Population Structure of Mesoclemmys vanderhaegei (Bour, 1973) (Testudines:Chelidae) in the Cerrado of Chapada dos Guimarães, Mato Grosso, Brazil

Estrutura de população de Mesoclemmys vanderhaegei (Bour, 1973) (Testudines: Chelidae) no Cerrado de Chapada dos Guimarães, Mato Grosso, Brasil

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Population Structure of Mesoclemmys vanderhaegei (Bour, 1973) (Testudines:Chelidae) in the Cerrado of Chapada dos Guimarães, Mato Grosso, Brazil

Estrutura de população de Mesoclemmys vanderhaegei (Bour, 1973) (Testudines: Chelidae) no Cerrado de Chapada dos Guimarães, Mato Grosso, Brasil

Elizângela Silva de Brito^I,II,*; Christine Strussmann^I,II,III; Jerry Magno Ferreira Penha^I,II

^IPrograma de Mestrado em Ecologia e Conservação da Biodiversidade, Instituto de Biociências, Universidade Federal de Mato Grosso - UFMT, Cuiabá, MT, Brazil

^IINúcleo de Estudos Ecológicos do Pantanal (NEPA), Instituto de Biociências, Universidade Federal de Mato Grosso - UFMT, Cuiabá, MT, Brazil

^IIIDepartamento de Ciências Básicas e Produção Animal, Faculdade de Agronomia e Medicina Veterinária, Universidade Federal de Mato Grosso - UFMT, Cuiabá, MT, Brazil

ABSTRACT

Studies on population structure of freshwater turtles belonging to the family Chelidae are scarce in Brazil. Herein we describe the structure of a population of the chelid Mesoclemmys vanderhaegei in a Cerrado area in the municipality of Chapada dos Guimarães, Mato Grosso, Brazil. A total of 80 individuals were captured in five collecting sites, from January to March 2007: 42 adult females, 27 adult males, and 11 juveniles with undetermined sex. Among 80 individuals, 24% were recaptured, at least once. This is the first attempt to estimate the size of a population of Mesoclemmys vanderhaegei. Our results suggest that additional sampling efforts are needed for more accurate estimates of population structure. Nevertheless, they surely provide minimum values of the number of individuals of M. vanderhaegei living in the locality sampled.

Keywords: Chelidae, Neotropics, sex ratio, population size, freshwater turtles.

RESUMO

Estudos sobre estrutura de populações de quelônios da família Chelidae são escassos no Brasil. Nós descrevemos a estrutura de uma população de Mesoclemmys vanderhaegei de uma área de Cerrado, no município de Chapada dos Guimarães, Mato Grosso, Brasil. Os indivíduos foram capturados entre os meses de janeiro e março de 2007, em cinco sítios de coleta. Foram capturados 80 indivíduos: 42 fêmeas, 27 machos e 11 indivíduos com sexo não determinado (jovens). Dos 80 indivíduos capturados, 24% foram recapturados pelo menos uma vez. Esta é a primeira tentativa de estimar o tamanho de uma população de Mesoclemmys vanderhaegei. Embora nossas estimativas sugiram a necessidade de maior esforço de captura, nossos dados permitem apontar números mínimos de indivíduos para a localidade amostrada.

Palavras-chave: Chelidae, Neotrópicos, razão sexual, tamanho de população, quelônios aquáticos.

Introduction

Information on structure and size of chelonian populations are essential to adequately evaluate their responses to impacts imposed by antropic activities and habitat changes and, ultimately, to evaluate their conservation status (Smith et al. 2006). However, data on population dynamics of such organisms with long life cycles are usually difficult to obtain (Gibbons 1987, Pough et al. 1998). According to Congdon et al. (1994), even long term studies on chelonians offer limited information regarding their life history traits.

Natural history data are lacking for the most majority of Brazilian species of freshwater turtles (Souza 2004), including those in the family Chelidae, which also occurs in Australia. It comprises more than 50 species, of which 17 were recorded in Brazil (Bonin et al. 2006). Among them is Mesoclemmys vanderhaegei, also recorded for Paraguay and Argentina. According to the literature, the species is restricted to the upper and medium Paraguay River Basin (Iverson 1992, McCord et al. 2001, Bour & Zaher 2005, Bonin et al. 2006), although additional records refer to its presence in rivers belonging to the Amazon Basin (Villaça 1999, Brandão et al. 2002). The species is considered as "near threatened" by the International Union for Conservation of Nature (IUCN 2008).

