ECOLOGY, BEHAVIOR AND SOCIAL ORGANIZATION OF THE BOTTLENOSE DOLPHIN: A REVIEW

The authors review the literature on bottlenose dolphin ecology, behavior and social organization, focusing on data collected on free-ranging animals. Most bottlenose dolphins studied to date have had definable home ranges, and behavioral, morphological and biochemical information indicates discrete stocks in some areas. Bottlenose dolphins appear to form relatively permanent social groups based on sex and age. Mother—calf bonds are long-lasting. Movement patterns are extremely variable from location to location but are relatively predictable at any given site. Food resources are one of the most important factors affecting movements. Bottlenose dolphin behavior is very flexible, and these dolphins are generally active day and night. Feeding peaks in the morning and afternoon have been observed at several sites. Social behavior is an important component of daily activities. Sharks are the most significant predator on bottlenose dolphins in most areas, but captive and wild studies show that dolphins and sharks frequently live in harmony as well. Human activities may be helpful, harmful or neutral to bottlenose dolphins, but interactions with humans are frequent for these coastal cetaceans.     

A QUANTITATIVE ASSESSMENT OF DOMINANCE RELATIONS AMONG BOTTLENOSE DOLPHINS

Agonistic behavior of bottlenose dolphins was studied at Brookfield Zoo for nearly 4.5 yr, and dominance relationships were determined using a quantitative technique adapted from primate behavioral research. Dominance relations among dolphins were influenced by the gender of participants. Male dolphins were clearly and consistently dominant to females, and intersexual agonism occurred at moderate rates with seasonal peaks in spring and fall. Dominance relationships among female dolphins were age-ordered and stable, even though agonism among females did occur at uniformly low rates. In contrast, the two males had a changeable dominance relationship in which periods of stability and low-level agonism were interspersed with episodes of intense competition. Zoo-based research revealed patterns of behavior that conformed to current knowledge about bottlenose dolphin social structure. Moreover, research in a zoo setting facilitated development of a quantitative technique that can be used to assess cetacean dominance relationships in field research.     

Progress report: Results of a long-range captive breeding program for the bottlenose dolphin,Tursiops truncatus and Tursiops truncatus gilli

There have been 36 bottlenose dolphins born in breeding colonies at Sea World, California, and Sea World, Florida, from 1978–1985. Significant features of this successful reproductive program are construction of a compatible breeding colony, early hormonal detection of pregnancy, pre- and postnatal association of inexperienced mothers with experienced females and their calves, and minimized handling of the mothers and calves until the calves are over 1 year of age. Inclusion in the breeding colonies of males old enough to be effective breeders is stressed. Females in the colonies have successfully bred from 8–9 years of age to 23–24 years of age. Calving intervals in the colonies vary from 2 to over 3 years. Calves are born all year round, with some peaking in calving activity in the spring and fall months. Serum progesterone levels greater than 6,000 pg/ml, maintained over a 4–6-week period, are considered indicative of pregnancy. Progesterone levels vary from less than 10,000 to over 50,000 pg/ml during gestation, averaging 25,000 pg/ml. The need for further study of variation in the pattern of progesterone levels during pregnancy is emphasized. A gestation period for Tursiops of 11.5 to 12 months is consistent with our cumulative progesterone data.     

THE OCULAR SECRETIONS OF THE BOTTLENOSE DOLPHIN TURSIOPS TRUNCATUS

A viscous ocular secretion streams from the eyes of bottlenose dolphins, Tursiops truncatuss , removed from the water. The chemical composition and environmentally adaptive implications of this substance continue to perplex curators and scientists. Secretions from three dolphins were examined for deviations from literature values for normal human and terrestrial mammalian tear film components. Osmolality (= 470 mOsm/kg), pH (= 8.44) and glucose (= 26.58 mg/100 ml) were significantly greater than human and terrestrial mammalian tear film values; while lysozyme (= 0.005 mg/ml) and total cholesterol (= 0.17 μg/μ1) were significantly less. Refractive index, sodium ion concentration, and potassium tear to plasma ratios in the dolphin ocular secretion did not deviate significantly from human or terrestrial mammalian tears. Electrophoresis of dolphin tears yielded four major protein bands (probably lactoferrin, serum albumin, teat specific prealbumin, and lysozyme) and high molecular weight glycoproteins similar to human tears. The chemical composition of the T. truncatus ocular secretion suggests that the circumorbital conjunctival gland may be analogous (relative to refractive index, electrolytes, and component protein) to the terrestrial lachrymal gland. At the functional level the T. truncatus ocular secretion may reduce hydrodynamic resistance on the cornea and protect it from bacterial invasion.     

