Tropical Zoology 23, 000-000, 2010 Morphological characterization of adults of Orbigny’s slider Trachemys dorbignyi (Duméril & Bibron 1835) (Testudines Emydidae) in southern Brazil A. Bager 1,1, T.R.O. Freitas 2 and L. Krause 3 1 Departamento de Biologia, Universidade Federal de Lavras, Campus Universitário, 37200-000 Lavras, Minas Gerais, Brazil 2 Departamento de Genética, Universidade Federal do Rio Grande do Sul, C.P. 15053, Porto Alegre-RS, Brazil, 91501 970 3 Departamento de Zoologia, I.B., Setor de Herpetologia, Prédio 43.435-S/105, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonçalves 9.500, Porto Alegre-RS, Brazil, 91501 970 Received 11 March 2010, accepted, 11 June 2010 We characterized a population of the Southern Orbigny’s slider turtle, Trachemys dorbignyi (Duméril & Bibron 1835), in its natural environment, focusing on the sex ratio, morphology and sexual dimor- phism. The male:female sex ratio was 1.02:1. The 16.8% difference in mean size between the sexes of T. dorbignyi is among the small- est in the family Emydidae. The female is larger than the male in all measurements except one (carapace and plastron terminal distance, CPD). CPD proved to be the most useful measurement to identify the gender of smaller individuals. The discriminant analysis was ca- pable of differentiating males and females with 100% accuracy. CPD was the most important variable in differentiating males, and the cara- pace height and width of the mouth were most important for females. key words: sexual dimorphism, morphometry, sexual maturity, population structure, Trachemys dorbignyi, Emydidae. Introduction ......................................................................................................... 2 Material and methods ........................................................................................... 3 Results .................................................................................................................. 6 Discussion ............................................................................................................ 9 Acknowledgements ............................................................................................. 11 References ........................................................................................................... 11 1 Corresponding author: Dr Alex Bager, (E-mail: [email protected]). 2 A. Bager, T.R.O. Freitas and L. Krause INTRODUCTION According to Ernst (1990), the most frequently studied genus of fresh- water turtles is probably Trachemys (Reptilia Testudines Emydidae), commonly known as sliders. Seidel et al. (1999) reported that the Trachemys scripta com- plex, including T. dorbignyi (Duméril & Bibron 1835), is an especially difficult group, with no clear taxonomic relationships among the species and subspecies because of a lack of sufficient genetic and morphological data. Phylogenetic anal- yses of the 26 taxa recognized in the genus Trachemys have suggested the existence of 15 species, 8 of them polytypic (Seidel 2002). Trachemys dorbignyi, the Southern Orbigny’s slider, is the southernmost species of the group, inhabiting Uruguay, northern Argentina and the state of Rio Grande do Sul in southernmost Brazil (Lema & Ferreira 1990). There is no explanation for the isolation of T. dorbignyi in southern South America. The geographically closest species is Trachemys adiutrix (Vanzolini 1995), which occurs 3000 km to the north in the Brazilian state of Maranhão (Vanzolini 1995). According to Seidel (2002), the relationships of T. adiutrix with T. dor- bignyi and the other South American species require further study. Freiberg (1969) described two subspecies: T. d. dorbignyi from Uruguay and Argentina, and the Brazilian T. d. brasiliensis. However, these subspecies have not been widely accepted by subsequent authors. Barco & Larriera (1991) repeated Freiberg’s analysis using larger samples and concluded that the suppos- edly diagnostic morphological characteristics separating the two subspecies were mainly a reflection of ontogenetic variation. Seidel (2002) continued to recog- nize T. d. brasiliensis as a subspecies of T. dorbignyi. Among the publications describing the biology of T. dorbignyi (Freiberg 1969; Krause et al. 1982; Silva et al. 1984; Cascone et al. 1991; Vanzolini 1997, Molina & Gomes 1998a, 1998b; Malvasio et al. 1999; Souza et al. 2000; Bager et al. 2007a, 2007b; Gonçalves et al. 2007), none has described the morphology of the species in detail. Biometric studies have been an impor- tant tool for the differentiation between species and subspecies (Lamb & Lovich 1990, Lovich & Lamb 1995, Seidel et al. 1999), geographical variation within a species (Iverson 1985, Lubcke & Wilson 2007), analysis of reproductive