The following account is modified from Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo (©2005 by the Regents of the University of California), used with permission of University of California Press. The book is available from UC Press.

Rana septentrionalis (Baird, 1854[b])
Mink Frog

Gary S. Casper¹

1. Historical versus Current Distribution. The southern limit of distribution of mink frogs (Rana septentrionalis), or "the frogs of the north,” is at the highest latitude of any North American anuran (43 ˚N; Hedeen, 1986). Historically, mink frogs were distributed from southern Labrador and the Maritime Provinces to Minnesota and southeastern Manitoba, south to northern New York and northern Wisconsin, with isolated colonies in northern Québec and northern Labrador. Hedeen (1977) reviews erroneous locality records. Western range limits are thought to be influenced by sufficient moisture (Hedeen, 1986). Hypotheses to explain the southern limits of the distribution include predation pressures from American bullfrogs (Rana catesbeiana; Moore, 1952; Bleakney, 1958; Schueler, 1975) and the limited tolerance of embryos to warmer water temperatures and consequent lower oxygen diffusion rates (Hedeen, 1986). No substantial changes in distribution have been reported, although global warming trends can be expected to have an effect in the future.

2. Historical versus Current Abundance. Mink frogs generally are considered to be locally common in suitable habitats, and range-wide abundance appears to be unchanging. Judge et al. (1999) performed a mark-recapture study in an Ontario population and found wide annual fluctuations in population estimates of mink frogs, American bullfrogs, and northern green frogs (Rana clamitans melanota). No directional trends could be discerned. Shirose and Brooks (1997) likewise detected no trend in a study spanning 9 yr in a central Ontario wilderness area, but noted marked short-term fluctuations in population sizes, with the percent of transforming individuals varying among years from 21.6–60.2% (mean = 38.4, s.d. = 17.2, among 6 yr). Mink frog density estimates in New Brunswick ranged from 0–88.8 frogs/100 m2 (McAlpine, 1997b). Mink frogs may benefit from utilizing aquatic habitat created by beaver (Castor canadensis) ponds, and recent increases in beaver may have increased breeding habitat availability in some areas (New Brunswick, McAlpine, 1997a; Apostle Islands, Wisconsin, personal observations, 1999).

3. Life History Features.

A. Breeding. Reproduction is aquatic.

i. Breeding migrations. Mink frogs do not have breeding migrations. Their entire life cycle is completed in relatively close proximity in aquatic shoreline habitats, typically in areas with high macrophyte abundance.

ii. Breeding habitat. Breeding takes place in permanent waters with a high abundance of aquatic macrophytes. Shortly after males begin calling, females arrive to mate and deposit egg masses on the submerged stems of macrophytes. The breeding period extends from the first week in June to the first week in August (Garnier, 1883; Wright and Wright, 1949; Bleakney, 1958; Hedeen, 1971a, 1972c). In Minnesota, male choruses run from late May to early August, calling males are not restricted to fixed areas, and males spend more time at breeding sites than females (Hedeen, 1972c; Hunter et al., 1992).

B. Eggs.

i. Egg deposition sites. The globular egg mass is attached to stems of submerged vegetation, often ≥ 1 m below the water surface (Vogt, 1981), in permanent aquatic habitats such as rivers, lakes, and ponds (Hedeen, 1971a; Hedeen, 1986). The subsurface placement of the egg mass avoids subsequent freezing of surface waters in northern latitudes, which would kill eggs (Moore, 1940; Herreid and Kinney, 1967). Egg masses may detach from vegetation and drop to the bottom of the pond or stream some time after laying, usually remaining viable and completing development (DeGraaf and Rudis, 1983; Hunter et al., 1992).

ii. Clutch size. Clutch size ranges from 500–4,000 eggs (Vogt, 1981), although Shirose and Brooks (1995b) report a maximum of 2,000 eggs/clutch. The upper limiting embryonic temperature is 30 ˚C (Moore, 1952). At warmer temperatures, oxygen diffusion is inadequate to supply embryos in the center of the egg mass, and dying embryos may produce decomposition products lethal to neighboring embryos (Moore, 1949a,b). The length of time needed for eggs to develop and hatch is apparently variable and more data are needed (Bleakney, 1958).

