Active during the day, the diamondback terrapin spends its time basking on land and feeding on a variety of worms, crabs, snails and fish (3) (5). The female, with its broader heads and crushing jaws, is well-adapted for eating hard-shelled prey, including salt marsh periwinkles (Littorina irrorata), small bivalves, barnacles, and crabs (4), while the smaller, narrower-headed male generally eats smaller prey and softer prey items such as worms, fish and some plant material (4) (6).
Courtship and mating occur from late March to May, and begin with a male approaching a female in the water and nuzzling or nudging her cloacal region with his snout. If the female remains still, the male mounts immediately and copulation occurs at the surface, but if the female swims away, the male may pursue her for long distances (3). Females lay their eggs from April through July (4), typically during the day, although females do not nest during heavy or prolonged rains (7). The nests, which are flask-shaped egg chambers, measuring up to 20 centimetres deep and 10 centimetres wide, are located above the high-tide mark along the sandy edges of salt marshes and rivers, in the dunes of sea beaches, and on offshore islands. Females produce at least two, and sometimes up to five, clutches a year, with each clutch containing from 4 to 22 leathery, dimpled, pinkish-white eggs. Female diamondback terrapins in the southern part of the range have been found to produce fewer but larger eggs than females in the north (3) (7). Research indicates that, like many other reptiles, the diamondback terrapin has temperature-dependent sex determination; that is, when eggs are incubated at low temperatures male hatchlings are produced, while incubation at higher temperatures results in female hatchlings (6). After 9 to 15 weeks, the eggs hatch, and the young may remain in the nest for the first winter, before emerging in April and May to head for brackish waters (6). If the hatchlings successfully avoid predation (for example, from gulls, crows and black-crowned night-herons), male diamondback terrapins mature before the end of their third year whereas females mature after their sixth year (4). Individuals may live for 25 to 40 years (6).