Rana aurora: Red-Legged Frog

Shashank Sharma

Rana aurora was first described from the Puget Sound region in 1852 by Baird and Girard. Rana aurora consists of twosubspecies: R. aurora aurora, the Northern Red-legged frog and R. aurora draytonii, the California Red-legged Frog.

Rana aurora aurora is the only subspecies found in the Pacific Northwest. Its distribution ranges from Southwest British Columbia through most of Oregon and Washington west of the Cascades. Its geographic distribution extends from sea level to 3800 ft, but is usually found below 2000 ft. (Brown, 1975; Dunlap, 1955; Leonard et al., 1993; Porter, 1961). Rana aurora is well adapted for the cool and wet climate of western Pacific Northwest. They prefer cool densely covered land areas to streams and ponds and usually occur in woodlands next to streams. They have been found up to 200-300 meters from the nearest water source and can be frequently found on roads during rainy nights. (Nussbaum et al., 1983; Porter, 1961). During the warm an dry summers, some Red-legged frogs move closer to the water sources (Nussbaum, 1983). Rana aurora are medium sized frogs among the ranid frogs. The females can reach a SVL of 100 mm (4 inches) and males about 70 mm (2 ¾ inches) (Nussbaum et al., 1983; Leonard et al., 1993). They are reddish brown with small black spots across the back and the side. The undersides of the hind legs are a pinkish-red color caused by a pigment that seems to be deep within the skin. As the frog grows in age, the red color can spread to the belly and sides (Leonard et al., 1993). The groin is covered with interspersed yellowish green and black patches. Compared to most ranid species, the hind legs are long and the toe webbing is reduced (Leonard et al., 1993). The skin is usually smooth and the eyes are oriented laterally (to the sides).

Rana aurora is a species that is very well adapted to breed in the cool waters of the Pacific Northwest. The breeding behavior, characteristics of eggs, the low temperature tolerance, and the late stage of hatching are very advantageous for the survival of the embryos in cool waters (Licht, 1971). Rana aurora breeds very early in the year, usually in late February and early March but has been observed breeding as early as late January (Storm, 1960). The breeding season is short, usually lasting less than two weeks and breeding begins soon after ice melts from the spawning areas (Licht, 1971; Storm, 1960). The breeding behavior of R. aurora is controlled by a number of environmental factors. Breeding activity is first initiated by moderate to heavy rains (Licht, 1971; Storm, 1960). Breeding will not take place unless the air temperature has been at least 5C for several days. If the air temperature falls below 5C after the breeding activities have been initiated, the frogs again become inactive (Licht, 1969b). Another limitation is the water temperature. R. aurora will not mate until the water temperature has been around 6C for several days. In regular season, adult auroras are inactive at temperatures below 10C. Yet in mating season, they are active at 6C. This is possible because the hormonal levels during breeding season are high enough to allow activity in the cool water (Licht, 1969b). Breeding activities usually take place in permanent pools but can occasionally occur in temporary ponds (Brown, 1975).

Licht’s (1969b) study sheds light on the breeding behavior of R. aurora. Following the precipitation and temperature discussed above, male auroras become active from “hibernation”. Immediately after emergence, males begin to migrate to the breeding site. The movement usually occurs at night, under precipitation. Males arrive first and remain hidden during daytime without feeding. Vocalization begins one week later. In the first “silent” week, other males migrate to the breeding site. The absence of vocal cues from other males indicates that olfactory and visual cues may be important in migrating to the breeding pond. Apparently vocalization does not signal the readiness for mating. In the first “silent” week, many male auroras are eager and ready to mate as they have been observed in amplexus with many objects such as Ambystoma gracile or even an apple. Chorusing is initiated by warmer temperatures as well. Males call while submerged in water and can be found calling at the depth of 5 ft. They are at least 3 ft from the edge of the water. Males call while motionless on the substrate. The call is very low in volume and can not be heard farther than 5 ft away. No sound of the male can be detected in air. Licht (1969b) could only detect the calling sounds in the water with the aid of a hydrophone. Furthermore, the male frogs calls are usually covered by the louder calls of other species such as the Bullfrog or the Pacific Tree Frog. These conditions lead one to believe that vocal cues are not used by the migrating female auroras. Instead, females probably rely on olfactory and visual cues like male auroras to migrate to the breeding site. The function of calling underwater may be a species specific isolating mechanism. Males call only during the night and the early morning. Any movement near a calling male stimulates him to call more often, sometimes resulting in nonstop calling.

