Animal Diversity Web

Geographic Range

Mink frogs (Lithobates septentrionalis) are found in the Nearctic region. Their range extends as far east as Labrador, Canada, as far north as southern Ontario and Quebec, and as far south 43 degrees north latitude in the United States. In Canada, they are native to Manitoba, New Brunswick, Newfoundland, Ontario, and Quebec. In the United States, they are considered native to Maine, Michigan, Minnesota, New Hampshire, New York, Vermont, and Wisconsin. (Popescu, 2009; Warkentin, et al., 2003)

• Biogeographic Regions
• nearctic
  • native

Habitat

Mink frogs are a boreal, primarily aquatic species. They are rarely away from water, and live in slow-moving, freshwater habitats at the margins of rivers, lakes, ponds, bogs and streams. Because their skin desiccates quickly, mink frogs only leave the water at night during rain events.

During their breeding season (early June through early August), they require cool, oxygen-rich water, at temperatures 21 °C or below in order for the submerged embryo to properly develop. Mink frogs are commonly found in bodies of water with circumneutral pH (6.5-7.5) to assure proper development of the embryo. High sulfate concentrations in the water negatively impact mink frog populations, making them sick and causing them to die.

American beaver (Castor canadensis) activity (damming and pond creation) creates suitable habitats for mink frogs. Specifically, creating permanent oxygen rich bodies of water and decreasing in the velocity of the water. The ponds increase surface area of water across the landscape, allowing mink frogs to traverse it without desiccation. These beaver activities help mink frogs sustain their populations during times of drought.

Mink frogs are commonly found in areas that have dense aquatic vegetation that provides shelter for egg masses, and provides cover from predators. (Bevier, et al., 2004a; Hedeen, 1986; Kelly, et al., 2017; Mcalpine, et al., 1998; Popescu, 2009)

• Habitat Regions
• temperate
• freshwater

• Aquatic Biomes
• lakes and ponds
• rivers and streams

• Wetlands
• bog

Physical Description

As adults, mink frogs are 45 to 76 mm in total length. Raised ridges of the skin that run down the side of their backs, called dorsolateral folds, are weakly developed or absent for mink frogs. Their toes are broadly webbed and reach the last joint of the fourth toe and tip of the fifth toe. Mink frogs' abdomens are a yellowish-white and their throat is yellow. Their dorsal side can vary from green to olive brown and contains spots or smears of dark brown. Their skin is smooth but responds to being touched by releasing a musky, mink-like odor from their glands. They have granular glands on their skin that release antimicrobial peptides to prevent growth of pathogens. Mink frogs have a body temperature of 28.8 °C when in the sun on rocks and 22 °C when not submerged in water or in the sun. If their average temperature reaches 36 °C when they are under water for an hour or more they will die.

Mink frogs have a tadpole stage that lasts 1 to 2 years and they metamorphose when they reach lengths of 29 to 42 mm. Their dorsal side is green or olive brown with dark spots and their ventral side is yellow. They have a tail that is a lighter shade of their dorsum with dark smudges. Tadpoles that are born in the summer and survive the winter months weigh an average of 0.02 g and are 9.7 mm long in July of the following year. By mid-October, they weigh 3.68 g and are 41.4 mm long. Adults with a SVL 48 mm or longer can be sexed by a snout-vent length to tympanum diameter ratio. They are males if the ratio is less than 10:1, and females if the ratio is at least 10:1.

Sexual dimorphism is evident in summer months where females grow at a rapid rate and outweigh the males. Males become sexually mature when they have a snout-vent length of 45 to 50 mm and also have a larger tympana than mature females. Juveniles do not experience seasonal changes. Typically, it takes about a year for males to become mature.

