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Lithobates grylioPig Frog

By Phillip Noel

Geographic Range

Pig frogs Lithobates grylio are located predominantly on the southeastern coast of the United States. They reside from as far north as the southern part of South Carolina to all of Florida. Their range stretches westward to southeast Texas including the southern regions of Louisiana, Mississippi, Alabama, and Georgia. This species was introduced to Caribbean islands such as the Bahamas and Puerto Rico. (Alix, et al., 2014; Dorcas and Gibbons, 2008; Hammerson, et al., 2008; Lannoo, 2005; Wilson, 1995)

Habitat

Pig frogs are found in a variety of wetland habitats including large, open, permanently freshwater lakes, ponds, marshes, temporary pools and swamps. Emergent vegetation is key factor for pig frogs' presence. They also are located on the banks of rivers and streams. The average depth pig frogs would dive under water is roughly 0.1 meters while there are some cases where they can dive up to 0.3 meters deep. Lannoo (2005) reported that pig frogs also live in freshwater with "moderate" salinity, but it's unknown if this salinity level would be considered brackish water. (Alix, et al., 2014; Dorcas and Gibbons, 2008; Lannoo, 2005; Thorson and Svihla, 1943; Wilson, 1995)

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams
  • temporary pools
  • Range depth
    0.30 (high) m
    0.98 (high) ft
  • Average depth
    0.10 m
    0.33 ft

Physical Description

Adult, mature pig frog snout-vent length ranges from 8.2 cm to roughly about 16.5 cm long, although most reach maturity around 10 cm. They weigh, on average, 20 grams as a young adult. Adult male and female pig frogs are the same size until maturity; then, females tend to grow larger than males. Pig frogs closely resemble the larger American bullfrogs Lithobates catesbeianus, but the snout of pig frogs is much more pointed. The smooth skin of pig frogs complements one of their physical features of being heterothermic. Pig frogs' color is the same species-wide, described as green, olive, or brown, with dark spots towards their hind end. They have a white-gray belly. Their hind legs are very strong and sturdy with feet that are almost 100% webbed. (Dorcas and Gibbons, 2008; Lannoo, 2005; Walkowski, 2015; Wilson, 1995; Zhang, et al., 2001)

  • Sexual Dimorphism
  • female larger
  • Average mass
    20 g
    0.70 oz
  • Range length
    8.2 to 16.5 cm
    3.23 to 6.50 in

Development

Two to three days after the deposition of eggs, tadpoles begin to hatch. At hatching, they average 110 mm long. Tadpoles usually start development in August and it takes, on average, 18 months to begin metamorphosis. Metamorphosis of pig frogs is not well-described, but it is likely similar to American bullfrogs (Lithobates catesbeianus). As tadpoles, American bullfrogs have external gills, no legs, elongated bodies, and fleshy tails. Gills develop first and are covered with gill sacs to develop lungs for out-of-water breathing. After that, the legs start to grow, and the tail is reabsorbed into the body and continue to grow determinately. Pig frogs typically metamorphose and have adult lengths of 3.2 cm or greater. Most reach sexual maturity at lengths of about 10 cm, but Lannoo (2005) cited studies in which mature adults were 8.2-8.5 cm long. (Altig and McDiarmid, 2015; Birge, et al., 1987; Elliott, et al., 2008; Hammond, 2010; Lannoo, 2005; Thibaudeau and Altig, 2012; Wood, et al., 1998)

Reproduction

Pig frogs are polygynandrous, with mating occurring between February-July. Males can make advertisement calls year-round. However, in their native range, calls are limited to February-September. Male pig frogs drift out on average, 40 meters from the shore-line into a body of water to call out to the females while the females usually stay on the shore in the breeding zone. The males' calls are 1-13 short, loud, pig-like sounding grunts. The male frogs swim back to the breeding zone after calling and on the rarest occasion, the females will emit a reply call back. Pig frogs use inguinal amplexus (a copulatory position) for external fertilization. Year-round, male pig frogs have mature sperm, but sperm count is at its peak during the month of June. Similarly, although females are able to mate from April to July, they hold the greatest number of eggs in June. A females' clutch, on average, is about 10,000 eggs. Eggs are deposited in a film floating on the surface of a body of water attached to vegetation. After the females deposit the clutch, they start to develop their next clutch. (Elliott, et al., 2008; Lamb, 1984; Lannoo, 2005; Walkowski, 2015; Wood, et al., 1998)