Rana aurora show highly amplectic behavior. Male auroras are so eager to form amplexus that they will attempt amplexus with other males. When this happens, the males will emit release calls and separate (Licht, 1969b). In amplexus, a male will tightly clasp the female in an axillary position. While clasping, the male vocalizes a very low intensity note once every second directly into the female’s ear. This note is so low intensity that it can only be heard by the amplectic female. This may be done so that nearby males don’t hear the note and try to break up the amplectic pair. When females try to release from amplexus, the males use a different vocalization similar to the release call of a clasped male. This may be done to deceive the nearby males that two males are attempting amplexus instead of a male and female. If deceived, the nearby males won’t jump on the amplectic pair and try to break them up. Unreceptive females can become free of amplexus within a minute by continually rolling over and emitting release calls. Persistent males can hold on as long as 15 minutes (Licht, 1969b).

The release behavior of an unreceptive aurora female is more extensive and elaborate than that of any other anuran species studied (Licht, 1969b). Females of most anuran species can attain release by uttering a release call accompanied by abdominal vibrations. In R. aurora, this behavior is not enough to get rid of an amplectic male. In addition to the release calls and abdominal vibrations, female auroras continually roll over. If this doesn’t work, they extend their back legs and roll to one side with body and limbs outstretched and stiff. Female R. aurora depend almost completely on tactile stimulus (rolling over) rather than auditory stimuli (release calls) for release (Licht 1969a). Licht (1969b) proposed that the elaborate release behavior of aurora females may have evolved because females may reach the breeding site before ovulation. Females migrate to the breeding site from different distances. Some arrive at the site before they are receptive. If a preovulating female is clasped too soon, she must be able to release herself since amplectic pairs are more vulnerable to predation. Female release behavior is most effective in shallow water. This can be explained by the fact that unreceptive females tend to stay in shallow water or by the shore and only ovulating females go to the deep water where mating occurs.

Male R. aurora amplex more aggressively than males of other Rana. They are unique among North American Rana species for amplectic behavior before vocalization. This may be because the R. aurora breeding season lasts less than two weeks whereas the breeding season of other ranid species extends for one month or more (Licht, 1969a).

Male R. aurora may or may not be territorial. Calling males are spaced several feet apart and always surface directly above their calling spot (Licht, 1969a). Furthermore, the amplectic call may be a territorial call since it is vocalized even with receptive females.

Mating occurs underwater and the female starts depositing the eggs soon after. Eggs will not be deposited unless the water is at least 7C (Licht 1969b; Storm, 1960). The clutch size varies from 550-1100 and variation in egg number is possibly due to the size and age of the female (Storm, 1960). All eggs are laid within two weeks of the first egg deposition. Females spawn only at night and attach their eggs to submerged vegetation. Eggs are placed in shallow quiet water with little or no flow and in areas exposed to sunlight for most of the day. The water must be at least 12 inches deep and the eggs are placed at least 3 ft. from the edges (Licht, 1969b). Water utilized for breeding must last until the beginning of June to avoid stranding the tadpole (Storm, 1960). Egg masses are usually placed two ft apart but are laid in the same general vicinity. The eggs are usually laid in the area where the fertilizing male had vocalized.

The time it takes the eggs to hatch is dependent on water temperature. Under normal field conditions, the eggs hatch in approximately 42 days. In the lab under a constant temperature of 65F, eggs hatched in 8.5-9 days (Licht, 1969b). This may be an evolutionary adaptation. As the water temperature increases, the likelihood that the pond will dry out also increases. Therefore, the embryos need to hatch earlier, metamorphose and leave the pond before it dries out (Licht, 1969b).

