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Turtle Studies on the University of Michigan's E. S. George Reserve


Justin Congdon

 

common snapping turtle
Three species of turtles are common on the ESGR: midland painted turtles, (Chrysemys picta marginata); Blanding's turtles, (Emydoidea blandingii); and common snapping turtles (Chelydra serpentina). Between 1953-1994, approximately 35,000 captures and recaptures have been made of 8,105 marked individuals (including hatchlings) on the ESGR, and an additional 1,105 individuals were marked in marshes immediately adjacent to the ESGR. Owen Sexton marked 913 painted turtles and 92 Blanding's turtles in five primary marshes studied (Southwest Swamp, Fishhook Marsh, Crane Pond, Cattail Marsh and the Canal (The Ditch) on the ESGR between 1953-1957. From 1968-1973 Henry Wilbur, working primarily in the same wetlands studied by Owen Sexton, marked approximately 600 painted turtles, 60 Blanding's turtles, and 12 common snapping turtles on the ESGR. Of the painted turtles captured by Henry Wilbur, 46 were recaptures of those marked during the 1950s. From 1975-1979, Donald Tinkle and Justin Congdon marked 1,216 painted turtles, 281 Blanding's turtles, and 356 snapping turtles. During the period from 1980-1994, the study was continued by Justin Congdon and an additional 2,687 painted turtles, 469 Blanding's turtles, and 1,291 snapping turtles were marked on the ESGR. Between 1975-1994, 13 and 95 painted turtles, and 21 and 37 Blanding's turtles were recaptured that had been marked by Sexton and Wilbur, respectively.

Methods used to capture turtles during the previous studies (muddling, dip net, baited traps, fyke nets, basking traps, and captures on land) were all used in the present study, with the major difference primarily related to the intensity of effort and inclusion of East Marsh as a primary study site, and Southeast Marsh and Hidden Lake as minor sites. The present study relied heavily on a 0.9 km drift fence that completely enclosed Crane Pond (1975-1982) and a 1.3 km fence that enclosed East Marsh (1983-1994), and at shorter drift fences installed between a marsh and adjacent nesting areas. In addition, during all nesting seasons (mid-May to the beginning of July) from 1976-1994 from four to seven people walked fences and searched nesting areas between 0600h and 2300h. As a result of nesting season efforts, about 3,00 x-radiographs of gravid females were taken and observations were made on over 2,500 nests of all three species.

The three common species on the ESGR have some features in common. Sex is determined by the incubation temperature of the embryos rather than genetically; therefore, nest site selection and annual variation in weather patterns can alter sex ratios of hatchlings. Females of all three species leave marshes and move into nesting areas to lay eggs from mid-May to early July; however, the nesting season for snapping turtles is substantially shorter than that of painted and Blanding's turtles. Across all years of the study on the ESGR, nest survivorship is highest in painted turtles, intermediate in Blanding's turtles, and lowest in snapping turtles; however, among year variation in nest survival ranges from 0-67% for all species. Eggs incubate throughout the summer, hatch during late August and September and then hatchling Blanding's and snapping turtles emerge in autumn and move to aquatic areas to overwinter. In contrast the majority of painted turtle hatchlings remain in the nest for the winter, emerging in the following April. Female painted turtles reach sexual maturity from 7-12 y of age and produce one or two clutches of about 7 eggs annually. Both clutch size and egg size increases significantly with body size of females. Males mature in their 4th or 5th year at smaller body size than do females.

Research on Blanding's turtles has increased in recent years because of their conservation concern. Blanding's turtles are gentle and seldom, if ever, attempt to bite while being handled by researchers. They reach maturity between 11 and 20 y and males and females are similar in body size. Blanding's turtles are extremely long-lived, with cohort generation times of approximately 37 years. Clutch size averages 10 eggs and one or fewer clutches are produced per year. They are primarily carnivorous with diets that are comprised of snails, crayfish, earthworms, insects and vertebrates.

Snapping turtles are the most widely distributed of the three species that occupy the ESGR. Snapping turtles are aggressive and the largest males are twice the body mass of large females. Females mature between 11 and 16 years of age, clutch size averages 28 eggs and clutch frequency is less than annual. Snapping turtles are primarily carnivorous in some habitats; however, a large portion of their diet can consist of plant material in some habitats and during some seasons.

