Dolphin Group Size Research Articles

Aspects of the ecology and behaviour of bottlenose dolphins (Tursiops truncatus) in Santa Monica Bay, California

The occurrence, distribution, site fidelity, group size and behaviour of common bottlenose dolphins (Tursiops truncatus) were assessed during a photo-identification study conducted between 1997-2001 in Santa Monica Bay, California. Bottlenose dolphins occurred yearround in the bay and were encountered on 56.8% of all surveys (n, total surveys=211). This species was found in waters within 0.5km of shore in 80.0% of the sightings (n=157), but sometimes found in deeper waters further offshore (>0.5km). No correlations between anomalies in sea surface temperatures during the 1997-98 El Niño event and sighting frequencies were observed. Group sizes varied significantly between schools observed inshore and offshore in the bay, with the largest groups sighted offshore. A total of 290 dolphins were individually photo-identified based on long-term natural marks on their dorsal fins. Forty-four individuals (15.2%) were encountered in both inshore and offshore waters, showing no exclusive fidelity to inshore waters. The low resighting rates of known individuals provided little evidence of long-term year-round site fidelity for Santa Monica Bay, revealing a range greater than the chosen study area. Several individuals, however, were resighted over one or two year periods, generally during more than one season. This suggested that these dolphins were highly mobile within the inshore waters of the Southern California Bight but they also spent time foraging and feeding in Santa Monica Bay, probably due to the presence of submarine canyons within this area. The behavioural budget for this species showed a predominance of activities characterised by travel and dive-travel (69.0%) and feeding (5.0%), indicating a fairly high proportion of time devoted to searching for prey and feeding in the study area.

Group size varies with climate and oceanographic conditions in bottlenose dolphins

An understanding of animal grouping patterns is essential to increase knowledge about their social systems. We used a 5-year continuous dataset to investigate grouping behavior in a free-ranging highly social mammal, the bottlenose dolphin. Our objective was achieved by examining the relationships between a set of oceanographic, climatic, topographic, anthropogenic and social variables and bottlenose dolphin group size. Modeling analysis revealed that bottlenose dolphin group dynamics was linked to both small-scale oceanographic variation and large-scale climatic variation. Larger groups of bottlenose dolphins were predicted with an elevated concentration of dissolved oxygen in the water column, a moderate concentration of chlorophyll-a 60 days before each encounter, and a neutral NAO index. The observed link between environmental variables and dolphin group size is likely associated to changes in dolphin prey availability. Bottlenose dolphin group dynamics are likely driven by multiple factors, and social variables may act synergistically with environmental parameters. Larger groups of bottlenose dolphins were indeed also predicted with a greater number of dependent calves, likely because of enhanced care towards the calves. This study illustrates the value of using multiple variables at different scales to explore the factors that shape animal societies. The current study therefore contributes to the growing body of literature on how environmental change, occurring at different spatio-temporal scales, is indirectly related to the social behavior of a marine top predator.

Interannual shifts in sea surface temperature and chlorophyl drive the relative abundance and group size of common bottlenose dolphins Tursiops truncatus on the Southeast Gulf of California

Despite being one of the most common odontocetes off Sinaloa (Mexican Pacific coast), basic studies on the ecology of common bottlenose dolphins (Tursiops truncatus) are scarce in the region. This study aimed to describe changes in the relative abundance, group size, and behavior of this species during 2007 – 2012. We used boat-based surveys and satellite images of sea surface temperature (SST) and chlorophyll-a (Chl-a) to model changes in dolphin relative abundance (RA) over time, using correlations in time series analyses. Overall, mean RA was 3.6 dolphins h-1 (SD = 8.0), and significantly higher RA (6.4 –16.7 dolphins h-1) occurred in 2008, 2011, 2012, which was concurrent with hydrographic effects of La Niña oceanographic conditions, as well as during the upwelling season (February – April) (SST: 17.3 – 25.0 °C; Chl-a: 3.7 – 21.4 mg m-3). Conversely, significantly lower RA values (0.5 – 3.8 dolphins h-1) occurred in 2007 and 2010 that were likely associated with El Niño effects on the biological productivity of the area (Chl-a: 0.3 – 7.6 mg m-3). We found significant correlations between monthly Chl-a and SST average values with mean bottlenose dolphins RA, and lags (22 – 29 days) in the trophic response to variations of the hydrographic parameters. Significantly larger dolphin groups were recorded during La Niña years possibly because of the higher availability of their prey. This hypothesis is supported by higher feeding frequencies (35 – 73%) observed during the upwelling seasons, especially during La Niña conditions, whereas the most frequent behavior throughout all other years was traveling (28 – 69%). Our results show that RA and group size of bottlenose dolphins inhabiting the waters off Sinaloa, Mexico, are likely influenced by the changes in hydrographic parameters, especially during extreme climatic events.