We here describe the structure and population size of M. vanderhaegei, after field studies in Cerrado (a huge savannah-like formation in central Brazil) habitats at Chapada dos Guimarães (Mato Grosso, Midwestern Brazil).

Material and Methods

Specimens of M. vanderhaegei were captured from January to March 2007, in five distinct sites (Table 1). These are mainly small, oligotrophic streams, two of them artificially dammed. Bottom substrate was rocky, sandy, or muddy, and riparian vegetation consists of grasses, shrubs, and small trees. The two dammed streams (Quineira and Monjolinho), and two marginal lagoons existing along the stream at Aldeia Velha also presented abundant aquatic vegetation, both submergent and floating.

A 1km section was sampled along each stream, where seven funnel traps (1 m long × 0.45 m external diameter × 0.20 m entrance diameter; plastic mesh 5.0 × 1.0 mm) baited with bovine meat were set near and parallel to the margins. Distance between each consecutive trap varied from 20 to 100 m. At Aldeia Velha, a single trap was installed on each of the two marginal lagoons, in addition to the seven traps installed along the main course of the stream. At each collecting site, traps were maintained in operation for six consecutive days, being revised three times a day (06:00-07:00 AM; 02:00-03:00 PM, and 08:00-09:00 PM). Trapped individuals were marked according to Cagle (1939), measured (maximum carapace length - CL, in mm), and sexed, after which they were released at the capture site. Sex was determined by external examination of secondary sexual characteristics: chelid males usually possess a longer and thicker tail than females. Individuals in which sex could not be determined (N = 11), all of them with CL measuring less than 100 mm, were treated as juveniles and grouped together in a single category (hereby treated as "SND").

Specimens positively sexed were used to test if sex ratio differed from 1:1 by means of the chi-square test. Due to the small number of degrees of freedom (only 1), Yates Correction for Continuity was employed to run this analysis (Zar 1999). We used MARK™ software to estimate population size (N) at each sampling site. We divided the six sampling days in two capture sessions of three days each, and ran the analyses considering the population as a closed one. We ran together all the estimates, considering each population as a group (a total of five groups analyzed). MARK is a flexible software which allows testing hypothesis about spacial and temporal variation influencing probabilities of capture (p) and of recapture (c) of individuals. Estimates were done by using the method of maximum likelihood. Because of the reduced number of capture sessions (only two) we could test only two models: a more general one (in which p and c do not differ between sites) and a most restrictive (p and c dependent on sites). In order to estimate N we constraint p = c for the second capture session (Cooch & White 2007). Selection of the models was based in the corrected Akaike Information Criterion (AIC_c) (Burnham & Anderson 2002). AIC_c is a statistics that combines a relative measure of adjustment (or deviance) of the models (dev), and the number of estimated parameters (K), where AIC_c = dev+2K (Burnham & Anderson 2002, Cooch & White 2007). The lower the AIC_c, the better the model, which therefore represents the best compromise between absence of precision in the estimates (models incorporating many parameters) and better adjustment (lower deviance) (Cooch & White 2007).

Results

A total of 80 individuals of M. vanderhaegei were captured, of which 42 (52.50%) were females, 27 (33.75%) were males, and 11 (13.75%) were SND (Figure 1). In females, mean length of the carapace (CML) was 168 ± 16 mm (variation from 106-193 mm), and mean mass was 453.29 ± 123.43 g (variation from de 120-650 g). In males, CML was 139 ± 18 mm (variation from 103-180 mm), and mean mass was 260.00 ± 107.59 g (variation from 110-600 g). In SND, CML was 81 ± 11 mm (variation from 67-97 mm), and their mean mass was 60.5 ± 22.59 g (variation from 30-100 g). Total length of the carapace in females was significantly higher than in males (p < 0.001; F_1.67 = 40.251) (Figure 2).

Sex ratio differed from 1:1. Although it was female-biased (1.55♀:1.00♂), the difference was not statistically significant (X²_c = 3.275; df = 1; 0.05 < p < 0.10).

