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Increasing boat traffic in estuarine environments poses growing challenges for the conservation of coastal cetaceans, yet how these pressures interact with habitat features to shape spatial use remains poorly understood. This study investigated the influence of boat traffic and habitat features on the spatial use and habitat selection of Guiana dolphins ( Sotalia guianensis ) in Pontal Bay, a northeastern Brazilian estuary, using shore‐based observations and theodolite tracking of dolphin and boat trajectories collected between April 2015 and January 2016. Utilization distribution and habitat selection analyses revealed substantial overlap between dolphins and boats, particularly in deeper areas. At finer scales, however, dolphins preferentially used areas with lower boat traffic, greater depths, and proximity to fishing centers, while avoiding shallow zones and river edges. These patterns indicate a scale‐dependent trade‐off between access to foraging opportunities and exposure to anthropogenic disturbance. Our findings highlight the importance of integrating environmental and human‐related factors into estuarine management and support the need for measures that regulate vessel activity within key dolphin habitats.

Populations usually considered foraging generalists may include specialized individuals that feed on a restricted subset of the prey spectrum consumed by the population. By analyzing the time series of δ13C and δ15N values in sequential growth layer groups within tooth dentin, we measured population- and individual-level variation in resource use of three populations of Guiana dolphins (Sotalia guianensis)—Caravelas River, Babitonga Bay, and Norte Bay—along a latitudinal gradient in the southwestern Atlantic Ocean. We show that the Guiana dolphin at Caravelas River is a generalist population consisting of individual dietary specialists, likely due to the absence of other resident dolphin populations thus allowing individuals to target prey across a wide range of habitats. The Babitonga Bay population is also composed of individual specialists potentially due to the selective foraging behavior of some individuals on high-quality prey sources within and near the bay. In contrast, the Norte Bay population comprises individual generalists, which likely reflects its distinctive cohesive social organization, coexistence with two other dolphin species, and an opportunistic foraging strategy in response to resource fluctuations inherent to the southern limit of the species distribution. Although the Guiana dolphin is generally considered to be a dietary generalist at the population level, our findings reveal that the total niche width of populations and the degree of individual diet specialization are highly context dependent, suggesting dietary plasticity that may be related to a latitudinal gradient in resource availability and environmental conditions.

This study describes the pathologic findings of 24 humpback whales (Megaptera novaeangliae) found stranded along the Brazilian coast from 2004 to 2016. Eighteen (75%) animals evaluated were found stranded alive. From these, 13 died naturally on shore and five were euthanized. Six died at sea and were washed ashore. Of the 24, 19 (79.2%) were calves, four (16.7%) were juveniles, and one (4.2%) was an adult. The most probable cause of stranding and/or death (CSD) was determined in 23/24 (95.8%) individuals. In calves, CSD included neonatal respiratory distress (13/19; 68.4%), infectious disease (septicemia, omphaloarteritis and urachocystitis; 3/19; 15.8%), trauma of unknown origin (2/19; 10.5%), and vehicular trauma (vessel strike; 1/19; 5.3%). In juveniles and adult individuals, CSD was: emaciation (2/5; 40%), sunlight-thermal burn shock (1/5; 20%); and discospondylitis (1/5; 20%). In one juvenile, the CSD was undetermined (1/5; 20%). This study integrates novel findings and published case reports to delineate the pathology of a South-western Atlantic population of humpback whales. This foundation will aid in the assessment of the population health and establish a baseline for development of conservation policies.

Cetaceans are considered environmental sentinels and their health often reflects either anthropogenic or natural spatio-temporal disturbances. This study investigated the pathological findings and mortality of small cetaceans with the aim of detecting hazards and monitoring health trends in a high-biodiversity area. Between 2007 and 2012, 218 stranded cetaceans were recorded on the Paraná coast, southern Brazil. Fifty-seven (26.1%) of these animals, including 50 Sotalia guianensis, 2 Pontoporia blainvillei, 2 Stenella frontalis, 1 Stenella longirostris, 1 Tursiops truncatus and 1 Globicephala melas were necropsied and samples were collected for histopathology. Causes of death were determined in 46 of the 57 (80.7%) animals and most (30 or 65.2%) were ascribed to anthropogenic activities, including fisheries bycatch (28/30) and trauma (2/30). The remaining 16 fatalities were considered natural, and attributed to pneumonia (10/16), emaciation (3/16), septicemia (1/16), neonatal pathology (1/16) and choking via food obstruction (1/16). Irrespective of the cause, bronchointerstitial pneumonia, associated with parasitism, lymphadenitis and membranous glomerulonephritis were common findings among all fatalities. These results suggest, that while anthropogenic activities are a leading cause of cetacean strandings in Paraná, underlying pre-existing diseases may contribute towards deaths. Although the studied area is considered a biosphere reserve by UNESCO, complex anthropogenic and natural interactions might be occurring, increasing cetacean susceptibility to hazards. This study may help facilitate developing an effective conservation plan for coastal cetaceans focusing on reducing fisheries interactions, habitat degradation and pollution as mechanisms for ultimately increasing species resilience.