Rana aurora may be adapted to low and fluctuating temperature of the Pacific Northwest. Since the eggs are deposited early in the year, they are exposed to freezing temperatures at night. However, the eggs are relatively warm during the day. Rana aurora embryos tolerate very low temperatures (4-21 C) (Licht, 1971). Both the minimum and the maximum limits are the lowest for any North American ranid. Furthermore, the embryos can survive at least an eight hour exposure to temperature as low as 1C. Since the eggs are submerged and attached to a substrate, they are well shielded from surface temperature fluctuations. Even if an egg mass breaks off the substrate and floats to the surface, it does not happen until the embryos have developed through several stage and can withstand higher temperature fluctuations (Licht, 1971). Since the eggs are laid at night, the embryos will have an opportunity to develop through several stages and are better able to withstand the warmer water temperature of the next day. Licht (1971) showed that embryos in stage 4 could tolerate 20C but not 23C whereas the embryos in stage 9 can survive at 23C.

The hatching time of R. aurora is longer than almost all other North American ranids. Longer hatching time means that embryos hatch at a later developmental stage. Since auroras breed in permanent pools in wet humid regions, it is advantageous for the embryos to develop to an advanced stage before hatching. Larger larvae will be eaten by fewer predators, can escape from predators more successfully, and can better survive extreme temperature fluctuations (Licht, 1971). A longer embryonic period means that the eggs must be larger to hold a large yolk supply. As expected, the eggs of R. aurora are much larger than the eggs of any other Rana (Licht, 1971). The large eggs are well adapted for the cool waters of the Northwest. Since embryos hatch in cool deep water, they require more yolk for energy to initiate swimming and feeding. Similarly, the oxygen demands of the large eggs can be supported since they are deposited in cool deep water which is enriched with oxygen (Licht, 1971).

After hatching, tadpoles remain crowded near the jelly mass for 1-2 days, feeding off the yolk (Storm, 1960). Tadpoles range from 8.8-10.3 mm at hatching and are black with golden guanophores scattered through most of the body (Brown, 1975; Storm, 1960). The tail tip is symmetrically rounded. The mouth opening is small and the gills are faintly evident. Development is slow for the first 2 weeks. Slowly eyes begin to form but the mouth parts are very slow to develop (Brown, 1975). The significance of the slow development of the mouth parts is not known.

Like slow embryonic development, the developmental rate for R. aurora tadpoles is slow. Brown (1975) observed that aurora tadpoles grow at .62-.99 mm a day and require approximately 110 days to metamorphose. Body length at metamorphosis is approximately 29 mm. Rana aurora draytonii tadpoles required an even longer time to metamorphose, 5-7 months, and metamorphosed at 18-30 mm (Rathbun et al., 1997). The slow rate of development and therefore the larger size at metamorphosis is advantageous because aurora tadpoles inhabit permanent pools in wet humid regions. The large size is advantageous in that as a frog gets larger, the ratio of surface area to volume decreases. During dry summer periods, the large frogs will be at an advantage since they have a low surface area to volume ratio and will lose less water.

Recently metamorphosed frogs have light sides with black spottings and a pale flesh color on the undersides of the hind legs (Storm, 1960). Growth in recently metamorphosed frogs occurs mainly during the rainy season. Frogs usually reach sexual maturity in 2-3 years after metamorphosis.

Wiens (1970) showed that the R. aurora tadpoles show a preference for a striped patterned substrate type. This may be because R. aurora usually breeds in shallow ponds which are filled with linear structures that cast linear shadows causing a striped background. These striped areas are usually interspersed with open areas of uniform substrate (Storm, 1960). It is clearly beneficial for the tadpoles to remain hidden in the vegetated striped habitat rather than the open areas where they could be spotted easily.

