With an average adult size of 3 to 3.5 inches (7.6 to 8.9 cm) and a record length of 4.5 inches (11.4 cm), the bog turtle or Muhlenberg's turtle (Clemmys muhlenbergii) is generally considered to be the smallest turtle native to the United States, although the flattened musk turtle (Sternotherus depressa) runs a close second. Bog turtles hatch with a carapace length of about 1 inch (2.5 cm), making them enormous hatchlings relative to their adult size.

The carapace of Muhlenberg's turtle is light brown to mahogany to black with a lighter center or, often, a yellowish or reddish sunburst pattern on each scute. A low dorsal keel is present. Carapacial growth annuli may or may not be present, but are never as raised as those of its close relative the wood turtle (Clemmys insculpta). Bog turtles are flat at hatching, and become more domed with age. The plastron is brown to black with small areas of a lighter hue.

The bog turtle's trademark is a large patch of orange, red or yellow on the temporal region of the head often extending onto the neck, as shown in the photograph. Occasionally, the patch is split into two parts. The dark limbs are usually suffused with red, orange, or yellow mottlings.

Male bog turtles have a larger head, a longer, thicker tail with a more posterior vent, longer front claws, and a wider and less high shell than females. Additionally, males have a concave plastron, whereas females have a flat plastron with a wide notch at its posterior margin. Sexual maturity is thought to be at 7.0 to 7.5 cm (about 3 inches), which in the wild occurs at 5 to 8 years.

Mating occurs from April to June. The male grasps the female's shell with all four of his feet and bangs his shell against hers in typical Clemmys fashion. Nesting occurs from late May to July. From one to six (typically two to four) eggs are laid at the base of tussocks or on top of sphagnum in a 2 inch deep cavity which is covered with vegetation. Occasionally, females may lay multiple clutches. The eggs hatch in 49-60 days. The young emerge in August to September or may over-winter in the nest and emerge in the spring.

The bog turtle has a very spotty distribution and is broken down into two "mega" populations: northern and southern. The many northern populations are unevenly scattered through New York, Massachusetts, Connecticut, Pennsylvania, New Jersey, Delaware, Maryland and, possibly, Rhode Island. The northern populations occur at low elevation (0 to 1000 feet) and relatively close to either the ocean or the Great Lakes. The few southern populations are found in select areas of Virginia, North and South Carolina, Georgia, and Tennessee. They are found at higher elevations (2000 to 4000 feet) and are far removed from the influence of any large body of water.

Muhlenberg's turtles are habitat specialists being found in bogs, swamps, and wet meadows. They require full sunlight, an abundance of grassy or mossy cover, and spring seepage. They prefer mucky-bottomed waters into which they can dive and quickly bury themselves.

Bogs, swamps and wet meadows are successional habitats, that is, habitats which by nature have short lifetimes. It is only a matter of time before most bogs will become overgrown with trees and shrubs or fill in and dry up. In the past, natural phenomena such as wild-fires and animals such as beavers helped to maintain or create new bogs. With man's interference in these processes (not to mention his direct draining and destruction of bogs), bogs are disappearing at a rate greatly exceeding the rate at which they are being formed. Additionally, roads and other obstacles, combined with the destruction of surrounding areas, isolate bogs and make it difficult for bog turtle populations to relocate naturally.

Because bog turtles are extremely secretive, they are difficult to find. Indeed, they were not confirmed to occur in several of the above states until the 1980's. On one level, they are more secretive than rare. On the other level, many entire populations are disappearing or have disappeared. In a recent survey by the state of New York on its 7 historically known bog turtle populations, 47% of the sites were now unsuitable habitat with 26% having been physically destroyed by man and 21% having been lost to the natural process of habitat succession. Only 14% of the remaining sites were judged to be of excellent quality, and the remaining 39% were in fair to good condition depending on the degree of succession.