Rana grylio (Stejneger, 1901)
Pig Frog, American Pigfrog
Taxonomic Notes: This species was placed in the genus Lithobates by Frost et al. (2006). However, Yuan et al. (2016, Systematic Biology, doi: 10.1093/sysbio/syw055) showed that this action created problems of paraphyly in other genera. Yuan et al. (2016) recognized subgenera within Rana for the major traditional species groups, with Lithobates used as the subgenus for the Rana palmipes group. AmphibiaWeb recommends the optional use of these subgenera to refer to these major species groups, with names written as Rana (Aquarana) catesbeiana, for example.
© 2011 Stephen Bennett (1 of 13)
Rana grylio Stejneger, 1901
Stephen C. Richter1
1. Historical versus Current Distribution. Pig frogs (Rana grylio) are endemic to the southeastern coastal plain of the United States, along the Atlantic Coast from southern South Carolina to the Everglades of Florida, and west along the Gulf Coast to extreme eastern Texas (Altig and Lohoefener, 1982; Conant and Collins, 1991). Though not native, pig frogs have been found on barrier islands off the Atlantic Coast (Martof, 1963; Kiviat, 1982). They have been introduced on Andros and New Providence islands in the Bahamas (Neill, 1964a; Conant and Collins, 1991) and are well established in northern Puerto Rico (Rios-López and Joglar, 1999). Fossils are known from the Pleistocene (Illinoinan–Sangamonian glacial age; 100,000–1,300,000 ybp) of north-central Florida (Tihen, 1952; Lynch, 1965).
2. Historical versus Current Abundance. No records that quantify historical and current abundance exist, but pig frogs have been and still are considered common throughout their range (Deckert, 1914a; Voice, 1923; Wright, 1932; Dixon, 1987). Populations do not appear to have been diminished; in fact, their geographic distribution has expanded (Neill, 1964b; Rios-López and Joglar, 1999). Pig frogs, unlike most other anuran species, are positively affected by residential development. For example, Delis et al. (1996) found higher abundances of pig frogs in a developed area that was once pine flatwoods than an adjacent pine flatwoods habitat. No long-term studies exist that quantify changes in population-level abundance over time. Wood et al. (1998) found that females were typically more abundant than males at most sampling locations and that juvenile abundance varied widely among sites, but because this study occurred over a 3-yr period, it could not address long-term variation in abundance.
3. Life History Features.
A. Breeding. Reproduction is aquatic.
i. Breeding migrations. In South Carolina, Lamb (1984) found that aquatic pig frogs were confined to the cypress-hardwood zone during the non-breeding season but moved into the grass-shrub zone to breed. Males made this movement earlier and remained within the grass-shrub zone longer than females. Males typically call at night but are often heard during the day (Deckert, 1914b; Barbour, 1920; Mount, 1975; Lamb, 1984; personal observations). Pig frogs are reproductively active throughout most of the year in Puerto Rico, where males have been heard calling from March–November (Rios-López and Joglar, 1999). Lamb (1984) observed calling males for 6–7 mo and gravid females for 4 mo. Carr (1940a) and Duellman and Schwartz (1958) reported males calling in every month of the year (rarely below 21 ˚C), however, calling is typically most intense from February–September in the continental United States (Wright and Wright, 1949; Lamb, 1984; Dundee and Rossman, 1989). Males have mature spermatozoa in their testes year-round; females have mature ova April–July, with development beginning in August (Lamb, 1984). The next clutch of eggs begins to develop soon after the first clutch is deposited, and females overwinter in this condition (Lamb, 1984).
ii. Breeding habitat. Pig frogs typically breed in open, permanent freshwater lakes, cypress ponds, marshes, brushy swamps, roadside ditches, and overflowed river banks containing emergent vegetation (Wright, 1932; Smith and List, 1955; Mount, 1975; Ashton and Ashton, 1988; Dundee and Rossman, 1989), though Voice (1923) found them in wetlands with moderate salinity. Pig frogs have also been seen in ephemeral ponds, streams, and spring runs (Carr, 1940a). Neill (1947b) found them breeding in old, flooded rice fields, and according to Uzzell (1952), their distribution may be limited in South Carolina by these old rice reserves and similar habitats where they are commonly found in the lower coastal area.