Coastal dolphins and marine megafauna in Exmouth Gulf, Western Australia: informing conservation management actions in an area under increasing human pressure

Context Exmouth Gulf is adjacent to the Ningaloo Marine Park, a UNESCO-listed area in Western Australia. The gulf remains largely unprotected, and is under increasing anthropogenic pressure from proposed industrial activities that pose threats to marine megafauna inhabiting the gulf. Threatened and near threatened species, such as the Australian humpback dolphin (Sousa sahulensis) and Indo-Pacific bottlenose dolphin (Tursiops aduncus), reside in the gulf; however, detailed information on their ecology and behaviour is lacking. Aims The aim was to (1) provide baseline data on the distribution, encounter rate, group size and behaviour of coastal dolphins over an area where current industrial developments are proposed, and (2) report on the occurrence of other marine megafauna within this area. Methods Boat-based photo-identification surveys were conducted on the western coastline of Exmouth Gulf along pre-determined line transects (150 km2) over austral autumn/winter 2021. Key results Across 809.35 km of surveyed waters (181 h), a total of 93 bottlenose dolphin, 15 humpback dolphin, and six interspecific dolphin groups were sighted. Bottlenose dolphin groups were encountered at a rate of 0.077/km, humpback dolphin groups at 0.015/km and interspecific dolphin groups at 0.005/km. Dolphins were predominantly recorded in shallow (mean 10 m) and warm (mean 21°C) waters, and were commonly travelling and foraging. In total, 199 individual bottlenose dolphins and 48 humpback dolphins were photo-identified (excluding calves). There were 30 bottlenose dolphin calves (including three newborns) and four humpback dolphin calves (including two newborns) identified. Other marine megafauna group sightings included humpback whales (Megaptera novaeangliae; n = 32), southern right whales (Eubalaena australis, n = 1), dugongs (Dugong dugon, n = 25), turtles (n = 54), sea snakes (n = 27), manta rays (Mobula alfredi, n = 13) and sharks (n = 2). Conclusions The presence of threatened marine species feeding, socialising, and resting highlights the importance of these waters for the identified species. Implications The information provided is applicable for the spatial management and conservation efforts of these species, and aids in informing environmental impact assessments of individual and cumulative pressures.

Group Size of Indo-Pacific Humpback Dolphins (Sousa chinensis): An Examination of Methodological and Biogeographical Variances

Observer-based counts and photo-identification are two well-established methods with an extensive use in cetacean studies. Using these two methods, group size has been widely reported, especially for small dolphins. Both methods may come with potential errors in estimating the group size, yet there is still a lack of comparison between both methods over a broad range of group size. Particularly, biogeographical variances in group size estimates were often mixed with methodological variances, making it difficult to compare estimates from different geographic regions. Here, group size estimates of a small, shallow-water, and near-shore delphinid species, Indo-Pacific humpback dolphins (Sousa chinensis), were simultaneously sampled using observer-based counts and photo-identification at three regions in the northern South China Sea. Data showed that dolphin group size from two methods were highly variable and associated with sampling regions. Generalized linear mixed models (GLMMs) indicated that dolphin group size significantly differed among regions. Statistical examinations further demonstrated dolphin group size could be affected by a complex combination of methodological and biogeographical variances. A common hurdle to examine potential factors influencing the estimation process is the inability to know the true group size at each sample. Therefore, other methods that could generate comparable estimates to represent true group size are warranted in future studies. To conclude, our findings present a better understanding of methodological and biogeographical variances in group size estimates of humpback dolphins, and help yield more robust abundance and density estimation for these vulnerable animals.

