Northern Leopard Frog
Rana pipiens
Summary: This is a relatively common frog in the Illinois and Wisconsin counties. Areas with dense herbaceous vegatation near marshes provide the best habitat, usually within former landscape-level savanna regions. In the Indiana counties observations are usually from the more open areas in the Kankakee drainage. In extensively forested parts of Indiana the species is generally rare.
Description: A sleek, medium-sized frog, 2.0 to 4.0 inches (50-100mm). Individual frogs with ground colors of tan, brown, and bright green are usually present in any given population. No matter what the ground color, rounded, oval or irregular dark brown spots with light borders are scattered over the dorsal and lateral surfaces; these may be in orderly rows, or arranged chaotically. Raised light dorsolateral folds are prominent. The underside is white. Breeding males have swollen thumbs and paired vocal sacs between the jaw and shoulder.
The only similar frogs are the pickerel frog, very rare in the Chicago region, and the plains leopard frog, known only from the extreme southwest of the region. The pickerel frog has bright yellow-orange flash colors under the hind legs, and usually two even rows of rectangular spots between the dorsolateral folds. The plains leopard frog usually has small round or oval dorsal spots with little or no light border. The standard reference for leopard frog taxonomy and variation is Pace (1974).
Distribution and Status: Northern leopard frogs are widespread and locally abundant in all of the northeastern Illinois counties, and in parts of Lake County, Indiana. In more heavily forested parts of Porter and LaPorte Counties the species becomes relatively rare and is sporadically distributed.
Northern leopard frogs have apparently declined in other parts of the range, and even in Illinois the species may exhibit cycles of relative abundance. In the 1970s observations of northern leopard frogs were relatively infrequent, but by the mid 1980s the species was once again relatively common.
Habitat: Northern leopard frogs are strongly associated with the presence of a dense herbaceous layer. Although they may occur in extensive prairie openings, at a landscape level they are a savanna species, at least within the Chicago region. In most of central Illinois and southwest across the great plains they are replaced by the plains leopard frog, a true grassland form. To the east, Rana pipiens becomes less common and is restricted to extensive wetland openings in the forest.
Breeding usually occurs in marshes or ponds which hold water into mid-summer. After the breeding season adults, and later juveniles, disperse into surrounding areas. Community types supporting large northern leopard frog populations include wet to mesic prairie, wet to mesic savanna, sedge meadow, and marsh. Populations of moderate size frequently are found around oxbow ponds in open floodplain forest. Small populations persist in some woodlands, especially around wetland openings, and in specialized habitats such as graminoid fen.
Northern leopard frogs are adaptable, and they will readily colonize restoration sites as long as emergent wetlands are present and holding water into mid-July most years, and surrounding areas with dense herbaceous vegetation are available. On several occasions small and isolated populations within degraded woodlands and savannas have recovered rapidly when buckthorn and other weedy shrubs and small trees are removed.
Voice: A low-pitched snore, with little carrying power; also a variable sound which can be imitated by rubbing a thumb over an inflated balloon. The call is difficult to hear much beyond the wetland margin.
Phenology: The earliest Chicago region observations of leopard frogs have been in March, but most breeding activity occurs in April. The large, globular to flattened egg masses are deposited in vegetated shallows, usually just below the surface of the water. The juveniles emerge from the ponds in mid-July, and disperse into surrounding areas.
Field Notes
July 29, 1984. Lake County IL, near the Wisconsin line. Along the Des Plaines River, many young Rana pipiens. Later at Illinois Beach State Park, more Rana pipiens near various ponds and marshes.
April 6, 1985. Lake County IL. Near Wadsworth, one freshly laid Rana pipiens egg mass in a shallow marsh.
April 26, 1989. McHenry County. Lots of Pseudacris triseriata and Rana pipiens calling from a large emergent marsh.
April 19, 1996. Lake County, IL. Wadsworth. Lots of Pseudacris triseriata and Rana pipiens calling from wetlands in open oak woodland. Crossed the railroad to a large marsh within wet prairie, large R.pipiens calling in marsh openings with sparse vegetation, photographed several. A few miles north, more of the same, P. triseriata and R.pipiens. To Zion, same two species calling.
May 8, 2000. Kane County IL. Near St. Charles, at a restored marsh. 80s, rain/thunderstorms starting just after dark. Bufo americanus, Pseudacris triseriata, Rana catesbeiana, Rana clamitans, Rana pipiens all calling.
July 15, 2000. Lake County IL. Degraded bur oak savanna near Grayslake, wooded groves interspersed with large weedy open fields and marshes. Numerous Rana pipiens juveniles around three wetlands, more than 100 frogs in all.
Literature Cited
Pace, A. E. 1974. Systematic and biological studies of the leopard frogs (Rana pipiens complex) of the United States. University of Michigan Museum of Zoology Misc. Publications 148:1-140.
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| Updated: October 9, 2007
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