Sex-specific impact of inbreeding on pathogen load in the striped dolphin.

The impact of inbreeding on fitness has been widely studied and provides consequential inference about adaptive potential and the impact on survival for reduced and fragmented natural populations. Correlations between heterozygosity and fitness are common in the literature, but they rarely inform about the likely mechanisms. Here, we investigate a pathology with a clear impact on health in striped dolphin hosts (a nematode infection that compromises lung function). Dolphins varied with respect to their parasite burden of this highly pathogenic lung nematode (Skrjabinalius guevarai). Genetic diversity revealed by high-resolution restriction-associated DNA (43 018 RADseq single nucleotide polymorphisms) analyses showed a clear association between heterozygosity and pathogen load, but only for female dolphins, for which the more heterozygous individuals had lower Sk. guevarai burden. One locus identified by RADseq was a strong outlier in association with parasite load (heterozygous in all uninfected females, homozygous for 94% of infected females), found in an intron of the citron rho-interacting serine/threonine kinase locus (associated with milk production in mammals). Allelic variation at the Class II major histocompatability complex DQB locus was also assessed and found to be associated with both regional variation and with pathogen load. Both sex specificity and the identification of associating functional loci provide insight into the mechanisms by which more inbred individuals may be more susceptible to the infection of this parasite. This provides important insight towards our understanding of the impact of inbreeding in natural populations, relevant to both evolutionary and practical conservation considerations.

Muscle biochemistry of a pelagic delphinid (Stenella longirostris longirostris): insight into fishery-induced separation of mothers and calves.

The length of time required for postnatal maturation of the locomotor muscle (longissimus dorsi) biochemistry [myoglobin (Mb) content and buffering capacity] in marine mammals typically varies with nursing duration, but it can be accelerated by species-specific behavioral demands, such as deep-diving and sub-ice transit. We examined how the swimming demands of a pelagic lifestyle influence postnatal maturation of Mb and buffering capacity in spinner dolphins (Stenella longirostris longirostris). Mb content of newborn (1.16±0.07 g Mb per 100 g wet muscle mass, n=6) and juvenile (2.77±0.22 g per 100 g, n=4) spinner dolphins were only 19% and 46% of adult levels (6.00±0.74 g per 100 g, n=6), respectively. At birth, buffering capacity was 52.70±4.48 slykes (n=6) and increased to 78.53±1.91 slykes (n=6) once a body length of 141 cm was achieved, representing 1.6- to 2.0-year-old dolphins. Based on the age of weaning (1.3-1.6 years post-partum), muscle maturation occurred just after weaning as described for coastal bottlenose dolphins (Tursiops truncatus). Thus, a pelagic lifestyle does not promote rapid maturation of muscle biochemistry. Rather, it promotes enhanced muscle biochemistry: newborn and adult spinner dolphins had four- and two-times greater Mb contents than newborn and adult bottlenose dolphins, respectively. Indeed, adult levels rivaled those of deep-diving cetaceans. Nonetheless, the relatively underdeveloped muscle biochemistry of calves likely contributes to documented mother-calf separations for spinner dolphins chased by the tuna purse-seine fishery.

Contrasted accumulation patterns of persistent organic pollutants and mercury in sympatric tropical dolphins from the south-western Indian Ocean.

Due to their high trophic position and long life span, small cetaceans are considered as suitable bioindicators to monitor the presence of contaminants in marine ecosystems. Here, we document the contamination with persistent organic pollutants (POPs) and total mercury (T-Hg) of spinner (Stenella longirostris, n =21) and Indo-Pacific bottlenose dolphins (Tursiops aduncus, n=32) sampled from the coastal waters of La Réunion (south-western Indian Ocean). In addition, seven co-occurring teleost fish species were sampled and analyzed as well. Blubber samples from living dolphins and muscle from teleosts were analyzed for polychlorinated biphenyls (PCBs), DDT and metabolites (DDTs), chlordanes (CHLs), hexachlorocyclohexanes (HCHs), hexachlorobenzene (HCB), and polybrominated diphenyl ethers (PBDEs). Methoxylated PBDEs (MeO-PBDEs), reported as having a natural origin, were also analyzed. T-Hg levels were measured in blubber and skin biopsies of the two dolphin species. Stable isotopes δ(13)C and δ(15)N values were determined in skin of the dolphins and in the muscle of teleosts. For PCBs, HCHs and T-Hg, concentrations were significantly higher in T. aduncus than in S. longirostris. For other POP levels, intra-species variability was high. MeO-PBDEs were the dominant compounds (55% of the total POPs) in S. longirostris, while PCBs dominated (50% contribution) in T. aduncus. Other contaminants showed similar profiles between the two species. Given the different patterns of POPs and T-Hg contamination and the δ(15)N values observed among analyzed teleosts, dietary and foraging habitat preferences most likely explain the contrasted contaminant profiles observed in the two dolphin species. Levels of each class of contaminants were significantly higher in males than females. Despite their spatial and temporal overlap in the waters of La Réunion, S. longirostris and T. aduncus are differently exposed to contaminant accumulation.

