A new level of complexity in the male alliance networks of Indian Ocean bottlenose dolphins (Tursiops sp.)

Male bottlenose dolphins in Shark Bay, Western Australia form two levels of alliances; two to three males cooperate to herd individual females and teams of greater than three males compete with other groups for females. Previous observation suggested two alliance tactics: small four to six member teams of relatives that formed stable pairs or trios and unrelated males in a large 14-member second-order alliance that had labile trio formation. Here, we present evidence for a third level of alliance formation, a continuum of second-order alliance sizes and no relationship between first-order alliance stability and second-order alliance size. These findings challenge the 'two alliance tactics' hypothesis and add to the evidence that Shark Bay male bottlenose dolphins engage in alliance formation that likely places considerable demands on their social cognition.     

Male reproductive success increases with alliance size in Indo-Pacific bottlenose dolphins (Tursiops aduncus)

1. Determining the extent of variation in male mating strategies and reproductive success is necessary to understand the fitness benefits of social and cooperative behaviour. 2. This study assesses the reproductive success of male Indo-Pacific bottlenose dolphins in a small embayment population where different behavioural strategies of males have previously been identified. Parentage for 44 sampled calves was examined using 23 microsatellite loci and one mitochondrial DNA marker. Our candidate parent pool of 70 males and 64 females contained individuals sampled from both the embayment and adjacent coastal populations. 3. A moderate level of polygyny was detected in our sample. We assigned paternity of 23 calves to 12 males at the strict 95% confidence level and an additional nine calves to two males at the 80% confidence level. The majority (92%) of successful males were identified as residents to the embayment, and 46% of offspring were located within the same social group or community as their father. 4. Our results suggest that the size of alliances was the best predictor of reproductive success for males in this population, while the strength of association among allied males, alliance stability and male ranging patterns had little influence. In line with predictions for male alliances formed between unrelated individuals, we found that reproductive skew within alliances was not large. 5. Together, our genetic and behavioural analyses demonstrate that alliance formation between male dolphins is a successful strategy to enhance reproductive output.     

The ecological conditions that favor tool use and innovation in wild bottlenose dolphins (Tursiops sp.)

Dolphins are well known for their exquisite echolocation abilities, which enable them to detect and discriminate prey species and even locate buried prey. While these skills are widely used during foraging, some dolphins use tools to locate and extract prey. In the only known case of tool use in free-ranging cetaceans, a subset of bottlenose dolphins (Tursiops sp.) in Shark Bay, Western Australia habitually employs marine basket sponge tools to locate and ferret prey from the seafloor. While it is clear that sponges protect dolphins' rostra while searching for prey, it is still not known why dolphins probe the substrate at all instead of merely echolocating for buried prey as documented at other sites. By 'sponge foraging' ourselves, we show that these dolphins target prey that both lack swimbladders and burrow in a rubble-littered substrate. Delphinid echolocation and vision are critical for hunting but less effective on such prey. Consequently, if dolphins are to access this burrowing, swimbladderless prey, they must probe the seafloor and in turn benefit from using protective sponges. We suggest that these tools have allowed sponge foraging dolphins to exploit an empty niche inaccessible to their non-tool-using counterparts. Our study identifies the underlying ecological basis of dolphin tool use and strengthens our understanding of the conditions that favor tool use and innovation in the wild.     

Blubber development in bottlenose dolphins (Tursiops truncatus)

