Repeated downsweep vocalizations of the Araguaian river dolphin, Inia araguaiaensis

Abstract

Araguaian botos (I. araguaiaensis) are known to produce pulsed as well as tonal sounds. Here, we present the first evidence for repetitive sequences of downsweep whistles in botos that appear to be shared between individuals and we investigate the context of their occurrence. Our study was conducted along the Tocantins River located in Eastern Amazon over a period of 42 days of boat surveys between 2012-2018. We observed 82 groups of Araguaian botos and acquired 43h of sound recordings. 632 downsweep whistles were recorded in 10 encounters. Four of these encounters contained downsweep bouts (21 bouts with ≥ 2 whistles) with short inter-call intervals (bout criterion 50s) and up to 161 whistles. We did not find a statistical relationship between downsweep occurrence and any of the contextual parameters we investigated including socializing, travelling, feeding, group size, presence of calves and socio-sexual displays. The rarity of these signals makes them unlikely candidates for individual or group identification. It is more likely that they are associated with very specific contexts such as nursing or mating, both of which were rarely observed in our study. Further studies are required to investigate context specificity and elucidate the function of these signals. I. INTRODUCTION All toothed whales (Odontoceti) produce pulsed calls for communication, with some species using these as their main social signals, e.g. orcas (Orcinus orca) (Ford, 1989; Deecke et al., 2010), northern right whale dolphins (Lissodelphis borealis) (Rankin et al., 2007), narwhals (Monodon monoceros) (Marcoux et al., 2012), and pilot whales (Globicephala sp.) (Sayigh et al. 2013; Pérez et al. 2017). Pulsed calls are discrete signals composed of series of individual clicks (Schevill and Watkins, 1966). Such pulsed calls often encode social group identity (Deecke et al., 2010; Ford, 1989) but are also used in immediate social interactions such as conflict negotiation (Overstrom, 1983). Many marine dolphins (family Delphinidae) also use whistles in their social communication (Janik 2005; May-Collado et al., 2007).

The Amazon River dolphin, Inia geoffrensis: What have we learned in the last two decades of research?

“…In the last couple of years, whistles and repetitive sequences of downsweep whistles have been described as part of the diverse vocal repertoire of I. araguaiaensis, but the functions of these sounds have not yet been elucidated (Melo-Santos et al, 2019. Due to the rarity of downsweep whistle sequences, these authors suggest that they are probably associated with specific contexts, such as nursing or mating (Melo-Santos et al, 2020).…”