) may act as reservoirs of viruses pathogenic to amphibians (Mao et al. 1999).

 

Physiology

The Red-legged Frog is adapted to breeding in cold conditions (Licht 1971). Adults are active early in the spring when air and water temperatures are low, and males may call at water temperatures as low as 4 – 5° C (Licht 1971, Calef 1973a, Brown 1975). The eggs can withstand exposure to similarly low temperatures, although egg-laying typically occurs in somewhat warmer water (see Section on Reproduction, above). The thermal tolerance of young embryos (up to Gosner developmental stage 11) ranges from 4 to 21°C (Licht 1971). Both the lethal maximum and minimum are the lowest reported for North American Rana, and the pattern most closely resembles that of the cold-adapted Wood Frog (Rana sylvatica) from Alaska. The thermal tolerance of embryos increases as development proceeds; hatching occurs at Gosner developmental stage 21 in this species. In nature, the eggs are protected within a gelatinous mass and are typically submerged in water; both factors buffer them from thermal fluctuations (Licht 1971). Because these frogs breed at night very early in the spring, it is unlikely that young embryos will ever be exposed to temperatures above the lethal limits in British Columbia.

The eggs of the Red-legged Frog are relatively large, with large yolk supplies, when compared to other species of Rana (Licht 1971). This trait appears to be an adaptation to northern climates and to breeding at low temperatures. A correlate of a large egg size is a slow embryonic developmental rate. The adaptive significance of large eggs in these frogs is unknown but may relate to advantages gained by correspondingly larger larvae, which can better escape predation.

These frogs are not known to be freeze-tolerant, as are the Wood Frog and a few other northern anurans. Instead, they over-winter in the bottom of pools or on the forest floor, presumably in microhabitats that are buffered from below-freezing conditions.

 

Movements and Dispersal

Adults undertake seasonal migrations between aquatic breeding sites and terrestrial foraging habitats in the spring, and metamorphs disperse away from the breeding sites from late summer to early autumn each year. Migrations to and from hibernation sites may also occur, but it is possible that hibernation takes place within foraging or breeding areas. The spatial extent of seasonal migrations and dispersal patterns for the Red-legged Frog are poorly known. In Washington, five female frogs equipped with radio-transmitters moved relatively long distances (up to 80 m day) during the spring migration period from breeding sites to foraging areas (Shean 2002). In contrast, their movements were shorter (< 3 m/day) and unidirectional while at the breeding site. The summer foraging sites were up to 312 m away from the breeding sites in straight-line distance. In Oregon, four adult frogs were found in April – May at a straight-line distance of 1.1 – 2.4 km from their capture points the previous December, indicating that at least some individuals undertake relatively long migration movements (Hayes et al. 2001). The migration movements of the California Red-legged Frog took place overland through routes that were up to 500 m from water (Bulger et al. 2003). The frogs traveled distances of 200 – 2800 m over several months over the wet season in winter to reach their breeding grounds. Interestingly, only a relatively small proportion (11 – 22%) of the breeding population migrated; most frogs remained in the vicinity of the breeding sites year-round. The California Red-legged Frog is more aquatic than the Northern Red-legged Frog and somewhat dissimilar in its ecology, morphology, and genetics from its southern form – therefore, these observations may not be applicable to the northern subspecies.

Within breeding sites in southern British Columbia, individual adult males typically moved distances of 100 – 300 m within and between weed-beds (Calef 1973a). Individual males showed site-fidelity to particular breeding sites from year to year, and about 20% of the males marked in one year were recaptured the following year. Furthermore, about 58% of the recaptured males returned to the same weed-bed, and many others occupied adjacent weed-beds within 100 m of their original capture locations. Site-fidelity of females is less well documented, most likely due to difficulties in obtaining sufficiently large sample sizes of females rather than differences in their behaviour.

Within terrestrial habitats on northern Vancouver Island, Chan-McLeod (2003a, b) and Chan-McLeod and Moy (in review) studied movements of adults of the Red-legged Frog in relation to various logging patterns. Frogs that were experimentally released under individual trees or into small forest patches within a logged matrix (cut area surrounding the patches) during the non-breeding season from May to October either remained in these patches, moving at average rates of less than 10 m/day, or left the patches and moved through the habitat, including the logged matrix, at average rates of 50 – 60 m/day. No tendencies to move towards the direction of original capture sites some distance away were found. In another experiment, frogs that were released at the clearcut – old growth forest edge moved straight-line distances of up to 221 m in 3 days and 191 m in two days across the clearcut under favourable, wet conditions (Chan-McLeod 2003b).

