Maternal Effects on Life-History Traits in the Amazonian Giant River Turtle Podocnemis expansa

Abstract

-Energy allocation to eggs and nest site selection by females can affect life-history variables such as offspring size, offspring number, developmental rate, survivorship, growth rate, and performance in oviparous reptiles. Nest site selection can affect offspring phenotype by altering incubation conditions. I present evidence of a positive effect of female size on clutch size, egg mass, and nest depth through the study of trackways left by female river turtles, Podocnemis expansa, on their nesting beaches. Larger females laid larger clutches composed of larger eggs, which were buried deeper than clutches laid by smaller females. The data suggest that P. expansa does not conform to optimal propagule size models. Neither egg size nor clutch size reached a plateau as female size increased. Females seem to allocate the extra energy (in absolute terms) gained allometrically with increasing size and age to both number and size of eggs. There was no evidence of a trade-off between egg size and number after removing the effect of female size. Larger eggs produced larger hatchlings that survived better but grew less than individuals of smaller initial size during the first two months of life, under unlimited food conditions. I suggest that fitness of female P. expansa increases by producing larger eggs because of the advantage that larger hatchlings have in survival. Deeper nests experience cooler temperatures and tend to produce a higher percentage of males than more superficial nests. Therefore, there is a potential for important effects of nest depth on sex ratios produced by different sized females within the population and possibly by single females throughout their lifetime. Constant temperature in artificial incubation experiments had an effect on the size of individuals at hatching, but differences vanished by the second month of age via the greater growth rate shown by individuals of smaller initial size. urnal of Herpetology, Vol. 35, No. 3, pp. 368-378, 2001 yright 2001 Society for the Study of Amphibians and Reptiles aternal Effects on Life-Histo y Trai s in the Amazonian Giant River rtle Podocnemis exp nsa Life-history variables such as offspring size, offspring number, developmental rate, survivorship, growth rate, and performance may be influenced by maternal factors such as energy allocation and nest site selection in oviparous reptiles. Because those parameters affect offspring fitness, females could maximize their own fitness by optimizing that of their offspring (Brockelman, 1975). On one hand, species with larger clutches and no parental care are expect1 Present Address: Department of Zoology and Genetics, Iowa State University, Ames, Iowa 50011, USA; E-mail: nvalenzu@iastate.edu. istory variables such as offspring size, ing number, developmental rate, survivor, rowth rate, and performance may be inced by maternal factors uch as en rgy alio and nest site sel ction in oviparous rep. ecause those parameters affect offspring ed to show the patterns of energy allocation predicted by optimality models (Smith and Fretwell, 1974), which may involve compromises between longevity and fertility, and trade-offs between offspring size and offspring number (Roff, 1992; Stearns, 1992; Bulmer, 1994; Sikes, 1998 and references therein). On the other hand, nest site selection can alter incubation conditions experienced by the offspring that can affect their phenotype (Shine and Harlow, 1996). Incubation conditions such as temperature vary with nest substrate characteristics, sun and wind exposure, and nest depth (Souza and Vogt, 1994; Janzen, 1994; Shine and Harlow, 1996). Incubation temperature has profound effects on s o the pa terns of energy allocation preoptimality models (Smith and Frel, 974), which may involve compromi es longevity and fert lity, and trade-offs offspring size and o fspring number 368 This content downloaded from 157.55.39.17 on Wed, 31 Aug 2016 04:39:41 UTC All use subject to http://about.jstor.org/terms LIFE HISTORY OF AMAZONIAN GIANT TURTLES reptiles, particularly for species with temperature-dependent sex determination (TSD). Females allocate energy to eggs and choose nest site locations within each reproductive season, and it has been suggested that certain combinations of both parameters (nest-site choice dependent on egg size) could be adaptive in turtle species (Roosenburg, 1996). Here, I report on the effects of female size of a freshwater turtle species (Podocnemis expansa) on egg size, egg number, and an aspect of nest location, namely, nest depth. Podocnemis expansa is a large species inhabiting the Amazon and Orinoco basins in South America. The size of adult females ranges between 50 and 80 cm carapace length (Hildebrand et al., 1997). Descriptions of the nesting behavior in P expansa report that females excavate a nest hole such that their head is level with the surface of the beach while ovipositing (Mosqueira-Manso, 1945; Ramirez, 1956; Vanzolini, 1967). Therefore, large females should dig deeper holes than smaller females. Ramirez (1956) suggested a positive relationship between female size and nest depth, but no data have been presented to test this prediction. Podocnemis expansa exhibits temperature-dependent sex determination (TSD), such that males are produced at low incubation temperatures and females at high temperatures (Alho et al., 1985; Valenzuela et al., 1997). Females nest in sandbars where temperature varies considerably with depth in the nesting substrate (Valenzuela, 1999). An experimental study in the same nesting area revealed a significant effect of the depth of artificial nests on offspring sex ratio (Valenzuela, 1999). In general, the deeper the nest the cooler and less variable the incubation conditions, which tended to produce more males (Valenzuela, 1999). The experimental effect of nest depth on sex ratio would be of great importance if similar nest depth variation were present in the wild. Particularly, if nest depth is correlated with female size within the population, or within a given female as her size increases with age, then the population sex ratio or a female's lifetime sex-ratio production could be affected. These considerations are especially important given the endangered status of P ex-