Blubber, the lipid-rich hypodermis of cetaceans, functions in thermoregulation, buoyancy control, streamlining, metabolic energy storage, and locomotion. This study investigated the development of this specialized hypodermis in bottlenose dolphins (Tursiops truncatus) across an ontogenetic series, including fetuses, neonates, juveniles, subadults, and adults. Blubber samples were collected at the level of the mid-thorax, from robust specimens (n = 25) that stranded along the coasts of North Carolina and Virginia. Blubber was dissected from the carcass and its mass, and the depth and lipid content at the sample site, were measured. Samples were prepared using standard histological methods, viewed by light microscopy, and digital images of blubber captured. Images were analyzed through the depth of the blubber for morphological and structural features including adipocyte size, shape, and numbers, and extracellular, structural fiber densities. From fetus to adult, blubber mass and depth increased proportionally with body mass and length. Blubber lipid content increased dramatically with increasing fetal length. Adult and juvenile blubber had significantly higher blubber lipid content than fetuses, and this increase was reflected in mean adipocyte size, which increased significantly across all robust life history categories. In juvenile, subadult, and adult dolphins, this increase in cell size was not uniform across the depth of the blubber, with the largest increases observed in the middle and deep blubber regions. Through-depth counts of adipocytes were similar in all life history categories. These results suggest that blubber depth is increased during postnatal growth by increasing cell size rather than cell number. In emaciated adults (n = 2), lipid mobilization, as evidenced by a decrease in adipocyte size, was localized to the middle and deep blubber region. Thus, in terms of both lipid accumulation and depletion, the middle and deep blubber appear to be the most metabolically dynamic. The superficial blubber likely serves a structural role important in streamlining the animal. This study demonstrates that blubber is not a homogeneous tissue through its depth, and that it displays life history-dependent changes in its morphology and lipid content.     

Why become speckled? Ontogeny and function of speckling in Shark Bay bottlenose dolphins (Tursiops sp.)1

To date, color patterns have been used to assess cetacean age and taxonomic status, but few studies have determined precise correlates of coloration with known age or investigated its function. Here, we examine the ontogeny of speckling in 88 bottlenose dolphins (Tursiops sp.) in Shark Bay, Australia, of known age, tracked from birth to age 34. Ventral speckles first appear in the genital area at a mean age of 10.2 ± 0.35 yr (range = 7.6–12.7 yr). Throughout their life span, speckles increase in number and density, particularly along the ventral and lateral sides. The timing of speckle onset does not significantly differ by sex but is related to sexual maturity in females. The age of speckle onset in the genital area correlates with the age of first known parturition. In terms of speckle function, we discuss two hypotheses commonly proffered to explain color variation, concealment, and communication. Concealment from predators or prey is unlikely to explain speckle development in Shark Bay Tursiops because the onset occurs long after peak predation risk and initial hunting success (at 3 mo of age). We suggest that speckle patterns offer reliable cues on reproductive status and/or condition and could, thus, serve a communicative or some other function.     

Diaphragm muscle development in bottlenose dolphins (Tursiops truncatus)

Being born directly into the aquatic environment creates unique challenges for the breathing muscles of neonatal cetaceans. Not only must these muscles be active at the instant of birth to ventilate the lungs, but their activities must also be coordinated with those of the locomotor muscles such that breathing takes place only at the water's surface. At least one major locomotory muscle of bottlenose dolphins (Tursiops truncatus) has been demonstrated to be well developed and, therefore, able to power the neonatal dolphin's early movements (Dearolf et al. [2000] J Morphol 244:203-215). Thus, because of the demands for coordinated behavior with the locomotor muscles, it is hypothesized that the breathing muscles of bottlenose dolphins, represented in this study by the diaphragm, will also demonstrate adult morphology at birth. However, histochemical and biochemical analyses demonstrate that neonatal dolphins have immature diaphragms, with only 52% of the adult slow fiber-type profile (neonates: 34% slow-twitch fibers; adults: 66% slow-twitch fibers). The developmental state of the dolphin diaphragm is compared to those of other neonatal mammals, using a muscle development index (% slow-twitch fibers in neonatal muscle / % slow-twitch fibers in adult muscle). Fiber-type profiles reported in the literature are used to calculate index values for the diaphragms of altricial rats, rabbits, and cats, intermediate baboons and humans, and precocial sheep and horses. The dolphin is not unique in having an immature diaphragm at birth; however, there is a positive relationship between the developmental state of the diaphragm and the overall developmental state of the neonate. The presence of type IIc ("undifferentiated") fibers in the diaphragms of altricial developers (e.g., rats, rabbits, and cats) is correlated with the slow contraction speeds recorded from their diaphragms. The diaphragms of neonatal horses and dolphins express little to no type IIc fibers and, thus, may have the ability to contract at the speeds required for their increased ventilation rates. These results lead to the modification of the criterion for evaluating the developmental state of a muscle at birth. Thus, the developmental state of a neonatal muscle should be based on both its value of Dearolf et al.'s (2000) developmental index, as well as the percentage of type IIc fibers found in that muscle.     