Whistle characteristics and daytime dive behavior in pantropical spotted dolphins (Stenella attenuata) in Hawai'i measured using digital acoustic recording tags (DTAGs).

This study characterizes daytime acoustic and dive behavior of pantropical spotted dolphins (Stenella attenuata) in Hawai'i using 14.58 h of data collected from five deployments of digital acoustic recording tags (DTAG3) in 2013. For each tagged animal, the number of whistles, foraging buzzes, dive profiles, and dive statistics were calculated. Start, end, minimum, and maximum frequencies, number of inflection points and duration were measured from 746 whistles. Whistles ranged in frequency from 9.7 ± 2.8 to 19.8 ± 4.2 kHz, had a mean duration of 0.7 ± 0.5 s and a mean of 1.2 ± 1.2 inflection points. Thirteen foraging buzzes were recorded across all tags. Mean dive depth and duration were 16 ± 9 m and 1.9 ± 1.0 min, respectively. Tagged animals spent the majority of time in the upper 10 m (76.9% ± 16.1%) of the water column. Both whistle frequency characteristics and dive statistics measured here were similar to previously reported values for spotted dolphins in Hawai'i. Shallow, short dive profiles combined with few foraging buzzes provide evidence that little spotted dolphin feeding behavior occurs during daytime hours. This work represents one of the first successful DTAG3 studies of small pelagic delphinids, providing rare insights into baseline bioacoustics and dive behavior.

Whistle comparison of four delphinid species in Southeastern Brazil.

The present study evaluates variations in frequency and duration parameters of whistles of four dolphin species (Sotalia guianensis, Steno bredanensis, Stenella frontalis, and Tursiops truncatus), recorded in the Rio de Janeiro State Coast, Southeastern Brazil. A total of 487 whistles were analyzed. Acoustic parameters of the whistles were classified to species by discriminant function analysis. Overall classification score was 72.5%, with the highest classification score obtained for whistles of S. bredanensis and the lowest obtained for S. frontalis. Most differences were among S. bredanensis and S. guianensis, species that did not have their repertoires compared in other studies.

Single-click beam patterns suggest dynamic changes to the field of view of echolocating Atlantic spotted dolphins (Stenella frontalis) in the wild.

Echolocating animals exercise an extensive control over the spectral and temporal properties of their biosonar signals to facilitate perception of their actively generated auditory scene when homing in on prey. The intensity and directionality of the biosonar beam defines the field of view of echolocating animals by affecting the acoustic detection range and angular coverage. However, the spatial relationship between an echolocating predator and its prey changes rapidly, resulting in different biosonar requirements throughout prey pursuit and capture. Here, we measured single-click beam patterns using a parametric fit procedure to test whether free-ranging Atlantic spotted dolphins (Stenella frontalis) modify their biosonar beam width. We recorded echolocation clicks using a linear array of receivers and estimated the beam width of individual clicks using a parametric spectral fit, cross-validated with well-established composite beam pattern estimates. The dolphins apparently increased the biosonar beam width, to a large degree without changing the signal frequency, when they approached the recording array. This is comparable to bats that also expand their field of view during prey capture, but achieve this by decreasing biosonar frequency. This behaviour may serve to decrease the risk that rapid escape movements of prey take them outside the biosonar beam of the predator. It is likely that shared sensory requirements have resulted in bats and toothed whales expanding their acoustic field of view at close range to increase the likelihood of successfully acquiring prey using echolocation, representing a case of convergent evolution of echolocation behaviour between these two taxa.

Whistle characteristics of the spinner dolphin population in the Comoros Archipelago.

