LOCAL AND GEOGRAPHIC-VARIATION IN THE REPRODUCTIVE-BIOLOGY OF THE SNAPPING TURTLE (CHELYDRA-SERPENTINA)

We studied female snapping turtles at two shallow lakes on the Crescen t Lake National Wildlife Refuge from 1990-1994, most intensively durin g the nesting seasons of 1993 and 1994. Females matured at 28-29 cm ca rapace length (CL), 22-23 cm plastron length (PL), 5.5-6.5 kg body mas s, and 10-12 winters of age. Nesting was recorded from 1-28 June, but in a given year season length was 13-18 days and its start may be inve rsely related to spring temperatures. Nesting began between 0400 and 0 930 h and between 1800 and 0100 h. During the warm June of 1994, nesti ng was equally divided between morning and evening, but during the rel atively cold June of 1993, smaller females apparently shifted predomin ately to evening nesting. II was not determined whether all females re produced every year. Reproducing females produced one clutch per year of 20-73 eggs ((x) over bar = 46.8) weighing 6.4-19.1 g each ((x) over bar = 11.4). Egg size, clutch size, and clutch mass were each positiv ely correlated with female size. However, relative to female body size , large females produce smaller eggs and smaller clutches, but the sam e total clutch mass, as small females. No differences in reproductive output were detectable between 1993 and 1994. Geographic variation in female size across the species range is significant. In temperate regi ons, mean female size is correlated with latitude, longitude, and elev ation, with the largest females found in Nebraska and South Dakota. Fe males in the tropics are as large as the largest temperate females. Eg g size does not vary with average female body size across populations or with latitude. Eggs in the tropics average nearly twice the mass of those in temperate regions, but relative to female body size, egg siz e in the tropics is not significantly different. Clutch size is positi vely correlated with female body size within and among populations, as well as with latitude. However, relative to body size, clutch size do es not vary among temperate populations, although it is larger there t han in the tropics. No trade-off between average egg size and clutch s ize was evident among populations. Clutch mass was correlated with fem ale body size and varied geographically along with body size. However, relative to body size, there was no significant geographic variation in clutch mass. Variation in growth rate to maturity among temperate p opulations suggests that at northern, western, and high elevation loca tions a greater selective advantage accrues from maturing at a larger body size than from maturing at an earlier age, perhaps because of the strong body size-clutch size correlation.