Despite the adaptations to the Northwest habitat, the mortality of R. aurora is high. There is relatively low mortality of the eggs. Since eggs are laid in deep water, which is not likely to recede, they are protected from extreme temperature fluctuations (Calef, 1973; Licht, 1974). Some eggs will be lost to predators and very few to freezing. Almost all embryos hatch into tadpoles. Mortality is most intense during the larval stage (Calef 1973; Licht, 1974). The mortality of tadpoles has two phases: a rapid decline during the first 4 weeks followed by a less rapid decline until approximately 5% of the original population remains at metamorphosis. Most of tadpole mortality is believed to be caused by predation (Calef, 1973; Licht, 1974). Tadpoles are usually not affected by disease. Food shortage is a very unlikely cause of death since tadpoles feed on all kinds of detritus, rotting organic material, and many types of algae. Furthermore, in the lab tadpoles survived several weeks without being fed and some even recovered from a short period of severe starvation (Licht, 1974). Most tadpoles living in densities 100 times normal density survived and metamorphosed in the absence of a predator.

Some of the primary predators of aurora tadpoles are salamander larvae of Ambystoma gracile and Taricha granulosa, Notonecta, Lethocerus, Dytiscus, dragonfly larvae, leeches, and garter snakes (Licht 1974; Calef, 1973). Mortality is most intense shortly after the tadpoles hatch because they tend to cluster around the egg masses to consume yolk and therefore make themselves highly visible to predators. At this time, they are not actively swimming and cannot escape from predators (Calef, 1973; Licht, 1974). As the tadpoles grow, their chances of survival increase. Predation of tadpoles is much lower at low densities (Calef, 1973).

Recently metamorphosed auroras are terrestrial and semi aquatic. They usually remain in or along the river and go out in the woods only on rainy nights (Calef, 1973; Licht, 1974). This restriction to the banks is the main cause for the mortality of young frogs.

By the third year when aurora are sexually mature, there are less than 1% frogs alive from the number of eggs deposited (Licht, 1974). In this sense, R. aurora follows a type III, or concave, survivorship curve. The large numbers of eggs laid by aurora compensates for the heavy predation. R. aurora are true opportunists. They produce enough eggs so that some will successfully hatch, metamorphose and survive to sexual maturity to breed (Licht, 1974).

Due to the heavy predation, R. aurora has developed a predator avoidance strategy. The strategy is to remain perfectly still to avoid revealing its presence to the predator. When the predator approaches too closely, the frog jumps away almost always in water, and almost never inland seeking cover (Gregory, 1979). The inconspicuous coloration may help the frog to escape before a predator can react. The water escape route is so preferred that most frogs face the water when sitting on shore (Gregory, 1979). However, Licht (1971) reported that most frogs escaped by seeking cover under vegetation and some that did jump in the water quickly returned to shore. Perhaps Lichts results were different because of the presence of large cut throat trout in the water in that study. It seems advantageous for frogs to escape by jumping in the water since frogs are better swimmers than most of the terrestrial predators (Gregory, 1979).

Along with predator avoidance behavior, R. aurora have evolved certain characteristics to reduce mortality caused by environmental factors. R. aurora eggs seem to be resistant to low level UV-B radiation (290-320 nm) and have the ability to repair UV damaged DNA in eggs (Blaustein et al., 1996; Ovaska et al., 1997). Photoreactivation is the most important mechanism for repair of UV damaged DNA and R. aurora have very high levels of photoreactivation enzyme, photolyase, activity. But, the egg and larval survival greatly decline as UV-B radiation level is increased. In 15% enhanced UV-B radiation, the egg survival went down to 56% as compared with 89.8% survival under normal conditions (Ovaska et al., 1997). The depletion of the ozone layer will cause an increase in the levels of UV-B radiation on Earth which will decrease the hatching success and larval survival of R. aurora.