Cultural transmission of tool use by Indo-Pacific bottlenose dolphins (Tursiops sp.) provides access to a novel foraging niche

Culturally transmitted tool use has important ecological and evolutionary consequences and has been proposed as a significant driver of human evolution. Such evidence is still scarce in other animals. In cetaceans, tool use has been inferred using indirect evidence in one population of Indo-Pacific bottlenose dolphins (Tursiops sp.), where particular dolphins (‘spongers’) use marine sponges during foraging. To date, evidence of whether this foraging tactic actually provides access to novel food items is lacking. We used fatty acid (FA) signature analysis to identify dietary differences between spongers and non-spongers, analysing data from 11 spongers and 27 non-spongers from two different study sites. Both univariate and multivariate analyses revealed significant differences in FA profiles between spongers and non-spongers between and within study sites. Moreover, FA profiles differed significantly between spongers and non-spongers foraging within the same deep channel habitat, whereas the profiles of non-spongers from deep channel and shallow habitats at this site could not be distinguished. Our results indicate that sponge use by bottlenose dolphins is linked to significant differences in diet. It appears that cultural transmission of tool use in dolphins, as in humans, allows the exploitation of an otherwise unused niche.     

Food-related bray calls in wild bottlenose dolphins (Tursiops truncatus)

Because cetaceans are difficult to study in the wild, little is known about how they use their sounds in their natural environment. Only the recent development of passive acoustic localization systems has enabled observations of the communication behaviour of individuals for correlation with their surface behaviour. Using such a system, I show that bottlenose dolphins in the Moray Firth, Scotland, produce low-frequency bray calls which are clearly correlated with feeding on salmonids. The production of these calls is followed by fast approaches by conspecifics in the area. In animals which use sound as a foraging tool, it is difficult to distinguish between food calls which have evolved because of their role in attracting conspecifics, and food manipulation or searching calls which may attract conspecifics as a by-product. However, the low-frequency structure of the bottlenose dolphin bray suggests that it evolved because of a role in manipulating prey rather than in attracting conspecifics. This conclusion suggests that dolphins exploit the perceptual systems of their prey to facilitate capture.     

Signature-whistle production in undisturbed free-ranging bottlenose dolphins (Tursiops truncatus)

Data from behavioural observations and acoustic recordings of free-ranging bottlenose dolphins (Tursiops truncatus) were analysed to determine whether signature whistles are produced by wild undisturbed dolphins, and how whistle production varies with activity and group size. The study animals were part of a resident community of bottlenose dolphins near Sarasota, Florida, USA. This community of dolphins provides a unique opportunity for the study of signature-whistle production, since most animals have been recorded during capture-release events since 1975. Three mother-calf pairs and their associates were recorded for a total of 141.25 h between May and August of 1994 and 1995. Whistles of undisturbed dolphins were compared with those recorded from the same individuals during capture-release events. Whistles were conservatively classified into one of four categories: signature, probable signature, upsweep or other. For statistical analyses, signature and probable signature whistles were combined into a 'signature' category; upsweep and other whistles were combined into a 'non-signature' category. Both 'signature' and 'non-signature' whistle frequencies significantly increased as group size increased. There were significant differences in whistle frequencies across activity types: both 'signature' and 'non-signature' whistles were most likely to occur during socializing and least likely to occur during travelling. There were no significant interactions between group size and activity type. Signature and probable signature whistles made up ca. 52% of all whistles produced by these free-ranging bottlenose dolphins.     

Site fidelity,residency, and abundance of bottlenose dolphins (Tursiops sp.) in Adelaide's coastal waters,South Australia

Little is known about the ecology and behavior of southern Australian bottlenose dolphins (Tursiops sp.). This hinders assessment of their conservation status and informed decision‐making concerning their management. We used boat‐based surveys and photo‐identification data to investigate site fidelity, residency patterns, and the abundance of southern Australian bottlenose dolphins in Adelaide's coastal waters. Sighting rates and site fidelity varied amongst individuals, and agglomerative hierarchical cluster analysis led to the categorization of individuals into one of three groups: occasional visitors, seasonal residents, or year‐round residents. Lagged identification rates indicated that these dolphins used the study area regularly from year to year following a model of emigration and reimmigration. Abundance estimates obtained from multisample closed capture‐recapture models ranged from 95 individuals (SE ± 45.20) in winter 2013 to 239 (SE ± 54.91) in summer 2014. The varying levels of site fidelity and residency, and the relatively high number of dolphins found throughout the study area highlights the Adelaide metropolitan coast as an important habitat for bottlenose dolphins. As these dolphins also appear to spend considerable time outside the study area, future research, conservation, and management efforts on this population must take into account anthropogenic activities within Adelaide's coastal waters and their adjacencies.     

