Snapping turtles

(Chelydridae)

Class Reptilia

Order Testudines

Family Chelydridae

Thumbnail description
Large, vicious turtles with a long tail, a hooked beak, and a greatly reduced plastron

Size
7.1–31.5 in (18–80 cm); up to 249 lb (113 kg)

Number of genera, species
2 genera; 4 species

Habitat
Freshwater ponds, lakes, swamps, and rivers

Conservation status
Vulnerable: 1 species

Distribution
Canada to Ecuador

Evolution and systematics

Chelydridae is most closely related to Platysternidae, the family of the big-headed turtles. Some authors have considered the two groups as subfamilies in the same family, although molecular evidence supports their separate family recognition. The fossil record dates from the Paleocene of North America and the Oligocene of Eurasia. The genus Chelydra (snapping turtles) is known from as far back as the Pliocene in North America, whereas the genus Macrochelys is known from as early the Miocene.

Physical characteristics

Snapping turtles are large aquatic turtles with a long tail with three rows of tubercles; a hooked beak; a keeled, posteriorly serrated carapace; a reduced, cruciform, hingeless plastron; and heavy claws. Males are larger than females. Only 11 marginal scutes are present on each side of the carapace. The abdominal scutes on the plastron are reduced and not in contact medially. The carapace and plastron are connected by a narrow bony bridge. The posterior skull roof is deeply emarginated.

Distribution

Snapping turtles occur in southern Canada across the eastern two-thirds of the United States, and in discontinuous populations from Veracruz, Mexico, to western Ecuador; from sea level to over 6,560 ft (2,000 m) elevation.

Habitat

These turtles are found in almost any kind of freshwater habitat within their range, but also occasionally enter brackish waters.

Behavior

They possess a vicious temperament (hence their common name) and direct their powerful snapping jaws at both their food and their predators. Snapping turtles are highly aquatic, but do leave the water to nest, and one species migrates between bodies of water. They may be active at any time of day or night, but nocturnal activity is rare in northern populations. These turtles hibernate at temperate latitudes, but presumably are active year-round at more tropical sites. They occasionally bask out of water.

Feeding ecology and diet

Snapping turtles are primarily carnivorous, but omnivorous to herbivorous in some populations.

Reproductive biology

Male snapping turtles are larger than females, and hence courtship is not elaborate. Females lay up to 109 eggs per clutch during the spring or early summer, with a maximum of one clutch produced per year. Eggs are spherical, hard-shelled (but not brittle), and 0.9–1.6 in (2.3–4 cm) in diameter. Snapping turtles exhibit temperature-dependent sex determination, the sex of hatchlings being determined by the temperature during the middle third of incubation.

Conservation status

Tropical forms are apparently not common; their status is uncertain, but they are apparently not yet endangered. The alligator snapping turtle (Macrochelys temminckii) has declined significantly due to overharvesting, and hence is classified as Vulnerable on the IUCN Red List.

Significance to humans

Snapping turtles are harvested primarily for their meat, although some may be removed for the pet trade.

Species accounts

List of Species

Snapping turtle

Chelydra serpentina

taxonomy

Testudo serpentina Linnaeus, 1758, "Calidus regionibus" (warm regions). Two subspecies are recognized.

other common names

English: Common snapping turtle; snapper; French: Chélydre serpentine; German: Schnappschildkröte; Spanish: Tortugalagarto común.

physical characteristics

The carapace of these large turtles, to 19.3 in (49 cm), bears three low, knobby keels (except in the oldest individuals). The shell is dark, although it may range from brown to olive to black. The head is large, the upper jaw is somewhat hooked anteriorly, and the eyes open dorsolaterally (toward the top and side of the head).

distribution

Ranges extend from southern Canada across the United States east of the Rocky Mountains to New Mexico, Texas, and

Florida. Tropical snapping turtle populations (until recently considered subspecies of the snapping turtle) range from Veracruz, Mexico, through Central America to western Ecuador in northwestern South America.

habitat

Snapping turtles inhabit nearly any body of freshwater within their range, although they also invade brackish water environments in many areas. They seem to prefer warm, shallow, still water, with soft bottoms and abundant aquatic vegetation.

behavior

Snapping turtles are highly aquatic, spending most of their time lying camouflaged in the mud in shallow water. Most active during the morning and early evening, these turtles are often active at night in the south, but rarely so in the north. When active, they are primarily bottom-walkers, slowly moving over the bottom in search of food or mates. Most thermoregulation is done by selecting warm, shallow, submerged sites; however, they occasionally bask by floating on the water's surface or even by climbing out of the water onto emergent logs or other objects. They may be active year-round in the south, but hibernate for half the year or more in the north. However, some individuals may be active in the north in midwinter under thick ice cover. In addition to nesting forays by females, snappers frequently travel great distances overland between bodies of water (often almost a mile). They also make equally impressive movements in the water, one observed female moved 2.1 mi (3.4 km) in just 10 days.

feeding ecology and diet

Although snapping turtles are probably carnivorous by preference, they can subsist on diets ranging from completely carnivorous to completely herbivorous. Animal foods include sponges, flatworms (planarians), earthworms, leeches, clams, snails, crustaceans, spiders, insects, amphibians and their eggs and larvae, snakes, other turtles, birds, small mammals, and carrion. Snapping turtles also eat algae, duckweed, and other submergent and emergent aquatic plants and their seeds.

reproductive biology

Mating is known to occur from April to November, but probably peaks in the spring and fall. Courtship is highly variable, from the male directly mounting the female from behind, to the male trailing along after the female for several minutes before mounting, to face-offs, head-swaying, and/or water gulping and belching before mounting. Eventually the male mounts atop the female's shell and swings his tail under hers to mate. At high latitudes, nesting begins in early June and may extend to early July, whereas at low latitudes it may extend from late April to early June. Females may nest within a few feet of the water's edge or travel overland as far as 9.9 mi (16 km) to nest. The nest is dug (and covered) with the hind feet, and because of its size and depth (to 9.8 in [25 cm]), even a covered nest is quite obvious. Consequently, nests are heavily predated by animals, most within 24 hours of construction.

