SPECIES ACCOUNT. Pickerel frog / Grenouille des marais (Lithobates palustris)
Department of Biology, Queen’s University, Kingston, ON Canada K7L 3N6
Taxonomy: Class: Amphibia. Order: Anura. Family: Ranidae. Genus: Lithobates. Species: Lithobates palustris (LeConte, 1825). (Changed from: Rana palustris based on DNA evidence from Frost et al., 2006). Some contention exists regarding the validity of these systematic changes (see Wiens, 2007).
Description: Commonly mistaken for the northern leopard frog (Lithobates pipiens, formerly Rana pipiens), this species can be distinguished by the
presence of seven pairs of dark brown, rectangular dorsal blotches on the head and between the dorsolateral folds (Schaaf and Smith, 1970)- See Figure 1. The inner thighs of L. palustris are bright yellow to orange in colouration (Figure 2), which differs from L. pipiens with white inner thighs (Schaaf and Smith, 1970). Females are significantly larger than males on average. One study found that females averaged ~64 mm in snout-vent-length and ~23g in mass, while males averaged ~53mm in snout-vent-length and ~12g in mass, albeit in a Louisiana population (Hardy and Raymond, 1991). Resetarits and Aldridge (1988) showed almost no overlap in the body sizes of mature male and females within Missouri cave populations, such that females were much larger than males. This sexual dimorphism is consistent across the range of the pickerel frog; however some variation in body size between populations has been noted (Schaaf and Smith, 1970). Male pickerel frogs produce vocalizations during the breeding season from the water or shoreline vegetation to attract mates or repel other males (see the Ecology section for a description of call types). Recently it was demonstrated that L. palustris also produces vocalizations which are inaudible to the human ear while submerged (Given, 2005).
This unique form of acoustic communication has been suggested to occur in response to advertisement calls from conspecifics, interference from heterospecific choruses, physical disturbances or reduced vegetation cover (Given, 2008). Variation in the frequency of underwater vocalizations has been noted between populations, suggesting that environmental factors may influence this behaviour (Given, 2008). The pickerel frog has skin secretions consisting of peptides that are toxic to predators and irritating to humans (Basir and Conlon, 2003). More information regarding this defense mechanism can be found below. Pickerel frog tadpoles are greenish in colour, with black dots covering the body and tail. The tail crests are dark or clouded, and the underside is cream-coloured. Tadpoles can grow to be 76mm long and undergo metamorphosis at 70-80 days after hatching (Wright and Wright, 1949). Distinguishing between the tadpoles of pickerel frogs and leopard frogs is extremely difficult using gross morphology; however they can be identified by examining their tooth ridges (Wright and Wright, 1949).
Distribution: The range of the pickerel frog extends from northern Georgia to western Texas, then northward to Ontario, Québec and New Brunswick. Though they can be found in southern Canada, the majority of their range occurs in the United States. In Ontario (Figure 3) the pickerel frog has a patchy distribution, but can be found throughout the south of the province. Previously, pickerel frogs had been sighted as far north as Sudbury; however these occurrence records date back to the early 1980’s, which could be indicative of a range contraction. This species is widely distributed over its range but seldom locally abundant, giving an illusion of rarity (Harding, 1997). In the vicinity of the Queen’s University Biological Station, pickerel frogs have been captured at Telephone Bay, Beaver Marsh, SRB, and along the margins at Indian Lake, however they are much less common in this region than other members of the genus Lithobates (northern leopard frogs, American bullfrogs, green frogs and wood frogs).
Habitat: Adult and larval pickerel frogs can be found in permanent wetlands, marshes, stream corridors, and beaver ponds, usually with nearby grassy meadows which are important for foraging during the summer months (Cunningham et al., 2007). Typically, males emit advertisement calls from ponds that contain emergent vegetation; however recent evidence has shown that some populations have adapted to produce underwater vocalizations to cope with reduced vegetation cover (Given 2008). The French name “grenouille des marais” literally means frog of the marshes, and was given to the pickerel frog due to the fact that they can be found foraging in shallow grassy areas, and are more terrestrial than many other North American frogs. The English name “pickerel frog” is unrelated to the ecology of the species; rather it comes from the practice of using pickerel frogs for fishing, as they were commonly used to catch yellow pickerel (Sander vitreus; also known as Walleye). Pickerel frogs require permanent bodies of water for overwintering; adults spend the winter months buried in the mud and debris at the bottom. Water depth is particularly important, as the ponds must be deep enough to prevent complete freezing. Pickerel frogs sometimes use limestone caves as hibernacula (Resetarits, 1986). Though other North American anurans have been found in cave systems, this behaviour in L. palustris is unique in North American anurans in that members of this species may remain within caves from September to March; however there are few cave systems present within the Ontario range of the species (Resetarits, 1986). The pickerel frog, because it is a cave dweller that cannot complete its entire life cycle within a cave, is referred to as a trogloxene species (Resetarits and Aldridge, 1988).
