Body Mass Shapes Most Life History Traits and a Fast-Slow Continuum in Amphibians

Amphibians have the least studied life histories among vertebrates, although they have unique and the most diverse life histories within this group. We compiled a new dataset on adult body mass and 16 other life history traits of 2069 amphibian species across three orders (1796 frogs, 236 salamanders, 37 caecilians). These traits characterise fecundity, offspring development from egg deposition to metamorphosis and adult life. We established allometric models on traits for all amphibians and each of the three orders to assess a potential scaling of traits to body mass and then checked whether allometric slopes were consistent with two different metabolic scaling exponents. Further, we examined a possible fast-slow continuum in all amphibians, as well as in each of the two orders frogs and salamanders by applying principal component analysis (PCA) to five traits. Our allometric models indicated a positive scaling to body mass for 11 traits across all amphibians, 12 in frogs, and 10 in salamanders, and for five out of eight traits analysed in caecilians. Allometric slopes on most traits characterising offspring development were not significant. All slopes did not support a three-quarter metabolic scaling exponent, whereas slopes on age at maturity and maximum longevity were consistent with an amphibian metabolic scaling exponent of 0.88. As in fishes, reptiles, birds, and mammals, the first axes of our PCAs indicated a body mass-dependent fast-slow continuum in amphibians. Amphibian species of slow life histories have larger body masses, later sexual maturities and longer lifespans and lay more and larger eggs than species of fast life histories, a pattern also known from reptiles. The second axes indicated a trade-off between egg size and number. As this trade-off was nearly independent of body mass, we hypothesise that amphibians have occupied a broad range of ecological niches without evolutionary changes in body mass.