Lateralized behavior in two captive bottlenose dolphins (Tursiops truncatus)

The study of behavioral laterality in humans and nonhumans can contribute to our understanding of brain evolution and functional similarities across species. Few studies have focused on cetaceans. This report exams lateralized behaviors in two captive bottlenose dolphins (Tursiops truncatus). Observations were made by videotaping through a 90 × 150 cm underwater one-way Plexiglass mirror. Directional bias in swimming, “barrel-roll” maneuvers, and circular head movements was assessed for each subject. There was a strong clockwise bias in swimming direction and direction of “barrel-rolls,” but not circular head movements. The clockwise bias in swimming direction and “barrel-roll” maneuvers may be consistent with a rightward turning bias. Zoo Biol 16:173–177, 1997. © 1997 Wiley-Liss, Inc.     

A novel mammalian social structure in Indo-Pacific bottlenose dolphins (Tursiops sp.): complex male alliances in an open social network

Terrestrial mammals with differentiated social relationships live in 'semi-closed groups' that occasionally accept new members emigrating from other groups. Bottlenose dolphins (Tursiops sp.) in Shark Bay, Western Australia, exhibit a fission-fusion grouping pattern with strongly differentiated relationships, including nested male alliances. Previous studies failed to detect a group membership 'boundary', suggesting that the dolphins live in an open social network. However, two alternative hypotheses have not been excluded. The community defence model posits that the dolphins live in a large semi-closed 'chimpanzee-like' community defended by males and predicts that a dominant alliance(s) will range over the entire community range. The mating season defence model predicts that alliances will defend mating-season territories or sets of females. Here, both models are tested and rejected: no alliances ranged over the entire community range and alliances showed extensive overlap in mating season ranges and consorted females. The Shark Bay dolphins, therefore, present a combination of traits that is unique among mammals: complex male alliances in an open social network. The open social network of dolphins is linked to their relatively low costs of locomotion. This reveals a surprising and previously unrecognized convergence between adaptations reducing travel costs and complex intergroup-alliance relationships in dolphins, elephants and humans.     

Fat distribution in the skin of bottlenose dolphins (Tursiops truncatus and Tursiops gilli)

Frozen sections stained with Oil-red-O and semithin (0.5 μm) plastic sections stained with toluidine blue revealed an abundance of fat globules of various sizes in all strata of the epidermis of bottlenose dolphins (Tursiops truncatus and T. gilli). The fat was rather evenly distributed but sometimes appeared as circumscribed areas of heavier concentration involving hundreds of cells (as seen in a single plane). Occasionally, there were smaller groups of epidermal cells heavily loaded with lipid. The dermis presented a unique phenomenon in the presence of abundant extracellular fat distributed among the collagen bundles as droplets of various sizes or as larger, irregularly shaped lipid particles that seemed to conform to the spaces between collagen bundles. These lipid particles were sometimes seen to be closely applied to the dermal surface of the stratum basale. Equally unusual was the presence of lipid particles of various sizes and shapes in the lumen of some of the vessels of the dermal papillae. Granular cells resembling mast cells were commonly seen in the papillary dermis and some were closely associated with lipid particles. Blood vessels of the reticular dermis tended to have collections of lipid droplets in the loose connective tissue often found adjacent to the tunica adventitia. It is postulated that the extracellular dermal lipids (probably mainly triglycerides) are broken down to free fatty acids that diffuse into the basal layer of the epidermis and are there resynthesized into triglycerides. Possible uses for the epidermal lipids are discussed.     

