Caracterização de uma população de trachemys dorbigni (duméril & bibron, 1835) (testudines, emydidae) em um corpo d’água em São Gabriel, Rio grande do Sul, Brasil

Abstract

In order to describe the structure of a small population of the emydid Trachemys dorbigni, we carried out captures and recaptures of specimens of this population from March 2009 to February 2010 in a reservoir occurring in the Fundação Estadual de Pesquisa Agropecuária (FEPAGRO), São Gabriel, Rio Grande do Sul. Fourty four individuals were collected: 34 females, seven males, and three juveniles. The density of individuals in this studied population was 0,050 turtles/m² (0,007 males/m² and 0,038 females/m²) and the total biomass was 51,02g/m² (7,40g/m² for males and 43,62g/m² for females). On average, females showed a tendency to be heavier and larger than males, but this difference was not statistically significant. The mean length of the carapace was 190.50 ± 22.81 mm (N = 41) and the mean mass was 1,210 ± 420 g (N = 37). A multiple regression analysis showed that none of the environmental factors herein used (temperature, relative humidity, and rainfall) had a significant influence on the rates of capture and recapture, even when analyzing sexes separately. Only mean values of the distance from the posterior border along the intersection of anal shells on plastron relative to the posterior margin of carapace (VAO), the length of the base of tail to cloacae (BAC), and the length of the base of tail to its apex (BAP) in females were lower than in males. BAC was statistically significant different among males and females. Differences in this measure might be an adaptation which has facilitated the mating in this species. Sex ratio differed from 1:1 (4.85 females: 1 male) in this population, which may be caused by the selectivity of traps. The lake of suitable areas available to thermoregulation at the monitored site, the suns be related to the high frequency of leeches observed on T. dorbigni.