Wood frog

Wood frog
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Amphibia
Order: Anura
Family: Ranidae
Genus: Lithobates
Species: L. sylvaticus
Binomial name
Lithobates sylvaticus
(LeConte, 1825)
Geographic range of the wood frog in North America (in blue)
Synonyms
  • Rana sylvatica
    LeConte, 1825

The wood frog (Lithobates sylvaticus or Rana sylvatica[2]) has a broad distribution over North America, extending from the southern Appalachians to the boreal forest with several notable disjunct populations including lowland eastern North Carolina. The wood frog has garnered attention by biologists over the last century because of its freeze tolerance, relatively great degree of terrestrialism (for a ranid), interesting habitat associations (peat bogs, vernal pools, uplands), and relatively long-range movements. The ecology and conservation of the wood frog has attracted research attention in recent years because they are often considered "obligate" breeders in ephemeral wetlands (sometimes called "vernal pools") that are themselves more imperiled than the species that breed in them. The wood frog has been proposed to be the official state amphibian of New York.[3]

Description

New Jersey Pine Barrens demonstrating lighter skin tones

Wood frogs range from 51 to 70 mm (2.0 to 2.8 in) in length. Females are larger than males.[4][5] Adult wood frogs are usually brown, tan, or rust-colored, and usually have a dark eye mask.[6] Individual frogs are capable of varying their color; Conant (1958) depicts one individual when light brown and dark brown at different times. The underparts of wood frogs are pale with a yellow or green cast.[7]

Mer Bleue Conservation Area, showing dark skin tones

A small brown frog with a dark eye mask in the woods is likely to be a wood frog. No other species has a similar appearance to the wood frog in North America. The first evasive leap is fast and long. Close observation will often glimpse a second short dive under the leaf litter, making the frog seem to disappear.

Geographic range

The contiguous wood frog range is from northern Georgia and northeastern Canada in the east to Alaska and southern British Columbia in the west.[8] It is the most widely distributed frog in Alaska. It is also located in the Medicine Bow National Forest.

Habitat

Wood frogs are forest-dwelling organisms that breed primarily in ephemeral, freshwater wetlands: woodland vernal pools. Long-distance migration plays an important role in their life history. Individual wood frogs range widely (hundreds of meters) among their breeding pools and neighboring freshwater swamps, cool-moist ravines, and/or upland habitats. Genetic neighborhoods of individual pool breeding populations extend more than a kilometer away from the breeding site. Thus, conservation of this species requires a landscape (multiple habitats at appropriate spatial scales) perspective.

Adult wood frogs spend summer months in moist woodlands, forested swamps, ravines, or bogs. During the fall, they leave summer habitats and migrate to neighboring uplands to overwinter. Some may remain in moist areas to overwinter. Hibernacula tend to be in the upper organic layers of the soil, under leaf litter. By overwintering in uplands adjacent to breeding pools, adults ensure a short migration to thawed pools in early spring. Wood frogs are mostly diurnal and are rarely seen at night, except maybe in breeding choruses. They are one of the first amphibians to emerge for breeding right when the snow melts, along with spring peepers.

Feeding

Wood frogs eat a variety of small, forest-floor invertebrates. Omnivorous, the tadpoles feed on plant detritus and algae, and also attack and eat eggs and larvae of amphibians, including those of wood frogs.[9]

The feeding pattern of the wood frog, basically similar to that of other ranids, is triggered by prey movement and consists of a bodily lunge that terminates with the mouth opening and an extension of the tongue onto the prey.[10] The ranid tongue is attached to the floor of the mouth near the tip of the jaw, and when the mouth is closed, the tongue lies flat, extended posteriorly from its point of attachment. In the feeding strike, the tongue is swung forward as though on a hinge, so some portion of the normally dorsal and posterior tongue surface makes contact with the prey. At this point in the feeding strike, the wood frog differs markedly from more aquatic Lithobates species, such as the green frog, leopard frog, and bullfrog.[10] The wood frog makes contact with the prey with just the tip of its tongue, much like a toad.[11] A more extensive amount of tongue surface is applied in the feeding strikes of these other frog species, with the result that usually the prey is engulfed by the fleshy tongue and considerable tongue surface contacts the surrounding substrate.