Observed variations in dolphin acoustic signals may be associated with behavior, social composition, and local differences in habitat features. This study aims at characterizing whistles emitted by the spinner dolphin population occurring in the waters of the main island of the Archipelago of Comoros (Mozambique Channel, Indian Ocean) and to assess factors possibly influencing the acoustic structure of signals. All parameters examined on 953 whistles significantly differed in relation to environmental conditions, group size, and behavior. By mixed model analysis, it was found that only habitat characteristics play a role in the variation of frequency parameters, and exerted on the acoustic structure of whistles stronger influence than socio-behavioral factors. Spinner dolphins occurring in the Comoros archipelago use higher frequencies and show longer signal duration compared to those from the Pacific and the Atlantic. Results suggest that frequency parameters are distinctive of the local population and reflect the habitat use of the species in the area. In conclusion, acoustic measurements may be crucial elements to be included in monitoring programs to identify local peculiarities of dolphins' populations.

Spinner dolphin whistle in the Southwest Atlantic Ocean: Is there a geographic variation?

Acoustic parameters for the spinner dolphins' bioacoustic sounds have previously been described. However, the dolphins in the Southwest Atlantic Ocean were only recently studied near the Fernando de Noronha Archipelago. Therefore, to contribute to additional knowledge of this cosmopolitan species, this study compares previous results with a Brazilian recording. Despite statistically significant differences, the mean value comparison indicated that Hawaiian and Southwest Atlantic Ocean spinners emit similar whistles. The fact that geographical isolation does not lead the dissemblance nor the similarity of the acoustic variations in this species raises the possibility of other factors influencing those emissions. Here those differences and similarities are discussed, thereby contributing to an understanding of how distinct populations and/or species communicate through different ocean basins.

Whistle source levels of free-ranging bottlenose dolphins and Atlantic spotted dolphins in the Gulf of Mexico.

Whistles of bottlenose dolphins (Tursiops truncatus) and Atlantic spotted dolphins (Stenella frontalis) in the eastern Gulf of Mexico were recorded and measured with a calibrated towed hydrophone array. Surveys encountered groups of both bottlenose (N = 10) and spotted dolphins (N = 5). Analysis of those data produced 1695 bottlenose dolphin whistles and 1273 spotted dolphin whistles with a high signal-to-noise ratio. Whistle frequency metrics were lower in bottlenose than spotted dolphins, while whistle duration was longer in spotted dolphins, data that may help inform automatic classification algorithms. Source levels were estimated by determining the range and bearing of an individual dolphin from the array and then adding the predicted transmission loss to the calculated received level. The median bottlenose dolphin source level was 138 dB re 1µPa at 1 m with a range of 114-163 dB re 1µPa at 1 m. The median spotted dolphin source level was 138 dB re 1µPa at 1 m with a range of 115-163 dB re 1µPa at 1 m. These source level measurements, in conjunction with estimates of vocalization rates and transmission loss models, can be used to improve passive acoustically determined dolphin abundance estimates in the Gulf of Mexico.

Combining whistle acoustic parameters to discriminate Mediterranean odontocetes during passive acoustic monitoring.

Acoustic observation can complement visual observation to more effectively monitor occurrence and distribution of marine mammals. For effective acoustic censuses, calibration methods must be determined by joint visual and acoustic studies. Research is still needed in the field of acoustic species identification, particularly for smaller odontocetes. From 1994 to 2012, whistles of four odontocete species were recorded in different areas of the Mediterranean Sea to determine how reliably these vocalizations can be classified to species. Recordings were attributed to species by simultaneous visual observation. The results of this study highlight that the frequency parameters, which are linked to physical features of animals, show lower variability than modulation parameters, which are likely to be more dependent on complex eco-ethological contexts. For all the studied species, minimum and maximum frequencies were linearly correlated with body size. DFA and Classification Tree Analysis (CART) show that these parameters were the most important for classifying species; however, both statistical methods highlighted the need for combining them with the number of contour minima and contour maxima for correct classification. Generally, DFA and CART results reflected both phylogenetic distance (especially for common and striped dolphins) and the size of the species.

An "ex vivo" model to evaluate toxicological responses to mixtures of contaminants in cetaceans: integumentum biopsy slices.