From the 80 M. vanderhaegei captured during the field study, 19 (24%) were recaptured at least once. Recapture rates in distinct sites varied from zero to near 54% (Table 1). In spite of having estimated the higher number of parameters (K = 15), the most restrictive model (p and c site dependent) returned the lower AIC_c (-158.131) and better explained our data (Table 2). The model allowed to estimate a population of 33.15 individuals in the Monjolinho stream (sd= 16; 95% CI = 20.77 to 100.64); 23.61 individuals in Aldeia Velha (sd = 12.93; 95% CI = 17.56 to 94.3); 24, in Quineira (sd= 0.0); 29691.05 in Independência (sd = 1485346.5; 95% CI = 131.37 to 7141613.5), and 13 individuals in the Congonhas stream (sd = 0.0). The more general model had no support (AIC_c = -113.218; ΔAIC_c = 44.91) but has produced more realistic estimates. Based on this model, the estimates for population sizes were: 26.72 individuals in the Monjolinho stream (sd = 4.85; 95% CI = 21.15 to 42.13); 25.21 individuals in Aldeia Velha (sd = 4.66; 95% CI = 19.91 to 40.13); 35.8 individuals in Quineira (sd = 5.96; 95% CI = 28.63 to 54.07); 11.6 in Independência (sd = 2.85; 95% CI = 8.91 to 22.13), and 19.16 individuals in the Congonhas stream (sd= 3.88; 95% CI = 14.98 to 32.14).

Discussion

Sex ratio diverging from expected values of 1:1 is usual among chelonian natural populations (Gibbons 1990), which may be caused by pressures exerted by distinct ecological processes. These may include differential mortality rates among sexes, differential activity, movements, and habitat use, as well as sex determination by temperature and trapping bias (Gibbons 1990, Gibbons & Lovich 1990, Smith & Iverson 2002, Georges et al. 2006). Traps used in the present study cannot be responsible for the sex ratio slightly biased for females, as they allowed the capture of individuals from distinct size classes and were baited with bovine meat, an item consumed in higher frequencies by males than by females of M. vanderhaegei (E.S. Brito, unpublished).

Bait efficiency, higher in lentic habitats than in faster moving waters, may be responsible for the relatively high recapture rate obtained during the study, particularly in the sites with floating and submerging aquatic vegetation, lagoons and dammed streams.

In spite of generating some unrealistic estimates of numbers of the chelid Mesoclemmys vanderhaegei, the better of the two models tested here included the parametrization of the spatial variation in capture rates. Hypothesis to explain this variation (which could not be tested due to our experimental design and/or small sample) include variation in habitat structure or quality, and differential habitat use by individuals from distinct size classes or sexes. Nevertheless, unrealistic estimates of N for some sites (namely, Independência and Congonhas streams) were clearly a product of the data matrix structure. Estimates of N in any mark-recapture model depend on data about the number of unique individuals encountered, and on probabilities of encounters (Lukacs 2009). The latter is estimated based on the proportion of marked individuals in the population, which in turn is calculated considering the number of recaptures and the number of new captures, since the second capture session and so on. Because there was not a single recapture in Independência stream, and no new capture on the second capture session in Congonhas stream, data were insufficient to obtain precise estimates of N for these two streams.

This is the first attempt to estimate population size for M. vanderhaegei in Brazilian cerrados. Low numbers of captures and recaptures in the second session suggest that capture effort was insufficient (number of traps/site lower than required). Even so, they provided estimates of the minimum number of individuals on each sampling site. To obtain more realistic data on population size and tendencies in subsequent field research, it will be important to broaden spatial and temporal scales, in order to obtain improved estimates for other essential demographic parameters, such as age-specific survival and reproduction. To improve techniques for evaluating and analyzing population sizes in freshwater chelonians is an essential step to conserve them, a major concern in habitats under high environmental pressure, such as olygothrophic Cerrado small streams (Wantzen 2003, Wantzen et al. 2006).

Acknowledgements

For logistical support, we thank to Instituto Chico Mendes de Conservação da Biodiversidade (and in particular to the environmental analysts and technicians working at Parque Nacional de Chapada dos Guimarães), to the employees and director board of the Escola Evangélica do Buriti, to the Mayors office of Chapada dos Guimarães, and to Louriza and her family, at Chapada. We also thank Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for partially supporting this project, through a master's scholarship for ESB, and a PRODOC (Programa de Apoio a Projetos Institucionais com a Participação de Recém-Doutores) fellowship to CS. Thanks are due also to Richard C. Vogt, for kindly reviewing our manuscript.

Recebido em 12/05/09

Versão reformulada recebida em 04/08/09

Publicado em 01/10/09

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Publication Dates

• Publication in this collection
  26 Mar 2010
• Date of issue
  Dec 2009

History

• Accepted
  01 Oct 2009
• Reviewed
  04 Aug 2009
• Received
  12 May 2009