ABSTRACT Lontra longicaudis Olfers, 1818 is a semi-aquatic carnivore widely distributed in the Neotropical region. Understanding their diet contributes to an indirect understanding of their ecology and the composition of the local fauna. To this end, we analyzed 109 fecal samples and identified 238 morphological structures; these samples were collected between May 2006 and September 2007 from the Santa Lúcia Biological Station (SLBS) in southeastern Brazil. The area is intersected by the Timbuí River, which arrives at the site after crossing the urban perimeter of the city of Santa Teresa in the state of Espírito Santo. We found a predominance of fish in the otters’ diets (82%), mainly cichlids (50%), which are fish with sedentary habits and low mobility levels. The crustacean Trichodactylus fluviatilis Latreille, 1828 was the third most consumed taxon; this occurred mainly during the rainy season, corresponding to the crustacean’s reproductive period, when it is more vulnerable to predation. Otters exhibited a seasonal variation in their prey selectivity. Furthermore, they displayed opportunistic foraging behavior, as the most preyed fish were those with both low mobility and a high frequency in the environment, followed by fish with high mobility and high frequency, and then those with low mobility and low frequency. We concluded that the feeding habits of the otters in the SLBS are in line with the optimal foraging theory since prey selection was optimized through the balance between net energy gained and the energy costs of foraging.

É proposta uma nova metodologia para o estudo das reações de alarme em peixes, utilizada aqui no estudo da "Schreckreaktion" de Mimagoniates lateralis (Nichols, 1913) e Mimagoniates microlepis (Steindachner, 1876), cujos resultados foram contextualizados à ecologia das espécies. O método baseia-se na interpretação cartesiana do comportamento, em que três baterias experimentais distintas são analisadas de forma segmentada nos eixos horizontal e vertical. Os dados foram analisados por estatística não-paramétrica e pela análise de dados longitudinais, a qual permite a interpretação gráfica de séries temporais, e integrados com os estados de agregação dos cardumes pelo uso de índices. A análise conjugada de todos os fatores permitiu uma mensuração menos subjetiva da intensidade de reação. Os resultados para o eixo horizontal não foram conclusivos e os do eixo vertical demonstraram significativas diferenças comportamentais entre baterias e espécies. Mimagoniates lateralis (Nichols, 1913) apresentou indivíduos mais estressados e agregados, com cardumes mais ao fundo, enquanto M. microlepis aparentou maior adaptabilidade às condições experimentais. Ambas as espécies demonstraram reação de alarme nítida, sendo que M. lateralis a apresentou cerca de três vezes mais intensa, em que demonstrou maior coesão dos cardumes e maiores período e amplitude de reação, o que é coerente com seu status de espécie endêmica e ameaçada.

A new method for analysis of alarm reaction of fishes is proposed by the study of "Schreckreaktion" from Mimagoniates lateralis (Nichols, 1913) and Mimagoniates microlepis (Steindachner, 1876), which results were discussed on species ecology context. The method is based on Cartesian interpretation of the behaviour, where three distinct experimental series are analyzed in segmented way on horizontal and vertical axes. Data were analyzed by non-parametric statistics and longitudinal data analysis, which allow the time series graphic analyses, and interpreted together the crowding states of the schools by use of indexes. The conjugated analysis of all factors allowed a less subjective measurement of the reaction intensity. The results obtained from the horizontal axis weren't conclusive and the vertical axis demonstrated significant behavioural differences between series and species. Mimagoniates lateralis (Nichols, 1913) showed more stressed and crowded individuals, with schools trending to bottom, while M. microlepis showed more fitness to experimental conditions. Both species exhibited clear alarm reaction, and M. lateralis showed it three times more intense, presenting larger cohesion of the schools and larger period and amplitude of reaction, what is coherent with its status of endemic and threatened species.