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Northern Leopard Frog (Rana Pipiens)

Introduction

Rana pipiens, the Northern Leopard frog or grenouille léopard (Fig. 1) is a member of the family Ranidae – or "true frogs" – which is composed of 46 genera and approximately 560 species (Hillis and Davis, 1986). All North American members of the family belong to the genus Rana. The species name "pipiens" is believed to come from the observation of an early collector who heard a bird-like peeping when he collected R. pipiens and assumed the call went with the frog (Pace, 1974). In all likelihood, the call was a different frog, Pseudacris crucifer (Spring Peeper). Other common names for R. pipiens include the meadow frog, grass frog and laboratory frog (Breckenridge, 1944).

Figure 1.  Northern Leopard frog, Rana pipiens (David M. Green photo).

Figure 1.  Northern Leopard frog, Rana pipiens (David M. Green photo).

Rana pipiensfrom North and Central America was once considered to be one wide-ranging species with considerable geographic variation (Moore, 1944). The complex was later recognized to be composed of several species, based primarily on differences in call structure (Littlejohn and Oldham, 1968; Pace, 1974) and morphology (e.g. Post and Pettus, 1966, in Hillis et al., 1983; Pace, 1974). Rana pipiens is the only member of the complex found in Canada. The species complex is commonly divided into four groups: 1) R. areolata group (Crawfish Frogs); 2) R. berlandieri group (Rio Grande Leopard Frogs); 3) R. montezumae group, including other Mexican species; and 4) R. pipiens group (Northern Leopard Frogs; Frost, 1985; Hillis and Frost, 1985, both in Hillis and Davis, 1986). A phylogenetic tree based upon ribosomal DNA for 32 species of Rana suggests (Hillis and Davis, 1986) that R. pipiens’ closest relatives are R. magnaocularis, R. palustris (Pickerel Frog) and R. sphenocephala (Southern Leopard Frog).

Rana pipiens is a medium-sized, semi-terrestrial frog characterized by conspicuous dark dorsal spots bordered with light coloured rings. It has a whitish belly and prominent light coloured dorsolateral folds. Rana pipiens is polymorphic for the background colour of the dorsum. It is commonly green, but may be brown. Background colour is inherited through a simple Mendelian system of two alleles at one locus with green dominant to brown; dorsal coloration is not sex-linked (Fogleman et al., 1980). The polymorphism has been recognized for over 100 years (Cope, 1889, in Corn, 1981). The brown morph can make up 2-68% of a given population (Corn, 1981; Schueler, 1982; Seburn et al., 1997). In general, green frogs appear to be more common in forested areas and brown in areas of extensive marsh and lakes (Schueler, 1982). Dark spotting appears to be more extensive in warmer, moister climates as compared with cooler, drier climates and may result from selection for crypsis by matching backgrounds (Ibid.). Two rarer colour morphs, “burnsi” and “kandiyohi”, were originally thought to be separate species (Weed, 1922; Merrell, 1972, in Schueler, 1982). Burnsi R. pipiens have few or no spots on their dorsum, while kandiyohi have dark interspot reticulations. The two variants are dominant to the normal R. pipiens pattern (Moore, 1942; Volpe, 1955). Burnsi and kandiyohi morph frequencies can be as high as 10% in some populations but are generally less than 5% (Merrell, 1970). The distribution of both forms appears to be centered in Minnesota and confined to recently glaciated areas (Ibid.), although speckled R. pipiens have been reported from Manitoba (Browder, 1968).

Females are larger than males with adults ranging from 50-100 mm svl. Maximum known size is 111 mm svl (Conant and Collins, 1991). Like most anurans R. pipiens is sexually dimorphic for forelimb musculature (Yekta and Blackburn, 1992). Although females are larger, most of the forelimb muscles are significantly heavier in males, particularly those involved in clasping the female during amplexus.

Despite the wide geographic range of R. pipiens in Canada there have been few in-depth studies of Canadian populations. The largest geographic study examined variation in skin pigmentation across Canada (Schueler, 1982). Other studies have been conducted in Nova Scotia (Gilhen, 1984), New Brunswick (McAlpine and Dilworth, 1989), Quebec (Leclair and Castanet, 1987; Leclair, 1990; Gilbert et al., 1994), Ontario (Emery et al., 1972; Cunjak, 1986; Licht, 1991; Pope, 1996), Manitoba (Eddy, 1976), and Alberta (Seburn et al., 1997).