Evolutionary Patterns of Diversity in Spadefoot Toad Metamorphosis (Anura: Pelobatidae)

The larvae of spadefoot toads exhibit extreme developmental/endocrinological diversity. For example, New World spadefoot toads (Scaphiopus and Spea) have the shortest larval periods known among anurans, and the tadpoles of Old World spadefoot taxa (Pelobates) are among the largest known. To analyze the patterns of this diversity in an evolutionary context, we generated comparable larval growth and development data from 10 of the 11 taxa of spadefoot toads and from one taxon of parsley frog (Pelodytes), the nearest spadefoot toad relative. We found dramatic differences in growth and development among taxa, which indicated that taxon-specific physiology, rather than phenotypic plasticity, underlies larval period diversity. For all eight response variables (development rate, three growth rates, time to forelimb emergence, time to tail resorption, mass at tail resorption, and body length at tail resorption), taxa within genera were similar to each other and were different from taxa in other genera. Larvae of Scaphiopus were small with short larval periods, larvae of Spea were large with short larval periods, larvae of Pelobates were large with long larval periods, and larvae of Pelodytes were small with long larval periods. Even though taxa within the same genus live in different environments, larval growth and development correlated with phylogenetic groupings rather than breeding habitat. Mapping larval data onto a molecular phylogeny indicated that short larval periods, as well as rapid embryonic development and high temperature tolerance, originated within the spadefoot toad family.

Fig. 1.

Comparison of spadefoot toad and parsley frog larval growth from beginning of the larval period to the end of metamorphosis at 24 C. Mass (A), body length (B), and total length (C) are compared across Gosner stage for each taxon. Each data point shows the average and standard deviation for 10 individuals

Fig. 2.

Comparison of spadefoot and parsley tadpole development and growth rates. (A) Development rates calculated from Gosner stage 27–46 are compared for tadpoles reared at 24, 28, and 32 C. Tadpole growth rates for mass (B), body length (C), and total length (D) were calculated from the size versus time curves between Gosner stages 27 and 38 for tadpoles reared at 24, 28, and 32 C. Only data for 24 C is available for tadpoles of Pelodytes. For clarity, each data point shows the average and standard deviation for each genus instead of each species. Uncircled data points and circled groups of data points represent significance groups based on Tukey-Compromise posthoc tests

Fig. 3.

Comparison of spadefoot and parsley frog larval period length and size at metamorphosis. Time to forelimb emergence (A), time to tail resorption (B), mass (C), and body length (D) at tail resorption are compared for tadpoles reared at 24, 28, and 32 C. Each data point shows the average and standard deviation from 10 individuals. Uncircled data points and circled groups of data points represent significance groups based on Tukey-Compromise posthoc tests

Fig. 4.

Phylogenetic relationships of the spadefoot toads and parsley frogs (Cannatella, 1985; Wiens and Titus, 1991). Underlined taxa live in desert environments. Thick lines indicate taxa with large larvae (> 1 g body mass) and thin lines indicate taxa with small larvae (< 1 g body mass). The bracket indicates taxa with short larval periods (< 20 days); the other taxa have long larval periods (30–40 days). Taxa with dashed lines were not examined in this study

Accepted: 17 Aug 2001