Apparent Risk of Predation by Dragonfly Naiads (Odonata: Libellulidae) Inhibits Tadpole Growth (Rana sphenocephala)

We reared 120 Rana sphenocephala tadpoles in 1.5 m long chambers (10 tadpoles per chamber) made from vinyl rain gutters. The chambers contained predatory late-instar dragonfly naiads (Odonata: Libellulidae) confined at one end in clear tubes drilled with aeration holes. The number of confined predators varied between control (n = 0), low (n = 1), and high density conditions (n = 5). Tadpoles were free to swim throughout each chamber, although food was isolated at one end adjacent to the predator tubes. In an additional high density predator condition, food was isolated at the opposite end of the chamber away from the predator tubes. There were a total of three replicates for each of the four conditions. Tadpoles from each condition were individually weighed (g) and their total length (mm) was recorded at regular intervals throughout the trial. Tadpoles raised in the high density condition with multiple predators surrounding their only food source were significantly shorter in length and weighed significantly less that those reared in all other conditions. When multiple predators and larval food were isolated at opposite ends of the chamber, tadpole growth did not statistically differ from that of control animals. Tadpoles raised with multiple predators surrounding their food also possessed slightly deeper tails, although these results were not statistically significant. Our results suggest that the inhibition of tadpole growth observed in this study may be linked to behavioral changes when perceived predation risks are high.