C. Larvae/Metamorphosis.

i. Length of larval stage. Metamorphosis usually takes place in July–August following a 1-yr larval period at body lengths ranging from 25–42 mm, although sometimes 2 yr are required and tadpoles can reach total lengths exceeding 72 mm (Garnier, 1883; Wright, 1932; Wright and Wright, 1949; Hedeen, 1971a, 1972c; Vogt, 1981). Tadpole growth slows during the winter (Hedeen, 1971a), and northern populations exhibit a prolonged larval period (Leclair and Laurin, 1996).

ii. Larval requirements.

a. Food. Larvae feed primarily on algae. Garnier (1883) reported mink frogs tadpoles feeding on dead fishes and dead green frog tadpoles, but this observation has not been corroborated. Hedeen (1972b) reports a primarily algal diet in Minnesota and no observations of carnivory despite carcasses being available. Feeding is suspended during metamorphosis when forelimbs first appear, and an adult diet begun when the tail is < 2.1 cm long (Hedeen, 1972b).

b. Cover. Tadpoles use both emergent and macrophytic vegetation for cover.

iii. Larval polymorphisms. Unknown and unlikely.

iv. Features of metamorphosis. As mentioned above (see "Length of larval stage"), metamorphosis usually takes place in July–August at least 1 yr after eggs are laid.

v. Post-metamorphic migrations. Rare; juveniles and adults share the same habitats.

D. Juvenile Habitat. Similar to adults.

E. Adult Habitat. Mink frogs are highly aquatic. Tolerance to desiccation is substantially lower than the tolerances of other northern anurans, and tolerance to hydration is lighter than in other anurans (Schmid, 1965b). Terrestrial activity is not commonly reported, and usually restricted to periods of nocturnal precipitation (Hedeen, 1986; Schueler, 1987; personal observations, 1990s). Adults typically occupy rivers, lakes, ponds, pools, puddles, ditches, and streams, avoiding rapid currents and large wave activity, and preferring quiet bays and protected areas with a high abundance of aquatic macrophytes, especially floating water-lily (Nymphaeaceae) and pickerel weed (Pontederia cordata) or the edges of sphagnum mats (Garnier, 1883; Jackson, 1914; Wright, 1932; Moore and Moore, 1939; Wright and Wright, 1949; Gorham, 1970; Hedeen, 1971a, 1972a,b; Stewart and Sandison, 1972; Vogt, 1981; Hedeen, 1986). A mean body temperature of 28.8 ˚C is reported for four frogs basking in the sun on 25.0 ˚C rocks in 22.0 ˚C air (Brattstrom, 1963). A median lethal temperature of 36 ˚C is reported for Minnesota mink frogs submerged in warm water for 1 hr (Dean, 1966). Field body temperatures of 27 Minnesota mink frogs ranged from 16.2–27.1 ˚C, and thermoregulation (basking and posturing to control body temperatures) is utilized (Hedeen, 1971b). The maximum voluntary temperature recorded is 30.5 ˚C (Brattstrom, 1963).

F. Home Range Size. Unknown. Dispersal movements through forested habitats typically occur on rainy nights, but dispersal distances are unknown (Schueler, 1987; personal observations, 1990s).

G. Territories. Territoriality has not been reported, although investigations are probably incomplete.

H. Aestivation/Avoiding Dessication. Aestivation has not been reported. Because mink frogs are highly aquatic and occupy the permanent waters of northern latitudes, aestivation is probably not a necessary behavior.

I. Seasonal Migrations. Schueler (1987) observed overland movements during rainy October nights in Ontario, which he interpreted as movements towards hibernation sites. Most mink frogs probably hibernate within the same waters they inhabit the rest of the year, however.

J. Torpor (Hibernation). Mink frogs are not freeze tolerant (Schmid, 1982); to escape freezing they hibernate in the bottom mud of permanent lakes and streams (Schmid, 1965b; Hunter et al., 1992; Harding, 1997). In the Great Lakes region, they may enter dormancy by late September and remain inactive until well into May (Harding, 1997).

