Body size

Amphibia-Reptilia 29 (2008): 541-553

Extreme reduction in body size and reproductive output associated with sandy substrates in two anuran species
Federico Marangoni1, ∗ , Miguel Tejedo1 , Iván Gomez-Mestre2
Abstract. Geographic variation in body size and reproductive traits has been reported in a wide range of organisms, including amphibians. Most studies have focused on latitudinal and/oraltitudinal variation where differences in temperature and duration of the growing season are the main causes for population divergence. We describe a steep variation in body size and reproductive traits in two anuran species in southwestern Spain, associated with changes in the geological substrate. Pelobates cultripes and Bufo calamita (= Epidalea calamita) drastically reduced their size (a71.6% and 76.1% reduction in body mass for P. cultripes and B. calamita, respectively) in just about 60 km. This extreme size reduction was more pronounced at the boundary between two different geological substrates (hercinic and sandy soil). Mean clutch mass, egg size, and clutch size were all smaller in B. calamita populations in the sandy environment. Likewise, clutch mass and egg size were bothsmaller in sandy P. cultripes populations. We observed a negative correlation between size-adjusted fecundity and egg size for both species, suggesting the existence of a reproductive trade-off that could explain the differences in reproductive allocation between populations and species. In P. cultripes, small-bodied populations had relatively higher fecundities and smaller eggs than large-bodiedones, whereas in B. calamita populations from the sandy area we found both populations with high fecundity and small eggs, and populations with low fecundity and large eggs. Common environmental effects associated with the sandy substrate produce a similar reduction in size in both toad species. Keywords: amphibians, body size, Bufo (= Epidalea) calamita, geographic variation, Pelobates cultripes,reproduction.

Introduction Body size is often closely related to fitness due to its association with longevity, fecundity, metabolic rate and tolerance to environmental stresses such as starvation and desiccation (Calder, 1984; Schmidt-Nielsen, 1984). Within-species latitudinal and altitudinal clines in body size are frequently found and are often explained by changes in temperature, duration ofthe growing season, or a combination of both (Van Voorhies, 1996; Atkinson and Sibly, 1997; but see Mousseau, 1997). In addition to latitudinal and altitudinal clines, instances of dwarfism

1 - Department of Evolutionary Ecology, Estación Biológica de Doñana (C.S.I.C.), Avda. de María Luisa s/n, Pabellón del Perú, 41013, Sevilla, Spain 2 - Department of Wetland Ecology, Estación Biológica deDoñana (C.S.I.C.), Avda. de María Luisa s/n, Pabellón del Perú, 41013, Sevilla, Spain *Corresponding author; present address: Centro de Ecología Aplicada del Litoral, Consejo Nacional de Investigaciones Científicas y Técnicas, C.C. 291, 3400 Corrientes, Argentina e-mail: fmarangoni@ebd.csic.es

and gigantism have been associated with insular population differentiation in reptiles (Pregill, 1986;Lomolino, 2005; Keogh et al., 2005). Geographical variation in adult female body size may subsequently affect clutch size and offspring size in many organisms, including amphibians (e.g. Tejedo, 1992; Castellano et al., 2004). However, a trade-off between clutch size and offspring size would cause these traits to co-vary since they have important fitness consequences in terms of survival tometamorphosis and population recruitment rates (Semlitsch et al., 1996; Kaplan, 1998). Most studies of geographical variation in body size and reproductive performance in amphibians have focused on latitudinal and altitudinal variation (Ashton, 2002; Morrison and Hero, 2003; Laugen et al., 2005). Low temperatures experienced at higher latitudes and altitudes result in a lower developmental rate causing...