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Population features

Within much of its range, the painted turtle is the most abundant turtle species. Population densities range from 10 to 840 turtles per hectare (2.5 acres) of water surface. Warmer climates produce higher relative densities among populations, and habitat desirability also influences density. Rivers and large lakes have lower densities because only the shore is desirable habitat; the central, deep waters skew the surface-based estimates. Also, lake and river turtles have to make longer linear trips to access equivalent amounts of foraging space.

Adults outnumber juveniles in most populations, but gauging the ratios is difficult because juveniles are harder to catch; with current sampling methods, estimates of age distribution vary widely. Annual survival rate of painted turtles increases with age. The probability of a painted turtle surviving from the egg to its first birthday is only 19%. For females, the annual survival rate rises to 45% for juveniles and 95% for adults. The male survival rates follow a similar pattern, but are probably lower overall than females, as evidenced by the average male age being lower than that of the female. Natural disasters can confound age distributions. For instance, a hurricane can destroy many nests in a region, resulting in fewer hatchlings the next year. Age distributions may also be skewed by migrations of adults.

To understand painted turtle adult age distributions, researchers require reliable methods. Turtles younger than four years (up to 12 years in some populations) can be aged based on " growth rings " in their shells. For older turtles, some attempts have been made to determine age based on size and shape of their shells or legs using mathematical models, but this method is more uncertain. The most reliable method to study the long-lived turtles is to capture them, permanently mark their shells by notching with a drill, release the turtles, and then recapture them in later years. The longest-running study, in Michigan, has shown that painted turtles can live more than 55 years.

Adult sex ratios of painted turtle populations average around 1:1. Many populations are slightly male-heavy, but some are strongly female-imbalanced; one population in Ontario has a female to male ratio of 4:1. Hatchling sex ratio varies based on egg temperature. During the middle third of incubation, temperatures of 23–27 °C (73–81 °F) produce males, and anything above or below that, females. It does not appear that females choose nesting sites to influence the sex of the hatchlings; within a population, nests will vary sufficiently to give both male and female-heavy broods.