Aspects of the Life History of the Texas Map Turtle (Graptemys Versa)
Lindeman, Peter V., The American Midland Naturalist
The Texas map turtle (Graptemys versa) is endemic to the Colorado River drainage in southcentral Texas. A study of its life history was undertaken using data collected in 1998-2000 from a population in the South Llano River, southernmost tributary of the Colorado drainage, and data from museum specimens that had been collected from the South Llano River in 1949. Compared to congeners, G. versa is a small-bodied species. Its small body size is, predictably, linked to relatively small clutch size, small egg size, rapid growth toward asymptotic size and early maturation. As many as four clutches may be laid during an active season, although the effects of follicular atresia on clutch frequency are not known. Both clutch size and egg width were positively correlated with female body size, with the former relationship having a log-log slope significantly less than the expected value of 3, probably due to the latter relationship. Analyses were consistent with the hypothesis of anatomical constraint on egg size, with at least smaller females laying eggs that are of less than optimal size. No differences were found in body size or clutch size between 1949 and 1998-2000 despite a large-scale change in diet associated with invasion of the river by Asian clams (Corbicula sp.). However, body size is substantially reduced in the South Llano River compared to other sections of the Colorado drainage, a finding mimicked by at least one other turtle species in the drainage, Pseudemya texana.
Map turtles (Graptemys) are medium-sized emydid turtles found primarily in eastern and central North American rivers. Twelve species are recognized (Ernst et al., 1994). In a study of over 2200 museum specimens (Lindeman, 2000), maximum plastron lengths of females ranged from 152 mm in G. oculifera to 239 mm in G. pseudogeographica (erroneously reported as 229 mm for the latter in Lindeman, 2000). Males exceeded 102 mm in plastron length only in northern populations of G. geographica, G. ouachitensis and G. pseudogeographica. The largest female body sizes are achieved by members of the megacephalic clade (G. barbowri, G. ernsti, G. gibbonsi and G. pulchra) in the southeastern United States and by northern populations of G. geographica, G. ouachitensis and G. pseudogeographica (Lindeman, 2000). The large sexual difference in body size is driven primarily by extreme bimaturism, with males maturing at 2-5 y of age and females maturing perhaps as late as 14 y of age (Lindeman, 1999). Among all species of Graptemys that have been studied, average clutch size ranged from 4.7-14.1 eggs (Table 1). Average egg sizes ranged from 22.0-29.5 mm in width and from 34.0-39.4 mm in length (Table 1).
The Texas map turtle (Graptemys versa) is endemic to the Colorado River drainage of the Edwards Plateau in central Texas, between 30°N and 32°N (Vogt, 1981). In an exhaustive compendium of literature citations on the 56 species of turtles native to North America (Ernst et al., 1994), G. versa has the distinction of having the shortest species account with the fewest citations of ecological studies. Such a lack of information on basic life history parameters and ecology is a serious impediment to management of a species. The paucity of information on G. versa in the published literature, together with the availability of a large series of museum specimens collected five decades ago, provided the impetus for the present study of G. versa life history.
I used part of a large series of museum specimens captured on one day in 1949 and field work in 1998-2000 with a population from the same river to study body size, maturation, growth in body size and reproduction of Graptemys versa. Based on its southern distribution and lack of megacephaly, the species may be expected to be a relatively small-bodied, early-maturing species, life-history attributes which may be associated with relatively small clutch and/or egg size compared to other Graptemys. …