K. Interspecific Associations/Exclusions. Sympatry is reported with green frogs (Kramek, 1972; Stewart and Sandison, 1972; Shirose and Brooks, 1995b; Leclair and Laurin, 1996), American bullfrogs (Bleakney, 1958; Stewart and Sandison, 1972; Werner and McCune, 1979; Shirose and Brooks, 1995b; Leclair and Laurin, 1996), northern leopard frogs (Rana pipiens; Hedeen, 1972b; Leclair and Laurin, 1996), and wood frogs (Rana sylvatica; Hedeen, 1972b). Hedeen (1972b) described seasonal niche overlap, where northern leopard frogs moved into the aquatic habitat of the mink frog in late summer. Diets differed, with mink frogs ingesting a greater proportion of aquatic insects, and northern leopard frogs ingesting more terrestrial items when terrestrial foraging habitat was available; however, northern leopard frog diets became similar to mink frog diets at sites where no terrestrial foraging habitat was available. Kramek (1972) suggested that interspecific competition with bullfrogs and green frogs may influence diet through feeding niche separation. Stewart and Sandison (1972) found differences in habitat preferences between mink frogs, bullfrogs, and green frogs in New York; these differences were reflected in their diets. Mink frogs primarily inhabited the aquatic zone, sitting on lily pads or other floating vegetation when not submerged, while green frogs and American bullfrogs are found primarily on water margins, with green frogs moving rarely into the aquatic zone and often into the terrestrial zone. Mink frogs ingested fewer food types than bullfrogs (26 versus 46, with only 6 items shared), ingested fewer aquatic invertebrate food types than bullfrogs (26% versus 32%), and frogs comprised a major component of the bullfrog diet while no frogs were recorded from the mink frog diet. Dietary overlap of food items ingested by mink frogs with sympatric green frogs was 48%, with volume of aquatic food groups representing 36% in mink frogs and 24% in green frogs. Courtois et al. (1995) found that mink frogs and green frogs were more abundant and had a more even distribution across different habitats when American bullfrogs were absent. Viable triploid hybrids have resulted from lab crosses of mink frog eggs with American bullfrog sperm (Elinson, 1993).

L. Age/Size at Reproductive Maturity. Mink frogs transform at a relatively larger size than other ranids; immediately following metamorphosis, froglets already have a body size representing nearly 60% of the mean adult size, whereas this value is somewhere between 30–40% in other ranids (Leclair and Laurin, 1996). Maturity is reached after 1 yr of post-metamorphic life in southern populations, and after 2 yr in the North (Leclair and Laurin, 1996). Males become sexually mature when about 45–50 mm long, approximately 1 yr after transformation, while females become sexually mature when about 54–59 mm long, 1–2 yr after transformation (Hedeen, 1972c). In central Ontario, sexual maturity is reached at 3 yr post transformation (mean standard length = 63.8 mm) in females, and 2 yr post transformation (mean standard length = 56.6 mm) in males (Shirose and Brooks, 1995a). Females grow faster and attain larger sizes than males (Shirose and Brooks, 1995a; 11% larger than males, Leclair and Laurin, 1996). Mean age and maximum longevity of females is higher than males in southern populations (Leclair and Laurin, 1996). Hedeen (1972c) concluded that only some females bred 1 yr after transforming, at 54–59 mm. Wright (1932) measured size classes of mink frogs from New York and Ontario without discriminating between sexes and localities and reported sizes of 44 mm (40–48 mm) in the first year after transformation, 53.5 mm (48–58 mm) in the second year, and 63 mm (59–72 mm) in the third year. Average SULs in Québec were 41.3–48.9 mm for first-year females, 41.2–46.5 mm for first-year males, 58.4–66.1 mm for second-year females, 52.8–60.6 mm for second-year males, 65.4–70.4 mm for third-year females, 63.1 mm for third-year males, and 70.0 mm for fourth-year females (Leclair and Laurin, 1996). Schueler (1975) demonstrated geographic variation in size, with mink frogs much larger in the North, and postulated that individuals in southern populations attained sexual maturity earlier and at a smaller size due to predation pressures. Leclair and Laurin (1996) found that specimens from a northern population were 17% larger than specimens from a southern population and had similar annual growth rates despite the longer growing season in the South. Growth rate, delayed maturity, greater mean ages, and size at transformation all contribute to larger size in adult mink frogs at northern localities (Leclair and Laurin, 1996). Sex ratios favored females in southern populations where males incurred greater mortality (Leclair and Laurin, 1996).

