Animal Field Guide

Northern Leopard Frog

Rana pipiens
(Ranidae)

Montana Species of Concern
Global Rank: G5
State Rank: S1S3

Agency Status
USFWS:
USFS: SENSITIVE
BLM: SENSITIVE

The backs of adult northern leopard frogs and juveniles are a green or brown base color (rarely light bluish) covered with large, oval dark spots, regular in outline, each of which is surrounded by a lighter halo or border. Ventral color is white to cream, with some pinkish patches on the feet. The skin is smooth, the dorsolateral folds are not inset toward the midline on the rump, the tympanum (eardrum) usually lacks a distinct light spot, and the hind toes have extensive webbing. Snout-vent length is 1.8 to 11.0 centimeters. The breeding call of males is a snoring sound lasting 2 to 3 seconds followed by a series of 2 to 3 stuttering croaks or chuckles.

Larvae (tadpoles) are dark brown to olive or gray on the back with a flecking of light gold and black, more concentrated on the sides, and then merging with a silvery-white or transparent belly. Tail length is less than 1.5 times the body length, the dorsal tail fin begins anterior to the tail musculature when viewed from the side. The anus is on the right side in front of the fin, not on the midline. The eyes fall within the outline of the head when viewed from above. Lateral oral papillae are strongly indented toward the corners of the mouth, and the lower mandible is noticeably thicker than the upper. The total length of tadpoles is 5.5 to 10.0 centimeters. The eggs are black above and white below, and are laid in large (orange- to grapefruit-sized) somewhat flattened globular masses; total diameter of individual eggs (including the two jelly layers) is less than 6.0 millimeters. Masses are usually attached to submerged vegetation.

Adult and juvenile northern leopard frogs differ from other Montana ranids by the dorsal spotting of dark ovals with light halos, rather than irregular dark patches with light center spots (Columbia spotted frog) or general absence of spotting (bullfrog); the presence of a lateral yellowish stripe on the side of the snout (absent in bullfrog); and lack of reddish coloration on the belly and undersides of the legs (present in Columbia spotted frog).

Leopard frog tadpoles have tails less than twice the length of the body, and lack the large, black flecks on the body and the metallic coppery sheen on the belly, all of which are present in spotted frog tadpoles. Bullfrog tadpoles have bright to creamy-yellow bellies and perfectly round, black spots on the back and tail. Spotted frog eggs are twice the size of leopard frog eggs because of the thicker jelly layers, and spotted frog egg masses tend to be at or near the water's surface and not attached to vegetation. Bullfrog egg masses are spread out over the surface of the water or bottom of a pond rather than in a globular mass typical of leopard frog egg masses. In Montana, extant populations of northern leopard frog overlap Columbia spotted frogs and bullfrogs in very few locations. Leopard frogs are present mostly across the prairies of the eastern two-thirds of the state; spotted frogs and most bullfrog populations are in the mountainous western third.

No northern leopard frog information is available for Montana. In other locations, leopard frogs usually remain in relatively small seasonal home ranges, but may range several hundred meters or more between seasons in the upper Midwest. In Michigan, average nightly movement during rain was 36 meters, and as much as 800 meters. Individuals in Colorado have been documented moving at least 3 kilometers between years, and 8 kilometers between-year movements have been reported in the Cypress Hills, Alberta; young-of-the-year moved 2.1 kilometers between natal and breeding ponds in the Cypress Hills (Wagner 1997, Hammerson 1999).

Habitats used by northern leopard frog in Montana are similar to those reported for other regions, and include low elevation and valley bottom ponds, spillway ponds, beaver ponds, stock reservoirs, lakes, creeks, pools in intermittent streams, warm water springs, potholes, and marshes (Brunson and Demaree 1951, Mosimann and Rabb 1952, Black 1969, Miller 1978, Dood 1980, Reichel 1995, Hendricks and Reichel 1996, Hendricks 1999). There is no evidence that this species in Montana has ever occupied high elevation wetlands, in contrast to Wyoming and Colorado (Baxter and Stone 1985, Hammerson 1999).

More specifically, northern leopard frogs require a mosaic of habitats to meet annual requirements of all life stages. Generally separate sites are used for breeding and overwintering, but this may occur in the same pond in some cases. They occupy a variety of wetland habitats of relatively fresh water with moderate salinity, including springs, slow streams, marshes, bogs, ponds, canals, flood plains, beaver ponds, reservoirs, and lakes, usually in permanent water with rooted aquatic vegetation. Habitats are often with few or no trees, but in Alberta and Colorado forested areas may be used. In summer, adults and juveniles commonly feed in open or semi-open wet meadows and fields with shorter vegetation, usually near the margins of waterbodies, and seek cover underwater; taller, denser vegetation seems to be avoided.

Eggs are laid and larvae usually develop in shallow warm and still water, generally in areas well exposed to sunlight. Generally eggs are attached to vegetation just below the surface of the water. In northern Minnesota, successful reproduction in acidic bog water either does not occur or is a rare event (Karns 1992). During winter, northern leopard frogs usually are found inactive underwater on the bottom of deeper streams and ponds or springs that do not freeze to the bottom and are well oxygenated, sometimes under bottom rubble and debris, in water as deep as 85 centimeters (Baxter and Stone 1982, Nussbaum et al. 1983, Russell and Bauer 1993, Wagner 1997, Hammerson 1999).