Females do not display overt physical features to signify when they are mature, but internal egg stages can help determine when females mature. Eggs of mink frogs have three stages: stage one contains unpigmented eggs that are 0.5 mm in diameter and are present in females with snout-vent lengths up to 50 mm. Stage two eggs are gray to black with a diameter of 0.4 to 1.0 mm, with polarity and are found in females with snout-vent lengths over 50 mm. Stage three eggs are polarized, 0.9 to 1.5 mm in diameter and were white on the bottom and black on the top. Females that have snout-vent lengths over 53 mm contain stage three eggs. Based on the stages of the eggs and the presence of stage three eggs, females are considered sexually mature when they have snout-vent lengths 54 to 59 mm.

Mink frogs are different from green frogs (Lithobates clamitans) in that the green frogs display cross banding on the dorsum and hind legs. The webbing on green frogs also does not reach the last joint of the fourth toe and tip of the fifth toe like the minks frogs do. (Ashcroft, et al., 2007; Bevier, et al., 2004a; Boulenger, 1920; Brattstorm, 1963; Hedeen, 1971a; Hedeen, 1971b; Hedeen, 1972a)

• Other Physical Features
• ectothermic
• bilateral symmetry

• Sexual Dimorphism
• female larger

• Range length

  45 to 76 mm

  1.77 to 2.99 in

• Average length

  60 mm

  2.36 in

Development

Mink frog tadpoles hatch from their egg masses four days after being laid. Eight days after hatching, they are mobile. They increase in length and weight from hatching until the winter months. They eat algae and live in the same habitats as adult mink frogs. Tadpole stomachs may contain dead fish and other dead tadpoles, but they have never been seen actively eating these items. In the winter months, they remain at a consistent weight and length. Mink frogs have a tadpole stage that lasts 1 to 2 years and they metamorphose when they reach lengths of 29 to 42 mm. Tadpoles that hatch in the summer weigh an average of 0.02 g and are 9.7 mm long. By mid-October, they weigh an average of 3.68 g and are 29-42 mm long. Adults with a SVL 48 mm or longer can be sexed by a snout-vent length to tympanum diameter ratio. They are males if the ratio is less than 10:1, and females if the ratio is at least 10:1. During metamorphosis, mink frogs do not eat. They begin eating the adult diet when their tail is less than 2.1 cm long.

Sexual dimorphism is evident in summer months where females grow at a rapid rate and outweigh the males. Males become sexually mature when they have a snout-vent length of 45 to 50 mm and also have larger tympana than mature females. Juveniles do not experience seasonal changes. Typically, it takes about a year for males to become mature.

Females do not display overt physical features to signify when they are mature, but internal egg stages can help determine when females mature. Eggs of mink frogs have three stages: stage one contains unpigmented eggs that are 0.5 mm in diameter and are present in females with snout-vent lengths up to 50 mm. Stage two eggs are gray to black with a diameter of 0.4 to 1.0 mm, with polarity and are found in females with snout-vent lengths over 50 mm. Stage three eggs are polarized, 0.9 to 1.5 mm in diameter and were white on the bottom and black on the top. Females that have snout-vent lengths over 53 mm contain stage three eggs. Based on the stages of the eggs and the presence of stage three eggs, females are considered sexually mature when they have snout-vent lengths 54 to 59 mm. (Boulenger, 1920; Hedeen, 1971b; Hedeen, 1972a; Lannoo, 2005)

• Development - Life Cycle
• metamorphosis

Reproduction

Mating season is from early June to early August and takes place in aquatic habitats. Male mink frogs use nocturnal choruses to attract females to mate with them. External fertilization occurs when males are on top of females in amplexus pairs and their egg masses (500-4,000 eggs) are deposited and submerged under vegetation. Males use rumble notes to attract the females and repel other males. When other males are present, males increase their call time, duration of calls and the complexity of calls. Males spend more time at the breeding sites than females.

