GEOGRAPHIC VARIATION IN RATES OF VOCAL PRODUCTION OF FREE-RANGING BOTTLENOSE DOLPHINS

Echolocation and whistle production, group sizes, and activities of free-ranging bottlenose dolphins were compared across four regions (Wilmington, NC Intracoastal Waterway [ICW]; Wilmington coastline; Southport, NC coastline; and Sarasota, FL inshore waters). Number of whistles and echolo-cation bouts differed significantly across sites. Dolphins whistled significantly more in Southport than in the other sites, independent of group size. Unlike at the other sites, dolphin vocalizations in Southport did not vary significantly across activities; this difference may be due to the fact that Southport animals were often found behind shrimp-trawling vessels, which may affect their behavior. Resident Sarasota dolphins vocalized significantly less than dolphins at the NC sites. At most sites, echolocation production per dolphin decreased as group size increased, supporting the idea that echolocation information is shared. In the ICW and Sarasota, echolocation production per dolphin was highest while feeding, indicating that echolocation is used in foraging. At all sites but Southport, whistle production per dolphin was highest while socializing, indicating that whistles are used in communication. Overall, these data show that dolphins have different vocal and activity patterns at different sites; thus, caution should be used when extrapolating results from one study site to another.     

DIURNAL BEHAVIOR OF FREE-RANGING BOTTLENOSE DOLPHINS IN THE KVARNERIĆ (NORTHERN ADRIATIC SEA)1

The diurnal behavior of a bottlenose dolphin (Tursiops truncatus) community was observed from small inflatable craft between 1987 and 1994. Following a preliminary ad libitum study 11,839 3-min behavioral samples were recorded in 1991-1994. The behavioral budget showed a predominance (about 80%) of activities characterized by long (>30 sec) dives, considered to be largely related to prey search or feeding. Obvious foraging near the surface was observed rarely. The frequent following of trawlers (accounting for 4.6% of the behavioral budget) was indicative of the presence of alternative strategies for finding food. Yearly and seasonal behavioral variation-particularly in feeding-related and travel behaviors-was consistent with the hypothesis of behavioral flexibility as a response to environmental changes and fluctuating prey kind and availability. Yearly shifts in social behavior appeared to be partly influenced by breeding cycles. Groups engaged in feeding-related activities were significantly smaller than traveling or socializing groups, and dramatic interannual group-size shifts seemed to be largely affected by environmental variables, rather than being entirely determined by behavioral activity changes. The remarkable behavioral flexibility of this bottlenose dolphin community may contribute to its survival in the shifting environmental conditions of the northern Adriatic Sea. However, the high proportion of time consistently devoted to feeding-related activities, as compared to other areas, suggests that food resources in the Kvarneri? were not only highly variable but also depleted.     

EXPERIMENTAL RETURN TO THE WILD OF TWO BOTTLENOSE DOLPHINS

In the first scientific experiment of its kind, two young male bottlenose dolphins ( Tursiops truncatus ) were captured in Tampa Bay, Florida, and then returned to the wild at the same locale in October 1990, after two years in captivity. The dolphins' age/sex class and the capture and release site were selected prior to their collection. The ranging and social association patterns of the host community were examined prior to, and, including the two animals, after release. The dolphins remained together for the first month, then began interacting more with other dolphins and less with each other. Within the first year, one dolphin returned to the waters near his capture site and has remained there at least through September 1993. The other dolphin has remained in his original home range at least through June 1996. Observations of each dolphin have shown them to be fully integrated into the local dolphin societies. They displayed typical behavioral, ranging, and social association patterns. Their body condition has been excellent at each observation. They have not been observed interacting with humans. The apparent success of this experiment cannot necessarily be generalized to all potential candidates for return to the wild, but the results can be used to guide future experiments.     

PRIMARY PRODUCTION REQUIRED TO SUPPORT BOTTLENOSE DOLPHINS IN A SALT MARSH ESTUARINE CREEK SYSTEM

We developed a model to estimate the proportion of annual primary production required to support bottlenose dolphins within the 32-km² North Inlet salt marsh creek system in South Carolina, U. S. A. The estimated annual prey consumption by dolphins was compared to the total annual production of prey available to dolphins, as determined from estimates of annual primary production, trophic transfer efficiencies, and the mean trophic level of prey. A best estimate range of 3.2%-6.8% of the total annual primary production of the North Inlet system was required to support an average population of only six dolphins (maximum range of 0.4%–7.0%). Dolphins were estimated to consume 11.1–14.2 metric tons of fish (wet weight) each year in North Inlet. The proportion of North Inlet primary production required to support dolphins increased dramatically during the winter months, when primary production declined but dolphin numbers remained similar. This period was marked by a decline in the abundance of available prey species and by a shift in the creek utilization patterns of dolphins. Despite the numerical scarcity of dolphins in the system, they appear to have a significant ecological impact and may be important predators of overwintering prey species.     

MOTHER-INFANT SPATIAL RELATIONS IN CAPTIVE BOTTLENOSE DOLPHINS,TURSIOPS TRUNCATUS

The prolonged nursing period and strong, extended mother-infant bond observed among bottlenose dolphins may reflect social and physical ontogeny critical for infant survival. This study was conducted to quantify ontogentic changes in mother-infant contact time and the amount of time infants spent in specific spatial states with their mothers from birth to age 12 mo. These behaviors were studied through a systematic, longitudinal study of six mother-infant pairs of captive bottlenose dolphins from three different social groups. There was a significant decrease in the time infants spent with their mothers (logistic regression, P < 0.001), following the general mammalian pattern of increasing independence with age. When with their mothers, the probability that infants would be found in “echelon” position, flanking the mother, decreased as the calf aged (logistic regression, P <0.001), possibly due to anatomical and hydrodynamic factors. The probability that infants would be found in “infant” position, underneath the mother, increased with calf age (logistic regression, P < 0.001). Results obtained in this study are consistent with similar studies of wild bottlenose dolphin mother-infant pairs, indicating a suite of ontogenetically comparable behaviors between wild and captive bottlenose dolphins.     

SURFACE BEHAVIOR OF BOTTLENOSE DOLPHINS IS RELATED TO SPATIAL ARRANGEMENT OF PREY

We tested the hypothesis that spatial arrangement and movements of bottlenose dolphins ( Tursiops truncatus ) are related to the spatial arrangement of their prey. From 65 groups we: (1) classified feeding behavior of dolphins according to spatial arrangement and movements of individuals; (2) assessed spatial arrangement and location of prey from direct observations, numbers of associated seabirds, and echosounder recordings; and (3) related feeding behavior of dolphins to spatial arrangement and location of prey. Four feeding categories were defined from cluster and principal component analyses: (1) moving rapidly with no diving, (2) milling with no diving, (3) diving in several locations, and (4) diving in one location. These feeding categories were related to spatial arrangement and location of prey.     

SEXUAL DIMORPHISM IN BOTTLENOSE DOLPHINS FROM THE EAST COAST OF FLORIDA

Skulls of 69 bottlenose dolphins (genus Tursiops ) from the Indian/Banana River on the east coast of Florida were examined for evidence of sexual dimorphism. The only sexual dimorphism shown by t -tests on 28 morphological and four meristic skull characters was that males have, on average, more teeth than females in all four arcades. Results of covariance analysis, employed to account for variation in size, indicate minor dimorphism in parietal width of the skull. Twenty body measurements of 29 Tursiops originating in the same area were also analysed for differences between males and females. Statistical results indicate the possibility of sexual dimorphism in the length from the snout to the umbilicus and in flipper width. No evidence was found for differences in overall skull or body length between the sexes.