This study quantifies the influence of the persistent variation in maternal investment in individual offspring on larval fitness in a population of the oriental fire-bellied toad Bombina orientalis. Offspring fitness was evaluated by exposing newly hatched larvae to interspecific tadpole predators and assessing the effect of ovum size on the probability of surviving predation. The experiment incorporates natural variation in temperature that occurs during development. Embryos that developed in colder ponds took longer to hatch and hatched at a larger size. Larger size afforded larvae significant protection from predation. In addition, larvae that developed from large eggs in cold ponds had a higher probability of surviving predation than larvae that developed from small eggs. A significant interaction between egg size and developmental temperature, however, resulted in larvae that developed from large eggs in warm ponds having a lower probability of surviving predation. Larger eggs resulted in larger snout-to-vent lengths at all temperatures. In warm ponds, however, tails of larvae from large and small eggs were of equal length. Thus, diminished locomotory ability due to larger amounts of inert yolk can be invoked to explain the increased susceptibility to predation of larvae that developed from large eggs in warm ponds. In cold ponds tail length was substantially greater than in warm ponds, and larvae from large eggs tended to have longer tails than those that developed from small eggs. This allowed larvae from large eggs to achieve the advantages of increased body size. Thermally induced plasticity during early development resulted in an unpredictability regarding the adaptive value of a particular egg size. This result fits within a growing theoretical literature that indicates that environmental uncertainty can result in the natural selection of developmental plasticity in egg size, even though the fitness ramifications are realized in subsequent stages. Thus, increased maternal investment in offspring is not necessarily associated with an increase in offspring fitness, and the evolution of developmental plasticity in egg size may be the result of environmental uncertainty during the larval stages of life.
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