ABSTRACT Ecological niche is the multidimensional space comprising the resources used by an organism. Intraspecific variation in resource exploitation is common in reptile populations to maximize coexistence of individuals. The use of stable isotope analysis is an effective tool when there are variations in resource exploitation, since it can provide quantitative information about food consumption and habitat use. Mesoclemmys vanderhaegei (Bour, 1973) is a medium-sized turtle with a limited distribution in south central Brazil and Paraguay. In spite of that, little is known about its ecology. In this study we used stable isotope analysis to understand the intraspecific trophic niche variation in M. vanderhaegei at Serra das Araras Ecological Station, state of Mato Grosso, Brazil. The isotopic ratios of δ15N and δ13C were determined in claw samples collected from 14 males and 14 females. Isotopic niche width values were not statistically different between the sexes, there was a high degree of overlap between sexual niches and there were no relationships between isotopic compositions and body size. These results suggest that individuals of both sexes and throughout their ontogenetic development exploit food resources with the same isotopic baseline.

The isotopic composition ( δ13C and δ 15N) of muscle, carapace and claw was determined from six wild individuals of Phrynops geoffroanus (Schweigger, 1812) in order to verify the variation between those tissues. The mean values of δ13C e δ 15N were, respectively, -19.48 ± 0.81‰ (-20.8 - -18.64‰) and 7.23 ± 0.67‰ (6.49 - 8.3‰) for muscle, -16.52 ± 0.98‰ (-17.88 - -15.43‰) and 7.29 ± 0.54‰ (6.74 - 7.97‰) for carapace and -18.57 ± 0.97‰ (-19.97 - -17.26‰) and 4.36 ± 0.33‰ (3.93 - 4.79‰) for claw. We found a significant difference for the tissues evaluated for both δ13C and δ15N. Muscle and claw were statistically similar and more depleted in 13C than the carapace. Nonetheless, claw was more depleted in 15N than muscle and carapace tissues. These results are likely related to differences in metabolic processes involved on each tissue formation. The description of the isotopic signatures variation in P. geoffroanus tissues provides a baseline for future investigations on the diet reconstruction of this species when more than one tissue is involved.

The innate immune activity of the freshwater turtle Phrynops geoffroanus (Schweigger, 1812) was investigated, using a sheep-red-blood cell hemolysis assay. The time- and concentration-dependent hemolytic activity of the turtle plasma was low compared to that reported for other reptiles. However the plasma of P. geoffroanus exhibited higher activity at elevated temperatures, resulting in temperature-dependent hemolysis. The sensitivity of turtle plasma to temperature could be interpreted as a mechanism by which freshwater turtles use basking behavior to elevate body temperature, thus enhancing the innate immune response. However, we cannot discard the possibility that environmental contaminants could be affecting the turtle's immune response, since the animals in this investigation were captured in a polluted watercourse.