M. Longevity. In populations studied by Leclair and Laurin (1996), most mink frogs survived only 1–2 yr past metamorphosis, with females sometimes reaching 3–4 yr of age and males only rarely entering a third year. In central Ontario, Shirose and Brooks (1995a,b) found mortality to be lowest at transformation and then increasing gradually, with an estimated longevity (maximum life span) of 5–6 yr past transformation and a mean life expectancy of 1.7–4.0 yr past transformation.

N. Feeding Behavior. As with most adult ranids, mink frogs are opportunistic feeders and prey upon anything of a proper size that moves, thus diet generally reflects the availability of prey within the foraging habitat, which is mostly the water surface. The diet is thus dominated by organisms such as dragonflies and damselflies, diving and whirlygig beetles, waterbugs, and aphids (Hedeen, 1972b; Kramek, 1976). Kramek (1976) studied feeding behavior of the mink frog. Actively feeding frogs usually assume an erect, “head up” posture on floating vegetation and wait for prey items to come within striking distance. Hunting frogs will orient toward prey before striking, and stalking of prey occurs if the prey is outside of the striking distance, sometimes including an underwater approach to within striking distance. If no prey approaches, the frog may change position within a 1–3 m hunting area. Mink frogs exhibit strong tendencies to return to certain places within a hunting area, and capture success for slow-moving prey (i.e., aphids) is high (84%) and low (16%) for fast-moving prey (i.e., aerial insects). In Minnesota, Hedeen (1972b) found plant material in 90.5% of the stomachs he examined (mostly duckweeds; Lemna sp.), but considered it to have been ingested incidental to predation on animals. Stomachs of Minnesota mink frogs contained Collembola, Odonata, Hemiptera, Homoptera, Neuroptera, Coleoptera, Trichoptera, Lepidoptera, Diptera, Hymenoptera, Araneida, Acarina, and Pulmonata (Hedeen, 1972b). Garnier (1883) reported food items of water insects, beetles (Carabidae), millepedes (Julidae), and small fish (chubs). Kramek (1972) reported leeches, snails, spiders, and 35 insect families from mink frog stomachs in New York. Odonata, Coleoptera, and Homoptera predominated, with aphids being consumed most often and in greatest volume (21.7%). There was no difference between the diets of similarly sized males and females. Stewart and Sandison (1972) recorded the following stomach contents from New York: Acarina, Araneae (7 families), Collembola, Plecoptera (larva), Odonata (2 families), Coleoptera (13 families), Hemiptera (5 families), Lepidoptera (larva), Mecoptera (larva), Diptera (5 families), and Hymenoptera (4 families). Dytiscids (diving beetles) and gyrinids (whirlygig beetles) were the most important food items. Frequency of both plant and animal matter was 100%; volume of plant and animal matter was 10.5% and 89.5%, respectively. The occurrence of plant matter in stomachs was interpreted as accidental ingestion of floating and moving material.

