Chelydra serpentina

(Linnaeus, 1758)
North American snapping turtle

This large, aggressive turtle (to 49.4 cm) has a tricarinate, posteriorly serrated, brown to olive-brown, slightly rounded carapace. In adults of some populations and in younger individuals the carapacial scutes may be streaked with red, or even yellow, radiations. The three keels become progressively lower with age. The anterior width of the 3rd vertebral is less than 33% of the maximum carapace width. The small plastron is yellowish to tan in adults, that of hatchlings and small juveniles is dark gray with white mottling. The bridge is 6-10% (usually less than 8%) of the carapace length; the gular scute is normally undivided. The abdominal scute is usually twice as broad as long, and the length of the plastral forelobe is less than 40% of the carapace width; inframarginal scales are usually absent; if present, only 1-2. The large head has a short, only slightly projecting snout, often a pattern of dark streaks on the jaws, and normally only two chin barbels. Tubercles on the dorsal surface of the neck can be either short and blunt or long and pointed (see Geographic Variation). Skin color is gray to black or tan above, yellowish below, with whitish flecks in some individuals.
Diploid chromosomes number 52: 24 macrochromosomes and 28 microchromosomes (Stock, 1972; Bickham and Baker, 1976a; Killebrew, 1977b; Haiduk and Bickham, 1982).
Males have the anal opening posterior to the carapacial rim. Their preanal tail length is commonly more than 120% of the length of the posterior plastral lobe, but that of females is generally less than 110% (R. J. Brooks, pers. comm.). Males are larger than females.

Chelydra serpentina ranges from Nova Scotia, New Brunswick, and southern Quebec westward in Canada to Southeastern Alberta, and southward in the United States east of the Rocky Mountains to southern Florida and the Texas coast; introduced in Nevada, Utah, Arizona and California (Stebbins, 1985).

Geographic Variation
Two subspecies are currently recognized. The common snapping turtle Chelydra serpentina serpentina (Linnaeus, 1758), ranges from southern Canada to the Texas coast and at least as far south as southern Georgia. The temporal region and back of the head are covered with flat juxtaposed scales; the dorsal surface of the neck has rounded, wartlike tubercles; and the width of the 3rd vertebral scute is much less than the height of the 2nd pleural scute and less than 33% of the combined length of the five vertebral scutes. C. s. osceola Stejneger, 1918, the Florida snapping turtle, seems restricted to peninsular Florida. The temporal region and back of the head are covered with granular scales and a scattering of low tubercles; the dorsal surface of the neck has long pointed tubercles; and the width of the 3rd vertebral is about as long as or more than the height of the second pleural and about 33% of the combined length of the five vertebrals.
Some researchers have considered C. s. osceola to be a separate species rather than conspecific with C. serpentina (Richmond, 1958). Recent treatment of this form has relegated it to subspecific rank (Medem, 1977; Gibbons et al., 1988) on the basis of a report of supposed intergradation between serpentina and osceola by Feuer (1971). However, Feuer examined relatively few turtles and found only seven he thought intergrade. A more thorough study is needed of the relationships between these two snapping turtles in northern Florida and southern Georgia.

The North American snapping turtle has been found in almost every type of freshwater habitat within its range, as long as the bottom is soft and abundant aquatic vegetation or an abundance of submerged hiding places are available. It is also known to occasionally enter brackish waters (Kinneary, 1993). It prefers slow-moving water with a soft mud or sand bottom and abundant aquatic vegetation or an abundance of submerged brush and tree trunks. Most of the water bodies in which it lives are shallow, but it may occur along the edges of deep lakes and rivers.