Females produce at most one clutch per year, with some females apparently skipping some years. They lay spherical, hard-shelled eggs that average 1.1 in (2.8 cm) in diameter and 0.4 oz (11 g) in mass. Egg size does not seem to increase significantly with female size. Clutch size is highly variable, ranging from six to 109, averaging about 32 across the range, and is positively correlated with female body size, latitude, longitude, and elevation (the largest clutches are laid in western Nebraska). Incubation in nature requires 55–125 days (more typically 75–95) depending on nest temperature (development being faster at higher temperatures) and geography (incubation times being longer in the south). Hatchling snapping turtles usually emerge from the nest in the late summer and fall (August to October) and move directly to the water. Hatchlings in northern populations that do not emerge in the fall before the onset of cold weather almost never survive the winter, probably because of their low tolerance of subfreezing body temperatures. High and low incubation temperatures result in the production of all-female offspring, and intermediate temperatures produce all males. Because their clutches are so large, eggs in different parts of the same nest may produce different sex ratios, e.g., all females at the top and all males at the bottom.

conservation status

This species has such an extensive range and is so prolific that it has so far been able to persist even in habitats significantly altered by humans.

significance to humans

Snapping turtles are exploited by humans primarily for their meat, although some small individuals make their way into the pet trade.

Alligator snapping turtle

Macrochelys temminckii

taxonomy

Chelonura temminckii Harlan, 1835, tributary stream of the Mississippi, which enters the river above Memphis, in west Tennessee. No subspecies are recognized.

other common names

English: Alligator snapper; French: Macroclémyde de Temminck; German: Geierschildkröte.

physical characteristics

Alligator snapping turtles are the largest freshwater turtles in North America (up to 31.5 in [80 cm]; 250 lb [113 kg]). The carapace bears three prominent knobby keels. The head is very large, has a strongly hooked beak, and eyes that open laterally (toward the side of the head). Three to eight small supramarginal scutes are present laterally between the marginal and pleural scutes. The turtles have a pink, wormlike projection on the tongue, which they can wriggle to lure prey.

distribution

Snapping turtles occur in the Gulf of Mexico drainages in the southeastern United States.

habitat

These turtles are primarily found in rivers and large streams, but also in lakes, ponds, swamps, and even brackish water.

behavior

Alligator snapping turtles are highly aquatic, almost never leaving the water except to nest. They are capable of making extremely long underwater movements, up to 4.2 mi (6.8 km) in a week, or to 18.7 mi (30 km) or more over several years.

feeding ecology and diet

These turtles are primarily carnivorous. In addition to luring fish with their tongue, they also eat crayfish, crabs, clams, snails, salamanders, turtles, snakes, small alligators, birds, and mammals, as well as plant roots, fruits, and seeds (e.g., grapes and acorns).

reproductive biology

Mating may occur in the spring or the fall. Courtship is poorly developed and males climb onto the female's shell from behind for mating. Nesting occurs between late April and early June, and most nests are laid within 236 ft (72 m) of the water. The eggs are spherical and 1.3–1.6 in (3.3–4.1 cm) in diameter. Females lay a single clutch each year of nine to 61 eggs (about 35 being typical), larger females producing larger clutches. Laboratory incubation of eggs to hatching requires 79–113 days, depending on temperature. Warm incubation temperatures result in the production of all females, low incubation temperatures produce nearly all females, and intermediate temperatures produce mostly males.

conservation status

Alligator snapping turtles have declined significantly due to overharvesting, and hence are cited as Vulnerable on the IUCN Red List.

significance to humans

Humans heavily exploit these turtles for their meat, and hatchlings are produced for the pet trade.

Resources

Books

Pritchard, P. C. H. The Alligator Snapping Turtle, Biology and Conservation. Milwaukee: Milwaukee Public Museum, 1989.

Periodicals

Congdon, J. D., A. E. Dunham, and R. C. van Loben Sels. "Demographics of Common Snapping Turtles (Chelydra serpentina): Implications for Conservation and Management of Long-Lived Organisms." American Zoologist 34, no. 3 (1994): 397–408.

Ewert, M. A., and D. R. Jackson. "Nesting Ecology of the Alligator Snapping Turtle (Macroclemys temminckii) along the Lower Apalachicola River, Florida." Florida Game and Fresh Water Fish Commission Non-Game Wildlife Program Final Report (1994): 1–45.

Iverson, J. B., H. Higgins, A. G. Sirulnik, and C. Griffiths. "Local and Geographic Variation in the Reproductive Biology of the Snapping Turtle (Chelydra serpentina)." Herpetologica 53 (1997): 96–117.

Phillips, C. A., W. W. Dimmick, and J. L. Carr. "Conservation Genetics of the Common Snapping Turtles (Chelydra serpentina)." Conservation Biology 10 (1996): 397–405.

Walker, D., P. E. Moler, K. A. Buhlmann, and J. C. Avise. "Phylogeographic Uniformity in Mitochondrial DNA of the Snapping Turtle (Chelydra)." Animal Conservation 1 (1998): 55–60.

John B. Iverson, PhD