The need for powerful new tools to detect the effects of chemical pollution, in particular of persistent organic pollutants (POPs) and polycyclic aromatic hydrocarbons (PAHs) on Mediterranean cetaceans led us to develop and apply a suite of sensitive biomarkers for integument biopsies of stranded and free-ranging animals. This multi-response ex vivo method has the aim to detect toxicological effects of contaminant mixtures. In the present study, we applied an ex vivo assay using skin biopsy and liver slices, combining molecular biomarkers [Western blot of Cytochrome P450 1A1 (CYP1A1) and Cytochrome P450 2B (CYP2B)] and gene expression biomarkers (Quantitative real-time PCR of CYP1A1, heat shock protein 70, estrogen receptor alpha and E2F transcription factor) in response to chemical exposure [organochlorines compounds (OCs), polybrominated diphenyl ethers (PBDEs), and PAHs] for stranded Mediterranean Stenella coeruleoalba. The main goal of this experiment was to identify the biomarker and/or a suite of biomarkers that could best detect the presence of a specific class of pollutants (OCs, PBDEs, and PAHs) or a mixture of them. This multi-response biomarker methodology revealed an high sensitivity and selectivity of responses (such as CYP1A and ER α mRNA variations after OCs and PAHs exposure) and could represent a valid future approach for the study of inter- and intra-species sensitivities to various classes of environmental contaminants.

Geographic variability in the acoustic parameters of striped dolphin's (Stenella coeruleoalba) whistles.

Geographic variation in the acoustic features of whistles emitted by the striped dolphin (Stenella coeruleoalba) from the Atlantic Ocean (Azores and Canary Islands) and the Mediterranean was investigated. Ten parameters (signal duration, beginning, end, minimum and maximum frequency, the number of inflection points, of steps, of minima and maxima in the contour and the frequency range) were extracted from each whistle. Discriminant function analysis correctly classified 73% of sounds between Atlantic Ocean and Mediterranean Sea. A cline in parameters was apparent from the Azores to the Mediterranean, with a major difference between the Canaries and the Mediterranean than between Azores and Canaries. Signal duration, maximum frequency, and frequency range measured in the Mediterranean sample were significantly lower compared to those measured in the Atlantic. Modulation parameters played a considerable role in area discrimination and were the only parameters contributing to highlight the differences within the Atlantic Ocean. Results suggest that the acoustic features constrained by structural phenotype, such as whistle's frequency parameters, have a major effect on the Atlantic and Mediterranean separation while behavioral context, social, and physical environment may be among the main factors contributing to local distinctiveness of Atlantic areas. These results have potential passive acoustic monitoring applications.

Better educational signage could reduce disturbance of resting dolphins.

Spinner dolphins on Hawai'i Island's west coast (Stenella longirostris longirostris) rest by day in protected bays that are increasingly popular for recreation. Because more frequent interactions of people with these dolphins is likely to reduce rest for dolphins and to explain recent decline in dolphin abundance, the National Oceanic and Atmospheric Administration (NOAA) proposed stricter rules regarding interactions with spinner dolphins near the main Hawaiian Islands and plans to increase enforcement. Simultaneous investment in public education about both interaction rules and their biological rationale has been and is likely to be relatively low. To test the hypothesis that more educational signage will reduce human-generated disturbance of dolphins, a paper questionnaire was distributed to 351 land-based, mostly unguided visitors at three dolphin resting bays on Hawai'i Island's west coast. Responses indicated that visitors wanted to see dolphins, were ignorant of interaction rules, were likely to read signs explaining rules and their biological rationales, and were likely to follow known rules. Therefore, investment in effective educational signage at dolphin resting bays is recommended as one way to support conservation of spinner dolphins on Hawai'i Island's west coast and similar sites in the Hawaiian archipelago.

Bottlenose dolphins (Tursiops truncatus) aggressive behavior towards other cetacean species in the western Mediterranean.

Aggressive behavior of bottlenose dolphins (Tursiops truncatus) towards conspecifics is widely described, but they have also often been reported attacking and killing harbour porpoises (Phocoena phocoena) around the world. However, very few reports exist of aggressive interactions between bottlenose dolphins and other cetacean species. Here, we provide the first evidence that bottlenose dolphins in the western Mediterranean exhibit aggressive behavior towards both striped dolphins (Stenella coeruleoalba) and Risso's dolphins (Grampus griseus). Necropsies and visual examination of stranded striped (14) and Risso's (2) dolphins showed numerous lesions (external rake marks and different bone fractures or internal organ damage by blunt trauma). Indicatively, these lessons matched the inter-tooth distance and features of bottlenose dolphins. In all instances, these traumatic interactions were presumed to be the leading cause of the death. We discuss how habitat changes, dietary shifts, and/or human colonization of marine areas may be promoting these interactions.

