Behavioral Ecology Advance Access originally published online on February 9, 2005
Behavioral Ecology 2005 16(4):675-685; doi:10.1093/beheco/ari028
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Spinner dolphins in a remote Hawaiian atoll: social grouping and population structure
a Marine Mammal Research Program, Texas A&M University, 4700 Avenue U, Building 303, Galveston, TX 77551, USA, b Oceanic Society, Fort Mason Center, Building E, San Francisco, CA 94123, USA, and c Midway Atoll National Wildlife Refuge, Pacific Remote Islands NWR Complex, U.S. Fish and Wildlife Service, 300 Ala Moana Boulevard, P.O. Box 50167, Honolulu, HI 96850, USA
Address correspondence to Leszek Karczmarski, who is now at the Institute of Marine Life Sciences, Texas A&M University, 4700 Avenue U, Bldg. 303, Galveston, TX 77551, USA. E-mail: karczmal{at}tamug.edu.
Spinner dolphins (Stenella longirostris) commonly use inshore island and atoll habitats for daytime rest and social interactions and forage over deep waters at night. In Hawaii, they occur throughout the archipelago. We applied photoidentification mark-recapture techniques to study the population structure of spinner dolphins associated with remote Midway Atoll, far-western Hawaii. At Midway, spinner dolphins live in stable bisexually bonded societies of long-term associates, with strong geographic fidelity, no obvious fission-fusion, and limited contacts with other populations. Their large cohesive groups change little over time and are behaviorally/socially discrete from other spinner dolphin groups. This social pattern differs considerably from the fluid fission-fusion model proposed previously for spinner dolphins associated with a large island habitat in the main Hawaiian Archipelago. These differences correspond to geographic separation and habitat variation. While in the main islands there are several daytime resting places available at each island habitat; in far-western Hawaii, areas of suitable habitat are limited and separated by large stretches of open pelagic waters with potentially high risk of shark predation. We hypothesize that with deepwater food resources in close proximity and other atolls relatively far away for easy (day-to-day) access, it is energetically more beneficial in the remote Hawaiian atolls to remain "at home" than to travel to other atolls, so there is stability instead of variability; there is no fission-fusion effect. Thus, the geographic isolation and small size of remote atolls trigger a process in which the fluidity of the fission-fusion spinner dolphin society is replaced with long-term group fidelity and social stability.
Key words: geographic insularity/connectivity, group dynamics, Hawaii, Midway Atoll, social evolution, social structure, Spinner dolphin Stenella longirostris.
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