Ontogeny of escape swimming performance in the spotted salamander

1. The life stage suffering the highest predation rate is expected to have the highest escape performance unless developmental or functional constraints interfere. Peak aquatic escape performance in ephemeral pond-breeding amphibians is expected to develop early in the larval period, and metamorphosis is expected to reduce or completely disrupt aquatic escape performance. In anurans, exceptionally low escape performance during metamorphosis creates selection favouring rapid metamorphosis – which minimizes the time individuals spend in the vulnerable transition between tadpole and frog. 2. We investigated the development of aquatic escape performance in the spotted salamander, Ambystoma maculatum (Shaw, 1802), from embryonic development through metamorphosis. We expected performance to peak early in the larval period as hatchlings face high rates of predation but embryos must first develop escape behaviours. We also tested whether escape performance during metamorphosis was intermediate, as predicted by tail fin resorption, or lower than larvae and adults indicating a major physiological disruption. 3. Escape performance shows a complex ontogeny that is first positively influenced by embryonic and early larval development and then negatively correlated with tail resorption and body size. Escape distance was the only performance metric not affected by life stage. In contrast, both escape velocity and duration showed ontogenetic peaks early in the larval period with the lowest performance found in early embryos and adults and intermediate performance during metamorphosis. 4. This pattern suggests that metamorphosis does not impose a major physiological disruption on escape performance. Because spotted salamanders do not pass through a frog-like ‘ontogenetic performance valley’ during metamorphosis, they may be less subject than anurans to selection favouring rapid metamorphosis. 5. Functional implications of phenotypic variation should be considered in an ontogenetic framework because the relationship between body size and escape performance can be reversed on either side of an ontogenetic performance peak. The assumption that metamorphosis radically disrupts basic functions such as predator evasion does not seem universally warranted and suggests examination of ontogenetic performance trajectories in a diversity of animals with complex life cycles.