A Comparison of Maternal and Temperature Effects on Sex, Size, and Growth of Hatchlings of the Magdalena River Turtle (Podocnemis lewyana) Incubated under Field and Controlled Laboratory Conditions Vivian P. Pa ´ez 1 , Juana C. Correa 1 , Amalia M. Cano 1 , and Brian C. Bock 1 During two nesting seasons we monitored 19 naturally incubated nests of Podocnemis lewyana obtained from two sites in the Mompos Depression of northern Colombia. We incubated another nine nests in the laboratory under similar humidity conditions, but at six different temperatures. We sexed the hatchlings obtained from all nests to confirm for the first time the occurrence of temperature-dependent sex determination in this species and quantify sex ratios and the pivotal temperature for this population. In both nesting seasons, the majority of the nests in the field produced hatchlings of both sexes, but sex ratios differed between study beaches/years. The pivotal temperature documented (33.4uC) appears to be among the highest reported for a turtle species. Incubation conditions in the naturally incubated nests also influenced hatching success rates and incubation periods, but not hatchling size or weight. Hatchlings obtained from the nests incubated in the laboratory were reared for one month in order to study the influence of pre- hatching factors on growth rates. In these nests we documented maternal effects on egg size, initial hatchling size, and weight and growth rates. Incubation temperature also influenced hatching success rates, sex ratios, growth rates, and hatchling size, but not hatchling weight. Durante dos estaciones reproductivas monitoreamos 19 nidos de Podocnemis lewyana incubados naturalmente en dos localidades de la Depresio ´n Momposina al norte de Colombia. Incubamos otros nueve nidos en el laboratorio bajo condiciones similares de humedad pero bajo seis temperaturas diferentes. Sexamos los neonatos obtenidos de todos los nidos para confirmar por primera vez la ocurrencia de determinacio ´n sexual dependiente de la temperatura en esta especie y cuantificar las proporciones sexuales y temperatura pivotal para esta poblacio ´ n. En ambas estaciones, la mayorı ´a de los nidos incubados bajo condiciones naturales produjeron neonatos de ambos sexos, pero las proporciones sexuales fueron diferentes entre playas de anidacio ´ n/an ˜os. La temperatura pivotal documentada (33.4uC) parece ser una de las ma ´s altas reportadas para una especie de tortuga. Las condiciones de incubacio ´n de los nidos naturales tambie ´ n influyeron en las tasas de eclosio ´ n y en los periodos de incubacio ´ n, pero no en el taman ˜ o de los neonatos o en su peso. Los neonatos obtenidos a partir de nidos incubados artificialmente en el laboratorio fueron criados por un mes con el propo ´ sito de estudiar la influencia de los factores pre-eclosio ´n en las tasas de crecimiento. En estos nidos, documentamos efectos maternos en el taman ˜ o de los huevos, en el taman ˜ o y peso inicial de los neonatos y en las tasas de crecimiento. Las temperaturas de incubacio ´ n de los nidos artificiales tambie ´n influyeron en las tasas de eclosio ´n, proporciones sexuales, tasas de crecimiento y taman ˜o pero no peso inicial de los neonatos. I N species with temperature-dependent sex determina- tion (TSD), the sex ratio of a nest not only depends on the microclimatic environmental conditions that the embryos experience during incubation, but also may be influenced by different forms of maternal effects (Bowden et al., 2000, 2001; Janzen and Morjan, 2001; St. Juliana et al., 2004). Studies have shown that threshold temperatures not only differ between species or different populations of the same species, but also among individual females from the same population, implying that the specific pivotal temper- ature (the constant incubation temperature producing a 1:1 sex ratio) for a clutch may be considered a maternal effect (Bull et al., 1982; Ewert and Nelson, 1991; Mrosovsky and Pieau, 1991; Janzen, 1992; Ewert et al., 2004). Thus, it should be possible to obtain different sex ratios at a given temperature within different nests from one or more populations, even in nests incubated under identical microclimatic conditions (Rhen and Lang, 1998; Mrosovsky et al., 2002; Morjan, 2003). Another potentially heritable maternal effect is female preferences for oviposition site and date, which may influence the specific microclimatic conditions experienced by the embryos, and hence also influence not only the sex and survivorship of the neonates, but also other important life history traits such as their initial sizes, growth rates, and physical condition (Janzen and Morjan, 2001; Valenzuela and Janzen, 2001; Morjan, 2003; Doody et al., 2004). Recently, a number of studies have shown that females may also influence the sex ratio of a clutch by means of allocating different levels of endoge- nous hormones in the yolk, which thereby affects the thermal sensitivities of the embryos (Janzen et al., 1998; Bowden et al., 2001, 2002; Elf, 2003, 2004). Finally, some studies (St. Juliana et al., 2004) have attempted to determine whether behavioral traits (oviposition site preference) and physiological traits co-vary. Although a considerable number of studies have docu- mented effects of incubation condition (mainly temperature and humidity) on sex ratios and phenotypic traits (Janzen and Paukstis, 1991; Ewert and Nelson, 1991; Valenzuela and Lance, 2004), and other studies have suggested females may be able to influence incubation conditions by means of their preferences for nest site locations and nesting dates (Doody et al., 2004), the evidence for other maternal effects on sex ratios or hatchling traits is more equivocal (Radder, 2007). One difficulty is that under field conditions it is hard to separate the effects of specific physical conditions of the nests versus effects of differences among clutches in threshold temperatures or steroid hormone contents, re- quiring that such studies be conducted under controlled laboratory conditions (Janzen, 1992; St. Juliana et al., 2004). 1 Instituto de Biologı ´a, Bloque 7-136, Universidad de Antioquia, A.A. 1226, Medellı ´n, Colombia; E-mail: (VPP) VivianPaez1@gmail.com. Send reprint requests to VPP. Submitted: 25 August 2008. Accepted: 18 May 2009. Associate Editor: J. W. Snodgrass. F 2009 by the American Society of Ichthyologists and Herpetologists DOI: 10.1643/CE-08-149 Copeia 2009, No. 4, 698–704