Morphological Plasticity in Four Larval Anurans Distributed along an Environmental Gradient

We investigated morphological plasticity to the presence of predators in the tadpoles of four ranid frog species distributed along a pond hydroperiod gradient in southeast Michigan. We first reared all four species (Wood Frog, Rana sylvatica; Leopard Frog R. pipiens; Green Frog, R. clamitans; and Bullfrog, R. catesbeiana) under identical laboratory conditions in the presence and absence of caged larval dragonflies (Anax spp.). We then reared wood frog and leopard frog in outdoor mesocosms to examine the predator-induced responses during ontogeny. Finally, we reared leopard frog with predators fed either leopard frog or wood frog larvae to determine whether prey responses depended upon predators consuming conspecific prey. All four ranids exhibited some degree of morphological change in the presence of Anax; these differences were species specific and fairly robust to different experimental conditions. The responses over ontogeny indicated that the changes were direct responses to the predator's presence and not an indirect result of the predator slowing anuran growth or development. Finally, larval leopard frog responded similarly to predators feeding on conspecifics and congenerics. Taken together, these results suggest that morphological responses to predators may be relatively common in larval anurans. Further, because many of the responses are known to be adaptive antipredator strategies, predator-induced morphological plasticity has important evolutionary and ecological implications.

Fig. 1.

Lateral and dorsal views of a wood frog tadpole showing the seven linear measures used in the analyses of morphological plasticity (BD = body depth, BL = body length, BW = body width, TD = tail depth, TL = tail length, MD = muscle depth, and MW = muscle width)

Fig. 2.

Relative morphological changes (mean residuals ±1 SE) in seven linear measures of four ranid larvae (W = wood frog, L = leopard frog, G = green frog, B = bullfrog) in the presence (filled circles) and absence (open circles) of caged Anax in the lab experiment. Effects of overall size were removed for by regressing all individuals against the first principle components scores

Fig. 3.

Change in overall size (PC-1) and relative morphology (mean residuals ± 1 SE) of wood frog larvae over time in the presence (filled circles) and absence (open circles) of Anax in the tank experiment. Differences in overall size were removed by regressing all individuals against the first principle components scores (PC-1)

Fig. 4.

Change in overall size (PC-1) and relative morphology (mean residuals ± 1 SE) of leopard frog larvae over time in the presence (filled circles) and absence (open circles) of Anax in the tank experiment. Differences in overall size were removed by regressing all individuals against the first principle components scores (PC-1)

Fig. 5.

Mass and relative morphology of leopard frog reared without predators (NP), with predators fed larval leopard frog (P-leop), and predators fed larval wood frog (P-wood). Differences in body sizes were removed prior to analysis; thus all morphological differences in tadpole shape are relative

Accepted: 21 Jun 1999