Resting behaviors of captive bottlenose dolphins (Tursiops truncatus).

In order to specify the resting behaviors in captive bottlenose dolphin, we quantitatively analyzed behavior of 16 dolphins in three aquariums in Japan. We observed their behaviors in the low-activity time (0 a.m.-3 a.m.), assuming that the resting behaviors would be most prominent and abundant at this time. We analyzed these behaviors based on the data of their diurnal activity rhythm that we have clarified by measuring swimming speed and breath frequency. The behavior patterns characteristic to the low-activity time could be categorized into the following three types: "bottom-rest" characterized by long immobile stay on the tank bottom, "surface-rest" characterized by long immobile stay at the water surface, and "swim-rest" characterized by slow circle-swim near the bottom along a fixed trajectory. During these behaviors, breath frequency was significantly lower than the daily mean and at least one eye tended to be closed. These three behaviors accounted for 86.6% of the total observation time in the low-activity time. The resting behaviors were often observed even in the high-activity time (0 p.m.-4 p.m.); these three behaviors accounted for 38.5% of the total observation time in the high-activity time. In swim-rest, frequency of sound emission was significantly lower than that of other behaviors with high-speed swimming, and both eyes or one eye, especially the eye facing the inner side of the swimming circle tended to be closed. The eye condition and the periodical change of circle-swim direction or position-exchange observed during swim-rest suggest a relation between this behavior and unihemispheric sleep. The change in the resting behavior observed in the dolphins under nervous situations suggests that the dolphins flexibly change the type and the quantity of the resting behaviors according to the situation.