Whistles emitted by Atlantic spotted dolphins (Stenella frontalis) in Southeastern Brazil.

The whistles of Atlantic spotted dolphins have been studied in a few localities of the North Atlantic Ocean and those studies revealed that the species emits whistles within a broad frequency range, with a high number of inflection points and presence of harmonics. In the South Atlantic Ocean, there is no information about the sounds produced by Atlantic spotted dolphins. A total of 1092 whistles emitted by free-ranging Atlantic spotted dolphins in Southeastern Brazilian coastal waters were analyzed. Whistles recorded in this study had a broad frequency range from 1.15 to 23.44 kHz. Whistles without harmonics were frequently emitted (N=701; 64.2%) and those signals with zero up to two inflection points corresponded to 94% of all whistles. Some differences in whistle characteristics (inflection points and duration) were found in relation to areas in North Atlantic Ocean and whistles were shorter and with a smaller number of inflection points in Brazil. Whistles produced by Atlantic spotted dolphins varied between the two behavioral states in which dolphins were engaged. Whistles were more frequently emitted when dolphins presented behaviors that included fast movement at surface, prey pursuit, aerial behavior, and physical contact. In these situations, whistles were on average longer and had higher frequency parameters than those emitted when animals were engaged in slowly and moderate traveling. The findings presented herewith reveal that dolphins modified whistle structures within behavioral states.

Discriminating features of echolocation clicks of melon-headed whales (Peponocephala electra), bottlenose dolphins (Tursiops truncatus), and Gray's spinner dolphins (Stenella longirostris longirostris).

Spectral parameters were used to discriminate between echolocation clicks produced by three dolphin species at Palmyra Atoll: melon-headed whales (Peponocephala electra), bottlenose dolphins (Tursiops truncatus) and Gray's spinner dolphins (Stenella longirostris longirostris). Single species acoustic behavior during daytime observations was recorded with a towed hydrophone array sampling at 192 and 480 kHz. Additionally, an autonomous, bottom moored High-frequency Acoustic Recording Package (HARP) collected acoustic data with a sampling rate of 200 kHz. Melon-headed whale echolocation clicks had the lowest peak and center frequencies, spinner dolphins had the highest frequencies and bottlenose dolphins were nested in between these two species. Frequency differences were significant. Temporal parameters were not well suited for classification. Feature differences were enhanced by reducing variability within a set of single clicks by calculating mean spectra for groups of clicks. Median peak frequencies of averaged clicks (group size 50) of melon-headed whales ranged between 24.4 and 29.7 kHz, of bottlenose dolphins between 26.7 and 36.7 kHz, and of spinner dolphins between 33.8 and 36.0 kHz. Discriminant function analysis showed the ability to correctly discriminate between 93% of melon-headed whales, 75% of spinner dolphins and 54% of bottlenose dolphins.

Morphological structure of the aortic wall in three Delphinid species with shallow or intermediate diving habits: Evidence for diving adaptation.

Some modifications in the vascular system of marine mammals provide adaptive advantages for diving. This study analyses the organisation of the aortic wall in dolphins, observing artery changes in volume and blood pressure for diving behaviour. Samples of three aortic segments (ascending, thoracic and abdominal) of three dolphin species were processed for histological and morphometric studies. The three dolphin species used, striped dolphin (Stenella coeruleoalba), Atlantic spotted dolphin (Stenella frontalis) and common dolphin (Delphinus delphis), have shallow or intermediate diving habits. Our results indicated that the components of the aortic wall of the dolphins had different dispositions in the three selected segments. The aortic wall decreased in thickness along its length due to a loss of the lamellar units in the tunica media and a thinning of the main elements of the lamellar units along the artery. The life stage had little influence on the thickness of the aortic wall except for the ascending aorta. The weight, body length, species or sex of the specimen did not significantly influence the thickness of the wall or the lamellar units. In summary, the histological and morphometric aortic structure in dolphins, in relation to the studied parameters, seems to be similar to that previously described of terrestrial mammals such as pigs, except for a larger difference in the proportion of lamellar units between the ascending and thoracic segments.

Broadband backscatter from individual Hawaiian mesopelagic boundary community animals with implications for spinner dolphin foraging.