Vegetation largely correlates with breeding sites; lots of lily pads allow for multiple individuals to be present, but still spatially separated. When other males are in close range, single cuks (calling chorus) are most often used to sort out spatial configuration with other male mink frogs. When lily pads are not as common, submerged objects, like fallen trees, take their place and male mink frogs aren't disrupted by other individuals as much and only one or two male frogs are present per object. (Bevier, et al., 2004b; Garnier, 1883; Lannoo, 2005; Patrick, et al., 2012)

• Mating System
• polygynous

Mink frogs' breeding season is early June to early August in aquatic habitats. They have breeding sites in cold, well-oxygenated water in permanent aquatic sites, with temperatures never greater than 21 °C. If the water is too warm, the available oxygen decreases; the embryos die due to lack of oxygen and the decomposing embryos become lethal to surrounding embryos. Females and males do not encounter each other until early morning, before sunrise, during peak calling period due to less predation present. Female and male mink frogs are in amplexus pairs (males on top of females to fertilize eggs) and oviposition (externally laying eggs) is typically done under water, roughly 1.5 m deep around submerged vegetation. The eggs are submerged to protect them from possible surface water freezing. Some egg masses will fall to the bottom of the streams or ponds but remain viable.

The egg masses contain 500-4,000 eggs and hatch within three to six days of being laid with a birth mass of 0.02 to 0.09 g. Tadpoles are initially motionless and are not mobile until eight days post-hatching. Most of tadpole movement for the mink frogs occurs during the night. Tadpoles grow in length and weight during the fall and growth tapers off through the winter months. Tadpoles surviving the winter months restart their growth beginning in May and complete their metamorphosis after a year of being in the tadpole stage. Male mink frogs reach sexual maturity after a year post-metamorphic transformation and are 45-50 mm long. Females take 1-3 years post-metamorphic transformation to reach sexual maturity, averaging about 2 years. They are 54-59 mm long when they reach sexual maturity.

Due to the increase in predation pressure, southern populations of mink frogs are overall smaller and reach sexual maturity sooner than mink frogs in the north. (Bevier, et al., 2004b; DeGraaf and Rudis, 1983; Garnier, 1883; Hedeen, 1971b; Lannoo, 2005; Moore, 1952; Patrick, et al., 2012; Vogt, 1981)

• Key Reproductive Features
• seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual
• fertilization
  • external
• oviparous

• Breeding interval

  Multiple times during the breeding season.

• Breeding season

  Mink frogs breed late June through early August.

• Range number of offspring

  500 to 4,000 per clutch

• Average number of offspring

  2,000

• Range time to hatching

  3 to 6 days

• Average time to hatching

  4 days

• Average time to independence

  0 days

• Range age at sexual or reproductive maturity (female)

  1 to 3 years

• Range age at sexual or reproductive maturity (male)

  1 to 2 years

• Average age at sexual or reproductive maturity (male)

  1 years

Mink frogs' have no parental involvement with their juveniles. Once the eggs are laid and fertilized in the water, the tadpoles hatch after three to six days and are on their own. They are in the same bodies of water as the adult mink grogs, but the parents do not invest any time into them. (DeGraaf and Rudis, 1983; Hedeen, 1971b; Hedeen, 1972a; Lannoo, 2005)

• Parental Investment
• no parental involvement

Lifespan/Longevity

Mink frogs have an average life span of 1-3 years after metamorphosis, which happens roughly a year after they are laid. Females have a longer lifespan and may live 3-4 years after metamorphosis. Males typically die two years after metamorphosis. In central Ontario, mink frogs were documented living 5-6 years and had an average lifespan of 1.7-4.0 years after metamorphosis. They are not kept in captivity. (Lannoo, 2005; Leclair Jr. and Laurin, 1996)

• Range lifespan
  Status: wild

  5 to 6 years

• Typical lifespan
  Status: wild

  1 to 4 years

• Average lifespan
  Status: wild

  1-3 years

Behavior

Mink frogs are primarily aquatic as they exhibit a low tolerance to desiccation. The only times they leave their aquatic areas are at night during precipitation events. They more commonly reside in areas with dense vegetation for protection and places to lay their eggs. Their embryos require oxygen-rich water with temperatures of less than 21 °C. Mink frogs do not migrate; to avoid freezing during the winter months, they hibernate in the muddy sediment of permanent ponds and lakes from September to May.