O. Predators. Schueler (1975) concluded that American bullfrogs are major predators on mink frogs. Stewart and Sandison (1972) also report bullfrog predation on mink frogs. Other predators include raccoons (Procyon lotor; Hedeen, 1972a), great blue herons (Ardea herodius) and other Ciconiiformes (Bent, 1926; Wright, 1932; adults and tadpoles, Hedeen, 1967), wood ducks (Aix sponsa; Mallory and Lariviere, 1998), the spruce grouse (Dendragapus canadensis; Applegate, 1978), eastern newts (Notophthalmus viridescens; eggs and embryos, Wright, 1932), green frogs (Moore, 1952), larvae of eastern tiger salamanders (Ambystoma tigrinum; tadpoles, Hedeen, 1972a), snakes (probably common garter snakes, Thamnophis sirtalis; Hoopes, 1938), five-spined sticklebacks (Culaea inconstans; tadpoles, Hedeen, 1972a), giant water bugs (Lethocerus americanus; tadpoles, Hedeen, 1972a), and leeches (Macrobdella decora; tadpoles, Hedeen, 1972a).

P. Anti-Predator Mechanisms. Hedeen (1972a) studied escape behavior in Minnesota and concluded that the usual response (+ 85%) of frogs to disturbance was to dive underwater and conceal themselves in mud or vegetation, remaining hidden for 30 s to 26 min. Upon returning to the surface, frogs remained wary, exposing only their eyes and snout above the water surface and diving again at the slightest disturbance. Vogt (1981) suggested that mink frogs give a warning cry when startled, but this vocalization, while characteristic of American bullfrogs, has not been observed by this author or by other mink frog researchers (D.F. McAlpine and S.E. Hedeen, personal communications, 2001). Presumably, the musky odor of the skin secretions is offensive to some predators (Vogt, 1981).

Q. Diseases. Red-leg (usually Aeromonas bacteria) has been reported (Hedeen, 1972a). The icosahedral frog erythrocytes virus (Iridoviridae) has been isolated from the cytoplasm of Ontario mink frogs (Gruia-Gray et al., 1989), as well as an icosahedral cytoplasmic virus in leukocytes (Desser, 1992). Malformations have been reported from several locations, with a marked increase in this phenomenon in the past decade (Wisconsin—supernumerary limbs, unresorbed or deformed tails, misplaced bony projections, underdeveloped digits, Robert DuBois, personal communication, 1996; Minnesota—over 40% malformation rates recorded, Hoppe, 1996). Investigations into causal agents of these malformations remain equivocal, but trematode parasites (Sessions and Ruth, 1990; Johnson et al., 1999; Sessions et al., 1999), chemical contaminants (Ouellet et al., 1997a; Gardiner and Hoppe, 1999), and UV-B light (Worrest and Kimeldorf, 1975; Long et al., 1995; Blaustein et al., 1997) are implicated.

R. Parasites. Parasites reported include leeches (Macrobdella decora on adults and tadpoles, Hedeen, 1972a; Barta and Desser, 1984), opalinid ciliate infusorians (Opalina sp.: Metcalf, 1923), protozoans (coccidians, Chen and Desser, 1989; the hematozoan Lankesterella, Desser et al., 1990; Aegyptianella ranarum [Rickettsiales, Anaplasmataceae], Desser, 1987), and nematodes (giant kidney worm Dioctophyma renale, Mace and Anderson, 1975; microfilaria, presumably Foleyella sp., Barta and Desser, 1984).

4. Conservation. Range-wide, mink frog abundance appears to be unchanging. Threats include pesticides, acidification, and increased UV-B radiation. Repeated applications of the broad spectrum insecticide fenitrothion (0, 0-dimethyl 0-[4-nitor-m-tolyl] phosphorothioate), commonly used for spruce budworm (Choristoneura fumiferana) control, may adversely affect mink frog populations in forest ponds (McAlpine et al., 1998). The susceptibility of mink frog breeding ponds to pH depression resulting from atmospheric acid deposition is considered moderate (Schreiber and Newman, 1988). Because mink frogs thermoregulate by basking in the sun, they may be susceptible to damage to the skin and eyes from ultraviolet (UV-B) radiation, especially as UV-B levels increase in northern latitudes with continuing ozone depletion in the upper atmosphere. However, no data are available on UV-B susceptibility.

¹Gary S. Casper
Great Lakes Ecological Services
PO Box 375
Slinger, WI 53086-0375
USA
gc@greatlakeseco.com

Citation: AmphibiaWeb. 2024. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 19 Apr 2024.

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