Ecology: In the Great Lakes region, the pickerel frog breeding season typically begins in late April and lasts until mid-May (Wright and Wright, 1949). In southern areas however, breeding may begin as early as January (Saenz et al., 2006). Males form breeding assemblages and produce snore-like advertisement calls to attract females. In addition to advertisement calls, male pickerel frogs produce territorial vocalizations in response to the presence of nearby male competitors (Given, 2005). These calls are at a lower frequency than the advertisement call, and have been described as “snicker” and “growl” (Given, 2005). As in many other anurans, males engage in physical combat (wrestling) during territorial disputes. Males possess enlarged thumbs during the breeding season, which aid in grasping a female during amplexus (Wright and Wright, 1949). Pickerel frogs are considered prolonged breeders because breeding endures several months. This means that, unlike explosive breeding species (such as the wood frog, Lithobates sylvaticus), abiotic environmental factors (e.g. temperature and precipitation) have a stronger influence on chorus strength among nights (Saenz et al., 2006). Male pickerel frogs call when the ambient air temperature is between 10 – 22˚C (Saenz et al., 2006).
During mating, females deposit egg masses containing 2000-3000 eggs on submerged vegetation in the shallows (Wright and Wright, 1949). Several pairs of pickerel frogs often lay in the same area, which suggests that there may be a limited number of appropriate sites that females actively seek (Wright and Wright, 1949) or that aggregating the eggs is advantageous (e.g. by reducing the risk of predation). The eggs are brown to yellow and hatch within 11-21 days of laying (Fisher et al., 2007). Tadpoles feed by filtering plant matter out of the water, and may remain in the larval stage for up to 140 days (Alford, 1989). Though largely herbivorous, tadpoles will scavenge dead animal matter when available (Harding, 1997).
Adult pickerel frogs consume a wide variety of invertebrate prey, both terrestrial and aquatic; this includes insects, spiders, mollusks, and worms (Harding, 1997). Similar to the northern leopard frog, pickerel frogs display active foraging behaviour, and may travel long distances from water to locate prey. The skin secretions of the pickerel frog have been shown to deter many predators; however its toxicity is limited (Basir and Conlon, 2003). These secretions are stored in glands within the skin in the form of peptides, and are secreted in a holocrine manner following stress or injury through the rupture of cells (Basir and Conlon, 2003). The secretions possess antimicrobial properties, and thus their primary function may be anti-pathogenic as opposed to deterring predators (Basir et al., 2000). Snakes such as the garter snake (Thamnophis sirtalis), ribbon snake (Thamnophis sauritus) and northern water snake (Nerodia sipedon) which do prey on ranid frogs, appear to avoid eating pickerel frogs; however American bullfrogs (L. catesbeianus), green frogs (L. clamitans), and mink (Mustela vison) are not deterred by the skin toxins (Harding, 1997, Fisher et al., 2007). The use of pickerel frogs in fishing also suggests that predatory fish will not refrain from eating a pickerel frog, despite the skin secretions.
Hardy (1964) once suggested that a distinct subspecies of the pickerel frog (Lithobates palustris mansuetii) existed in the lower Mississippi valley and Coastal North Carolina; this was based on a smaller number of dorsal spots (often fused into continuous narrow stripes), darkened colouration on the venter, and melanophore stippling associated with the vomerine teeth (“black teeth”) within individuals of that region. Hardy’s suggestion prompted an extensive study by Schaff and Smith (1970) on the phenotypic variation throughout the range. The results suggested that much variation existed within the species, but the presence of individuals with intermediate patterning (particularly in cave systems) implied that L. palustris mansuetii is not phenotypically diagnostic (Schaff and Smith, 1970). Rather, geographical variation in water quality (dark swamps versus clear water) mediated selection for darker pigmentation as camouflage (Schaff and Smith, 1970). Further, Resetarits and Aldridge (1988) found no difference in body size or reproductive biology between cave dwellers and other members of the species, again suggesting that L. palustris manusetii should remain without subspecies designation.