Both male and female pig frogs are amphibians that reproduce sexually with external fertilization. They both also attain sexual maturity at the age of two years (range 1-3). When males begin their callings for female mates. Breeding can take place during anytime that the weather is warmer than 21°C usually during the range of months of March through September. Data show that female pig frogs deposit one clutch per year during the months of April-July. Females deposit between a range of 8,000 to 15,000 eggs. Usually 2-3 days later, tadpoles begin to hatch around the size of 110 millimeters in total length. Immediately after hatching, tadpoles are independent. Tadpoles typically start development in August and it takes on average 18 months to begin metamorphosis. (Dorcas and Gibbons, 2008; Lannoo, 2005; Walkowski, 2015; Wilson, 1995; Wood, et al., 1998)

  • Breeding interval
    Pig frogs breed anytime during the days, when it is warmer than 21°C.
  • Breeding season
    Pig frogs breed from March through September.
  • Range number of offspring
    8000 to 15000
  • Average number of offspring
    10000
  • Range time to hatching
    2 to 3 days
  • Average time to independence
    0 minutes
  • Range age at sexual or reproductive maturity (female)
    1 to 3 years
  • Range age at sexual or reproductive maturity (male)
    1 to 3 years

Parental care in pig frogs is negligible and males have no parental investment. In rare cases, the female frog will defend the clutch from potential predators for no longer than a few hours after the depositing. (Birge, et al., 1987; Lannoo, 2005; Walkowski, 2015; Wood, et al., 1998)

  • Parental Investment
  • no parental involvement
  • female parental care
  • pre-hatching/birth
    • protecting
      • female

Lifespan/Longevity

Information regarding the lifespan and longevity of pig frogs has been limited. Dissection is used to depict a pig frog's age. Once dissected, diagnostics are performed by measuring organs and body size to determine the age of the frog. Female pig frogs are usually larger than male pig frogs of the same age. The lifespan for both male and female pig frogs is the same, with a recorded maximum of six years in the wild.

Pig frogs are kept in captivity in some cases, but information on the average and longest lifespan have not been documented. (Huang, et al., 2006; Lannoo, 2005; Thorson and Svihla, 1943; Wood, et al., 1998)

  • Range lifespan
    Status: wild
    6 (high) years

Behavior

Pig frogs move around by hopping, leaping, and swimming. They are fully aquatic, and are usually confined to wetlands. However, they can move among wetland complexes until they find a suitable microhabitat. When a wetland becomes dry, pig frogs tend to burrow into mud until the wetland is saturated again.

Pig frogs also burrow in the mud at the bottom of ponds or wetlands when hibernating (November-March) in colder regions. Pig frogs are nocturnal, but typically call during the day. Male frogs are very territorial, especially during calling season and in most cases male pig frogs will fight on behalf of territories, mates, or both. Male frogs will call from the water to the breeding area on the banks where the females are located. After amplexus, males and females will deposit their clutch and provide no further care. Eggs usually hatch in a couple days into tadpoles and after a year and half they begin metamorphosis. (Dorcas and Gibbons, 2008; Lannoo, 2005)

Home Range

Home range has not been reported for pig frogs, but males can be territorial during the breeding season. Male pig frogs will wrestle and grapple for territory. Male frogs also call with high posture to distinguish territory. (Dorcas and Gibbons, 2008; Lannoo, 2005)

Communication and Perception

As their name suggests, pig frogs make pig-like grunts as their calls. When calling (typically during the day), the multiple choruses of 1-13 short, loud, pig-like grunts combine together to sound like a roar. During the breeding season, male frogs tend to call from the water to find their mates. Usually, only male frogs call, but there are cases where the female frogs will call back. If they do so, it is nearly inaudible by human observers.

Pig frogs mostly use their eyes for perception of the environment. They use their eyes to seek out prey and to avoid predation. The eyes also are helpful in finding mates and finding the most suitable environment to make their calls to attract said mates. Mating involves tactile senses, with some form of amplexus and external fertilization. (Dorcas and Gibbons, 2008; Elliott, et al., 2008; Gerhardt, 1975; Gorman and Haas, 2011; Lamb, 1984)