Cetaceans of the Northern Bismarck Sea, Papua New Guinea1

There is little known about cetaceans in the waters of Papua New Guinea (PNG). Here we present findings from the first two sets of cetacean sighting and acoustic surveys conducted within the Admiralty Island group in the northern Bismarck Sea. More than 1,000 cetaceans were sighted during our boat-based surveys–the most common being spinner dolphins (Stenella longirostris), followed by pantropical spotted dolphins (Stenella attenuata), short-finned pilot whales (Globicephala macrorhynchus), and sperm whales (Physeter macrocephalus). The relative group size of spinner dolphins was different between years with group sizes in 2010 being significantly lower (median = 15, range = 2–50) than those recorded in 2013 (median = 50, range = 5–100). The presence of large aggregations of spinner dolphins (including calves) as well as sightings of Vulnerable sperm whales suggest the northern Bismarck Sea as an area of conservation importance for cetaceans. Our surveys also provide useful baseline data for a variety of marine protected area planning processes and management initiatives that are ongoing in PNG.

Intra-Population Variability in Group Size of Indo-Pacific Humpback Dolphins (Sousa chinensis)

Group size is a key social trait influencing population dynamics of group-living animals. The Indo-Pacific humpback dolphins (IPHDs), Sousa chinensis, a shallow water delphinid species, display a fission-fusion social system. Yet little is known about how social organization of this species vary with temporal scales and behavioral state. In this study, we sampled group size estimates from the world’s second largest population of humpback dolphins (Sousa spp.), which inhabit the eastern waters of Zhanjiang, China. IPHD group sizes changed seasonally and inter-annually, but not with tidal phases. Group sizes also changed with behavioral state of IPHD groups and with number of mother-calf pairs present. IPHDs formed larger groups in the autumn than in other seasons, which might be related to seasonal changes in food availability and reproductive cycle. Of the groups observed, we recorded the presence of mother-calf pair in 85 groups (i.e., nursery groups: 47 ones with one pair, 25 ones with two pairs, and others with three pairs). Notably, nursery groups were about 2–4 times larger than non-nursery groups. In addition, group sizes greatly increased with the number of mother-calf pairs. Living in relatively large groups, more protection, food, and resources might be available for IPHD mothers and calves, and such social strategy provide higher reproduction efficiency and survival success for this species. During our observations, feeding (45.5%) and traveling (25.2%) represented the majority of IPHD’s behavioral budget, while socializing (8.4%) and resting/milling (6.8%) were not frequently observed. Resting/milling groups were approximately 50% smaller than feeding, traveling, or socializing groups, while the latter three types had a similar mean group size. Large groups when IPHDs foraged, traveled, or socialized, might provide more added group benefits. For the first time, our findings clearly revealed intra-population variability in IPHD group sizes across different behavioral and temporal variables, and provided a better understanding of IPHDs’ adaptations to various biological processes and ecological constraints.

Spatial and Temporal Effects of Whale Watching on a Tourism-Naive Resident Population of Bottlenose Dolphins (Tursiops truncatus) in the Humboldt Penguin National Reserve, Chile

Standardized measures of behavior can be powerful tools for assessing the impact of whale watching activities on natural populations of cetaceans. To determine the possible impact of tourism on dolphins between a period without whale watching (1989–1992) (T1) and a period with whale watching (2010–2020) (T2), we examined the changes in the rate of surface behaviors, the group size of long-time resident bottlenose dolphins living in the waters of the Humboldt Current off Chile, and for T2 alone, we compared these differences between two localities, the Punta de Choros and Chañaral de Aceituno coves. We observed a significant decrease in the group size of the resident population and in the frequency of surface events associated with the absence and presence of tourism. For T2, we observed significant differences for the frequency of surface events between the Chañaral de Aceituno and Punta de Choros coves and differences in the frequency of surface events at different hours of the day. This was associated with the number of vessels at the time of the encounter. In addition, we observed for T2 that the most observed instantaneous response of the dolphins to the presence of tourist vessels was to avoid the boats, while approaching the boats was the least observed response. The number of vessels present in each dolphin encounter was the most important variable for our model as it explains these differences. These results show that tourism vessels have a significant impact on dolphin behavior and sociability, while the same population of dolphins have different spatial and temporal responses to different impacts of tourism. Further studies are needed to establish whether changes in the rate of surface behaviors are associated with higher levels of stress in dolphins and with effects on their health and reproductive success in the long term.

