Age and Body Size in Populations of Two Syntopic Spadefoot Toads (Genus Pelobates) at the Limit of Their Ranges
Age and size at sexual maturity are major life-history traits that affect growth rate and reproductive output. The differences in these life-history traits can be better assessed in syntopic populations of related species, where environmental variability in time and space is removed. The Spadefoot Toads (Pelobates spp.) are highly specialized burrowing and nocturnal species with a narrow ecological niche. We tested if age-related parameters are responsible for sexual size dimorphism and size differences in two syntopic populations from the Danube Delta Biosphere Reserve (Romania). The two populations differed significantly in size and body mass, and showed distinct sexual dimorphism, with females significantly larger in P. fuscus, and males slightly larger in P. syriacus. We also found differences in age structure: P. fuscus was on average 5.0 yr old, whereas P. syriacus was on average 7.4 yr old. Both species reached sexual maturity at a similar age. Different growth rates before sexual maturity and differences in energy allocation between growth and reproduction after sexual maturity are responsible for the differences in adult size between these syntopic populations. Our results suggest that growth allocation between species and sexes cannot be explained solely on the basis of age-related parameters.Abstract
SVL and body mass relationships in the two species of Spadefoot Toads (genus Pelobates) in females (A) and males (B).
Cross-sections through the diaphysis of the phalange of Pelobates syriacus and P. fuscus. The black arrows mark the number of LAGs; EL, eroded LAG; D, double LAG; MC, marrow cavity. The juveniles were used to mark endosteal erosion and the missing LAG (EL), necessary for the correction of the age estimation. (A) P. syriacus female at 9 yr old and 4 yr sexual maturity, with one partly EL and three D. (B) P. syriacus male at 8 yr old and 4 yr sexual maturity, with two EL and one D. (C) P. fuscus female at 5 yr old and 4 yr sexual maturity, with two EL and two D. (D) P. fuscus male at 5 yr old and 4 yr sexual maturity, with two EL. (E) P. syriacus juvenile at 0.5 yr old. (F) P. fuscus juvenile at 2 yr old.
Population age structure in males and females of the two studied species. Males are represented by solid bars and females are represented by open bars. Pelobates fuscus and P. syriacus are represented by (A) respectively (B).
Growth curves in Pelobates fuscus (A) and P. syriacus (B). We used individuals of the two studied species from syntopic populations in southeastern Europe. x-axis = age in years assessed by skeletochronology; y-axis = SVL in millimeters. Growth curves were fitted to von Bertalanffy's growth equation. Parameters are available in Table 2. Open dots, females; black dots, males.
Sexual size dimorphism of SVL for the two studied species. We assessed the sexual size dimorphism of SVL in 37 populations of Pelobates fuscus and 20 populations of P. syriacus on the basis of published and our own data (see Appendix 1).
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