Montana Field Guide

Snapping Turtle - Chelydra serpentina

The snapping turtle shell is hard and very serrated ("saw-toothed") on the rear edge of the carapace; the plastron is relatively small. The tail is as long or longer than the carapace, with a crest of large, bony scales. The head is large, with a hooked upper jaw and two barbels on the chin. The limbs are strong, with webbed toes and robust claws. Skin and shell color is black to tan. Maximum carapace length is 50 centimeters, but usually is less than 36 centimeters; average weight of wild individuals is 16 kilograms. In mature males, the anal opening extends farther beyond the base of the tail than in females, and is usually beyond the rear margin of the carapace (under the rear edge in females). In adults, the carapace is relatively smooth, and the longitudinal ridges are not prominent. In juveniles, there are three longitudinal ridges on the carapace; in hatchlings the carapace is rough with conspicuous ridges. Eggs are moderately pliable, somewhat brittle, and average 28 x 27 millimeters.

The snapping turtle differs from other Montana turtles by the presence of a plastron (ventral shell) that is reduced to a cross-like structure, covering perhaps only half of the ventral surface; the presence of keeled scutes or scales on the carapace (dorsal shell); and the presence of a tail at least as long as the carapace. The shell is hard, not soft and leathery, nor is it flattened or pancake-like, as is the case with the spiny softshell.

Montana Range

No specific information for Montana is available.

Research from other locations indicates that the snapping turtle may migrate up to several miles between the permanent water of usual residence and nesting areas. Some may travel up to a few kilometers between summer range and winter hibernation sites; others hibernate within summer range (Brown and Brooks 1994). In Ontario, the maximum distance traveled from the nesting site was 370 to 2020 meters (mean 1053 meters); movements were greatest from spring to mid-July (Pettit et al. 1995).

Habitat use by snapping turtles in Montana is probably similar to elsewhere in the range, but studies are lacking and there is little qualitative information available. They have been captured or observed in backwaters along major rivers, at smaller reservoirs, and in smaller streams and creeks with permanent flowing water and sandy or muddy bottoms (Reichel 1995, Hendricks and Reichel 1996, P. Hendricks personal observation). Nesting habitat and nest sites have not been described.

Elsewhere, snapping turtles occur in all types of shallow freshwater habitats, such as streams, rivers, reservoirs, and ponds, especially those with a soft mud bottom and abundant aquatic vegetation or submerged brush and logs (Hammerson 1999), and in brackish water in some areas. Although found most often in shallower water, they have been reported on the bottom of lakes in water up to 10 meters deep. Temporary ponds may also be occupied. Hatchlings and juveniles tend to occupy shallower sites than mature individuals in the same water bodies. They are mostly bottom dwellers, where they spend much of their time. Although highly aquatic, they may make long movements overland if their pond or marsh dries (Baxter and Stone 1985, Ernest et al. 1994, Hammerson 1999). They hibernate singly or in groups in streams, lakes, ponds, or marshes; in bottom mud, in or under submerged logs or debris, under an overhanging bank, or in muskrat tunnels; often in shallow water; sometimes in anoxic sites (Brown and Brooks 1994). Sometimes they bask out of water, especially younger individuals and in the northern extremes of the global range.

Nests are built in soft sand, loam, vegetation debris, or even sawdust piles, most often in open areas and often a hundred meters or more from water (Congdon et al. 1987, Ernst et al. 1994, Hammerson 1999); they also nest in beaver and muskrat lodges.

Snapping turtle diets have not been studied in Montana, but they are known to eat about anything that can be captured while foraging in the water. They eat many kinds of vertebrates (fish, amphibians, reptiles, aquatic birds, small mammals), invertebrates (insects, spiders, crustaceans, mollusks, leeches, sponges), algae and various aquatic vascular species (Ernst et al. 1994). Carrion is also consumed. Young snapping turtles actively forage for food, but adults often lie in ambush to seize their prey. This species is known to eat nine orders of insects (ants, beetles, and moths the most abundant), and spiders, scorpions, ticks, and mites have been reported in the diet (Hammerson 1999). Adults sometimes eat hatchling lizards.

Snapping turtles are most active at night in the southern parts of the range, but this is not so at northern locations, where activity is more pronounced in the morning (Ernest et al. 1994). They may be active year round in the southern United States, but cold weather there will drive them into dormancy. Most snapping turtles in more northern populations do not emerge until April, and enter hibernation by late October. In Colorado, snapping turtles emerge from hibernation in March and remain active through October (Hammerson 1999); there they are active day or night. The period of activity in Montana is poorly documented, with records mostly from early July through September (Reichel 1995, Hendricks and Reichel 1996); active individuals have been seen during mid-day in small streams (P. Hendricks personal observation).