Mink frogs are not typically social and use their calls to defend their territory. When they feel threatened, they secrete a musky, mink-like odor from their skin. They rely on solar radiation to warm them up.

Mink frogs mate during the night hours and rely on their calls to find a mate. They do not spend time with their offspring once they lay their eggs.

Mink frogs eat available prey that are within range. They keep their eyes just above the surface of the water to find their prey. (Bevier, et al., 2004a; Bevier, et al., 2004b; Hedeen, 1971a; Lannoo, 2005; Popescu, 2009)

• Key Behaviors
• natatorial
• nocturnal
• motile
• sedentary
• hibernation
• territorial

Home Range

Home ranges of mink frogs are unknown. Dispersal distances are unknown for movements that occur during rainy nights through forested habitats. Tenneson (1980) studied a population of mink frogs Minnesota, and discovered 9.75 calling sites in a 272-m^2 area. In that area, territorial behaviors between males were documented- but only when males were within 9.3 m of one another. (Lannoo, 2005; Tenneson, 1980)

Communication and Perception

Mink frogs have nocturnal calling periods to minimize acoustic competition with other species. Male mink frogs call in choruses more frequently during breeding season and depend on their direct social environment. Notes produced by males are called cuk and rumble notes. Cuk notes are pulses of sounds that are single and short and used during aggressive male to male interactions. The males aggression is just a competition of audio produced and they normally try and stay away from previously-identified grounds. Rumble notes are sound pulses with variable repetitions and increasing intensity and are used to get the attention of females and repel other males.

If the mink frogs eat insects infected with digenean flatworm (Halipegus eccentricus), the parasite inhabits the eustachian tube of the mink frogs. Mink frogs then have limited hearing and cannot respond to calls.

When mink frogs feed, they use their vision primarily to catch their prey. They keep their eyes just above the water's surface, with the rest of their body submerged and catch insects that are on or near the surface.

When mink frogs get stressed or are touched, they release a musky, mink-like odor from peptides on their skin. All mink frogs (male, female, juvenile) have the ability to release the odor as a form of communication. (Bevier and Gorman Gelder, 2018; Bevier, et al., 2004a; Bevier, et al., 2004b; Bevier, et al., 2006)

• Communication Channels
• visual
• tactile
• acoustic
• chemical

• Other Communication Modes
• choruses
• pheromones
• scent marks

• Perception Channels
• visual
• tactile
• acoustic
• chemical

Food Habits

Mink frogs' diet includes small fish, millipedes, insects (dragonfiles, whirligig beetles (Gyrinus natator), waterbugs and aphids), small frogs, leeches, snails, and spiders. Their diet depends on mink frogs being opportunistic feeders and consuming anything that is available, of proper size and that can be caught. Tadpoles' stomachs have been examined and contained animal tissue of dead fish, other tadpoles and "muddy matter." When tadpoles were directly observed, they were only seen eating algal diets, despite dead fish and tadpoles surrounding them.

Early stages of metamorphosis begin the adult diet for mink frogs. To catch their food, they either rest with only their eyes and snout above the water or a slightly greater amount of their body exposed. They typically catch their food at the surface or near the water. Vegetation in the stomachs typically is incidental and feeding on it is not purposeful. (Hedeen, 1972b; Lannoo, 2005)

• Primary Diet
• carnivore
  • insectivore
  • molluscivore
  • scavenger
• herbivore
  • algivore

• Animal Foods
• amphibians
• fish
• carrion
• insects
• terrestrial non-insect arthropods
• mollusks
• terrestrial worms

• Plant Foods
• algae

Predation

When mink frogs face predation, they most commonly dive under water and hide under vegetation or mud for 30 seconds to 26 minutes. When they return to the surface, they only put their eyes and snout above water initially and if they are spooked at all, they immediately dive completely under water. They also secrete a musky, mink like odor from their skin that is off-putting to some predators.

Predators to their eggs and embryos are eastern newts (Notophthalmus viridescens).

