Delayed hatching of salamander eggs in response to enhanced larval predation risk.

To our knowledge, this article presents the first test of the hypothesis that variation in size-dependent predation risk on hatchings can cause adaptive shifts in the timing of egg hatching and thus on the size and developmental stage of new hatchlings. Earlier field experiments documented heavy predation by flatworms (Phagocotus gracilis) on smaller, less developed hatchling salamander larvae (Ambystoma texanum and Ambystoma barbouri) but little or no predation on larger, more developmentally advanced larvae. Here, we divided sibships into groups of 12 eggs and compared hatchling traits (time, size, and stage of hatching) for eggs reared in control fresh water versus water with flatworms, flatworm chemicals, or nonpredatory isopods. Both flatworms and flatworm chemicals induced eggs to delay their hatching to a later time and thus a larger size and more advanced developmental stage. In particular, sibships that tended to hatch early in control conditions delayed hatching in response to flatworms and flatworm chemicals, while sibships that hatched late in controls showed no response to either treatment. Nonpredatory isopods did not cause a significant change in hatching traits. Adaptive plasticity in hatching traits can provide excellent, unexploited opportunities for studying the evolution and ecological consequences of a life-history switch point.