Stress hormone-mediated antipredator morphology improves escape performance in amphibian tadpoles

Abstract

Complete functional descriptions of the induction sequences of phenotypically plastic traits (perception to physiological regulation to response to outcome) should help us to clarify how plastic responses develop and operate. Ranid tadpoles express several plastic antipredator traits mediated by the stress hormone corticosterone, but how they influence outcomes remains uncertain. We investigated how predator-induced changes in the tail morphology of wood frog (Rana sylvatica) tadpoles influenced their escape performance over a sequence of time points when attacked by larval dragonflies (Anax junius). Tadpoles were raised with no predator exposure, chemical cues of dragonflies added once per day, or constant exposure to caged dragonflies crossed with no exogenous hormone added (vehicle control only), exogenous corticosterone, or metyrapone (a corticosteroid synthesis inhibitor). During predation trials, we detected no differences after four days, but after eight days, tadpoles exposed to larval dragonflies and exogenous corticosterone had developed deeper tail muscles and exhibited improved escape performance compared to controls. Treatment with metyrapone blocked the development of a deeper tail muscle and resulted in no difference in escape success. Our findings further link the predator-induced physiological stress response of ranid tadpoles to the development of an antipredator tail morphology that confers performance benefits.

Figure 1

Treatment of wood frog tadpoles with corticosterone (CORT) or nonlethal exposure to a predator increased tail muscle depth, while treatment with the corticosteroid synthesis inhibitor metyrapone (MTP) blocked the tail muscle response to the predator. Shown are boxplots (median and interquartile range) of mass-adjusted tail muscle depth in each treatment combination (n = 4 replicates) on Day 0 (a), Day 4 (b), and Day 8 (c). Mean individual tadpole mass (± s.e.m.) is given below each box. Asterisks indicate statistically significant differences compared to the No predator/No hormone treatment (Bonferroni-corrected P < 0.05). Other statistically significant pairwise contrasts are given in Supplementary Table S1.

Figure 2

Treatment with corticosterone (CORT) or constant exposure to nonlethal predators increased the ability of wood frog tadpoles to escape predator attack. Shown are boxplots (median and interquartile range) of percentage of Anax attacks from which tadpoles successfully escaped in each treatment combination (n = 4 replicates) on Day 0 (a), Day 4 (b), and Day 8 (c). NA indicates a treatment in which no data were recorded due to technical problems. Asterisks indicate significant differences compared to the No predator/No hormone treatment (P < 0.05).

Figure 3

Treatment of wood frog tadpoles with nonlethal exposure to a predator resulted in decreased activity. Shown are boxplots (median and interquartile range) of swimming activity of tadpoles in each treatment combination (n = 4 replicates) on Day 0 (a), Day 4 (b), and Day 8 (c). Asterisks indicate significant differences compared to the No predator/No hormone treatment (Bonferroni-corrected P < 0.05). Other significant pairwise contrasts are noted in Supplementary Table S2.