As mentioned above, pickerel frogs inhabit caves within the Ozarks of western Illinois, Missouri, and Arkansas; central Kentucky and Tennessee; and south-central Mississippi (Schaff and Smith, 1970). They use these cave systems as thermal refugia in both winter and summer; however, the number of individuals present peaks in winter when outside conditions are most severe (Resetarits, 1986). A stable isotope analysis of the stomach contents of cave dwelling individuals revealed no important role of pickerel frogs as predators within caves. Indeed many individuals suspend feeding during their stay, relying on fat stores to survive the winter months (Fenolio et al., 2005). Winter mortality is higher in juveniles within cave populations; however cave use may provide benefits to survival which regular aquatic hibernation does not offer (Resetarits, 1986).
Conservation Status: Though the pickerel frog is currently not at risk globally or nationally, habitat destruction, modification, and fragmentation represent the greatest threats to the species (Hecnar and M’Closkey, 1996).
The following provides a summary of conservation status for the pickerel frog. This information is summarized from the Natural Heritage Information Centre (NHIC, http://nhic.mnr.gov.on.ca/nhic_.cfm):
GRANK (global rank across the entire range): G5 = globally secure – very common; demonstrably secure under present conditions. GRANK DATE: 1996-10-18. GRANKS are determined collectively by conservation data centres and scientific experts.
NRANK (national rank): N5 = nationally secure.
SRANK (provincial or sub-national level): S4 = common and apparently secure in Ontario; usually more than 100 occurrences in the province.
Ontario General Status: SECURE Ontario General Status Date: 01-Nov-99. For amphibians and reptiles, Sranks are based largely on the Ontario Herpetofaunal Summary project (Oldham and Weller 2000).
Committee on the Status of Endangered Wildlife in Canada (COSEWIC): Not at Risk (NAR), assessed in April, 1999.
Research Needs: As one of the least studied members of the genus Lithobates, the pickerel frog requires much basic research on its ecology and geographic variation. Wide-scale comparisons of colouration and morphology have been conducted in the past (Schaff and Smith, 1970); however, the conclusions from this research have not been expanded into further work. Schaff and Smith (1970) supposed that variation in head colouration between northern and southern populations may relate to the heat budget of the organism, yet this hypothesis remains untested. Studies examining the possible adaptive significance of colour and pattern in southern cave dwelling populations (e.g. Hardy, 1964) would be informative. Further research on how interactions of males are mediated by territorial and aquatic vocalizations should be conducted (e.g. Given, 2005, 2008). The mating system is believed to be driven by female choice via selection of calls; yet the preferences for particular calling aspects by females remain unknown. While the abiotic factors influencing chorusing behaviour have been quantified generally (see Saenz et al., 2006), the mechanisms by which males form choruses are not completely known. Cave use, unique in North America to this species, is not fully understood, and represents a significant stage in the life history of southern populations of pickerel frogs (Resetarits, 1986; Resetarits and Aldridge, 1988; Fenolio et al., 2005). The skin secretions of the pickerel frog, and their composition and function in pathogen or predator defense merit further study (e.g. Basir and Conlon, 2003). Its widespread, yet patchy distribution, consisting of geographically isolated populations, suggests that a detailed, range-wide study of the genetic population structure of L. palustris populations would be fruitful. Within Ontario, additional monitoring and inventory of population sizes would be useful in determining if this species is in decline. Considering the distribution of L. palustris within the province, and the rarity of the species, more research needs to be conducted to fully understand its place in the ecosystems of Ontario.
Literature and Further Reading:
- Alford, R.A. 1989. Competition between larval Rana palustris and Bufo americanus not influenced by variation in reproductive phenology. Copeia 1989: 993-1000.