Predators to their tadpoles are larvae of eastern tiger salamanders (Ambystoma tigrinum), five-spined sticklebacks (Culaea inconstans), giant water bugs (Lethocerus americanus), and North American leeches (Macrobdella decora).

The major predators to adult mink frogs are American bullfrogs (Lithobates catesbeianus). Other predators include green frogs (Lithobates clamitans), eastern garter snakes (Thamnophis sirtalis), great blue herons (Ardea herodias), wood ducks (Aix sponsa), spruce grouse (Dendragapus canadensis), and raccoons (Procyon lotor). (Hedeen, 1972a; Lannoo, 2005)

• Known Predators

  • American bullfrogs (Lithobates catesbeianus)
  • Raccoons (Procyon lotor)
  • Great blue herons (Ardea herodias)
  • Wood ducks (Aix sponsa)
  • Spruce grouse (Dendragapus canadensis)
  • Eastern newts (Notophthalmus viridescens)
  • Green frogs (Lithobates clamitans)
  • Eastern tiger salamanders (Ambystoma tigrinum)
  • Eastern garter snakes (Thamnophis sirtalis)
  • Five-spined sticklebacks (Culaea inconstans)
  • Giant water bugs (Lethocerus americanus)
  • North American leeches (Macrobdella decora)

Ecosystem Roles

The parasites for mink frogs are leeches (Macrobdella decora) on adults and tadpoles.

Parasites in the SAR clade include the genus opalinid ciliate infusorians (Opalina), and the species Aegyptianella ranarum in the Rickettsiales order and Anaplasmataceae family. It also includes the genus Lankesterella in the Apicomplexa phylum and the subclass Coccidia from the class Conoidasida in the Apicomplexa phylum.

The parasites in the nematode phylum that infect adult mink frogs include the giant kidney worm (Diotophyma renale), and Foleyella.

The parasite in the trematode phylum that infect adult mink frogs is the digenean flatworm (Halipegus eccentricus). (Desser, 1987; Lannoo, 2005)

Economic Importance for Humans: Positive

Mink frogs do not have positive economic benefits for humans.

Economic Importance for Humans: Negative

Mink frogs do not have negative economic benefits for humans.

Conservation Status

The IUCN Red List has mink frogs as a species of "Least Concern." The US Federal List, CITES, and the State of Michigan List all have no special status for mink frogs.

The bacteria, Aeromonas, causes red-leg syndrome, an infection of the legs and abdomen of mink frogs. The infection causes these body parts to turn red and is a threat to mink frogs because it causes rapid damages to internal organs, which ultimately kills them. Mink frogs in Ontario have been affected by the frog erythrocytes virus and the virus in the cytoplasm of leukocytes, both caused by icosahedral cytoplasmic virus, Ranovirus. Low levels of the virus primarily attack the kidneys and causes the mink frogs to die.

Other threats that could explain malformations seen in mink frogs include parasites and UV-B light and could lead to decline in mink frog populations.

Chemicals, such as estrogen ethinylestradiol used for sewage treatment, being added to waters housing mink frogs are also threats to mink frog populations. They disrupt development and growth of mink frogs in embryos causing a decrease in hatching success. The estrogen inhibits the thyroid from properly working, preventing mink frogs from being able to undergo metamorphosis. (Gruia-Gray and Desser, 1992; Gruia-Gray, et al., 1989; "Lithobates septentrionalis", 2015; Lannoo, 2005; Park and Kidd, 2005)

• IUCN Red List

  Least Concern
  More information

• IUCN Red List

  Least Concern
  More information

• US Federal List

  No special status

• CITES

  No special status

• State of Michigan List

  No special status

Contributors

Marianna Griggs (author), Radford University, Alex Atwood (editor), Radford University, Lindsey Lee (editor), Radford University, Karen Powers (editor), Radford University, Joshua Turner (editor), Radford University, Tanya Dewey (editor), University of Michigan-Ann Arbor.

References

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