i. Egg deposition sites. Eggs are deposited as a surface film, typically attached to vegetation, from March–September (Wright, 1932; Carr, 1940a; Wright and Wright, 1949). Wells (1976) suggested that pig frogs might deposit > 1 clutch/yr, but data are lacking.
ii. Clutch size. Females deposit 8–15,000 eggs in a surface film (typically attached to vegetation) that ranges in diameter from 305–762 mm (Wright, 1932; Wright and Wright, 1949; Dundee and Rossman, 1989). Eggs are surrounded by two gelatinous capsules and take 2–3.5 d to develop and hatch (Wright and Wright, 1924, 1949; Wright, 1932; Ashton and Ashton, 1988).
Larvae are large and known to reach 110 mm total length and 35.6 mm body length (Wright, 1932; Wright and Wright, 1949).
The larval period ranges from 365–730 d—thus, tadpoles overwinter twice in some populations (Wright, 1932; Wright and Wright, 1949; Dundee and Rossman, 1989); transformation typically occurs from late April to mid July (Wright, 1932). Gosner (1959) described tooth morphology of larval pig frogs, and Altig and Pace (1974) provided scanning electron photomicrographs and measurements of individual labial teeth (length, width = 0.55, 0.36 mm). Wright (1932) found average size at metamorphosis to be 44 mm (range = 32–49 mm).
D. Juvenile Habitat. No studies have addressed ontogenic shifts in habitat preference/characteristics, but juveniles probably utilize habitats similar to adults.
E. Adult Habitat. Pig frogs are largely aquatic, typically remaining within permanent habitats (as described in “Breeding habitat" above) throughout the year (Wright, 1932; Wright and Wright, 1949; Lamb, 1984). Other than Lamb (1984), no data exist regarding non-breeding season activity, so it is possible that periodic land migrations occur.
F. Home Range Size. Unknown.
G. Territories. Agonistic behavior was observed by Lamb (1984), who observed males wrestling for > 3 min and noted that grappling terminated when one individual retreated. Lamb (1984) also observed calling males with “high posture,” as described for other territorial ranids by Wells (1977). These observations suggest that male pig frogs are territorial and that the mating system is complex.
H. Aestivation/Avoiding Dessication. As drought conditions approach, pig frogs will burrow in mud and peat (Ligas, 1960; Wood et al., 1998).
I. Seasonal Migrations. Wood et al. (1998) found that pig frogs tend to remain in one location when food and water conditions are suitable, but that substantial movement is possible when water conditions change. Short, breeding movements occur; see “Breeding migrations" above.
J. Torpor (Hibernation). No data exist addressing slowed metabolism or inactivity in the winter. Based on lack of sightings, Wright (1932) estimated a hibernation period of 4–5 mo, typically occurring from November–March, though geographic variation is probable.
K. Interspecific Associations/ Exclusions. Birkenholz (1963) observed and trapped pig frogs in the “houses” of round-tailed muskrats (Neofiber alleni) in Florida.
L. Age/Size at Reproductive Maturity. Post-metamorphic pig frogs range in size from 32–161 mm SVL (Wright, 1932). Growth rates of males and females are similar until about 100 mm SVL, when females begin growing faster and reach larger maximum sizes (Wright, 1932; Ligas, 1960). Wright (1932) estimated size at maturity at 91–102 mm; however, Wright and Wright (1949) reported a minimum size at maturity as 82 mm for males and 85 for females. Mount (1975) observed two calling males 52 mm and 59 mm SVL, one of which was dissected and found to be sexually mature. Maximum SVL records vary: 150 mm in Florida (Ashton and Ashton, 1988); 152 mm (male) and 161 mm (female) in Georgia (Wright, 1932); 162 mm in Louisiana (Dundee and Rossman, 1989); and 165 mm in Alabama (Mount, 1975).