Association patterns of bottlenose dolphins (Tursiops truncatus) in Bahía San Antonio,Argentina

This study aims to investigate the association patterns of a small and resident population of bottlenose dolphins (Tursiops truncatus) in Bahía San Antonio (Argentina), and assess any seasonal variation in view of the reported seasonal variation in the population's grouping behavior. The estimation of social differentiation (S) was 0.29 (SE = 0.08), suggesting a homogenous society. The half‐weight index (HWI) averaged 0.23 (± 0.06), with evidence of long‐term preferred associations. However, data showed seasonal variation in the association patterns. In winter, when large groups were reported in this population, HWI values averaged 0.30 (± 0.09) with no indication of preferred or avoided associations. However, during summer, when group size was generally small, HWI values averaged 0.14 (± 0.07), with an indication of preferred/avoided associations. This study indicates that the social structure of the bottlenose dolphins in Bahía San Antonio seems relatively homogenous and flexible over time, with the formation of a random social network at times when large aggregations are formed, and a more disconnected network made up of strongly connected components when the cost of grouping is high.     

Isolation of Toxoplasma gondii from bottlenose dolphins (Tursiops truncatus)

Toxoplasma gondii infection in marine mammals is intriguing and indicative of contamination of the ocean environment and coastal waters with oocysts. In previous serological surveys, >90% of bottlenose dolphins (Tursiops truncatus) from the coasts of Florida, South Carolina, and California had antibodies to T. gondii by the modified agglutination test (MAT). In the present study, attempts were made to isolate T. gondii from dead T. truncatus. During 2005, 2006, and 2007, serum or blood clot, and tissues (brain, heart, skeletal muscle) of 52 T. truncatus stranded on the coasts of South Carolina were tested for T. gondii. Antibodies to T. gondii (MAT 1:25 or higher) were found in 26 (53%) of 49 dolphins; serum was not available from 3 animals. Tissues (heart, muscle, and sometimes brain) of 32 dolphins (26 seropositive, 3 seronegative, and 3 without accompanying sera) were bioassayed for T. gondii in mice, or cats, or both. Tissues of the recipient mice were examined for T. gondii stages. Feces of recipient cats were examined for shedding of T. gondii oocysts, but none excreted oocysts. Toxoplasma gondii was isolated from hearts of the 3 dolphins (2 with MAT titers of 1:200, and 1 without accompanied serum) by bioassay in mice. Genotyping of these 3 T. gondii isolates (designated TgDoUs1-3) with the use of 10 PCR-RFLP markers (SAG1, SAG2, SAG3, BTUB, GRA6, c22-8, c29-2, L358, PK1, and Apico) revealed 2 genotypes. Two of the 3 isolates have Type II alleles at all loci and belong to the clonal Type II lineage. One isolate has a unique genotype. This is the first report of isolation of viable T. gondii from T. truncatus.     

Patterns of growth in wild bottlenose dolphins, Tursiops truncatus

The growth of bottlenose dolphins is described from observations made during a capture release programme that has operated in coastal waters of the eastern Gulf of Mexico from 1970 to the present. Measurements of standard length, girth and body mass were recorded from 47 female and 49 male dolphins, some captured as many as nine times. Ages were known from approximate birth dates or estimated from counts of dentinal growth layers. In all three measurements. females grew at a faster initial rate than males, but reached asymptotic size at an earlier age. This extended period of growth in males resulted in significant sexual dimorphism in length, girth and mass at physical maturity. The growth of both sexes was well described by three-parameter Gompertz models using either cross-sectional data or a mixture of longitudinal and cross-sectional data. There was considerable variation in size-at-age for both sexes in all year classes. Residuals of size measurements were used to derive measures of relative size for individual dolphins; most dolphins demonstrated little ontogenetic change in relative size. Body mass was adequately predicted by multiple regression equations that incorporated both length and girth as independent variables.     

Individual patterns of tonal (whistling) signals of bottlenose dolphins (Tursiops truncates) kept in relative isolation

The results of study of the subaqueous acoustic signaling of bottlenose dolphins (Tursiopsis truncatus) in a dolphinarium are shown. Individual patterns of whistling signals of four individuals were described and analyzed. To determine which animal produced the sound, the method of ??relative isolation?? was used. It was found that the pattern included whistles specific for a particular individual (??signature whistles??). The percentage of these whistles in the patterns varied considerably among individuals. The signatures had variable frequency and duration. The phenomenon of ??imitation?? was typical for animals at higher positions in the hierarchy: generating ??signatures?? specific for other individuals. Besides the signatures, variable signals and whistles with simple frequency contours were found in the patterns.