Evoked potential audiometry of 13 Pacific bottlenose dolphins (Tursiops truncatus gilli)

Evoked potential audiograms were measured in 13 Pacific bottlenose dolphins (Tursiops truncatus gilli) to determine the variability in hearing sensitivity and range of hearing. The auditory evoked potential system used a transducer embedded in a suction cup to deliver sinusoidal amplitude modulated tones to each dolphin through the pan region of the lower right jaw. Evoked potentials were recorded noninvasively using surface electrodes, and hearing thresholds were estimated by tracking the amplitude of the envelope following response, an evoked potential that is phase‐locked to the stimulus modulation rate. Frequencies tested ranged from 10 to 180 kHz in each animal. Variability in the range of hearing and age‐related reductions in hearing sensitivity and range of hearing were consistent with those observed in Atlantic bottlenose dolphins. Comparison of audiograms to a captive population of Atlantic bottlenose dolphins demonstrated that the Pacific bottlenose dolphins tested in this study had significantly lower thresholds at frequencies of 40 and 60–115 kHz. Differences in thresholds between the groups are unlikely to be due to methodological factors.     

Ontogenetic diet changes in bottlenose dolphins (Tursiops truncatus) reflected through stable isotopes

The ability of stable isotope analysis to provide insight into ontogenetic dietary changes was examined using bottlenose dolphin tooth and skin samples. Teeth were subsampled to compare tissue produced early in life (outer tooth) to that produced later in life (inner tooth). Outer tooth had significantly higher δ¹⁵N values than the corresponding inner sample from the same tooth (n= 60, P= 0.0041), indicating that there was a temporal shift to a lower δ¹⁵N diet. There were no significant δ¹³C differences. Higher δ¹⁵N values in young have previously been attributed to the period of suckling. Analysis of skin tissue from stranded animals of different developmental stages similarly indicated that the δ¹⁵N values were significantly higher in young animals. Further comparisons indicated that the primary influence for this difference was animals with lengths less than or equal to the largest neonatal dolphin. This difference likely reflects an ontogenetic dietary shift from a sole reliance on milk to a combination of milk and prey species during the first year of life.     

Bottlenose dolphins exchange signature whistles when meeting at sea

The bottlenose dolphin, Tursiops truncatus, is one of very few animals that, through vocal learning, can invent novel acoustic signals and copy whistles of conspecifics. Furthermore, receivers can extract identity information from the invented part of whistles. In captivity, dolphins use such signature whistles while separated from the rest of their group. However, little is known about how they use them at sea. If signature whistles are the main vehicle to transmit identity information, then dolphins should exchange these whistles in contexts where groups or individuals join. We used passive acoustic localization during focal boat follows to observe signature whistle use in the wild. We found that stereotypic whistle exchanges occurred primarily when groups of dolphins met and joined at sea. A sequence analysis verified that most of the whistles used during joins were signature whistles. Whistle matching or copying was not observed in any of the joins. The data show that signature whistle exchanges are a significant part of a greeting sequence that allows dolphins to identify conspecifics when encountering them in the wild.     

Population structure of island-associated dolphins: Evidence from photo-identification of common bottlenose dolphins (Tursiops truncatus) in the main Hawaiian Islands

Management agencies often use geopolitical boundaries as proxies for biological boundaries. In Hawaiian waters a single stock is recognized of common bottlenose dolphins, Tursiops truncatus , a species that is found both in open water and near-shore among the main Hawaiian Islands. To assess population structure, we photo-identified 336 distinctive individuals from the main Hawaiian Islands, from 2000 to 2006. Their generally shallow-water distribution, and numerous within-year and between-year resightings within island areas suggest that individuals are resident to the islands, rather than part of an offshore population moving through the area. Comparisons of identifications obtained from Kaua'i/Ni'ihau, O'ahu, the "4-island area," and the island of Hawai'i showed no evidence of movements among these island groups, although movements from Kaua'i to Ni'ihau and among the "4-islands" were documented. A Bayesian analysis examining the probability of missing movements among island groups, given our sample sizes for different areas, indicates that interisland movement rates are less than 1% per year with 95% probability. Our results suggest the existence of multiple demographically independent populations of island-associated common bottlenose dolphins around the main Hawaiian islands.     