Wright (1932) estimated age at maturity (based on body size) as 3 yr with no intersexual differences. Controversy exists regarding the correlation between size and age (Halliday and Verrell, 1988; Platz and Lathrop, 1993), so caution should be taken in applying this estimate. A skeletochronology and/or multi-year mark-recapture study is necessary to obtain accurate data on population age structure.
Wood et al. (1998) found the mean monthly survival estimate for adult females (0.83 ± 0.058) was higher than for adult males (0.75 ± 0.088) and juveniles (0.72 ± 0.095; all estimates reported as mean ± SE).
M. Longevity. Wright (1932) estimates maximum age based on body size as 6 yr. Again, controversy exists in predicting age from body size (see “Age/Size at Reproductive Maturity" above).
N. Feeding Behavior. Lamb (1984) found the diet of pig frogs consisted primarily (95%) of arthropods (Coleoptera 24.3%, Decopoda 19.8%, and Odonata 11.9%). Crayfish are typically the most commonly taken single food item: 20% of diet (Lamb, 1984), 40% of diet (Carr, 1940a), and 75% of diet (Ligas, 1960). Carr (1940a) found sailfin mollies (Poecilia latipinna), smaller frogs, and aquatic insects in stomach contents. Other known food items include leeches (Placobdella rugosa), cyprinodont fishes, green treefrogs (Hyla cinerea), southern leopard frogs (Rana sphenocephala), and northern water snakes (Nerodia sipedon); adult pig frogs will also cannibalize juveniles (Duellman and Schwartz, 1958). Lamb (1984) found that males fed infrequently during the breeding season and attributed intersexual differences in diet during the breeding season primarily to intersexual differences in behavior and habitat.
O. Predators. Wright (1932) reported predation by water snakes (Nerodia spp.), cottonmouths (Agkistrodon piscivorous), herons (Ardeidae), and ibises (Threskiornithidae). Coward (1984) reported predation by an osprey (Pandion haliaetus).
P. Anti-Predator Mechanisms. When captured or handled, pig frogs occasionally emit what Allen (1932) described as “an unmistakable dank, musty odor and a slime that is bitter to the taste.” The biochemical components of this “slime” are unknown, but the secretion of closely related dusky gopher frogs (Rana sevosa), which also have a musty smell and bitter taste, contains complex, irritant compounds (C. Graham, S.C.R., P. Flatt, and C. Shaw, unpublished data).
Q. Diseases. Unknown.
R. Parasites. Christian and White (1973) reported 56 specimens of Allassostomoides louisianaensis (Trematoda: Paramphistomidae) from the large intestines of 10 pig frogs in Louisiana. Boyce (1985) dosed a single pig frog with 20 nymphs of Sebekia mississippiensis (a blood-sucking, endoparasitic arthropod and natural parasite of Alligator mississippiensis); soon after infection, the frog became lethargic and died within 24 hr.
4. Conservation. Pig frogs are not considered Threatened, Endangered, or of Concern by any state or federal agency. In Florida, pig frogs are the second most abundant frog, and the frog most commonly gigged by humans (Carr, 1940a; Ashton and Ashton, 1988). The only legal “protection” given pig frogs is a state fishing license, which is required for Louisiana’s limited hunting season (Dundee and Rossman, 1989). Populations do not appear diminished; in fact, their geographic distribution has expanded (Neill, 1964b; Rios-López and Joglar, 1999). Pig frogs, unlike most other anuran species, are positively affected by residential development (see "Historical versus Current Abundance" above). Although pig frogs appear to be doing well, more data are needed that address populations' status across their range.
1Stephen C. Richter
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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Citation: AmphibiaWeb. 2023. <https://amphibiaweb.org> University of California, Berkeley, CA, USA. Accessed 11 Dec 2023.
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