parameters (Congdon & Van Loben Sels 1991, Daza & Páez 2007) and evaluation of the impact of commercial exploitation (Close & Seigel 1997, Gamble & Simons 2004). Thus, a more detailed understanding of morphologi- cal variation in Trachemys dorbignyi will be critical to elucidating the systematic relationships of this species. For example, Stephens & Wiens (2003) excluded T. dorbignyi from a phylogenetic study of the Emydidae because of a lack of data on its morphology and diet. In the present study, we characterized a wild population of T. dorbignyi in southern Brazil in order to complement the available morphological data and make it possible to compare this population with other species of the Trachemys complex. We analyzed the sex ratio, morphological variables in both genders and sexual dimorphism. Trachemys dorbignyi morphology 3 MATERIAL AND METHODS The study area lies on the coastal plain of Rio Grande do Sul, with a subtropi- cal climate (Cfa) (Köeppen 1948) characterized by well-distributed rainfall through- out the year and mean annual precipitation of 1252 mm. The maximum temperature exceeds 22 °C in summer (December to March) and oscillates between 18 and – 3 °C in winter (June to September) (Moreno 1961). The area is called the Verde La- goon Complex (UTM 22H 385820mE 6445000mN) and includes the Verde La- goon itself, the Senandes and Bolacha streams, and associated swamps, palustrine forest and fields (Fig. 1). This complex of aquatic, terrestrial and transitional habitats is an area of estuarine swamps of about 3500 ha, with a maximum water depth of 80 cm, salinities up to 17‰ and extensive aquatic macrophytes. Adults of T. dorbignyi were captured by hand via snorkeling (Moll & Moll 2004, Lubcke & Wilson 2007). Males and females occupy the same habitat, mak- Fig. 1. — Verde Lagoon Complex, state of Rio Grande do Sul, Brazil. Te nearest large town is Rio Grande; urban areas are in gray. Axes are in UTM grid units. 4 A. Bager, T.R.O. Freitas and L. Krause ing it possible to collect an unbiased sample of individuals of all length classes larger than 130 mm CL. The greatest advantage of sampling by diving is the elimination of errors produced by the use of baited traps. Traps can select individuals for sex or size, and trap samples may be influenced by diet, differing mobility between the sexes and other unknown factors (Boundy & Kennedy 2006). All the turtles were marked by notching the marginal scutes using Cagle’s (1939) method. Only data taken the first time each animal was captured were used. We combined the principal morphometric measurements of the carapace and plastron described in previous studies of freshwater turtles (Legler 1990, Daza & Paéz 2007, Rivera 2008) and established others. Up to 29 measurements were taken on each individual. Fig. 2 shows how the measurements were made and their acronyms. Circular measurements such as circular carapace length (CCL) or circular carapace width (CiCW) were taken with a non-stretching tape. They were measured at the same points for carapace length and maximum carapace width, respectively. Cephalic and mouth width were measured over the tympanic mem- branes and the maximum width of the lower rhamphotheca, respectively. Males were sexed on the basis of secondary sexual characteristics: tail length (longer in males) and carapace melanization (males have darker carapaces) (Seidel 1990, Molina 2001). Only females over 180 mm in carapace length were included in the descriptive statistics, because this is the shortest length recorded for nesting females in the area (A. Bager pers. comm.). Another 16 non-reproductive females with carapace lengths between 154 and 179 mm were included in the regression analysis. This allowed comparisons between males and females of the same size, and also estimation of the carapace length at which the allometry between the sexes begins. Original, non-transformed data were used for the statistical analyses, except in the discriminant analysis which was executed using the values of each variable divided by the maximum length of the carapace. This approach was used to reduce the effect of size and to improve the analyses of allometry of shape between males and females. Some investigators have questioned the use of data obtained from the ratio between two variables in morphometric analyses (Peres-Neto et al. 1995), but Seidel et al. (1999) and Aresco & Dobie (2000) used this technique for chelonians, with some success. Sexual size dimorphisms (SSD) were quantified as recommended by Stephens & Wiens (2009), according to the equation: SSD = (Mean of Female CL / Mean of Male CL) – 1 The sex ratio of males and females was compared by χ² analysis, using the Yates correction and equal expected proportions. The fit to a normal distribution