The herbicide diuron also adversely affects R. aurora. Exposure to diuron concentrations of 7.6 g/mL or greater for 14 days or more can cause hindlimb bud and forelimb developmental retardation (Scheytema et al., 1997). Diuron works to increase the time for limb development in R. aurora tadpoles. Mortality, deformity, and growth inhibition in embryos can often occur at concentrations much less than those that affect the adult auroras. This is a serious danger since longer developmental time means the pool can dry up, leaving the tadpoles stranded. Long term diuron exposure can cause reproductive stress leading to elimination of R. aurora (Schuytema et al., 1997).

Another threat to the R. aurora population comes from the introduced species of Rana catesbeiana, the Bullfrog. The disappearance of R. aurora from its traditional habitat is attributed mainly to predation and competition from R. catesbeiana. R. aurora may be susceptible to predation by R. catesbeiana because it does not recognize the introduced species as a predator and therefore doesn’t execute antipredatory behavior (Hayes et al., 1986; Moyle, 1973). Both adult and tadpoles of R. catesbeiana are highly aquatic and are known to prey on the tadpoles of R. aurora (Kiesecker et al., 1997). Bullfrogs have ecological requirements similar to R. aurora but can get larger, outcompete, and prey on R. aurora. The Bullfrog larvae may transmit pathogens, stress aurora larvae through chemical interference, or alter food resources (Kiesecker et al., 1997). The recent listing of R. a. draytonii as “threatened” emphasizes the effect of Bullfrog on the R. aurora population.

Alternative hypothesis concerning the reduction of the aurora subspecies have been put out. One of suggests that the overharvesting of R. a. draytonii by the frogging industry in the 1800s is partially responsible for its decrease and the introduction of the Bullfrog. Besides commercial exploitation, R. a. draytonii suffered from habitat alteration. This theory suggests that Bullfrogs were introduced into the state as the stock of R. a. draytonii became depleted. The open niche left by the depletion of the draytonii helped bullfrogs establish themselves (Jennings et al., 1985). Other possibilities are predation by introduced fishes, pathogens and parasites, acid rain, and catastrophic mortality. Declines in the population of R. aurora are most likely caused by the interplay of all these factors.

The phylogenetic relations of R. aurora are unclear. One study proposed R. aurora and R. sylvatica as sister species (Dumas, 1966). Another study proposed R. aurora and R. temporaria as sister species (Wallace et al., 1973). Dumas (1966) study of electrophoretic separation of blood proteins and precipitin rings indicated that R. aurora and R. sylvatica may even be sister species. He goes on to suggest that the speciation of these two frogs and their distribution are the result of paleoclimatic changes during the last glacial advance. The common ancestor of R. aurora and R. sylvatica probably lived in the Pliocene. The uplift of the Cascade-Sierra chain in the Miocene and Pliocene may have resulted in speciation of R. sylvatica and R. aurora. According to Dumas (1966) R. aurora advanced Northward into the Puget Basin and Vancouver Island immediately after the last glacial advance, the Wisconsin stage, while the sea level was still depressed to allow land travel to Vancouver Island. The subspecies R. aurora draytonii was isolated in California by adverse Wisconsin conditions in the mountains of Northern California. Electrophoretic separations of the proteins of the blood sera show that R. aurora and R. sylvatica exhibit only a single alpha globulin fraction whereas other Rana species exhibit two.

Comparisons of blood serum albumins provide a measure of the degree of amino acid sequence difference and show that R. aurora and R. temporaria may be sister taxa, indicating that the R. aurora and R. temporaria lineage split 33-37 mya (Wallace et al., 1973).

Rana aurora is a frog that is native to the Pacific Northwest. If we don’t take measures to protect it, it will cease to exist in the Northwest.

References:

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Brown, H.A. 1975. Reproduction and Development of the Red-Legged Frog, Rana aurora, in Northwestern Washington. Northwest Science 49: 241-252.

Calef, G.W. 1973. Natural mortality of tadpoles in a population of Rana aurora. Ecology 54: 741:758.

Dumas, P.C. 1966. Studies of the Rana species complex in the Pacific Northwest. Copeia 1966(1): 60-74.

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