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I examined fission-fusion sociality in dusky dolphins (Lagenorhynchus obscurus), and investigated aspects of social convergence between dolphins and great apes. I used boat-based group focal follows and photo-identification to collect data in Admiralty Bay, New Zealand during 2005-2006. I used generalized estimating equations to examine relationships between party (group) size, rate of party fission-fusion, activity, and location; and relationships between leaping frequency and behavior. Using photo-identification images from 2001-2006, I analyzed the strength and temporal patterning of associations, short- and long-term association patterns, preferred/avoided associations, and behaviorally-specific preferred associations. To analyze social convergence between dolphins and great apes, I compared female bottlenose dolphin (Tursiops spp.) and chimpanzee (Pan troglodytes) social strategies through literature review.
I conducted 171 group focal follows, totaling 157 observation hours. Mean party size was 7.0±6.0 individuals. Party size changed every 5±.47.6 min on average. The most frequent activity was resting (37%), followed by traveling (29%), foraging (18%), and socializing (15%). Foraging was positively related to party size and rate of fission-fusion. Near mussel farms, foraging increased, traveling decreased, and rate of party fusion increased. "Clean" leaps were the most frequent leap type (84%) and were positively related to party size and foraging. Noisy and coordinated leaps were positively related to party size; noisy leaps were negatively related to foraging.
Associations during 2001-2006 (N = 228 individuals) were nonrandom for 125 days; associations within one field season were nonrandom for 60 days. Individuals formed preferred/avoided associations during most years. The strongest associations occurred during foraging and socializing; the weakest associations occurred during traveling. Individuals formed preferred associations during foraging, resting, and socializing.
Review of female bottlenose dolphin and chimpanzee sociality revealed that: 1) females form weaker bonds and are less social than males, 2) females associate mostly with other females, 3) mothers are often alone with their offspring, 4) mothers (vs. non-mothers) and non-cycling (vs. cycling) females associate less with males, and 5) non-cycling (vs. cycling) females occur in smaller parties. Female dolphins may be more social than female chimpanzees due to decreased scramble competition, increased predation risk, and decreased cost of transport for dolphins vs. chimpanzees. |
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