The Guiana dolphin ( Sotalia guianensis ) is a common species along Central and South American coastal waters. Although much effort has been made to understand its behavioral ecology and evolution, very little is known about its brain. The use of ultra-high field MRI in anatomical descriptions of cetacean brains is a very promising approach that is still uncommon. In this study, we present for the first time a full anatomical description of the Guiana dolphin’s brain based on high-resolution ultra-high-field magnetic resonance imaging, providing an exceptional level of brain anatomical details, and enriching our understanding of the species. Brain structures were labeled and volumetric measurements were delineated for many distinguishable structures, including the gray matter and white matter of the cerebral cortex, amygdala, hippocampus, superior and inferior colliculi, thalamus, corpus callosum, ventricles, brainstem and cerebellum. Additionally, we provide the surface anatomy of the Guiana dolphin brain, including the labeling of main sulci and gyri as well as the calculation of its gyrification index. These neuroanatomical data, absent from the literature to date, will help disentangle the history behind cetacean brain evolution and consequently, mammalian evolution, representing a significant new source for future comparative studies.

Ecological niche models (ENMs) predict where species can occur in accordance with environmental factors. Suitability maps are generated through models to identify habitats more or less adapted to the species. Published works on the distribution and habitat use of Guiana dolphin, Sotalia guianensis, are limited to fine spatial scales. Here, we aimed to predict the potential geographical distribution of Guiana dolphins through ENMs and generate a map of suitable habitats for the species. Data were collected between 1997 and 2015 in Brazil, French Guiana, and Colombia. The environmental data were obtained from MARSPEC database with a cell resolution of 10 × 10 km. For modeling, 99 of the 859 initial occurrence points of the species were considered after rarefaction. Seven environmental variables were selected through factorial analysis: bathymetry, distance to shore, bathymetric slope, sea surface salinity (minimum monthly and annual range), and sea surface temperature (mean annual and annual range). Results from five distinct algorithms were assembled to generate the distribution model. Our findings show potential areas in shallow platforms of the continental margin of South and Central America, including regions where the species has never been reported, such as the Pacific Ocean, the Gulf of California, the Gulf of Mexico, and the oceanic islands in the Caribbean Sea. The absence of Guiana dolphins in these regions may be due to geographical (linking of North and South America), physical (water temperature), and biological (competition, limited ability to dispersal) limiting factors. The models suggest that the presence of other species of coastal dolphin may be an important limiting factor for the Guiana dolphin at both extremes of its distribution. The Guiana dolphin is habitat specialist with a clinal potential geographic distribution concentrated in tropical and subtropical shallow and coastal waters of the continental shelf of the western Atlantic Ocean. This more restricted distribution than reported by IUCN and other studies suggests a cautionary approach to its conservation status due to limited dispersal abilities and high overlap with human activities.

Marine apex predators, such as coastal dolphins, are key components of ecosystem function and dynamics; understanding their spatial use therefore is important for conservation planning. Here, we describe spatial use of the threatened Guiana dolphin, Sotalia guianensis, within a mosaic of protected areas at their southern range in the Paranaguá estuarine complex (PEC), Brazil. Based on environmental characteristics and S. guianensis population densities, individual photo-identification surveys were focused on two sectors of the northern mouth of the PEC between 2013 and 2015. We photo-identified 107 individuals and recorded 351 groups of Guiana dolphins; nearly 50% of which contained calves. Group sizes were small (mean ± SD: Sector 1 = 3.4 ± 2.7; Sector 2 = 3.6 ± 2.0) and ranged from 1 to 30 individuals. Clustering algorithms were applied to resighted, photo-identified Guiana dolphins to classify individuals as “residents” (sighted in 87% of surveys, n = 11) and “transients” (two clusters; 62%, n = 22; 95%, n = 72). All individuals predominantly were observed foraging (90% in Sector 1 and 76% in Sector 2), but spatial use by transient individuals differed from the residents in terms of environmental (slope, water depth, and rainfall) and behavioral factors (presence of calves and foraging behavior). Compared to transients, resident dolphins ranged over smaller areas, typically more suitable for feeding, calf caring, and more protected from predators and vessels. Because resident Guiana dolphins exhibit small home ranges, they are particularly vulnerable to local anthropogenic activities and habitat loss. These characteristics imply a strong need for regionalized management plans to reduce negative impacts and to maintain the ecological functionality of the species.

