Evaluating The Tradeoff Between Offspring Number And Survivorship Across Fishes, Amphibians, Reptiles And Mammals

Species differ widely in their strategies of resource allocation to offspring mass and number, ranging from teleost fishes and amphibians that produce many tiny offspring to reptiles and mammals that produce relatively few large offspring. Tradeoffs between offspring survivorship and fecundity are thought to limit the success of any particular reproductive strategy, but these tradeoffs have not been evaluated quantitatively across the full range of variability in offspring size and number. Here we examine the relationship of offspring size to reproductive success (i.e. fitness) within and across teleost fishes, amphibians, reptiles and mammals. To do so, we evaluate the relationships of offspring mass to survivorship (proportion of offspring surviving to maturity) and to fecundity (no. offspring/time). We show that survivorship tends to increase in proportion with relative offspring mass (offspring mass/adult mass), whereas fecundity, normalized to offspring biomass production rate, tends to decrease in proportion with offspring mass. Consequently, the product of survivorship and fecundity - reproductive success - is generally independent of offspring mass. Thus, our results show quantitatively how survivorship and fecundity tradeoff across diverse taxa to limit reproductive success.