Signature whistles in free‐ranging populations of Indo‐Pacific bottlenose dolphins,Tursiops aduncus

Common bottlenose dolphins (Tursiops truncatus) use individually distinctive signature whistles which are highly stereotyped and function as contact calls. Here we investigate whether Indo‐Pacific bottlenose dolphins (T. aduncus) use signature whistles. The frequency trace of whistle contours recorded from three genetically distinct free‐ranging populations was extracted and sorted into whistle types of similar shape using automated categorization. A signature whistle identification method based on the temporal patterns in signature whistle sequences of T. truncatus was used to identify signature whistle types (SWTs). We then compared the degree of variability in SWTs for several whistle parameters to determine which parameters are likely to encode identity information. Additional recordings from two temporarily isolated T. aduncus made during natural entrapment events in 2008 and 2009 were analyzed for the occurrence of SWTs. All populations were found to produce SWTs; 34 SWTs were identified from recordings of free‐ranging T. aduncus and one SWT was prevalent in each recording of the two temporarily isolated individuals. Of the parameters considered, mean frequency and maximum frequency were the least variable and therefore most likely to reflect identity information encoded in frequency modulation patterns. Our results suggest that signature whistles are commonly used by T. aduncus.     

Fine-scale population structure of bottlenose dolphins (Tursiops truncatus) in Tampa Bay, Florida

Some populations of coastal bottlenose dolphins ( Tursiops truncatus ) comprise discrete communities, defined by patterns of social association and long-term site fidelity. We tested the hypothesis that bottlenose dolphins in Tampa Bay, Florida, form a single community. The longitudinal study of dolphins in Sarasota Bay, adjacent to Tampa Bay, allowed us to ground-truth the definition of community and test whether our approach was robust to small sample sizes of resightings. We conducted photo-identification surveys in Tampa Bay during 1988–1993, and identified 102 dolphins with 10 or more sightings. We used hierarchical cluster analysis to examine the locations and association indices of these dolphins. We used analysis of variance (ANOVA) to test for differences in mean locations and determine whether mean coefficient of association (CoA) values within a community were higher than among communities. Dolphins in Tampa Bay clustered into five putative communities differing significantly in location and CoA values. Kernel estimates of the ranges of these five communities exhibited little overlap; some communities had no overlap at all. We conclude that five discrete communities of bottlenose dolphins exist in Tampa Bay and that such fine-scale structure may be a common feature of bottlenose dolphin populations throughout the southeastern United States.     

Ecology and conservation of common bottlenose dolphins Tursiops truncatus in the Mediterranean Sea

• 1 Bottlenose dolphins Tursiops truncatus are amongst the best‐known cetaceans. In the Mediterranean Sea, however, modern field studies of cetaceans did not start until the late 1980s. Bottlenose dolphins have been studied only in relatively small portions of the basin, and wide areas remain largely unexplored.
• 2 This paper reviews the ecology, behaviour, interactions with fisheries and conservation status of Mediterranean bottlenose dolphins, and identifies threats likely to have affected them in historical and recent times.
• 3 Whilst intentional killing was probably the most important cause of mortality until the 1960s, important ongoing threats include incidental mortality in fishing gear and the reduced availability of key prey caused by overfishing and environmental degradation throughout the region. Additional potential or likely threats include the toxic effects of xenobiotic chemicals, epizootic outbreaks, direct disturbance from boating and shipping, noise, and the consequences of climate change.
• 4 The flexible social organization and opportunistic diet and behaviour of bottlenose dolphins may allow them to withstand at least some of the effects of overfishing and habitat degradation. However, dolphin abundance is thought to have declined considerably in the region and management measures are needed to prevent further decline.
• 5 Management strategies that could benefit bottlenose dolphins, such as sustainable fishing, curbing marine pollution and protecting biodiversity, are already embedded in legislation and treaties. Compliance with those existing commitments and obligations should be given high priority.

     

Maternal aggressive contact vocalizations in captive bottlenose dolphins (Tursiops truncatus): Wide-band,low-frequency signals during mother/aunt-infant interactions

The mother-infant bond in bottlenose dolphins is critical to infant survival and has been reported to last from 3-10 years in both captive and wild populations. Little information on mother-infant communication during early development has been collected. This paper reports on a newly discovered dolphin vocalization, termed thunk, which is predominantly used by mothers toward infants. Four mother-infant pairs and one aunt-infant pair were the subjects for this study. Methods included a focal animal sampling technique using 2.5 min interval and event/continuous sampling regimes on audio- or videotape. Results indicated that thunks are produced by mothers or aunting females during infant departures (distances greater than 5 feet) and are frequently followed by disciplinary activity by the mother or aunt. In addition, thunks appeared to cease at approximately 9-10 months after birth, concurrent with a decrease in infant dependence. Thunks also were analyzed acoustically for frequency and duration parameters. Thunks have a harmonic structure with an energy peak in the 273–350 Hz range and ranged from 129–5,556 Hz in frequency and from 21–171 ms in duration. They appear to function as aggressive contact vocalizations produced by mothers and other adult females toward infants in order to maintain infant proximity. © 1995 Wiley-Liss, Inc.     

