Reproduction in the Suwannee cooter, Pseudemys concinna suwanniensis

Abstract

The Suwannee cooter, Pseudemys concinna suwanniensis, is the largest emydid turtle in North America. From 1988 to 1993, we studied the reproductive and nesting biology of a population of this turtle in the headwaters of the Wakulla River in northern Florida. Based on 239 marked females, we estimated a population of about 305 adult females in ca 5 km (41 ha) of river. Nesting females ranged in plastron length (PL) from 304 to 383 mm and in body mass from 4.5 to10.5 kg. We estimate that female body size at maturity varies by as much as 50 mm, with a mean size at maturity of ca 330 mm PL, or only about 1 cm less than the mean PL. The negligible post-maturational growth of adult females, coupled with their size range, suggests that sexual maturity is less a function of size than of age, which we estimate minimally at 10 years. Nesting begins in late March or early April and extends into early August. Females may lay as many as five or more clutches at intervals of approximately 16-25 days. Most, if not all, mature females nest annually. Nesting is diurnal and frequently coincides with rainfall. Typical clutches contain 8-27 eggs (mean 17.5). Average annual reproductive potential may approach 70 eggs, with a maximum for individuals of >100. Normal egg and hatchling sizes span ranges of 9.8-21.7 g and 6.8-14.6 g, respectively, and are, at most, only weakly correlated with female body size. Clutch mass tends to increase with body size, but relative clutch mass, which averages ca 0.05. does not. These results suggest this species is an egg number maximizer rather than an egg size optimizer. We estimate for the population an annual production of ca 26,700 eggs equivalent to 10.6 kg/ha/yr. Hatchlings may emerge from nests in the fall or may overwinter underground and delay emergence until the following spring. Sex determination of hatchlings is temperature-dependent, with a pivotal temperature at constant incubation temperatures of ca 28.4°C. Although this population occurs in a state park, the concentration of nesting activity principally along a linear corridor of artificial habitat has fostered nest predation approaching 100 percent; chief egg predators are raccoons (Procyon lotor) and fish crews (Corvus ossifragus). Some nesting females are also depredated, principally by raccoons. Continuing forest growth threatens to shade out the main nesting site and to skew the sex ratios of hatchlings toward males. We present management recommendations both for this population and for the species throughout Florida.