The bog turtle is small (to 11.4 cm) with a large, bright blotch on each side of the head. Its elongated, rough carapace is moderately domed and has an inconspicuous keel. Sides of the carapace are nearly parallel or slightly divergent behind, and its posterior rim is smooth or only slightly serrated. All vertebrals are broader than long; the 5th is longest. Color varies from light brown through mahogany to black; each scute usually has a light center. Lower sides of the marginals and bridge are the same color as the rest of the carapace. The plastron is dark brown to black, with a few irregularly dispersed light marks. The plastral formula is: abd > an > gul >< fem > hum >< pect; there is a posterior notch on the hindlobe. The small head is brown with a large, usually yellow or orange but sometimes red blotch above and behind the tympanum. The snout is nonprotruding and the upper jaw medially notched. Other skin is brown and may be mottled with red above and orange or red below.
The diploid chromosome number is 50 including 8 pairs of macrochromosomes with median or submedian centromeres, 5 pairs of macrochromosomes with terminal to subterminal centromeres, and 12 pairs of microchromosomes (Bickham, 1975).
Males have long, thick tails, with the vent posterior to the carapacial margin; concave plastra; and thick foreclaws. Females have high, wide carapaces and flat plastra.
Clemmys muhlenbergii has a discontinuous range in the eastern United States. The main range is from western Massachusetts, Connecticut, and eastern New York southward through eastern Pennsylvania and New Jersey to northern Delaware and Maryland. It is also found in northwestern New York, northwestern Pennsylvania, southern Virginia and adjacent western North Carolina, northern South Carolina and Georgia, and eastern Tennessee. There is a doubtful record from Rhode Island.
Sphagnum bogs, swamps, and marshy meadows having clear, slow-moving streams with soft bottoms are the preferred habitat. Clemmys muhlenbergii occurs from sea level to elevations of more than 1200 m (in the Appalachians of North Carolina).
Bog turtles in Pennsylvania mature at a plastron length of about 7 cm when at least six years old (Ernst, 1977a). In New Jersey, males may mature in the 4th year at a minimum plastron length of 6.5 mm, and most females mature in years 4-5 at a minimum plastron length of 7 cm (Holub and Bloomer, 1977). Reproductive cycles have not been described for either sex.
Mating usually occurs in the afternoon from March to June. Courtship is usually in water, but may also take place on land. Females remain secluded in the mating season: the males must search them out. The male approaches the female and determines her sex through visual and olfactory cues. He than circles her, probing her tail and cloacal area with his nose, and may bite at her head and neck. If she moves away, he may or may not pursue her, or he may bite her legs and head to stop her. Chases usually take some time. If the female is receptive, the male gently bites her head and neck before mating, and she often withdraws her head. He than moves rearward and mounts her carapace from behind, bites her head and neck, and hooks all four feet onto the rim of her carapace. If under water, bubbles often escape from the male's nostrils. Copulation may last 5-20 minutes (Zappalorti, 1976; Arndt, 1977; Holub and Bloomer, 1977; Carl H. Ernst, pers. obs.). Males and females will mate with more than one partner during a breeding season (Carl H. Ernst, pers. obs.).
Nesting occurs from May to July, and the female does not always dig a cavity, but may instead tuck the eggs up under moss or grass tussocks.
Probably only one clutch of 1-6 (typically 3) eggs is laid each season. The elliptical, white, flexible shelled eggs are 21.8-36.0 x 13.0-19.0 mm. Incubation time varies as a function of temperature, taking 42-80 days under natural or artificial conditions. Most hatchlings emerge in late August and September; some may overwinter in the nest (Carl H. Ernst, pers. obs.).
Hatchlings are 21.1-30.1 mm in carapace length; the width (19.5-23.4 mm) is about 80% of this. They have dark-brown, immaculate carapaces and yellow plastra with a large, dark central figure. The bright head blotches are well-developed. Details of captive propagation are summarized by Herman and George (1986), Herman (1991), and Tryon and Herman (1991).
Clemmys muhlenbergii is omnivorous, and it feeds both on land and underwater. Surface (1908) found that the stomach of one individual contained 80% insects and 20% berries. Barton and Price (1955) reported the stomach contents of two adults consisted primarily of insects, with one lepidopterous larva constituting nearly half the total amount in each stomach. Beetles were the next most common food item, followed by the fleshy seeds of pondweed (Potamogeton). Large numbers of sedge (Carex) seeds had been consumed by these turtles and by nine others subsequently examined. Also represented were caddisfly larval cases and the cocoons of a parasitic hymenopteran or dipteran. The upper intestine of one turtle contained the shells of several young snails (Succinea ovalis); that of the other had pieces of a millipede and a cranefly wing. The most common item found in the faeces of six turtles caught in mid-August was the exoskeleton of the Japanese beetle (Popillia japonica). Other food items in its broad diet include ranid frogs (Rana sp.), earthworms, crickets, slugs, nestling birds, crayfish, dead water snakes, butterflies (Euphydryas phaeton), salamanders, mice and voles, skunk cabbage (Symplocarpus foetidus), cattails (Typha sp.), duckweed (Lemna), and berries (Ernst et al., 1994).
Clemmys muhlenbergii is active only during the warmer parts of the day. After emerging from its nocturnal shelter, it basks for a time before foraging. Our observations indicate that the bog turtle requires more heat to initiate and support activity than does C. guttata, Chrysemys picta, or Terrapene carolina; on many days sufficiently warm and sunny to arouse these turtles, C. muhlenbergii remains in seclusion. A muskrat burrow is often used as a hibernaculum. Bog turtles emerge from hibernation in April and return in mid-autumn. They are most active in April, May, June, and September, and they may aestivate during the dry months (July and August). An aestivating individual was found embedded in hard clay under a board on 8 August. The burrowing habit is well-developed: when alarmed the bog turtle rapidly digs into the mucky substrate with which it usually is associated. When handled it retires into its shell, and it rarely makes any attempt to bite or scratch.
IUCN Red List Status (1996)
Endangered (A1cd+2cd). The fragmented range of Clemmys muhlenbergii indicates that its distribution was undoubtedly broader and more continuous post-glacially, when its preferred sedge-meadow habitat was more abundant. Range contraction may have begun in the pre-colonial period, as forests re-invaded open boggy wetlands, but human activities have greatly accelerated this habitat loss. More recently, the unfortunate demand for these turtles in the commercial pet trade has reduced or eliminated many populations, with illegal collectors even removing them from protected habitats. Bog turtles are legally protected or regulated in virtually all states where they occur, are listed on Appendix I of the CITES treaty, and are a candidate species for listing by the U. S. Department of the Interior as a threatened species (January 1997).