Cold tolerance

Lithobates sylvaticus found in southern Quebec

Similar to other northern frogs that enter dormancy close to the surface in soil and/or leaf litter, wood frogs can tolerate the freezing of their blood and other tissues.[12][13] Urea is accumulated in tissues in preparation for overwintering, and liver glycogen is converted in large quantities to glucose in response to internal ice formation. Both urea and glucose act as cryoprotectants to limit the amount of ice that forms and to reduce osmotic shrinkage of cells.[14][15] Frogs can survive many freeze/thaw events during winter if no more than about 65% of the total body water freezes. Wood frogs have a series of 7 amino acid substitutions in the sarco/endoplasmic reticulum Ca2+-ATPase 1 (SERCA 1) enzyme ATP binding site that allows this pump to function at lower temperatures relative to less cold-tolerant species (e.g. Lithobates clamitans).[16]

Reproduction

L. sylvaticus primarily breeds in ephemeral pools rather than permanent water bodies such as ponds or lakes.[17] This is believed to provide some protection of the adult frogs and their offspring (eggs and tadpoles) from predation by fish and other predators of permanent water bodies. Adult wood frogs emerge from hibernation in early spring and migrate to nearby pools. There, males chorus, emitting duck-like quacking sounds. A male approaches a female and clasps her behind her fore arms before hooking his thumbs together around her in a hold called “amplexus” which is continued until the female deposits the eggs.[5][17] Females deposit eggs attached to submerged substrate, typically vegetation or downed branches. Most commonly, females deposit eggs adjacent to other egg masses, creating large aggregations of masses.[5][17][18] Some advantage is conferred to pairs first to breed, as clutches closer to the center of the raft absorb heat and develop faster than those on the periphery, and have more protection from predators.[5][17] If pools dry before tadpoles metamorphose into froglets, they die.[5] This constitutes the risk counterbalancing the antipredator protection of ephemeral pools. By breeding in early spring, however, wood frogs increase their offspring's chances of metamorphosing before pools dry. The larvae undergo two stages of development: fertilization to free-living tadpoles, and free-living tadpoles to juvenile frogs.[19][20] During the first stage, the larvae are adapted for rapid development, and their growth depends on the temperature of the water and has a higher mortality rate.[20][21] The second stage of development features rapid development and growth, and depends on environmental factors including food availability, temperature, and population density.[20] Following metamorphosis, a small percentage (less than 20%) of juveniles will disperse, permanently leaving the vicinity of their natal pools. The majority of offspring are philopatric, returning to their natal pool to breed.[19] Most frogs breed only once in their lives, although some will breed two or three times, generally with differences according to age.[17][19][22] The success of the larvae and tadpoles is important in populations of wood frogs because they affect the gene flow and genetic variation of the following generations.[19]

Conservation status

The wood frog is not endangered or threatened. In many parts of its range, urbanization is fragmenting populations. Several studies have shown, under certain thresholds of forest cover loss or over certain thresholds of road density, wood frogs and other common amphibians begin to "drop out" of formerly occupied habitats. Another conservation concern is that wood frogs are primarily dependent on smaller, "geographically isolated" wetlands for breeding. At least in the United States, these wetlands are largely unprotected by federal law, leaving it up to states to tackle the problem of conserving pool-breeding amphibians.[1]

The wood frog has a complex lifecycle that depends on multiple habitats, damp lowlands, and adjacent woodlands. Their habitat conservation is, therefore, complex, requiring integrated, landscape-scale preservation.[1]

