The giant otter (Pteronura brasiliensis) is a social species that defends territories along water bodies. Although some researchers have visually monitored otters during long periods at night, no nocturnal activity of the species has been recorded and giant otters are currently believed to be strictly diurnal. In this study, we present information about the activity patterns of groups of giant otters in the Brazilian Pantanal, using radio telemetry and camera trap data. We captured, implanted transmitters in, and monitored three male giant otters from different groups in the Miranda and the Vermelho Rivers between November 2009 and June 2011. The locations and behavior of the group were recorded at 30-min intervals from 05:00 to 19:00. Camera traps were positioned at the active dens and latrines of eight groups of giant otters in the Miranda, Vermelho and Negro Rivers between June 2010 and October 2011. The groups of giant otters were mostly crepuscular and diurnal, but 31% of the camera-trap recordings were nocturnal. Fishing was the most frequent (64%) behavior recorded by telemetry. Giant otters were recorded exiting the den mostly in the early morning (06:00) and entering the den at the end of the day (16:00 to 19:00). Nocturnal activity appeared to be associated with the need to defecate, prey availability nearby the den and predation risk. © 2013 Dipartimento di Biologia, Università di Firenze, Italia.
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