Shipping is the principal source of anthropogenic noise in the aquatic soundscape of many coastal areas. Noise can affect temporally or permanently the physiology, behavior, and ecology of marine mammals. As noise pollution is considered to be an important threat to Guiana dolphins (Sotalia guianensis), we focused the present study in the effects of noise on occurrence and acoustic behavior in Sepetiba Bay, southeastern Brazil. We modeled the impacts of anthropogenic noise on the acoustic behavior and distribution of these dolphins in an estuarine area. We considered the acoustic parameters (maximum frequency, minimum frequency, delta frequency, duration of whistles, and whistle rates) and occurrence of Guiana dolphins as the response variables and sound exposure levels (SEL) and environmental factors (sea surface temperature, depth, bottom heterogeneity, and bottom type) as the explanatory variables. While dolphin occurrence was explained by environmental variables, mainly SST, the whistle rate was explained by SEL (contribution = 52.4%). The dolphins used noisier areas but communicated mainly in less noisy areas. Although the dolphins did not appear to avoid the noisiest areas, noise levels were the most important variable to explain the reduction in whistle rates. Our results are particularly important since they indicate the effects of noise on an endangered species living in a region with high cumulative impacts.

Passive electroreception is a widespread sense in fishes and amphibians, but in mammals this sensory ability has previously only been shown in monotremes. While the electroreceptors in fish and amphibians evolved from mechanosensory lateral line organs, those of monotremes are based on cutaneous glands innervated by trigeminal nerves. Electroreceptors evolved from other structures or in other taxa were unknown to date. Here we show that the hairless vibrissal crypts on the rostrum of the Guiana dolphin (Sotalia guianensis), structures originally associated with the mammalian whiskers, serve as electroreceptors. Histological investigations revealed that the vibrissal crypts possess a well-innervated ampullary structure reminiscent of ampullary electroreceptors in other species. Psychophysical experiments with a male Guiana dolphin determined a sensory detection threshold for weak electric fields of 4.6 µV cm−1, which is comparable to the sensitivity of electroreceptors in platypuses. Our results show that electroreceptors can evolve from a mechanosensory organ that nearly all mammals possess and suggest the discovery of this kind of electroreception in more species, especially those with an aquatic or semi-aquatic lifestyle.

Top marine predators present high mercury concentrations in their tissues as consequence of biomagnification of the most toxic form of this metal, methylmercury (MeHg). The present study concerns mercury accumulation by Guiana dolphins (Sotalia guianensis), highlighting the selenium-mediated methylmercury detoxification process. Liver samples from 19 dolphins incidentally captured within Guanabara Bay (Rio de Janeiro State, Brazil) from 1994 to 2006 were analyzed for total mercury (THg), methylmercury (MeHg), total organic mercury (TOrgHg) and selenium (Se). X-ray microanalyses were also performed. The specimens, including from fetuses to 30-year-old dolphins, comprising 8 females and 11 males, presented high THg (0.53-132 µg/g wet wt.) and Se concentrations (0.17-74.8 µg/g wet wt.). Correlations between THg, MeHg, TOrgHg and Se were verified with age (p<0.05), as well as a high and positive correlation was observed between molar concentrations of Hg and Se (p<0.05). Negative correlations were observed between THg and the percentage of MeHg contribution to THg (p<0.05), which represents a consequence of the selenium-mediated methylmercury detoxification process. Accumulation of Se-Hg amorphous crystals in Kupffer Cells was demonstrated through ultra-structural analysis, which shows that Guiana dolphin is capable of carrying out the demethylation process via mercury selenide formation.