In Ontario, males occupied relatively stable, overlapping home ranges; summer ranges were 0.4 to 2.3 hectares (Galbraith et al. 1987). Also in Ontario, July to August foraging home ranges in three sites during one year were 2.3 to 18.1 hectares (means fell between 5 and 9 hectares); home range length was about 550 to 1990 meters; home range size did not vary with habitat productivity (Brown et al. 1994). In another Ontario study, home range size over a year was 1.0 to 28.4 hectares, averaging about 9 hectares for females and about 2 to 3 hectares for males (Pettit et al. 1995).

Snapping turtles frequently incur high rates of nest predation (30 to 100% in Michigan) by various predators, especially skunks, raccoons, foxes, bears, crows, and snakes (Congdon et al. 1987, Ernst et al. 1994, Hammerson 1999). Coyotes, river otters, bears, and often humans prey on adults; herons and bitterns, large hawks, eagles, various predatory fish, and bullfrogs prey on hatchlings and juveniles. Predators in Montana, other than humans (P. Hendricks personal observation) have not been reported. See Iverson (1991) for a compilation of survivorship data (egg survival low, not more than 0.22; adult survival generally high, over 0.90). A population in Ontario, Canada, was characterized as stable, with adult female annual survivorship greater than 0.95; later, a great increase in adult mortality occurred, apparently due primarily to otter predation on hibernating turtles; there was no compensatory density-dependent response in reproduction and recruitment (Brooks et al. 1991). In Michigan, actual annual survivorship of juveniles was over 0.65 by age 2 and averaged 0.77 between ages 2 and 12 years; annual survivorship of adult females ranged from 0.88 to 0.97; population stability was most sensitive to changes in adult or juvenile survival and less sensitive to changes in age at sexual maturity, nest survival, or fecundity (Congdon et al. 1994).

In eastern Ontario 42 of 257 clutches emerged in fall. Of those overwintering, one survived. The species may be limited in range in the north by overwintering mortality (Obbard 1981). Densities in marshes in South Dakota reached one per 2 acres. Predators destroyed 59% of nests. Emergence in undisturbed nests was less than 20% (Hammer 1969).

There is essentially no information about the reproductive biology of this species in Montana. Juveniles (2.8 centimeters snout-vent length, 5.8 centimeters total length) have been collected in southern Carbon County in early September (P. Hendricks personal observation).

In southern Utah, reproduction occurred between mid-May and early July (Tinkle et al. 1993). Eggs are laid in June to July in Colorado, and May to July in west-central California. Extremes in clutch size are 1 and 8 eggs, but throughout the range clutch size averages between 3 and 5 eggs (Tinkle et al. 1993). Eggs hatch in 45 to 75 days (beginning in early to mid-August in Colorado and Utah, mid- to late August in west-central California). In Colorado and Utah, most adult females produce two clutches annually. Sexual maturity is attained in the first (south) or second (north) year (10 or 11 months in west-central California). In southern Utah, most females produce their first clutch at an age of about 22 to 24 months (some matured in about one year under uncommon optimal conditions). Males and females in southern Utah can live for at least six years (Tinkle et al. 1993).

Montana populations of the snapping turtle are poorly understood, making management of them more difficult. It is likely that dams and large reservoirs on rivers (e.g. Fort Peck Dam and Reservoir) are detrimental to population continuity to some degree, judging by the apparent lack of viable populations on the Missouri River in Montana (Maxell et al. 2003), although this species can travel overland and may be able to bypass some dams. Impacts of other habitat disturbances are not clear, but this species occupies man-made water bodies throughout its range that provide necessary resources and habitat characteristics. Studies of nesting success, population structure, dispersal, and population size need to be conducted throughout the range of the snapping turtle in Montana. Routine surveys for snapping turtles in appropriate habitats could be made a standard part of the field duties of agency fishery biologists. Records should be maintained of the incidental "take" by anglers, who should be encouraged to report any captured individuals; killed animals should be examined by agency fishery or wildlife biologists if possible so that data on sex, size, and food habits can be gathered and a base of information developed on the biology of this species in Montana. It is possible that even moderate harvest of adults by anglers in most localities will result in population declines, similar to Colorado (Hammerson 1999), because the life history of this species indicates recruitment of juveniles into breeding populations is low, and population densities in western states is probably low. Identified nesting sites should be monitored and protected from disturbance by humans.