Effects of Coastal Construction on Indo-Pacific Humpback Dolphin (Sousa chinensis) Behavior and Habitat-Use Off Hong Kong

Construction-related loss of habitat, degradation of existing habitat, noise pollution, and vessel activity are growing issues for Indo-Pacific humpback dolphins (Sousa chinensis) that occur in the shallow, near-shore, highly industrialized waters off Lantau Island, Hong Kong. We studied the occurrence of dolphins in discrete locations, fine-scale movement patterns, and dolphin behavioral activity states. Potential explanatory variables varied and included year, season, time of day, dolphin group size and behavioral activity state, proximity to construction activity, and vessel type and number. Land-based observations and theodolite tracking of dolphins and vessels were conducted from seven locations to the north of Lantau Island, Hong Kong, and marine construction activities near survey sites were identified. A total of 636 groups of dolphins were recorded, totaling 150.91 h of tracking, from 405 days of observation effort. Hurdle models were used to analyze dolphin occurrence, multivariate generalized additive models were used to analyze fine-scale movement patterns, and log-likelihood ratio and binomialzscorepost hoctests were used to analyze behavioral activity states. Dolphin occurrence was lower in historically important areas near long-term, low-intensity construction activity, and dolphin swimming speed was higher in response to vessel presence. Overall, foraging and traveling were the most frequently observed behavioral activity states and resting behavior was observed off only one location that was not in proximity to construction activities. Temporal overlap in adjacent marine construction areas may displace animals for extended periods and nearby ecologically similar habitats should be identified and designated as marine protected areas to mitigate effects of such disturbance.

Responses of bottlenose dolphins (Tursiops spp.) to small drones

Abstract Recent advances in aerial drones offer new insights into the biology, ecology and behaviour of marine wildlife found on or near the ocean’s surface. While opening up new opportunities for enhanced wildlife monitoring, the impacts of drone sampling and how it might influence interpretations of animal behaviour are only just beginning to be understood. The capacity of drones to record bottlenose dolphin (Tursiops spp.) behaviour was investigated, along with how the presence of a small drone at varying altitudes influences dolphin behaviour. Over 3 years and eight locations, 361 drone flights were completed between altitudes of 5 and 60 m above the ocean. Analyses showed that dolphins were increasingly likely to change behaviour with decreasing drone altitude. A positive correlation was also found between time spent hovering above a group of dolphins and the probability of recording a behavioural response. Dolphin group size also influenced the frequency of an observed behavioural change, displaying a positive correlation between behaviour change and group size. Overall, although drones have the potential to impact coastal dolphins when flown at low altitudes, they represent a useful tool for collecting ecological information on coastal dolphins owing to their convenience, low cost and capacity to observe behaviours underwater. To maximize benefits and minimize impacts, this study suggests that drones should be flown 30 m above coastal bottlenose dolphins.

Influence of artisanal fisheries on the behaviour and social structure ofTursiops truncatusin the South-western Gulf of Mexico

AbstractBehavioural plasticity in animals is tested whenever competitive interactions for space and/or food resources occur between wildlife and human activities. This study uses the concepts of operational and non-operational interactions between bottlenose dolphins (Tursiops truncatus) and artisanal fisheries in Alvarado, to search for differences in behaviour, age structure and group size. We conducted 20 surveys between 2015 and 2016, and recorded 64 groups by means of scan sampling from either a research boat or a fixed vantage point. Average dolphin group size was small (${\bar{\rm x}}$= 3.2, SD = 2.2 individuals) and fewer individuals were commonly present when interaction with fisheries occurred. Operational interactions were defined within the first 30 m and occurred mainly with lone individuals (54% recorded from the lighthouse and 82% during surveys); this benchmark also accounted for higher frequencies in locomotion and feeding (χ2= 83.10; df = 7;P< 0.001). We found a higher rate of new behavioural events for dolphin groups furthest from human activities, as well as a decrease in behaviours that imply greater body exposure as dolphins approach the fishing spots. Age structure and dolphin group size were not different during and in the absence of interaction with fisheries, but most interactions involved male dolphins. Behavioural variations in the dolphins' repertoire are likely a strategy to reduce the risk of injuries or death when interacting with human activities; these dolphins seem to have habituated to or at least tolerate fishing activities within the study area, possibly constituting a sex-biased pressure.

