Junichi Okuyama

Ethogram of Immature Green Turtles: Behavioral Strategies for Somatic Growth in Large Marine Herbivores

Animals are assumed to obtain/conserve energy effectively to maximise their fitness, which manifests itself in a variety of behavioral strategies. For marine animals, however, these behavioral strategies are generally unknown due to the lack of high-resolution monitoring techniques in marine habitats. As large marine herbivores, immature green turtles do not need to allocate energy to reproduction but are at risk of shark predation, although it is a rare occurrence. They are therefore assumed to select/use feeding and resting sites that maximise their fitness in terms of somatic growth, while avoiding predation. We investigated fine-scale behavioral patterns (feeding, resting and other behaviors), microhabitat use and time spent on each behavior for eight immature green turtles using data loggers including: depth, global positioning system, head acceleration, speed and video sensors. Immature green turtles at Iriomote Island, Japan, spent an average of 4.8 h feeding on seagrass each day, with two peaks, between 5∶00 and 9∶00, and between 17∶00 and 20∶00. This feeding pattern appeared to be restricted by gut capacity, and thus maximised energy acquisition. Meanwhile, most of the remaining time was spent resting at locations close to feeding grounds, which allowed turtles to conserve energy spent travelling and reduced the duration of periods exposed to predation. These behavioral patterns and time allocations allow immature green turtles to effectively obtain/conserve energy for growth, thus maximising their fitness.

Difference in Activity Corresponding to Changing Water Temperature among Juvenile Green Sea Turtles

In this study, a tank experiment was conducted to investigate the change in the activity level of juvenile green turtles reared for three months when exposed to water at a range of temperatures (13 to 25 C). The results of the experiment show that the activity level of juvenile green turtles declined with lower water temperature. Moreover, in cold waters at temperatures of 13-15 C, unusual swimming activity was observed. For example, the frequency of stroking was not constant, or the timing of stroking with both flippers did not coincide. These results indicate that staying in cold waters for a long period would be critical for juvenile green turtles.

Fine-Scale Monitoring of Routine Deep Dives by Gravid Leatherback Turtles during the Internesting Interval Indicate a Capital Breeding Strategy

The dive behavior of gravid leatherback turtles (Dermochelys coriacea) was studied during the internesting interval in two western Pacific nesting regions: Papua Barat, Indonesia, and the Solomon Islands in 2006, 2007 and 2010. We used three types of dive data: time-at-depth data (Papua Barat: N = 4; Solomon Islands: N = 6), intermittent dive data (Papua Barat: N = 6) obtained from ARGOS satellite transmitters, and continuous dive data obtained from recovered semi-archival tags (Papua Barat: N = 1, Solomon Islands: N = 1). All dive data demonstrated that the leatherback turtles routinely dove to deep waters (around 150 m) throughout the internesting interval. The continuous dive data showed that turtles spent 37.3% of their time in routine deep dives and that they stayed in cold waters below the thermocline. Fine-scale monitoring (1-s interval, 0.5 m of resolution) suggested that these routine deep dives were not accompanied with any wiggles (up-and-down undulations in the depth profile) or flat-bottom phases, and they reached deep waters by gliding, which suggests that these dives may have served to conserve energy and/or to thermoregulate. Comparison with the dive behavior in other regions (Costa Rica, French Guiana, Grenada, Malaysia, and St. Croix) suggests that gravid leatherback turtles in all regions except French Guiana assume an energy-saving strategy during the internesting interval that involves gliding to or resting on the sea floor in colder water. The behavioral tactics (dive patterns) they use, however, differ because of bathymetric constraints.

Difference in Flipper Beating Frequency of Green Turtles in Water and on Land

Sea turtles spend most of their lives in marine habitats, but they require a terrestrial environment for oviposition. In both conditions, they use limbs for thrust production. We attached animal-borne data loggers on green turtle and calculated the stroke frequency during swimming in water and crawling on land from surging acceleration. Stroke frequency was compared during swimming and crawling. The results showed that stroke frequency during terrestrial crawling is significantly higher than during swimming. This contrasts with previous studies of animals performing drag-based swimming. Because green turtles are considered to be lift-based swimmers that produce thrust mainly by dorsoventral excursion, one hypothesis is that anteroposterior excursion may be restricted despite its importance in terrestrial crawling and drag-based swimming. Small anteroposterior excursion resulting in short stride length may be complemented by higher stroke frequency during crawling.