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• W2324596757 abstract "-We examined whether intrapopulational variation in egg size was correlated with developmental rates and metamorphic traits of natterjack toads (Bufo calamita) grown at different experimental densities. Mean egg size of siblings was positively correlated with both mean size of hatchlings at the initiation of feeding and mean developmental rates to the feeding stage. However, size at and survival to metamorphosis were not related to ovum or hatchling size. At high densities tadpoles had slower growth rates, longer larval periods, and lower survival, and were smaller at metamorphosis. Length of larval period was inversely related to early tadpole growth rate for all densities. Mass at metamorphosis was positively correlated to tadpole growth rate, except at high densities. Size at metamorphosis was inversely related to length of larval period at low densities, but relationships were inconsistent at higher densities. Egg size in amphibians has been viewed as a resource tradeoff for females with limited energy for reproduction. This notion is derived in part from optimality models predicting that for each environment there is an optimal offspring size (Smith and Fretwell, 1974; Brockelman, 1975). Others have suggested that optimality theory is contradicted by the high degree of intraclutch egg size variability observed in amphibians (Kaplan, 1980; Crump, 1981). Egg size variability may be adaptive in unpredictable environments such as temporary ponds where the intensity of competition may vary annually. Larvae from larger eggs would be favored in competitive environments due to their initial size advantage, while in noncompetitive environments larger and smaller hatchlings would be equally successful. By producing eggs of mixed sizes females may increase their fitness relative to those following a pure strat146 This content downloaded from 207.46.13.57 on Fri, 09 Sep 2016 04:31:37 UTC All use subject to http://about.jstor.org/terms METAMORPHOSIS IN NATTERJACK TOADS egy (Crump, 1984; Kaplan and Cooper, 1984). This reasoning assumes a positive correlation between egg size and larval fitness in competitive environments. Hence, under this view the covariation is environmentally mediated, and the effect of egg size on larval fitness should be related to population density or food availability (Kaplan, 1985; Berven and Chadra, 1988). In temporary ponds, the length of larval period may be an important fitness component because it is inversely related to larval survival. In some amphibians, individuals that grow relatively fast metamorphose earlier (Wilbur and Collins, 1973; Collins, 1979; Smith-Gill and Berven, 1979; Travis, 1981, 1983, 1984) and may be less susceptible to size-selective predators (Heyer et al., 1975; Brodie and Formanowicz, 1983). Further, time to and size at metamorphosis may have strong ecological implications related to long-term fitness since both can affect juvenile survival, age at first reproduction, or level of adult fecundity (Smith, 1987; Semlitsch et al., 1988; Berven, 1990). The relationship between these two traits is controlled largely by the environment (Travis, 1984), and may vary in consistency and direction. A gradual drop in water temperature during a cohort's development may produce a positive covariation between traits (Smith-Gill and Berven, 1979). Likewise, covariation may result from competitive release as fast-growing tadpoles undergo metamorphosis and leave the pond. Remaining tadpoles may then have higher growth rates as a result of an increase in the per capita food ration (Collins, 1979). Negative or random covariations may be the result of an environmental decline in per capita food level that is not counterbalanced by competitive release of metamorphs (Alford and Harris, 1988). It also may be the result of heterogeneous environmental effects (Travis, 1984). The natterjack toad (Bufo calamita) breeds in temporary ponds, and is often the only amphibian species present. Tadpole developmental time is short and may last less than 30 d. However, under crowded conditions individuals may extend their larval development up to 80 d. This implies a greater mortality and total absence of metamorphic success in dry years (Tejedo, 1992a); hence, faster growth rate may be highly adaptive. Much variation in egg size exists within populations, and this variation is related to female body size (Tejedo, 1992a). In this report we examined in the laboratory the relationships between two maternally induced variables (egg size and early hatchling size) and four fitness traits: size at metamorphosis; time of development; growth rate; and survival. The relationship was assessed for larvae of different sibships from a single population, under a range of density treatments. Moreover, growth trajectories and covariation between body size at metamorphosis and developmental time were analyzed across the different densities. MATERIALS AND METHODS A total of 41 egg clutches of B. calamita was sampled from a breeding pond at Sierra Morena, C6rdoba, Spain, during January 1988, ca. 12 h after oviposition, and the eggs were between midand late cleavage (stage 8-9; Gosner, 1960) when they were measured. Average egg size was determined by measuring the diameter of 10 randomly-selected eggs from each clutch. For each egg, the diameter was measured to the nearest 0.02 mm with a dissecting microscope and ocular micrometer. From each clutch, we selected 20 eggs within 0.05 mm of the average diameter. These were allowed to develop to Gosner stage 25 (initial feeding phase), at which time the tadpoles were measured and randomly assigned to four experimental densities consisting of either 1 (D1), 2 (D2), 4 (D4), or 8 (D8) individuals per container. Each group of tadpoles was placed in 15 cm diameter plastic pans that contained 250 ml of dechlorinated tap water. The range of experimental densities was similar to that observed in the pond (5-1000 tadpoles/m2; Tejedo, unpubl. data). Tadpoles were fed and the water changed three times per week. Each pan initially received 30 mg of Purina rabbit chow per feeding. Food rations were increased to 70 mg after 30 d of growth. The ration was kept constant throughout the experiment despite the fact that tadpoles in some treatments died or reached metamorphic climax. Food levels at the lower densities (1-2 tadpoles per pan) allowed unlimited growth. However, tadpoles at higher densities (4 and 8 individuals per pan) normally consumed their food by the second and first day after feeding, respectively. The experiment was conducted in laboratory conditions with natural light. Temperature oscillated around 1416 C. We measured snout-to-tail base and total length of tadpoles 15, 30, and 45 d after the start of the experiment. Both length measures minimized handling of tadpoles and were highly correlated with tadpole weight (total length, r2 = 0.801; snout-to-tail base length, r2 = 0.781, n = 53, P 0.20). Consequently, data from different clutches were used as replicates of the density experiment. Tadpole cohorts were considered to be an experimental unit and analyses were performed on their mean values. All larval fitness variables were logetransformed to meet the assumption of homogeneity of variances among densities. Tadpole survivorship was arcsin-transformed to meet the assumptions of a normal distribution and homogeneity of variances among densities. Heterogeneity in variance among densities was observed for metamorphic size and growth rate and therefore a Kruskal-Wallis test was used (Zar, 1984). Linear regression was used to analyze the relationship between metamorphic size and developmental time. The best linear model was fitted with the aid of the Statgraphic Statistical package (STSC Inc., 1986). Analysis of intraclutch egg size variability was determined by measuring a randomly selected sample of 90-110 eggs from each of 10 clutches collected in the same population during the 1989 breeding season. Coefficients of variation (CV) for egg size were calculated. In addition, skewness (g,) and kurtosis (g2) were computed; these values were compared to the expected parametric values in a normal distribution (Sokal and Rohlf, 1981)." @default.
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• W2324596757 title "Effects of Egg Size and Density on Metamorphic Traits in Tadpoles of the Natterjack Toad (Bufo calamita)" @default.
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