Density, abundance and group size of river dolphins (Inia geoffrensis and Sotalia fluviatilis) in Central Amazonia, Brazil

Given the difficulties and costs often associated with surveying cetaceans, enlisting members of the public to collect data offers a promisingalternative approach. Comparison of cetacean ‘participatory science’ (also known as ‘citizen science’) data with data collected during traditionalscientific studies helps reveal the strengths and weaknesses of a participatory science approach. With a large number of vessel operators on thewater throughout the year, including dolphin-oriented tour boats, the Hawaiian Islands offer an ideal study site to employ such a dual-methodcomparison. The study aimed to enhance understanding of nearshore dolphin distributions relative to bathymetry. Operators of tour and fishingvessels within the shallow Maui Nui basin of the Hawaiian Islands were recruited to report delphinid sightings. Researchers conducted standarddolphin surveys within the same region. The participatory science approach was successful in generating a large sample size of sightings from fivedifferent species. Findings here demonstrate the potential value of participatory science and of using a multimethod approach to infer odontocetedistribution trends relative to bathymetry in areas where both methods are feasible. Important refinements for future projects are highlighted.

Accuracy and precision of dolphin group size estimates

AbstractEstimating the number of dolphins in a group is a challenging task. To assess the accuracy and precision of dolphin group size estimates, observer estimates were compared to counts from large‐format vertical aerial photographs. During 11 research cruises, a total of 2,435 size estimates of 434 groups were made by 59 observers. Observer estimates were modeled as a function of the photo count in a hierarchical Bayesian framework. Accuracy varied widely among observers, and somewhat less widely among dolphin species. Most observers tended to underestimate, and the tendency increased with group size. Groups of 25, 50, 100, and 500 were underestimated by <1%, 16%, 27%, and 47%, respectively, on average. Precision of group size estimates was low, and estimates were highly variable among observers for the same group. Predicted true group size, given an observer estimate, was larger than the observer estimate for groups of more than about 25 dolphins. Predicted group size had low precision, with coefficients of variation ranging from 0.7 to 1.9. Studies which depend on group size estimates will be improved if the tendency to underestimate group size and the high uncertainty of group size estimates are included in the analysis.

Another dolphin in peril? Photo‐identification, occurrence, and distribution of the endangered Indian Ocean humpback dolphin (Sousa plumbea) in Algoa Bay

Abstract In South Africa, the humpback dolphin (Sousa plumbea) has been recognized as the most endangered marine mammal, with a low abundance, a discontinuous distribution, and numerous threats. This research was initiated in 2008 to estimate the number of individual humpback dolphins in Algoa Bay, as well as studying their residency patterns and distribution. The last boat‐based study on humpback dolphins, conducted 24 years ago, formed an important reference against which to compare current findings. This study reveals that since the 1990s the number of identified animals decreased from 70 to 50 individuals, and the mean group size of humpback dolphins has also decreased, from seven to three animals. Humpback dolphin behaviour was predominantly that of foraging and travelling, although the overall frequency of foraging still seems to have decreased since early 1990s. Although the species was seen almost all year round, the number of sightings per survey was lowest in January, March, and April. The number of sightings per kilometre surveyed substantially decreased from 0.018 sightings per kilometre in 2008 to 0.004 in 2011, and the number of animals per kilometre also decreased from 0.042 to 0.009. The re‐sighting rates were low, ranging from one to eight times, with 52% of identified dolphins seen only once throughout the study period. Only 6% of the individuals were seen more than three times. Reduced humpback dolphin numbers and sighting rates could be indicative of the rise in anthropogenic activities, such as shipping and recreational boating, as well as a rise in pollution from vessels and urban development. Conservation strategies, such as a Biodiversity Management Plan for the species, are urgently needed in Algoa Bay, especially in the south‐west part of the bay, between Cape Recife and the Port Elizabeth Harbour, where the species occurs most frequently.

Largest reported groups for the Indo‐Pacific bottlenose dolphin (Tursiops aduncus) found in Algoa Bay, South Africa: Trends and potential drivers

AbstractThis study investigates how group size of Indo‐Pacific bottlenose dolphins (Tursiops aduncus) changes temporally, spatially, and/or with predominant behavior at two discreet sites along the Eastern Cape coastline of South Africa: Algoa Bay and the Wild Coast. The mean group size of bottlenose dolphins was large with an average of 52 animals. Significantly larger groups were observed in Algoa Bay ( = 60, range = 1–600) than off the Wild Coast ( = 32.9, range = 1–250). In Algoa Bay, the mean group size increased significantly over the study period, from an average 18 animals in 2008 to 76 animals in 2016. Additionally, the largest average and maximum group sizes ever reported both in South Africa and worldwide, were recorded in Algoa Bay (maximum group size = 600). Neither season nor behavior had a significant effect on mean group size at both sites. Similarly environmental variables such as the depth and substrate type also had no influence on group size. It remains unclear which ecological drivers, such as predation risk and food availability, are leading to the large groups observed in this area, and further research on abundance and distribution of both predators and prey is necessary.

