Case Report: Agonistic interaction between Amazonian manatee (Trichechus inunguis) and Tucuxi dolphins (Sotalia fluviatilis) in Brazil

Introduction

Living in groups is fundamental to the survival of many social species; however, despite its clear advantages, group living can also generate negative outcomes such as competition and conflict (Alexander, 1974; Ruxton, 2002; Majolo and Huang, 2022). Ecological interactions among animals are therefore of particular interest to ecologists, although they are often difficult to observe in natural populations.

Studies describing social interactions are common in the literature, but research on wild aquatic mammals has predominantly focused on intraspecific interactions, especially among cetaceans, addressing social structure, cooperation, and dominance relationships within groups (Frantzis and Herzing, 2002; Stensland et al., 2003; Bearzi, 2005; Cords and Würsig, 2014).

Intraspecific interactions among cetaceans include both affiliative and agonistic behaviors occurring between individuals of the same species. Affiliative interactions often involve synchronized swimming, play, and cooperative hunting or feeding, and are generally associated with social bonding, learning, and reproductive strategies (Fertl and Leatherwood, 1997; Scheer, 2010; Sakai et al., 2011; Senigaglia et al., 2012). In contrast, agonistic intraspecific interactions may include repulsion, fighting (e.g., hitting and biting), and competition, frequently linked to dominance hierarchies and sexual selection (Barrows, 2011; Robinson, 2014).

Interspecific interactions, on the other hand, involve individuals from different species and are less frequently documented in cetaceans. These interactions are driven by distinct ecological and evolutionary processes, such as resource competition, territorial overlap, or mistaken identity, and may result in affiliative or agonistic outcomes (Pérez-Gil et al., 2011; Methion and Díaz López, 2021; Volker and Herzing, 2021).

Here we report on rare interspecific agonistic interaction between Tucuxi dolphins (Sotalia fluviatilis - Cetacea) and an Amazonian manatee (Trichechus inunguis – Sirenia) calf. Little is known about the interspecific interactions between cetaceans and sirenians, however, the few documented interactions are atypical and associated to cooperative and altruistic behaviors (Kiszka, 2007; Knowles, 2015). The Tucuxi is known to have a riverine distribution, occupying the same areas as the Amazonian manatee, and sharing similar habitats, such as river channels and floodplains in the Amazon basin (Da Silva and Best, 1989; Da Silva and Best, 1996; Da Silva et al., 2010). Despite this overlap, there is no evidence of direct interactions between the two species (Da Silva and Martin, 2017). The information presented here was obtained from the rescue and necropsy reports registered at Centro Nacional de Pesquisa e Conservação de Mamíferos Aquáticos (ICMBio/CMA), carried out in partnership with the Fundação Mamíferos Aquáticos (FMA).

Sirenia is a group of aquatic mammals composed of species that exhibit herbivorous diets and low metabolism (O’Shea and Reep, 1990). They have complex behaviors but often perform slow movements in their turbid and shallow waters (O’Shea and Powell, 2001). Additionally, they have a limited visual acuity and a presumed reduced ability to respond to predators (Hartman, 1979; Marshall and Reep, 1995; Bauer et al., 2003). These animals are mainly threatened by human activities such as entanglement by fishing nets, alterations in habitat, and hunting for meat consumption (Denkinger, 2010; Brice et al., 2011; Utreras et al., 2013; Marmontel et al., 2016). While some human cultures hunt sirenians for food or ritualistic reasons, in certain regions, sirenians may occasionally fall opportunistically as prey to jaguars, caimans, sharks, crocodiles, and killer whales (Anderson and Prince, 1985; Ridgway and Harrison, 1985; Reynolds and Odell, 1991; Reep and Bonde, 2006). In response to predation risk, sirenians tend to avoid areas where opportunistic predators are present, even if it means feeding on less nutritious seagrasses elsewhere (Wirsing et al., 2007).

Case description

On April 12^th, 2004, the ICMBio/CMA was notified by the IBAMA Superintendence in Amapá State (SUPES/AP), a local environmental governmental institution, about the rescue of an Amazonian manatee at Pirativa Creek, Municipality of Macapá, Amapá State, Brazil (Figure 1). It was reported that the animal was a male calf of Amazonian manatee found by fishermen while being attacked and injured by Tucuxi dolphins. The calf was sent to the Reserva Particular do Patrimônio Natural/RPPN Revecom (Private Reserve of Environmental Heritage). Municipality of Santana, Amapá State, where it received its first medical treatment and appropriate nutrition. Although the geographic coordinates of the attack and the rescue specific locations were not recorded, the creek name was stated on the eyewitnesses’ testimonies.

On April 19^th, the Veterinary team from ICMBio/CMA received a call informing that the health condition of the calf was significantly compromised. The animal did not respond to the medical procedures with any improvement. Thus, the ICMBio/CMA team decided to travel to the location on April 22^nd. The clinical condition of the animal worsened, and it began to show apathy and difficulties in submerging (a common behavior in manatees), with the right lateral region being exposed on the surface of the water. When the team arrived in Macapá/AP, the animal collapsed and died and thus, a necropsy was performed on the same day (Table 1).

Table 1. Timeline summarizing the main events of the case report, from rescue and rehabilitation to clinical worsening and necropsy of the Amazonian manatee calf.

Timeline

Diagnostic assessment

Biometric measurements were performed, along with an extensive external assessment (Supplementary Material 1). Furthermore, the animal had an advanced state of thinness, with prominent cranial bones. Cone-shaped marks were found on the left lateral-abdominal region, consistent with the animal’s history of being attacked by Tucuxi dolphin prior to rescue by local fishermen (Figure 2).

