Population ecology and demography of the freshwater turtles chrysemys picta and Sternotherus odoratus

Records of over 6000 captures of about 1950 individual turtles were analyzed to determine ecological and demographic characteristics of syntopic populations of the freshwater turtles Chrysemys picta (Emydidae) and Sternotherus odoratus (Kinosternidae). Populations of these turtles were studied over a three year period in an urban lake near Richmond, Virginia, U.S.A. Monthly samples from a nearby lake provided information on reproductive cycles and their variations between species and years.

Male reproductive cycles were synchronous between species and years except for onset of spermatogenesis; S. Odoratus began about one month later than C. Picta. Peak spermiogenesis was in August, as were maximum testis weights. Female cycles were generally concordant but S. Odoratus were ovigerous about three weeks longer than C. Picta. Both species exhibited size specific fecundity and produced multiple clutches. Age specific fecundity was demonstrated by S. Odoratus. Mean clutch size was 4.2 for C. Picta and 3.1 for S. Odoratus. There was little variation between years. Female minimum age at maturity at this site was six years for C. Picta and four years for S. Odoratus. Males matured in their third and second years, respectively.

Chrysemys picta exhibited age and sex specific differences in growth rates, but male and female S. Odoratus grew at almost identical rates. Most individuals of both species grew significantly less during drought years. Delayed emergence from the nest was documented for this area for C. Picta, but results suggest most S. Odoratus emerge the year of egg deposition. Population sizes were estimated to be 517 for C. Picta and 534 for S. Odoratus. Both populations appear to be growing. Sex ratios were not significantly different from unity. Female C. Picta at this site mature at age eight years and males at age four, whereas female S. Odoratus mature at age four and males at two years of age. The differences in age at maturity between sites is discussed from the standpoint of life history evolution. Survivorship during the study period was high for all sex and age groups, except for juvenile C. Picta. The population ecology and life histories of these species are compared and discussed in relation to urban environments.