Broadband simulated dolphin echolocation signals were used to measure the ex situ backscatter properties of mesopelagic boundary community (MBC) in order to gain a better understanding of the echolocation process of spinner dolphins foraging on the MBC. Subjects were captured by trawling with a 2-m-opening Isaacs-Kidd Midwater Trawl. Backscatter measurements were conducted on the ship in a 2000 L seawater tank with the transducer placed on the bottom pointed upwards. Backscatter measurements were obtained in both the dorsal and lateral aspects for seven myctophids and only in the dorsal aspect for 16 more myctophids, six shrimps, and three squids. The echoes from the myctophids and shrimps usually had two highlights, one from the surface of the animal nearest the transducer and a second probably from the signal propagating through body of the subject and reflecting off the opposite surface of the animal. The squid echoes consisted mainly of a single highlight but sometimes had a low amplitude secondary highlight. The backscatter results were used to estimate the echolocation detection range for spinner dolphins foraging on the mesopelagic boundary community. The results were also compared with multi-frequency volume backscatter of the mesopelagic boundary community sound scattering layer.

Using stable isotope analysis to assess trophic relationships between Atlantic bluefin tuna (Thunnus thynnus) and striped dolphin (Stenella coeruleoalba) in the Strait of Gibraltar.

Stable isotope analysis (δ13C and δ15N from liver and muscle) was used to assess trophic relationships between Atlantic bluefin tuna (ABFT) (Thunnus thynnus) and striped dolphin (SC) (Stenella coeruleoalba) in the Strait of Gibraltar (SoG). δ15N values from ABFT muscle and liver tissues were significantly different from those of dolphin samples, but no for δ13C values. Diet estimation by MixSIAR models from muscle and liver revealed that ABFT fed mainly on squids (Todaropsis eblanae and Illex coindetii). The shrimp Pasiphaea sp. was estimated to be the most important prey-species in the diet of SC. Trophic positions estimated from muscle and liver isotopic data suggested that ABFT occupy a higher trophic level than SC. Estimations of isotopic niche, as measured by the standard ellipse area, indicated that ABFT show a broader trophic niche than SC; furthermore, SEAc did not show trophic overlap between both predators. The results of this study suggest that resource partitioning occurs between ABFT and SC in the SoG ecosystem.

[Distribution and environmental conditions related to the behavior in the bottlenose dolphin (Tursiops truncatus) and the spotted dolphin (Stenella attenuata) (Cetacea: Delphinidae) in Golfo Dulce, Costa Rica]. / Distribución y condiciones ambientales asociadas al comportamiento del delfín bufeo (Tursiops truncatus) y el delfín manchado (Stenella attenuata) (Cetacea: Delphinidae) en el Golfo Dulce, Costa Rica.

Habitat characteristics influencing behavior in animal species vary locally. The influence that a particular environmental characteristic can have on a species depends not only on other variables, but on morphological, physiological and social conditions of that species. In this study, developed from June 1996 to July 1997, I studied whether specific behaviors are related to particular distribution areas and environmental factors in the bottlenose (Tursiops truncatus) and the spotted dolphin (Stenella attenuata). The study area was covered along oblicuous linear transects, and the behavior of single groups was observed from 15 min to 5 h. Environmental factors such as depth, temperature, salinity and distance from shore, among others, were considered. For the bottlenose dolphin, foraging/feeding activities showed exclusive coincidence with river mouths, coral reef and mangrove areas, while social and milling activities where seen close to feeding areas. Traveling occurred along different points parallel to the coast, with a low percentage of cases across the gulf (16.56 %), suggesting that the bottlenose rarely crosses from one side to the other. In the spotted dolphin, several behaviors were observed simultaneously in the schools and it was not possible to associate areas with particular behaviors. The lack of significant relationships among activities and particular environmental variables (ANOVA tests) is attributed to three aspects: (a) transitions among activities generally occurred into a low variable area, (b) dolphins often traveled along large areas without changing activities and (c) environmental conditions in Golfo Dulce are homogeneous. In the two species the highest average in the number of individuals per group corresponded to the category of active socializing, followed by traveling, passive socializing and feeding. In the case of the bottlenose dolphin, the smallest group size was associated with feeding activities (ANOVA, F= 2.624, p=0.037, n=156, df=4), while in the spotted dolphin the smallest group size corresponded to milling activities (ANOVA, F=3.817, p=0.009, n=51, df=4).