Detection of Wave Direction by Juvenile Green Sea Turtles

(Still under construction)

Abstract

Laboratory studies have shown that hatchling turtles can detect the direction of the orbital motion of waves. This ability may play a role in their offshore migratory behavior. However, it is not known whether older turtles maintain this ability. To test whether juvenile turtles can determine wave direction, we observed the behavior of six juvenile green turtles as they were subjected to the orbital motion of simulated waves in the absence of other sensory information. Each turtle was tested under conditions that simulated waves approaching from one side for 5 minutes and then from the other side for five minutes. All turtles alternated between short periods of vigorous activity and longer periods of relative inactivity. During the active periods, turtles often made powerful strokes with only one of their front flippers (a behavior consistent with turning). The number of strokes made with each flipper was compared for each turtle. All turtles made significantly more strokes with one flipper than with the other during each phase of the test and, with all turtles, the flipper that was used more switched when the direction of simulated waves was reversed. The stroking behavior of five of the turtles was consistent with turning into oncoming waves while the behavior of one turtle was reversed. Therefore, while these results demonstrate that juvenile green turtles can determine the direction of waves, it is not clear how this information is used in the natural environment.

Introduction

Swimming hatchling sea turtles can orient using information from approaching waves (Salmon and Lohmann 1987, Wyneken et al. 1990). Furthermore, even when not in the water, hatchlings can detect and orient to the orbital or surge motion of simulated waves (Lohmann et al. 1995, Manning et al. 1997, Wang et al. 1998). This ability likely plays an important role in the offshore migration of hatchlings soon after emerging from nests and entering the water (Reviewed by Lohmann and Lohmann 1998). Whether older turtles maintain this ability is not known.

We observed the behavior of juvenile green turtles as they experienced the orbital motion of simulated waves. The number of unilateral flipper strokes made by each front flipper was compared as turtles experienced simulated waves approaching from their right and left sides. The results demonstrate that juvenile green turtles can determine the direction of oncoming waves.

Materials and Methods

Animals

Juvenile green turtles were collected by hand during daylight from near-shore reef habitat near Sebastian Inlet, FL, USA and were transported a short distance to a field site in Melbourne Beach, FL. Turtles were held overnight and used in magnetic orientation studies prior to being tested on the wave simulator in this study. Each turtle was tested only once and was released on the beach near the capture site a short time later (Fig. 1). Turtles (n=6) ranged from 17-33 kg.

Procedure

Turtles were tested on a wave simulator designed reproduce the orbital motions of waves (Fig. 2) (see Lohmann et al. 1995 for further details). Simulated waves had a period of 5 sec and an amplitude of 0.6 m. For each trial, a turtle was mounted on the simulator, plastic sheeting was placed around the test area to exclude light and the simulator was started. Three of turtles first experienced simulated waves from the left while the other three first experienced simulated waves from the right. Turtles were allowed one minute to acclimate and then the number of unilateral strokes made by each from flipper was counted for five minutes (Fig. 3 and 4). The direction of simulated waves was then reversed, turtles were allowed one minute to acclimate, and the number of strokes was again counted for five minutes.

Data analysis and statistics

For each turtle, the number of unilateral strokes made by each front flipper during the five minute test periods were compared using a chi-squared test.
The photograph above shows a juvenile green turtle supported beneath the camera on the wave simulator. The series of photographs below shows a turtle being moved in a motion that simulates a 0.6 m wave with a period of 5 sec. approaching from the turtle’s right. (Photos are taken at ~0.3 sec. intervals.) These photographs were taken in daylight; however, experiments were conducted in darkness.
The series of photos above shows a left flipper stroke indicating right turning behavior.
The series of photos above shows a right flipper stroke indicating left turning behavior

Results and Discussion

When simulated waves approached from the turtles’ right, 5 of the 6 turtles made significantly more flipper strokes with their left flipper while 1 turtle made significantly more strokes with it’s right flipper. When simulated waves approached from the turtles’ left, 5 of the 6 turtles made significantly more flipper strokes with their right flipper while 1 turtle made significantly more strokes with it’s left flipper.
Blue bars indicate the number of right flipper strokes. Red bars indicate the number of left flipper strokes.

Five of the turtles made significantly more flipper strokes on the side opposite the side from which the simulated waves approached. Under natural conditions, such flipper strokes would turn the turtles into the oncoming waves. One turtle, however, made significantly more flipper strokes on the same side as that from which simulated waves approached. Under natural conditions, these strokes would turn the turtle to orient in the same direction as wave propagation. All six turtles reversed which flipper they used more when the direction of simulated waves was reversed.

These data demonstrate that, similar to hatchlings, juvenile sea turtles can determine the direction of wave propagation. However, all turtles did not use this ability for the same apparent purpose. Five turtles attempted to turn into oncoming waves while one turtle attempted to turn with oncoming waves. Further studies will be necessary to determine how juvenile turtles use wave information in the natural environment.

Literature Cited


Lohmann, K. J. (1992). How sea turtles navigate. Sci. Am. Jan., 100-106.

Lohmann, K. J. and Lohmann, C. M. F. (1992). Orientation to oceanic waves by green turtle hatchlings. J Exp Biol 171: 1-13.

Lohmann, K. J. and Lohmann, C. M. F. (1998). Migratory guidance mechanisms in marine turtles. Journal of Avian Biology 29: 585-596

Lohmann, K. J., Swartz, A. W. and Lohmann, C., M, F. (1995). Perception of ocean wave direction by sea turtles. J Exp Biol 198: 1079-1085.

Manning, E. L., Cate, H. S. and Lohmann, K. J. (1997). Discrimination of ocean wave features by hatchling loggerhead sea turtles, Caretta caretta. Mar Biol 127: 539-544.

Salmon, M. and Wyneken, J. (1987). Orientation and swimming behavior of hatchling loggerhead turtles Caretta caretta L. during their offshore migration. J Exp Mar Biol Ecol 109: 137-153.

Wang, J. H., Jackson, J. K. and Lohmann, K. J. (1998). Perception of wave surge motion by hatchling sea turtles. J Exp Mar Biol Ecol 229: 177-186.

Wyneken, J., Salmon, M. and Lohmann, K. J. (1990). Orientation by hatchling loggerhead sea turtles Caretta caretta L. in a wave tank. J Exp Mar Biol Ecol 139: 43-50.

Page last updated 04/20/05