Food Habits

Pig frogs feed nocturnally, and 95% of adults' diets consist of arthropods. Specifically, crayfish are most common, and make up 20-75% of total diet. Other decapods, beetles, dragonflies also make up >10% of the diet. Annelids such as leeches (specifically, Placobdella rugosa reported) and molluscs such as slugs also contribute to their diet. Rarely, pig frogs eat small fish (minnows and shiners), snakes (northern water snakes, Nerodia sipedon, reported) and other frogs (reported are green treefrogs Hyla cinerea and southern leopard frogs Lithobates sphenocephalus). Some adults even show cannibalistic characteristics and eat smaller pig frogs. Tadpoles eat primarily algae and other aquatic plants. (Dorcas and Gibbons, 2008; Lamb, 1984; Lannoo, 2005; Ugarte, et al., 2007; Wilson, 1995)

  • Animal Foods
  • amphibians
  • reptiles
  • fish
  • insects
  • mollusks
  • terrestrial worms
  • Plant Foods
  • algae

Predation

Pig frog predators include large water snakes (Nerodia), cottonmouths Agkistrodon piscivorus, fish, birds such as herons (family Ardeidae), ibises (family Threskiornithidae), and ospreys Pandion haliaetus, American alligators Alligator mississippiensis and humans Homo sapiens. To avoid predation, pig frogs use the color of their body as camouflage. If captured, pig frogs emit a musty, bitter-tasting mucus, which sometimes causes the predator to release them. (Dorcas and Gibbons, 2008; Hammerson, et al., 2008; Lannoo, 2005; Wood, et al., 1998)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Parasites of pig frogs include both nematodes (Rhabdias ranae, Oxysomatium americana) and trematodes (Haematoloechus longi, Clinostomum marginatum, Allassostomoides louisianaensis). Haematoloechus longi shows up in the longitudinal uterine folds, extending to the pharynx region from the testes region. Clinostomum marginatum is located as cysts along the skin and extends to the posterior edge of the acetabulum of pig frogs. An acetabulm is where the head of the femur fits in a the socket of the hipbone. Allassostomoides louisianaensis was confined to the large intestine of infected frogs. (Lannoo, 2005; Manter, 1938; Walton, 1938)

Commensal/Parasitic Species
  • Nematode Rhabdias ranae
  • Nematode Oxysomatium americana
  • Trematode Haematoloechus longi
  • Trematode Clinostomum marginatum
  • Trematode Allassostomoides louisianaensis

Economic Importance for Humans: Positive

Pig frogs are hunted and harvested for food by humans and the main source of nourishment comes from the legs of pig frogs. Humans also recreationally hunt pig frogs, but a state fishing license is required to hunt them in the state of Louisiana. (Hammerson, et al., 2008; Huang, et al., 2006; Lannoo, 2005)

  • Positive Impacts
  • food

Economic Importance for Humans: Negative

There are no known negative economic effects of Lithobates grylio on humans.

Conservation Status

According to the IUCN Red List, pig frogs are a species of "Least Concern." There are no special statuses on any other international, federal, or state lists. Lannoo (2005) cited papers that indicated pig frogs are the second most abundant frog in Florida. Pig frogs also are positively affected by urbanization. For example, Lanoo (2005) summarized research that indicated pig frogs had a higher abundance where pine flatwoods once existed (since urbanized) rather than being adjacent to existing pine flatwoods.

They are being hunted by humans recreationally and consumed by humans, but it does not appear that hunting is having a detrimental impact on populations. The geographic distribution of pig frogs seems to be expanding. (Elliott, et al., 2008; Hammerson, et al., 2008; Lannoo, 2005)

Contributors

Phillip Noel (author), Radford University, Alex Atwood (editor), Radford University, Marisa Dameron (editor), Radford University, Karen Powers (editor), Radford University, Tanya Dewey (editor), University of Michigan-Ann Arbor.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

choruses

to jointly display, usually with sounds, at the same time as two or more other individuals of the same or different species

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

diurnal
  1. active during the day, 2. lasting for one day.
ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

external fertilization

fertilization takes place outside the female's body

female parental care

parental care is carried out by females

fertilization

union of egg and spermatozoan

food

A substance that provides both nutrients and energy to a living thing.

freshwater

mainly lives in water that is not salty.

herbivore

An animal that eats mainly plants or parts of plants.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

insectivore

An animal that eats mainly insects or spiders.

introduced

referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

marsh

marshes are wetland areas often dominated by grasses and reeds.

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

molluscivore

eats mollusks, members of Phylum Mollusca

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

piscivore

an animal that mainly eats fish

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

saltatorial

specialized for leaping or bounding locomotion; jumps or hops.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

swamp

a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.

tactile

uses touch to communicate

visual

uses sight to communicate

References

Alix, D., C. Anderson, J. Grand, C. Guyer. 2014. Evaluating the effects of land use on headwater wetland amphibian assemblages in coastal Alabama. Wetlands, 34/5: 917-926.