In the ventral region incision, upon observing the aspect of the abdominal cavity, it was possible to visualize the congested mesentery with seromuscular edematous hemorrhagic infiltration. Fibrinopurulent exudate was present on the surface of the intestinal loops, which was collected with a sterile swab for further microbiological analysis. However, due to the elapsed time between sample collection and its submission for laboratory processing, there was no bacterial growth, rendering the sample non-viable and yielding a negative result.

In the large intestine, there was a necrotic lesion with rupture of the intestinal wall, causing a leakage of feces (Figure 3) into the abdominal cavity. The stomach contained a small amount of liquid food already in advanced digestion stage and exhibited three (3) hemorrhagic lesions measuring 4mm, 3mm, and 2mm.

The lungs were edematous (Figure 4A), characterized by the presence of foam in the trachea and bronchi, hyperinflated lungs, with the left lung being completely inflated, and whitish areas in the cranial-lateral borders of the left and right lungs (Figure 4B).

No parasites were observed in the respiratory and digestive tracts. The other organs did not present significant macroscopic alterations to characterize pathological changes.

Discussion

The presumptive diagnosis was based on clinical history reporting an attack by Tucuxi dolphins (Sotalia fluviatilis) and the macroscopic findings observed during necropsy, which revealed severe external and internal trauma. Histopathological analyses confirmed neurogenic shock as the cause of death, a condition commonly associated with intense trauma and inflammatory mediator release (Dave and Cho, 2022). These findings confirm that the interaction between S. fluviatilis and the Amazonian manatee calf (Trichechus inunguis) was aggressive and fatal. This represents the first and only documented case of aggression by Sotalia fluviatilis toward Trichechus inunguis in Brazil.

Although clinical and pathological findings clearly document the outcome of this interaction, their broader relevance becomes apparent when considered from an ethological perspective. Accordingly, this case report follows a classical ethological approach (Tinbergen, 1963).

Several proximate factors may have contributed to the emergence of agonistic behavior. In delphinids this is frequently associated with social dynamics, physiological states, and environmental stimulus. Although no direct information on the physiological condition or social structure of the dolphins involved was available, heightened arousal related to social context, reproductive activity, or dominance interactions may have facilitated the aggressive behavior observed. In other delphinids, particularly Tursiops truncatus, calf-directed aggression and interspecific attacks have been linked to dominance assertion, coalitionary behavior, and sexual competition (Samuels and Gifford, 1997; Scott et al., 2005; Robinson, 2014). Additionally, spatial overlap between S. fluviatilis and T. inunguis in riverine environments may increase encounter rates and the likelihood of behavioral escalation (Methion and Díaz López, 2021).

Considering functional explanations, the adaptive significance of this interspecific aggression remains uncertain but can be explored through several non-exclusive hypotheses. One possibility is that the behavior represents an indirect form of competition reduction, whereby aggression toward vulnerable heterospecific individuals decreases future competition for shared space or resources. Alternatively, the interaction may reflect misdirected social or sexual behavior, a phenomenon documented in delphinids where courtship-like or play behavior escalates into harmful aggression (Scott et al., 2005; Volker and Herzing, 2021). Aggression toward calves has also been interpreted as infanticide-related behavior associated with dominance or reproductive strategies, even outside strictly conspecific contexts (Patterson et al., 1998; Robinson, 2014; Methion and Díaz López, 2021). Given the scarcity of documented interactions between cetaceans and sirenians, interpreting isolated events such as this one necessarily involves a degree of speculation.

The ontogenetic dimension of this interaction is particularly relevant given that the victim was a calf. Neonatal and juvenile individuals are inherently more vulnerable due to limited locomotor abilities, reduced escape responses, and incomplete behavioral development. In cetaceans, agonistic, exploratory, and social play behaviors vary across life stages and are strongly influenced by experience and social learning (Sakai et al., 2011). Although the age and developmental stage of the dolphins involved could not be determined, immature or socially inexperienced individuals may be more prone to engaging in risky or escalated interactions. Incorporating an ontogenetic perspective helps contextualize both the vulnerability of the manatee calf and the potential contribution of developmental processes to the dolphins’ behavior.

From an evolutionary and comparative perspective, interspecific aggression is not unique to Sotalia fluviatilis but has been documented across several delphinid taxa, including Tursiops truncatus, Grampus griseus, Stenella frontalis, and Sotalia guianensis (Wedekin et al., 2004; Methion and Díaz López, 2021; Volker and Herzing, 2021). A similar agonistic encounter involving S. fluviatilis and Inia geoffrensis has also been reported in the Amazon basin (Paschoalini, 2016). The recurrence of such behaviors across phylogenetically related taxa suggests that aggressive responses toward heterospecific may form part of an evolutionarily conserved behavioral repertoire within Delphinidae. Alternatively, these interactions may arise convergently in socially complex mammals occupying overlapping ecological niches.

Overall, this case allows the observed interaction to be discussed across different ethological levels of explanation, linking immediate mechanisms to broader ecological and evolutionary considerations (Tinbergen, 1963). Proximate mechanisms likely involve social and physiological triggers; functional interpretations include competition, dominance, or misdirected social behavior; ontogenetically, vulnerability and behavioral immaturity are central to understanding the outcome; and evolutionary, comparative evidence suggests either shared ancestry or ecological convergence in agonistic responses among delphinids. By embedding this observational record within an explicit ethological framework, this case report extends its relevance beyond descriptive natural history and contributes to broader discussions on aggression and interspecific interactions in aquatic mammals.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fetho.2026.1717951/full#supplementary-material

Supplementary material Supplementary material at the source

Footnotes

Edited by: Fabio Santos Nascimento, University of São Paulo, Brazil

Reviewed by: V. Wensley Koch, Retired, Loveland, CO, United States

Artur Andriolo, Juiz de Fora Federal University, Brazil