Natural History
Male C. serpentina mature at 4-5 years of age and plastron lengths of 123-145 mm, depending on location (White and Murphy, 1973; Mosimann and Bider, 1960; Christiansen and Burken, 1979). Active spermatogenesis has been recorded from late June to mid-November in Tennessee (White and Murphy, 1973) and mid-May to September in Wisconsin (Mahmoud and Cyrus, 1992). Testes are inactive at other times at these locations. In Tennessee and Iowa, females mature at about 123-175 mm plastron length between 4-7 years (White and Murphy, 1973; Christiansen and Burken, 1979), but the youngest known mature female from Michigan is 12 years old (Congdon et al, 1987). Females in Ontario become mature at 11-13 years, and the mean first nesting age is 17-19 years (Galbraith et al., 1989). The Tennessee ovarian cycle is divided into three phases; a pre-productive phase from August to mid-May during which follicles enlarge by yolking and are eventually ovulated, a reproductive or nesting phase from mid-May to mid-June, and a post reproductive phase from mid-June through late July (White and Murphy, 1973).
Mating may occur from April to November, and each encounter may be preceded by courtship, but precoital behavior varies (Legler, 1955). A male may directly approach a submerged females and mount her at once, or he may crawl or swim after her for several minutes and then mount her near the surface, with the pair slowly sinking to the bottom. The male and female may face each other on the bottom with necks extended and snouts almost touching. The hind quarters of each are usually elevated, the anterior edges of their plastrons touching bottom. The heads and necks may be swung sideways in opposite directions and then brought slowly back to the original position. This sequence is usually repeated about 10 times at intervals of approximately 10 seconds (Legler, 1955). Courting snappers have also been seen with their heads close together in shallow water gulping water and violently expelling it out of their nostrils, causing upheaval of the water surface above their heads (Taylor, 1933). Sperm may possibly remain viable in the female reproductive tract for several years (Gist and Jones, 1989).
Nesting usually occurs during a one month period from 15 May to 15 June, but the actual dates of oviposition vary with latitude, with females from more northern localities nesting later in the season. Females are usually rather quiescent in the immediate weeks prior to oviposition, but become quite active at the start of the nesting season (Brown and Brooks, 1993). An open site is selected, which may be up to 1 km from water, and the nest is dug with the hind feet in relatively loose sand, loam, or vegetable debris. Muskrat and beaver lodges, sawdust piles, and earthen dams and levees are often used as nesting sites. Females have also been observed nesting in ant mounds (Burke et al., 1993). The nest is a disturbed bowl or flask-like depression: a narrow opening descends at an angle to a large egg chamber below. The depth of the nest generally ranges from 7-18 cm, and its dimensions vary with the size of the female.
The number of eggs in a clutch may be as low as 6 or over 100 eggs, but typically a clutch has 20-40 eggs. Larger females lay more eggs, and clutch size seems to increase with latitude. Female productivity may also be influenced by the resources available in the local habitat (Brown et al., 1994; Iverson et al., 1997). Only one clutch is laid a season. The white, tough, spherical eggs are 23-33 mm in diameter and weigh 5-15 g. Natural incubation takes 55-125 days, but 75-95 days are more common; clutches from northern populations seem to have longer incubation periods (Obbard and Brooks, 1981; Congdon et al., 1987).
Hatchlings are about 24-31 mm in carapace length and 22-29 mm in carapace width. A buttonlike yolk sac 7-10 mm in diameter is attached to the center of the plastron at the time of hatching. The sex of the hatchling is determined by its incubation temperature. Clutches incubated at 20°C produce only females, eggs incubated at 21-22°C produce hatchlings of both sexes, those incubated at 23-24°C produce only males, eggs incubated at 25-28°C produce hatchlings of both sexes but mostly males at the lower end of this range, and eggs incubated at 29°C or higher produce only females (Yntema, 1976; Dimond, 1983; Crews et al., 1989). Exposure to at least four hours a day at 30°C ensures female development, but shorter exposures produce males if a male-determining temperature is used over the remaining interval (Wilhoft et al., 1983). In natural nests, eggs on top of the clutch are much warmer (>30°C) than those at the bottom (<30°C), resulting in all females from top eggs, and commonly all males from the bottom eggs (Wilhoft et al., 1983).
Chelydra serpentina is omnivorous, essentially consuming anything it can fit into its mouth. It eats both a wide variety of invertebrates, vertebrates of all classes, both as live prey or carrion, and both algae and parts of higher plants. Ernst has even seen them grazing in fields on fresh Carex stems much like cattle feed on grass.
Snapping turtles have been found in waters of Long Island Sound with salinities ranging from freshwater (0 ppt) to approximately 70% seawater (100% seawater = 35 ppt). Salinity data and field observations of distribution seem to indicate that they are restricted to salinities hypoosmotic to their blood plasma (308 milliosmolal) (Kinneary, 1993). Snappers are unable to osmoregulate when forced to remain in relatively saline waters (mean salinity = 13.9 ppt) over periods ranging from 11-27 days (Kinneary, 1993). Snapping turtles have recently been found to leave the water and to scratch and lie in ant mounds, possibly to have the ants aid in the removal of leeches attached to their skin (Burke et al., 1993).
Members of the genus Chelydra are aggressive and when disturbed can put up a good fight. A bite by a large individual is very painful, but does not usually break the skin.

IUCN Red List Status (1996)
Not listed.