Predators Induce Morphological Changes in Tadpoles of Hyla andersonii
Predators can affect the development, fitness, and behavior of prey species in myriad ways. In response to the threat of predation, tadpoles can alter growth rate, morphology, and foraging behavior. Changes to tadpole development have the potential to alter life history characteristics and are therefore of interest in species of conservation concern. Using experimental mesocosms, we explored how non-lethal predators affected the larval development of the Pine Barrens Treefrog, Hyla andersonii, a near-threatened species in the United States. We found that caged dragonflies (Anax junius) induced darker tail coloration and deeper tail fins in tadpoles of H. andersonii, but the dragonflies did not affect tadpole behavior, survival, or size at metamorphosis. Non-lethal predator presence also induced greater within population variation in the tail color trait compared to populations without predators. This result suggests that there may be underlying genetic variation in the ability to express phenotypically plastic traits, a concept that should be explored further because it has implications for the evolution of inducible defenses. These findings support the existence of an adaptive syndrome among hylid tadpoles, where tadpoles develop conspicuous tail morphology in response to larval dragonfly predators.
Photograph of H. andersonii depicting the locations of tail measurements. BL = Body length, TL = tail length, TMD = tail muscle depth, TFD = tail fin depth. Body length was measured from tadpole snout to start of tail muscle. Tail length was measured along the tail muscle from start of tail muscle to tail tip. Tail muscle depth was measured at the base of the tail, and tail fin depth was measured from fin top to bottom at the deepest location along tail length. Total tadpole length (TTL) was determined by adding TL to BL.
Representative photos of tadpoles of H. andersonii reared without predators (top) and with predators (bottom) and their corresponding ImageJ pixel color histograms. Lower values indicate darker pixels, and higher values indicate lighter pixels in the image. The y-axis of the histograms indicates the frequency of pixels for each darkness score.
Tadpole tail color (mean gray value) was significantly lower (=darker tails) in predator treatments compared to non-predator controls (LMM: χ2 = 16.2, df = 1, P < 0.001). Sampling date had no effect on tail color (χ2 = 2.34, df = 2, P = 0.31).
Box and whisker plots showing range of tadpole tail mean gray values (a measure of tail darkness). Lower mean gray values indicate darker tail coloration. (A) Day 32 (+predator treatment mean gray value±SD = 92±12.82; –predator control = 103.88±8.54), (B) Day 39 (+predator = 93.11±15.22; –predator = 106.54±12.95), and (C) Day 46 (+predator = 88.97±14.46; –predator = 100.68±12.63). Individuals were pooled across ponds within treatments on each sampling date. Variance was significantly greater in predator treatments compared to non-predator controls (LMM: χ2 = 5.7, df = 1, P = 0.017).
Comparison of relative tadpole size across sampling dates and predator treatments. All measurements were standardized against total tadpole length to account for size differences among individual tadpoles. *P < 0.05 between treatments.
Principal component analysis of the tail morphological traits total tadpole length (TTL), tail color (col1), and standardized body length (sBL), standardized tail muscle depth (sTMD), standardized tail length (sTL), and standardized tail fin depth (sTFD) sampled on Day 32.
Contributor Notes
Associate Editor: D. S. Siegel.