Whereas the total number of adult turtles on the ESGR has grown from about 1,200 to 1,600 individuals from 1975-1994, the relative numbers of turtles of each species have remained fairly constant. The long-term turtle studies on the ESGR provide encouraging news with respect to conservation and management programs, since the turtle populations and community composition have been essentially stable over at least two decades. Stability has been maintained with no other provisions other than those of: 1) undisturbed and unpolluted wetland habitats and associated terrestrial areas that are necessary for nesting, 2) corridors for movement among wetlands both on and off of the ESGR, and 3) protection from direct exploitation by man.

 

Some papers from the E. S. George Reserve study:

Breitenbach, G. L., J. D. Congdon, and R. C. van Loben Sels. 1984. Winter temperatures of Chrysemys picta nests in Michigan: effects on hatchling survival. Herpetologica 40:76-81.

Burke, V. J., R. D. Nagle, M. Osentoski, and J. D. Congdon. 1993. Common snapping turtles associated with ant mounds. Journal of Herpetology 27:114-115.

Congdon, J. D., G. L. Breitenbach, R. C. van Loben Sels, and D. W. Tinkle. 1987. Reproduction and nesting ecology of snapping turtles (Chelydra serpentina) in southeastern Michigan. Herpetologica 43:39-54.

Congdon, J. D., A. E. Dunham, and D. W. Tinkle. 1982. Energy budgets and life histories of reptiles. pp. 233-271. In. Biology of the Reptilia, Vol. 13. G. Gans (Ed.) Academic Press, New York.

Congdon, J. D., A. E. Dunham, and R. C. van Loben Sels. 1993a. Delayed sexual maturity and demographics of Blanding's turtles (Emydoidea blandingii): implications for conservation and management of long-lived organisms. Conservation Biology. 7:826-833.

Congdon, J. D., A. E. Dunham, and R. C. van Loben Sels. 1994. Demographics of Common snapping turtles (Chelydra serpentina): implications for conservation and management of long-lived organisms. American Zoologist 34:397-408.

Congdon, J. D. and R. E. Gatten, Jr. 1989. Movement and energetics of nesting Chrysemys picta. Herpetologica 45:94-100.

Congdon, J. D. and J. W. Gibbons. 1987. Morphological constraint on egg size: a challenge to optimal egg size theory? Proceedings National Academy of Science, U.S.A. 84:4145-4147.

Congdon, J. D. and J. W. Gibbons. 1989. Biomass productivity of turtles in freshwater wetlands: a geographic comparison. Pp. 583-592. in Freshwater Wetlands and Wildlife. (Eds.) R. R. Sharitz and J. W. Gibbons.

Congdon, J. D., T. E. Graham, and B. Brecke. 1995. Emydoidea blandingii (Holbrook, 1842), Blanding's Turtle. In: Conservation of Freshwater Turtles. A. Rodin and P. Pritchard (eds.) IUCN Publication.

Congdon, J. D., S. W. Gotte, and R. W. McDiarmid. 1993b. Ontogenetic changes in habitat use by juvenile turtles, Chelydra serpentina and Chrysemys picta. Canadian Field Naturalist 106:241-248

Congdon, J. D., T. E. Graham, and B. Brecke. 1995. Emydoidea blandingii (Holbrook, 1842), Blanding's turtle. In: Conservation of Freshwater Turtles. A. Rodin and P. Pritchard (eds.) IUCN Publication.

Congdon, J. D., J. L. Greene, and J. W. Gibbons. 1986. Biomass of freshwater turtles: a geographic comparison. American Midland Naturalist 115:165-173.

Congdon, J. D. and D. W. Tinkle. 1982. Reproductive energetics of the painted turtle (Chrysemys picta). Herpetologica 38:228-237.

Congdon, J. D., D. W. Tinkle, G. L. Breitenbach and R. C. van Loben Sels. 1983. Nesting ecology and hatching success in the turtle Emydoidea blandingi. Herpetologica 39:417-429.

Congdon, J. D. and R. C. van Loben Sels. 1991. Growth and body size variation in Blanding's turtles (Emydoidea blandingii): relationships to reproduction. Canadian Journal Zoology 69:239-245.

Congdon, J. D. and R. C. van Loben Sels. 1993a. Relationships of reproductive traits and body size with attainment of sexual maturity and age in Blanding's turtles (Emydoidea blandingii). Journal Evolutionary Biology 6:547-557.

Tinkle, D. W., J. D. Congdon, and P. C. Rosen. 1981. Nesting frequency and success: implications for the demography of painted turtles. Ecology 62:1426-1432.

Wilbur, H. M. 1975. The evolutionary and mathematical demography of the turtle Chrysemys picta. Ecology 56:64-77.

 
 
 
 
 
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