In the uncut forest on northern Vancouver Island, individual frogs occupied small home ranges in forested riparian areas along streams and typically moved only short distances when monitored in May – June (for 3 – 41 days) and in September – October (for 3 – 13 days; Chan-McLeod 2003a). At four sites, the average daily movements of radio-tracked 68 adults were less than 5 m between locations. Individual frogs were site-tenacious and moved back and forth within a small, defined area. The frogs remained within 36 m or closer to the water’s edge. Two of the frogs undertook directional, longer movements. One frog moved about 260 m along the riparian channel within a period of three weeks in early summer; another moved perpendicular to the stream at least 200 m. These results suggest that although typically sedentary in riparian habitats, these frogs are capable of and occasionally undertake relatively long-distance movements. Chan-McLeod (2003a) suggested that, although infrequent, longer movements might be important for connecting subpopulations and maintaining spatial structure of metapopulations.

Young juveniles remain on the shores of breeding habitats for some time after transformation (Licht 1969), but nothing is known of the dispersal movements of metamorphs into the terrestrial habitat. Movements of older juveniles, until maturation several years later, are also unknown.

The potential of a rescue effect for Canadian populations due to dispersal from nearby U.S. populations is very limited. There are several records from near the U.S. border on the Lower Mainland but most of these date from before 1960 and it is unknown whether these populations still exist. Dispersal across the border from the US could potentially occur through the lowlands west from the Columbia Valley near Cultus Lake, but this area is highly fragmented and heavily modified by agriculture, residential developments, and roads. Some forested areas remain in the immediate vicinity of the border. Immediately east of the Columbia Valley, the high peaks of the Cascade Mountain Range pose barriers to dispersal.

 

Nutrition and Interspecific Interactions

The diet of the Red-legged Frog consists of a wide variety of small invertebrates. In marshes in southern British Columbia, the dominant prey items of adults and juveniles, in terms of percentage of stomachs where present, were spiders (Arachnida), beetles (Carabidae, Staphylinidae, Chrysomelidae, and Curculionidae), leaf hoppers (Cicadellidae), damsel bugs (Nabidae), and minute moss beetles (Limnebiidae) (Licht 1986). Adult flies (Muscidae) and fly larvae of the family Syrphidae were also numerically abundant in some stomachs. Newly metamorphosed frogs had consumed spittlebugs (Cercopidae), spiders, leafhoppers, slugs, larvae of syrphid flies, and various other small prey. Metamorphosed frogs foraged in the riparian habitat, typically very close (within 1 m) to the water’s edge, but moved further into the terrestrial habitat during and after rains, foraging in patches of dense vegetation and along margins of rain-puddles.

Licht (1986) reported on the diet and foraging behaviour of adults and juveniles of the Red-legged Frog and Oregon Spotted Frog from marshes where the two species were syntopic. The Red-legged Frog foraged mostly on land on terrestrial prey, whereas the Oregon Spotted Frog commonly foraged in water and included a larger proportion of aquatic prey in its diet. The availability of food appeared not to be a factor limiting growth of either species. Barnett and Richardson (2002) found complex, indirect effects on the development of tadpoles of these two species in the presence and absence of a predator (dragonfly larvae) and each other in artificial pond experiments. Exposure of the  Red-legged tadpoles to the odonate predator or tadpoles of the Oregon Spotted Frog resulted in increased body size at metamorphosis; developmental time also decreased in the presence of Spotted Frog tadpoles. Tadpoles of both species exhibit avoidance behaviour when exposed to a caged odonate predator.

Tadpoles of the Red-legged Frog respond behaviourally to alarm cues from injured conspecifics and predators. They also alter their pattern of development in response to chemical cues of predators but in a complex way. Kiesecker et al. (2002) found that tadpoles exposed to predators fed a diet of conspecific tadpoles metamorphosed earlier and at smaller body size than did control tadpoles.

Tadpoles of the Red-legged Frog feed largely on filamentous green algae. Experiments in enclosures indicated that feeding by tadpoles of this species altered both the composition and abundance of periphyton (Dickman 1968). Dickman (1968) suggested that feeding by tadpoles might initiate seasonal succession of periphyton in water bodies, which in turn could result in widespread effects within the food-web.

 

Behaviour and Adaptability

Because males call from under water, even large breeding concentrations may remain undetected, unless special techniques such as hydrophones or snorkelling are used. Adults show fidelity to particular breeding sites and may attempt to return to them across modified landscapes where the risk of mortality is high, such as across busy roads. Where sufficient forest cover remains, these frogs can be found near human habitations and in backyard pools. Their ability to use a variety of habitats for breeding and other seasonal activities facilitates their occupancy of human-modified landscapes. However, their tolerance limits and exact spatial requirements, particularly in terrestrial habitats, are largely unknown.