Graduation Year

2006

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Marine Science

Major Professor

Joseph J. Torres, Ph.D.

Keywords

Grouping patterns, Separations, Whistle, Echolocation, Sarasota Bay

Abstract

The bottlenose dolphin exhibits a fission-fusion social structure characterized by temporary associations lasting from minutes to hours. Although social structure has been described for some dolphin communities, the selective pressures affecting fission-fusion patterns and their consequences on dolphin communication are not well understood. The goals of the present study were three-fold: 1) to quantify the rate with which fission-fusion occurred and identify the selective pressures influencing an individual's decision to leave and join a temporary group; 2) to examine the communication signals produced during temporary separations; and 3) to estimate the distances over which dolphins could remain in acoustic contact while separated.

It was found that a dolphin's decision to join or leave a group was related to social considerations such as the class of individual encountered (e.g., mothers with calves, adult single females, adult males, and juveniles) as dolphins move in different environments. The decision was also influenced by ecological characteristics such as the habitat where a dolphin was found. The two aspects in turn determined the rate of fission-fusion. Mothers with calves regularly using deep waters had high rates of fission-fusion. Those females encountered other females in the same reproductive condition frequently and associated with them. In contrast, mothers with calves using shallow waters had lower fission-fusion rates. Those females encountered juvenile dolphins often but they did not associate with them frequently.

Temporarily separated dolphins did not always produce the sounds typically used for long-distance communication, and sometimes they did not use any detectable acoustic signal to find each other. On average, this absence of communication occurred at distances less than 50 m. When both whistles and echolocation produced, they were apparently involved in maintaining contact between mothers and their calves and other associates. Estimates of active spaces defined by whistle transmission indicated that communication range varied between habitats. Shallow seagrass areas had the smallest active space while channels had the greatest active space. Findings indicated that the distances over which dolphins remain in acoustic contact and can be considered members of groups are much greater than has been described from observations of dolphin spacing and activity alone.

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