- Babbit, K.J., M.J. Baber, and T.L. Tarr. 2003. Patterns of larval amphibian distribution along a wetland hydroperiod gradient. Can. J. Zool. 81: 1539-1552.
- Basir, Y.J. and J.M. Conlon. 2003. Peptidomic analysis of the skin secretions of the pickerel frog Rana palustris identifies six novel families of structurally-related peptides. Peptides 24: 379-383.
- Cunningham, J.M., A.J.K. Calhoun and W.E. Glanz. 2007. Pond-breeding amphibian species richness and habitat selection in a beaver-modified landscape. J. Wild. Manage. 71: 2517-2526.
- De Solla, S.R., K.J. Fernie, G.C. Barrett, and C.A. Bishop. 2006. Population trends and calling phenology of anuran populations surveyed in Ontario estimated using acoustic surveys. Biodivers. Conserv. 15: 3481-3497.
- Fenolio, D.B., G.O. Graening and J.F. Stout. 2005. Seasonal movement patterns of Pickerel Frog (Rana palustris) in an Ozark cave and trophic implications supported by stable isotope evidence. Southwest. Nat. 50: 385-389.
- Fisher, C., A. Joynt and R.J. Brooks. Reptiles and Amphibians of Canada. Lone Pine Publishing. Edmonton, AB.
- Frost, D. R., T. Grant, J. Faivovich, R.H. Bain, A. Haas, C.F.B. Haddad, R.O. De Sa, A. Channing, M. Wilkinson, S.C. Donnellan, C.J. Raxworthy, J.A. Campbell, B.L. Blotto, P. Moler, R.C. Drewes, R.A. Nussbaum, J.D. Lynch, D.M. Green and W.C. Wheeler. 2006. The amphibian tree of life. Bull. Am. Mus. Nat. Hist. 297: 8-370.
- Given, M.F. 2005. Vocalizations and reproductive behavior of male Pickerel Frog, Rana palustris. J. Herpetol. 39: 223-233.
- Given, M.F. 2008. Does physical or acoustical disturbance cause male pickerel frogs, Rana palustris, to vocalize underwater? Amphibia-Reptilia 29: 177-184.
- Harding, J. 1997. Amphibians and Reptiles of the Great Lakes Region. Univ. Michigan Press. Ann Arbor, MI 160-161.
- Hardy, J.D., Jr. 1964. A new frog, Rana palustris mansuetii, subsp. nov. from the Atlantic Coastal Plain. Chesapeake Sci. 5:91-100.
- Hardy, L.M. and L.R. Raymond. 1991. Observations on the activity of the Pickerel Frog, Rana palustris (Anura: Ranidae), in Northern Louisiana. J. Herpetol. 25: 220-222.
- Hecnar, S.J. and R.T. M’Closkey. 1996. Regional dynamics and the status of amphibians. Ecology 77: 2091-2097.
- Oldham, M.J. and W.F. Weller. 2000. Ontario Herpetofaunal Atlas. Natural Heritage Information Centre, Ontario Ministry of Natural Resources. http://www.mnr.gov.on.ca/MNR/nhic/herps/ohs.html (updated 15-01-2001).
- Resetarits, W.J. Jr. 1986. Ecology of cave use by the frog, Rana palustris. Am. Midl. Nat. 116: 156-166.
- Resetarits, W.J. Jr. and R.D. Aldridge. 1988. Reproductive biology of a cave-associated population of the frog Rana palustris. Can. J. Zool. 66: 329-333.
- Saenz, D., L.A. Fitzgerald, K.A. Baum, and R.N. Conner. 2006. Abiotic correlates of anuran calling phrenology: the importance of rain, temperature and season. Herpetol. Monogr. 20: 64-82.
- Schaff, R.T. Jr. and P.W. Smith. 1970. Geographic variation in the Pickerel Frog. Herpetologica 26: 240-254.
- Wiens, J.J. 2007. Review: The Amphibian Tree of Life. Q. Rev. Biol. 82: 55-56
- Wright, A.H. and A.A. Wright. 1946. Handbook of Frogs and Toads of the United States and Canada. 3rd ed. Comstock Publishing Co. Ithaca, NY.
Reviewers: G. Blouin-Demers (Univ. Ottawa) and M. Oldham (Natural Heritage Information Centre)