References

  1. 1 2 3 IUCN SSC Amphibian Specialist Group (2014). "Lithobates sylvaticus". IUCN Red List of Threatened Species. Version 2014.3. International Union for Conservation of Nature. Retrieved 2 February 2015.
  2. ↑ Yuan, Z.-Y.; et al. (2016). "Spatiotemporal diversification of the true frogs (genus Rana): A historical framework for a widely studied group of model organisms.". Systematic Biology. 65: 824–42. PMID 27288482. doi:10.1093/sysbio/syw055.
  3. ↑ Mahoney, Bill (17 June 2015). "Senate backs the wood frog — barely". Capital New York. Retrieved 18 June 2015.
  4. ↑ Monnet J-M; Cherry MI (2002). "Sexual size dimorphism in anurans". Proceedings of the Royal Society B. 269 (1507): 2301–2307. PMC 1691160 Freely accessible. PMID 12495496. doi:10.1098/rspb.2002.2170.
  5. 1 2 3 4 5 Howard RD (1980). "Mating behaviour and mating success in woodfrogs, Rana sylvatica". Animal Behaviour. 28 (3): 705–716. doi:10.1016/S0003-3472(80)80130-8.
  6. ↑ Conant R, Collins JT. (1998). A field guide to reptiles & amphibians: eastern and central North America. Third edition. New York (NY): Houghton Mifflin Company ISBN 0395904528.
  7. ↑ Conant, Roger. (1958). A Field Guide to Reptiles and Amphibians. Houghton Mifflin Company, Boston.
  8. ↑ Wilbur HM (1977). "Interactions of food level and population density in Rana sylvatica". Ecology. 58 (1): 206–209. JSTOR 1935124. doi:10.2307/1935124.
  9. ↑ Redmer, Michael and Trauth, Stanley E. (2005). Amphibian Declines: The Conservation Status of United States Species M. Lannoo, ed. University of California Press ISBN 0520235924.
  10. 1 2 Cardini, F. (1974). Specializations of the Feeding Response of the Bullfrog, Rana catesbeiana, for the Capture of Prey Submerged in Water. M.S. Thesis, U. of Massachusetts, Amherst, MA
  11. ↑ Cardini, F. (1973). Characteristics and Adaptedness of Feeding Behaviors of North American Anurans, Paper presented at June 1973 meetings of the Animal Behavior Society, Amherst, MA
  12. ↑ Storey KB; Storey JM (1984). "Biochemical adaption for freezing tolerance in the wood frog, Rana sylvatica". Journal of Comparative Physiology B. 155: 29–36. doi:10.1007/BF00688788.
  13. ↑ Wilbur HM (1997). "Experimental ecology of food webs: complex systems in temporary ponds". Ecology. 78 (8): 2279–2302. doi:10.1890/0012-9658(1997)078[2279:EEOFWC]2.0.CO;2.
  14. ↑ Kenneth B. Storey (1997). "Organic solutes in freezing tolerance". Comparative Biochemistry and Physiology A. 117 (3): 319–326. PMID 9172388. doi:10.1016/s0300-9629(96)00270-8.
  15. ↑ Costanzo JP; Lee RE Jr.; DeVries AL; Wang T; Layne JR Jr. (1995). "Survival mechanisms of vertebrate ectotherms at subfreezing temperatures: applications in cryomedicine". The FASEB Journal. 9 (5): 351–358. PMID 7896003.
  16. ↑ Dode, L; Van Baelen, K; Wuytack, F; Dean, WL (2001). "Low temperature molecular adaptation of the skeletal muscle sarco(endo)plasmic reticulum Ca2+-ATPase 1 (SERCA 1) in the wood frog (Rana sylvatica)". Journal of Biological Chemistry. 276 (6): 3911–9. PMID 11044449. doi:10.1074/jbc.m007719200.
  17. 1 2 3 4 5 Berven KA (1981). "Mate choice in the wood frog, Rana sylvatica". Evolution. 35 (4): 707–722. JSTOR 2408242. doi:10.2307/2408242.
  18. ↑ Seale DB (1982). "Physical factors influencing oviposition by the woodfrog, Rana sylvatica, in Pennsylvania". Copeia. 1982 (3): 627–635. JSTOR 1444663. doi:10.2307/1444663.
  19. 1 2 3 4 Berven KA; Grudzien TA (1990). "Dispersal in the wood frog (Rana sylvatica): implications for genetic population structure". Evolution. 44 (8): 2047–2056. doi:10.2307/2409614.
  20. 1 2 3 Herreid CF II; Kinney S (1967). "Temperature and development of the wood frog, Rana sylvatica, in Alaska". Ecology. 48 (4): 579–590. doi:10.2307/1936502.
  21. ↑ Berven KA (1990). "Factors affecting population fluctuation in larval and adult stages of the wood frog (Rana sylvatica)". Ecology. 71 (4): 1599–1608. JSTOR 1938295. doi:10.2307/1938295.
  22. ↑ Berven KA (1988). "Factors affecting variation in reproductive traits within a population of wood frogs (Rana sylvatica)". Copeia. 1988 (3): 605–615. JSTOR 1445378. doi:10.2307/1445378.

Further reading

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