Spinner dolphins exhibit very characteristic social schedules that revolve around using coastal, inshore and atoll habitats during the day time for resting and social interactions, while foraging occurs offshore in the depths of the night. Within this social timetable, marked differences in social and population dynamics are evident between conspecifics of Hawaiian Spinner dolphins that reflect geographical separation and habitat differences. The Hawaiian archipelago is comprised of varying habitats including calm, coastal bays in the main Hawaiian Islands to sporadic habitat out in the atolls of the North Western Hawaiian Islands (NWHI).
A study by Karczmarski et al., (2005) pointed out that there is limited area in the North Western Hawaiian Islands (NWHI), namely Midway atoll, that is suitable habitat for resting dolphins. These areas are far and few between and present a high predation risk within open pelagic stretches of ocean between them. As a result, the dolphins that inhabit these areas possess more stable, long-term social associations with one another. Dolphins in this group appear to have strong group fidelity and expressed no fission-fusion dynamics. This means that their social system was very secure and rarely did members of the group separate out into subunits. Any alterations to the social group were only facilitated by basic population demographics.
Fission-fusion behavior is driven by the activity level of dolphins and the availability and distribution of resources. As the Main Hawaiian Islands (MHI) present a greater diversity in habitat within each island, dolphins that reside here appear to have more variability in their social dynamics. This is probably due to the diversity of habitat associated within the Main islands. It is clear that social variability occurs in response to obvious differences in changes in the environment and is particularly inherent when conspecifics occupy different ecologies.
Karczmarski noted that because of the environmental constraints of Midway atoll and limited critical available habitat, it was metabolically easier for atoll-dwelling spinner dolphins to remain within the atoll network than to travel to another. This trade off then facilitates the geographical isolation between the two groups of dolphins. However, considering the distance between Midway Island and the MHI, 100% geographical isolation between populations is unlikely however occasional overlapping interactions between both groups would occur.
For more information see Karczmarski L, Wursig B, Gailey G, Larson K W, Vanderlip C (2005) Spinner dolphins in a remote Hawaiian atoll: social grouping and population structure. Behaviouraly Ecology pp. 675-685.
Photo Credit: Photo from http://commons.wikimedia.org/wiki/File:Midway_Atoll_aerial_photo_2008.JPG. This photo was taken for the U.S. Fish and Wildlife Service Papahānaumokuākea Marine National Monument Management Plan, Volume IV – Midway Atoll Conceptual Site Plan