Herpetological Conservation and Biology 4(3):306-312. Submitted: 24 September 2008; Accepted: 16 September 2009. 306 NEST TEMPERATURE, INCUBATION TIME, HATCHING, AND EMERGENCE IN THE HILAIRE’S SIDE-NECKED TURTLE (PHRYNOPS HILARII) CLÓVIS S. BUJES AND LAURA VERRASTRO Programa de Pós-Graduação em Biologia Animal, Departamento de Zoologia, Universidade Federal do Rio Grande do Sul, Av Bento Gonçalves, 9500 Prédio 43435, Sala 105, Campus do Vale, CEP 91501-970. Porto Alegre, RS – Brazil, e-mail: chelonia_rs@hotmail.com Abstract.—The nest dimensions, physical characteristics of the eggs, thermal exposure, incubation period, and hatchling emergence were investigated. A total of 12 nests were monitored: six (N = 50 eggs) in natural conditions and six (N = 28 eggs) in artificial conditions. Nests constructed by females have an aperture, a neck, and an incubation chamber with mean dimension of 143 by 126 mm. Eggs (N = 78) were characterized as spherical, 34 x 32 mm, with a calcareous shell and mean mass of 21.5 g. The incubation period varied from 157 to 271 days, in natural conditions, and from 130 to 191 days under artificial conditions; whereas, hatching success varied from 43 to 75%, and from 50 to 100%, respectively. The mean temperature inside the natural nests ranged from 24.2 to 27.3°C; whereas, under artificial conditions it ranged from 18.8 to 28.6°C. Significantly more hatchlings emerged from eggs incubated under artificial conditions than from natural nests. Key Words.—Chelidae; development; emergence; freshwater turtles; hatching; incubation; nest temperature; Phrynops hilarii INTRODUCTION All chelonians are oviparous and lay their eggs in nests constructed by females in sandy substrates, or under dry leaves and detritus. Chelonians can build their nests in shady areas, or open areas with higher exposure to solar radiation (Ewert 1979). A combination of favorable location, substrate type (e.g., sandy, muddy, or decomposing vegetation), and egg depth can lead to an optimum environment for embryonic development. As chelonian eggs do not receive any direct parental care, embryonic development (i.e., incubation, hatching, and juvenile emergence) depends on the environmental conditions to which the eggs are exposed. The thermal and moisture conditions of the nest are important for egg incubation, influencing the development rate, incubation time (Yntema 1978), and survivorship of the embryo (Packard and Packard 1988; Booth 1998). Moreover, thermal and hydric conditions determine sex in many chelonian species (Bull 1980; Ewert and Nelson 1991), play an important role in the size of neonates (Packard et al. 1987; Packard and Packard 1993), and on their locomotor development (Miller et al. 1987; Janzen 1995). Iincubation temperature, however, has a greater influence on species that lay eggs with a hard shell (Janzen 1993) than water content of the soil (Leshem and Dmi’el 1986). Humidity does not appear to affect the embryo or hatchling’s metabolism in these species (Packard et al. 1979, 1981; Packard and Packard 1991). Hilaire´s Side-necked Turtle, Phrynops hilarii (Fig. 1), is the second most abundant chelonian species in the delta of the Jacuí River of Brazil inhabiting lentic and lotic environments (Bujes and Verrastro 2008). In this region, nesting activity occurs from September to October and February to March, being associated with a minimum mean air temperatures of 26°C. Females produce 10–22 eggs with hard shells, and present daily bimodal nesting activity with peaks at sunrise and sunset (Bujes 1998). The objectives of our study were to examine the influence of soil temperature on the incubation period and on the phenotypic variation of the hatchlings; as well as, the hatching and emergence behavior from the nest in natural and artificial conditions, eliminating potential effects of the substrate humidity. MATERIALS AND METHODS We identified 18 nests of Phrynops hilarii and randomly selected six, which we monitored from FIGURE 1. Female Hilaire’s Side-necked Turtle, Phrynops hilarii (Testudines, Chelidae). (Photographed by Clóvis Bujes)