Metamorphosed frogs eat various small invertebrates, including various insects, spiders, leeches, and snails obtained along the water's edge or in nearby meadows or fields. They rarely eat small vertebrates such as small frogs, fish, birds, and snakes, and are sometimes cannibalistic (Nussbaum et al. 1983, Russell and Bauer 1993, Wagner 1997). Larvae eat algae, plant tissue, organic debris, and probably some small invertebrates. In Montana, adults have been documented feeding on 10 orders of insects, spiders, mites, harvestmen, centipedes, millipedes, snails, and newly metamorphosed boreal toads (Miller 1978), but larval food habits have not been described.

Northern leopard frogs are active during the day and night. The active period extends from March to November in Colorado (Hammerson 1999). In Wyoming and the Pacific Northwest, adults emerge in March or April (Nussbaum et al. 1983, Baxter and Stone 1985, Russell and Bauer 1993) when water temperatures exceed 10 degrees C. In Montana, the active period of adults is reported to extend from mid-March to early October (Brunson and Demaree 1951, Roedel and Hendricks 1998, Hendricks 1999). In all cases, activity begins when ice melts. Predators of adults and juveniles include Great Blue Heron, Burrowing Owl, snakes (including garter snakes), some mammalian carnivores, and game fish. Tadpole predators include Pied-billed Grebe, tiger salamander, garter snakes, and bullfrog tadpoles (Nussbaum et al. 1983, Russell and Bauer 1993, Hammerson 1999). Predators in Montana have not been reported.

R. pipiens apparently out-competed R. pretiosa at low elevations in Montana (Black 1969). Differential tadpole mortality may be the primary mechanism of displacement of R. pretiosa by R. pipiens (Dumas 1966).

Information on reproduction in Montana is limited, and no detailed studies of the reproductive biology of any population have been conducted. Timing appears variable, and depends on the year and location. Calling males have been reported in April and May. Near Tiber Reservoir, in Toole and Liberty counties, females have been collected with relatively undeveloped eggs in mid-June and moderately developed to fully developed eggs in early and late July; recently transformed juveniles also were noted in late July (Mosimann and Rabb 1952). Eggs and tadpoles have been reported at breeding sites across eastern Montana during early April to late July, with a peak in May and June; sometimes tadpoles are observed in August and September (Reichel 1995, Hendricks and Reichel 1996, Hendricks 1999, Hossack et al. 2003). Recently metamorphosed juveniles with small tail stubs measured 2.6 to 3.4 centimeters snout-vent length.

In general, males gather at breeding sites of shallow, quiet water in spring and vocalize on warm sunny days (water temperatures of 14 to 23 degrees C.) while floating at the surface of the water. In favorable habitat, 20 to 25 or more males may gather in a 20 square meter area. Females begin laying eggs a few days after calling begins. The time of egg deposition varies with latitude and elevation. Egg deposition occurs typically in April in southern Quebec, New York, and the Great Lakes region, late April to late May farther north in Manitoba and Nova Scotia (Gilbert et al. 1994). In Colorado, eggs are laid mainly in late March or by mid-April at low elevations, and in May in the mountains (Corn and Livo 1989, Hammerson 1999). Breeding often peaks when water temperatures reach about 10 C. At a particular site, egg deposition generally occurs within a span of about 10 days. Egg masses include several hundred to several thousand ova; the clutch size of 68 Colorado egg masses was 645 to 6272 eggs (Corn and Livo 1989). The density of egg masses often reaches a few hundred per hectare in favorable habitat, sometimes more than 1000 per hectare, but is usually less than 100 in Colorado.

Eggs hatch in about 1 to 2 weeks; the larval (tadpole) period is about 10 to 12 weeks (58 to 105 days). Hatching may occur over several weeks at a single site. Recently metamorphosed juveniles appear in late June and early July at lower elevations, and in mid-July to September at higher elevations (Hammerson 1999). Size at metamorphosis is 2.1 to 3.6 centimeters snout-vent length. Aquatic larvae usually metamorphose in summer, but they may overwinter as tadpoles in some areas (Baxter and Stone 1985). Females are sexually mature usually in two years in most areas, three years in high elevation populations. Breeding males in Colorado are usually more than 5.0 centimeters snout-vent length, and breeding females more than 6.0 centimeters.

No special management needs are currently recognized for populations in eastern Montana. However, at permanent and semi-permanent water bodies (reservoirs and stock ponds) where breeding has been observed, portions of shorelines where emergent vegetation is present or might develop could be fenced to exclude access by livestock and thereby protect breeding adults, eggs and tadpoles from trampling and the removal of emergent cover by livestock. Another option would be the creation of ponds designed for use by prairie amphibians as breeding sites, with the perimeter surrounded by fencing to prevent access by livestock. Game fish should not be introduced to any of these ponds, nor should chemical fertilizers, pesticides and herbicides be used within 100 meters of the shoreline. All breeding sites west of the Continental Divide should be protected from livestock, and organic and chemical (pesticide and herbicide) contamination. Game fish and bullfrogs should not be introduced to these sites. Care should be taken to avoid introducing parasites and fungal, bacterial, and viral pathogens when monitoring these sites (see suggestions in Maxell 2000, Maxell et al. 2003). Any populations discovered in the western region should be reported to the Native Species Biologist of the Montana Department of Fish, Wildlife & Parks or the Program Zoologist of the Montana Natural Heritage Program.