Altig, R., R. McDiarmid. 2015. Handbook of Larval Amphibians of the United States and Canada. Ithaca, New York: Cornell University Press.

Birge, W., J. Black, A. Westerman, B. Ramey. 1987. Fish and amphibian embryos — A model system for evaluating teratogenicity. Fundamental and Applied Toxicology, 3/4: 237-242.

Dorcas, M., W. Gibbons. 2008. Frogs and Toads of the Southeast. Athens, Georgia: University of Georgia Press.

Elliott, M., J. Gibbons, C. Camp, J. Jensen. 2008. Amphibians and Reptiles of Georgia. Athens, Georgia: University of Georgia Press.

Gerhardt, H. 1975. Sound pressure levels and radiation patterns of the vocalizations of some North American frogs and toads. Journal of Comparative Physiology, 102/1: 1-12.

Gorman, T., C. Haas. 2011. Seasonal microhabitat selection and use of syntopic populations of Lithobates okaloosae and Lithobates clamitans clamitans. Journal of Herpetology, 45/3: 313-318.

Hammerson, G., B. Hedges, R. Joglar. 2008. "Lithobates grylio" (On-line). The IUCN Red List of Threatened Species 2008: e.T58611A11810060. Accessed September 12, 2016 at http://dx.doi.org/10.2305/IUCN.UK.2008.RLTS.T58611A11810060.en.

Hammond, P. 2010. Atlas of the World's Strangest Animals. New York, New York: Cavendish Square Publishing.

Huang, X., Y. Huang, X. Yuan, Q. Zhang. 2006. Electron microscopic examination of the viromatrix of Rana grylio virus in a fish cell line. Journal of Virological Methods, 133/2: 117-123.

Lamb, T. 1984. The influence of sex and breeding condition on microhabitat selection and diet in the pig frog Rana grylio. The American Midland Naturalist, 111/2: 311-318.

Lannoo, M. 2005. Amphibian Declines: The Conservation Status of United States Species. Berkeley, California: Regents of the University of California.

Manter, H. 1938. A collection of trematodes from Florida Amphibia. Transactions of the American Microscopical Society, 57/1: 26-37.

Thibaudeau, G., R. Altig. 2012. Coloration of anuran tadpoles (Amphibia): Development, dynamics, function, and hypotheses. ISRN Zoology, 2012/725203: 16. Accessed September 15, 2016 at http://dx.doi.org/10.5402/2012/725203.

Thorson, T., A. Svihla. 1943. Correlation of the habitats of amphibians with their ability to survive the loss of body water. Ecology, 24/3: 374-381.

Ugarte, C., K. Rice, M. Donnelly. 2007. Comparison of diet, reproductive biology, and growth of the pig frog (Rana grylio) from harvested and protected areas of the Florida everglades. Copeia, 2007/2: 436-448.

Walkowski, W. 2015. Endogenous and Exogenous Factors That Affect Calling Behavior in Male Pig Frogs Lithobates grylio (Master's Thesis). Hammond, Louisiana: Southeastern Louisiana University.

Walton, A. 1938. The Nematoda as parasites of Amphibia. IV. Transactions of the American Microscopical Society, 57/1: 38-53.

Wilson, L. 1995. The Land Manager's Guide to the Amphibians and Reptiles of the South. Durham, North Carolina: The Nature Conservancy.

Wood, K., J. Nichols, F. Percival, J. Hines. 1998. Size-sex variation in survival rates and abundance of pig frogs, Rana grylio, in northern Florida wetlands. Journal of Herpetology, 32/4: 527-535.

Zhang, Q., F. Xiao, Z. Li, J. Gui, J. Mao, V. Chinchar. 2001. Characterization of an iridovirus from the cultured pig frog Rana grylio with lethal syndrome. Diseases of Aquatic Organisms, 48/1: 27-36.

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Classification

Classification

  • Kingdom Animalia animals
  • Phylum Chordata chordates
  • Subphylum Vertebrata vertebrates
  • Class Amphibia frogs, salamanders, and caecilians
  • Family Ranidae Ranid Frogs, Ranids, Riparian Frogs, True Frogs
  • Genus Lithobates
  • Species Lithobates grylio Pig Frog