Spatial and temporal variation in abundance, group size and behaviour of bottlenose dolphins in the Florida coastal Everglades

Bottlenose dolphins (Tursiops truncatus) are abundant in many coastal ecosystems, including the coastal Everglades. Understanding spatial and temporal variation in their abundance and group sizes is important for estimating their potential ecological importance and predicting how environmental changes (e.g. ecosystem restoration) might impact their populations. From August 2010 to June 2012, we completed a total of 67 belt transects covering a total of 2650 linear km and an area of 1232 km2. Dolphin densities varied spatially and temporally. The highest densities of dolphins were found in coastal oceans and inland bays and were lowest in rivers. Use of rivers, however, increased during the dry season while densities in other habitats remained similar across seasons. Dolphins appeared to prefer portions of bays close to mangrove-covered islands over open waters. A resighting rate of 63.6% of individuals across the 2-year study suggests that at least a portion of the population is probably resident within study regions over long time periods. The largest groups (mean 6.28, range 1–31) were found in open waters and bays despite apparently low predation pressure. Indeed, shark bite scars – likely the result of unsuccessful predation attempts – were conclusively observed on only 1% of individuals. Although further studies are warranted, the high densities of dolphins suggest that they are an important upper trophic level predator in the coastal Everglades, but their ecological importance probably varies in space and time.

Temporal and spatial variation in group size of bottlenose dolphins (Tursiops truncatus) in the Shannon Estuary, Ireland

Temporal and spatial variation in group size of bottlenose dolphins (Tursiops truncatus) in the Shannon Estuary, Ireland

Temporal and spatial variation in group size of bottlenose dolphins (<em>Tursiops truncatus</em>) in the Shannon Estuary, Ireland

Temporal and spatial variation in group size of bottlenose dolphins (<em>Tursiops truncatus</em>) in the Shannon Estuary, Ireland

Sixteen years later: Occurrence, group size, and habitat use of humpback dolphins (Sousa plumbea) in Algoa Bay, South Africa

AbstractCoastal dolphins are more prone to cumulative impacts of environmental and anthropogenic changes than pelagic species. However, few studies use historical comparisons to evaluate those impacts across a temporal scale. The study presented here describes the pattern of occurrence, group size, behavior, and habitat use of humpback dolphins in Algoa Bay and compares these to a similar study conducted in the early 1990s. Results indicate a considerable change in the frequency of occurrence. Furthermore, the mean group size has decreased from six to three individuals, mainly due to an increase in occurrence of solitary individuals (36.3% vs. 15.4%). Foraging, which was previously the predominant activity (64%), has now been replaced by traveling (49%). Moreover, dolphins showed a negative reaction towards a variety of watercrafts and swimmers. These observed differences could be due to a change in prey abundance, direct anthropogenic disturbance, and/or population decline. We emphasize the need for long‐term environmental and biological data series and long‐term monitoring of the demographics of this population to accurately assess any changes observed in future.

Long‐term interspecies association patterns of Atlantic bottlenose dolphins, Tursiops truncatus, and Atlantic spotted dolphins, Stenella frontalis, in the Bahamas

AbstractWith the exception of primates, detailed interspecies behavioral studies are rare in mammalian species and for cetaceans, most are anecdotal descriptions. This study is the first long‐term study on interspecies associations of regularly interacting groups. In the Bahamas Atlantic bottlenose (Tursiops truncatus) and spotted dolphins (Stenella frontalis) regularly form mixed species encounters (MSE). Both species show strong site fidelity with high resighting rates. During MSE, the majority (>65%) of spotted dolphins (especially males) were continually resighted; however bottlenose dolphins had comparatively low resighting rates (<17%). During MSE group size was significantly larger for spotted dolphins (13.0 ± 11.0) than bottlenose dolphins (4.8 ± 3.5) (F = 93.803, df = 1, P < 0.001). This difference was largest during aggressive encounters, due to the increased spotted dolphin group size (t‐test, t = 4.75, df = 184, P < 0.0001), but no difference in bottlenose dolphin group size. Strong associations (greater than twice the community average) were primarily between male spotted dolphins. Male alliances were prevalent for spotted dolphins but rare for bottlenose dolphins. These species differences were also observed in lagged association rates. These results highlight the differences involved in alliance formation and function for regularly interacting sympatric species and reveal insights into possible ecological and social reasons for these group formations.