MECHANICAL-PROPERTIES OF THE TADPOLE TAIL FIN

The tadpole tail fin is a simple double layer of skin overlying loose connective tissue. Collagen fibres in the fin are oriented at approxim ately +/-45 degrees from the long axis of the tail. Three tests were c onducted on samples of the dorsal tail fin from 6-10 Rana catesbeiana tadpoles to establish the fin's viscoelastic properties under (1) larg e-deformation cyclic loading at 1 and 3 Hz, (2) small-deformation forc ed vibration at 1 and 3 Hz, and (3) stress relaxation under a 0.1 s lo ading time. The fin was very fragile, failing easily under tensile loa ds less than 7g, It was also strikingly viscoelastic, as demonstrated by 72+/-1% hysteresis loss (at 3Hz), 16+/-3 % stress remaining after 1 00 s of stress relaxation and a phase angle of 18+/-1 degrees in force d vibration. As a consequence of its viscoelastic properties, the fin was three times stiffer in small than in large deformation. This may a ccount for the ability of the fin to stay upright during normal undula tory swimming, despite the absence of any skeletal support. Tadpoles i n nature are often found with damaged tails. We suggest that the unusu ally viscoelastic and fragile nature of the fin helps tadpoles escape the grasp of predators. Because the fin deforms viscoelastically and t ears easily, tadpoles can escape predators and survive otherwise letha l attacks with only minor lacerations to the fin. Recent studies have shown that certain tadpoles develop taller fins in the presence of pre dators. This developmental plasticity is consistent with the tail fin acting as a protective but expendable 'wrap' around the core muscle ti ssue.