Estimating the impact of interactions between bottlenose dolphins and artisanal fisheries around the Balearic Islands

Interactions between marine mammals and fisheries are a growing problem, and effective management requires assessment of the factors driving the interaction and of the impacts on fisheries. We used data from interactions between artisanal fisheries and bottlenose dolphins around the Balearic Islands to assess these factors and impacts. Observers collected data during 1,040 fishing operations over 3 yr. Location and year were important factors affecting interaction probability, with some areas showing large increases over the study period. We estimated the combined cost of catch loss and net damage as 6.5% of the total catch value (95% CI ?12.3%, ?1.6%), and the annual loss to be 3.4% (95% CI ?6.5%, ?0.1%) of the total catch by weight. This weight equates to the dietary needs of ～12 dolphins (95% CI 0.2, 22), suggesting the fishery is not a vital food source for the dolphin population. Two dolphins died through entanglement during the observed fishing operations. We observed 3% of the total fishing activity, by weight, in 2003; scaling up this mortality directly suggests that as many as sixty dolphins may be dying in nets each year. This interaction likely has serious conservation implications for the dolphin population.     

LOW FREQUENCY NARROW-BAND SOUNDS PRODUCED BY BOTTLENOSE DOLPHINS

We present a new sound type recorded from bottlenose dolphins, Tursiops truncatus , in eastern Australian waters: low-frequency, narrow-band (LFN) harmonic sounds (defined as less than 2 kHz). Most of these sounds were of frequencies less than 1 kHz and were recorded commonly from socializing dolphins. These sounds differ significantly from narrow-band whistles, which are higher in frequency and longer in duration. The absence of these sounds in most studies of the acoustic behavior of bottlenose dolphins may reflect geographic differences in repertoires or result from insufficient sampling. Alternatively, these sounds may have been ignored where the focus of research was on other sound types.     

Do sampling method and sample size affect basic measures of dolphin sociality?

Although the fission–fusion social patterns of bottlenose dolphins have been described, defining and measuring sociality is difficult. No study to date has investigated how much data are necessary to accurately depict social patterns in a fission–fusion species and whether this amount differs depending on the measure used. Using the long-term Shark Bay bottlenose dolphin study we examined four different measures of sociality for mothers and dependent calves ( n = 89). Our objectives were to determine (1) the amount of observation time necessary to accurately depict sociality using these measures, (2) correlations among these measures, and (3) if sampling method affects the validity of sociality measurements. A minimum of 10 h and 5 h of observations, respectively, were necessary to accurately capture calf associate numbers and percent time alone. A pairwise comparison of calf sociality measurements from focal and survey data found that calves spent from 0% to 70.2% of their time alone in focal data, but these same calves spent only 0%–7.7% alone in survey data. These findings indicate that (1) results differ depending on the type of sampling method and (2) the amount of observation time necessary to adequately capture individual variation differs depending on the measure of sociality.     

A VIDEO SONAR AS A NEW TOOL TO STUDY MARINE MAMMALS IN THE WILD: MEASUREMENTS OF DOLPHIN SWIMMING SPEED

The use of a multibeam sonar for fine-scale studies of the foraging activity of bottlenose dolphins ( Tursiops truncatus ) was tested in a coastal environment where a group of 14 individuals is known to be resident. The sonar was set on a rubber boat and the signals were received continuously as the dolphins were foraging in a pass swept by strong tidal currents. These signals were recorded in HI 8 video standard. The system produced real-time undistorted images of the dolphins and of their movements within the limits of the sonar beam (15° vertical width, 90° horizontal width, range set at 50 m). The average swimming speed relative to the bottom was 1.8 m ·sec⁻¹ but swimming speed relative to the water was 2.2 m-sec⁻¹ when allowance was made for the average 1.2 m·sec⁻¹ tidal current. Maximum swimming speed relative to the water was as high as 4.8 m·sec⁻¹. It is estimated from studies of swimming energetics that continuously foraging in a current vein flowing at 1.2 m·sec¹ would represent only small additional costs compared to foraging out of these currents. However, previous observations of the same pod have shown that these dolphins keep foraging in this site when current speed reaches 2.7 m·sec⁻¹; under such circumstances, the cost of foraging in this site was calculated to increase by 96%. This suggests that foraging efficiency should be dramatically enhanced during rising tide since the dolphins consistently forage in these currents year round.