A community split among dolphins: the effect of social relationships on the membership of new communities

Little is known about community splitting among dolphins because such events are rare in dolphin populations. A case of a community split was confirmed in a population of Indo-Pacific bottlenose dolphins (Tursiops aduncus) in Japan, where a group of approximately 30 dolphins moved to a new habitat some 60?km from the original habitat. We examined the associations among the dolphins before the community split to determine whether the new community members were already socially different before the split, using 7-year identification data. Before the split, the males in the same community after the split more often associated with each other than they did with those in different community. In contrast, the association patterns among females and between sexes showed no relationships with their post-split community membership. These results indicate that the males of new community were socially different from the other males for a long time before the split, but the females might not have been different. Our findings suggest that at time of the community split, the factors determining the memberships of the subsequent communities are sex-linked. The long-term social relationships among males could be maintained in the subsequent communities

Alliance Formation of Indo-Pacific Bottlenose Dolphins (Tursiops aduncus) off Amakusa, Western Kyushu, Japan

Indo-Pacific bottlenose dolphins (Tursiops aduncus) off Amakusa-Shimoshima, approximately 200 individuals, form relatively large groups frequently exceeding 100 individuals and show high site fidelity to the area around Tsuji Island, northern coast of Amakusa-Shimoshima. This suggests that individual dolphins may have long interaction times with many other individuals. Consequently, competition between males is likely to be high and formation of alliances may be expected. However, this has not yet been confirmed. With photo-identification data collected between 2010 and 2014, we examined individual associations. Pairs of males formed significantly non-random associations for multiple years, and were seen surrounding females, many of whom were considered to be receptive at that time. Our results suggest that male Indo-Pacific bottlenose dolphins form alliances in this population, where dolphins form large groups and show high site-fidelity

Space Charge Distribution of Surface Discharge in SF₆ Gas

Surface space-charge distributions produced by streamers in SF₆ are investigated using the probe method with a high speed temporal resolution. Probes which also act the role of a plane electrode, are used for measuring the space-charge field. The fields are measured oscilloscopically and converted into space-charge densities by a numerical calculation. This inverstigation has revealed the charge distribution before the disturbance caused by the well known “back-discharge”. This method is also applied for surface discharges in air, and its results are compared with that obtained in SF₆