Environmental drivers of habitat use by common bottlenose dolphins (Tursiops truncatus) in the Indian River Lagoon,Florida, USA

Anthropogenic impacts in estuarine systems can influence marine mammal habitat use, population dynamics, fitness, and mortality events. The objective was to examine habitat use among the resident common bottlenose dolphin (Tursiops truncatus) population inhabiting the Indian River Lagoon, Florida, and the influences of variation in environmental factors and prey availability in 2003–2015. We utilized photo-identification surveys, stratified random samples of prey, and environmental variables collected monthly. Kernel density estimation was used to determine the magnitude-per-unit area of dolphins across the IRL by wet and dry seasons each year, the values were used as a response variable in classification and regression tree analyses with fish community and environmental variables as predictors. Spatial patterns in dolphin density in the IRL were associated with salinity and dissolved oxygen levels, which are in part associated with freshwater discharges of nutrient and algae laden waters from the region's storm water management system. These findings isolate locations of concern for management of dolphin habitat, and anthropogenic drivers of dolphin distributions requiring further research.     

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.     

Linear skin markings in common bottlenose dolphins (Tursiops truncatus) from the Indian River Lagoon,Florida

A previously undescribed skin abnormality, referred to as “linear skin markings” (LSM), has been identified in free-ranging common bottlenose dolphins (Tursiops truncatus) in the Indian River Lagoon, Florida (IRL). The lesions were identified during photo-identification surveys conducted from 2002 and 2015. LSM presented as distinct, parallel lines running dorso-ventrally on the torso and varied in length and width. The goals of this study were to determine (1) prevalence of the condition in IRL dolphins, (2) age and sex distribution of affected animals, (3) spatial and temporal distribution patterns, (4) duration of the condition, and (5) development of hypotheses regarding the etiology of the condition. Among 1,357 individual dolphins identified during the study period, 96 (7.0%) showed evidence of LSM. Nearly all (98.8%) cases with an established home range occurred in the northern and central regions of the IRL. The majority of cases of known sex were female (85%), of which 100% had given birth to one or more calves. The mean age of animals with LSM when first observed was 7.3 with a range of 1–20 years. The maximum observed duration of LSM was 15 years. Once observed, the condition persisted indefinitely. The etiology of LSM has not been established.     

Seasonal abundance and adult survival of bottlenose dolphins (Tursiops truncatus) in a community that cooperatively forages with fishermen in southern Brazil

A subgroup of a population of Tursiops truncatus in southern Brazil is known for a cooperative behavior with artisanal fishermen whereby the dolphins shoal fish towards net‐casting fishermen. Combining photo‐identification data collected between September 2007 and 2009 with mark‐recapture and Pollock's robust design models, we assessed abundance within seasons and survival and temporary emigration rates of dolphins between seasons. We also reanalyzed a previous data set collected during 1989–1991, and Cormack‐Jolly‐Seber models were applied to estimate survival rates for each of the study periods. The abundance of marked “cooperative” dolphins varied between seasons from 18 (CI: 17–24) to 21 (CI: 20–24). The total abundance varied from 59 in the winter of 2008 (CI: 49–72) to 50 in the autumn of 2009 (CI: 40–62). The annual adult survival was estimated to be 0.917 (CI: 0.876–0.961), close to that estimated from data collected in the 1990s (0.941; CI: 0.888–0.998). The emigration probability was low (0.031; CI: 0.011–0.084) and different capture probabilities between the “cooperative” and “noncooperative” dolphins indicated a degree of behavioral segregation. The precision of our estimates is likely to provide sufficient power to detect population change, but we recommend a precautionary management approach to protect this vulnerable dolphin community and its unique cooperative feeding tradition.     

Long-term passive acoustics to assess spatial and temporal vocalization patterns of Atlantic common bottlenose dolphins (Tursiops truncatus) in the May River estuary,South Carolina

Passive acoustics has been used extensively to study bottlenose dolphins; yet very few studies have examined the spatial, temporal, and environmental influences on vocalization types (echolocation, burst pulse sounds, and whistles), and few are long-term and provide high temporal resolution over multiple years. We used data from 2013 to 2018 to establish baseline acoustic patterns for bottlenose dolphins in the May River estuary, South Carolina. We deployed acoustic recorders at six stations during 2013–2014 and three stations during 2015–2018, with locations spanning the entire estuary (headwaters to the mouth). We discovered that acoustic detection of dolphins varied not only spatially, but also yearly, monthly, and tidally. Higher numbers of echolocation bouts, burst pulse sounds, and whistles were detected at the mouth as compared to the headwaters. At the mouth, vocalization detections were greatest in fall and winter for multiple years, and echolocation detection was greatest during falling and low tides. This study provides an example of another tool, long-term passive acoustics monitoring, to better understand spatial and temporal distribution of dolphins in a typical salt marsh estuary, that can be applied to other ecosystems throughout the southeastern United States and globally.     

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.