Rostrum abnormalities in the endangered Indian Ocean humpback dolphin (Sousa plumbea) in South Africa

Abstract

Morphological abnormalities in wild animals can be indicators of the underlying health of a population and may be determined through routine photographic surveys. Here, we assess unusual rostrum conditions in Indian Ocean humpback dolphins (Sousa plumbea) inhabiting South African coastal waters to understand the rate of prevalence of abnormal rostrums and formulate hypotheses on potential causes. Photographic data were collated from systematic boat surveys and opportunistic sightings, obtained between April 1998 and March 2021 in various regions along the South African coast. Overall, 31 unique individuals were found with abnormal rostrum conditions, varying from slight misalignments to severe wounds and/or aberrant morphologies. In most cases, injuries were likely caused by natural events during the animal's life history such as interactions with sharks and/or reef-associated hunting strategies. Mark-recapture data indicated that individuals had survived with these injuries for up to 10 years. This study reports the highest incidence of rostrum abnormalities in the species. As numbers reflect only those that have survived their injuries, they are considered a minimum estimate. A better understanding of the cause(s) of these injuries is important given the endangered status of this species.

Skull and mandible alterations in Amazon river dolphins (de Blainville, 1817) (Cetacea, Iniidae, Inia spp.)

“…Due to the elongated rostrum in Odontoceti, resulting from the projection of maxillary and premaxillary bones, it is common for the rostrum to show some deviation (da Silva, 1994). Rostrum deviation has been reported for some species, such as M. grayi (Montes et al, 2004), Pontoporia blainvillei (Denuncio et al, 2016; Gerholdt, 2006; Montes et al, 2004), Delphinus capensis (Montes et al, 2004; Van Bressem et al, 2006), and Sousa plumbea (Frainer et al, 2023). Among these, only P. blainvillei has a narrow and elongated rostrum similar to the Amazon river dolphin.…”

“…Rostrum deviation has been reported for some species, such as M. grayi (Montes et al, 2004) et al, 2006), and Sousa plumbea (Frainer et al, 2023). Among these, only P. blainvillei has a narrow and elongated rostrum similar to the Amazon river dolphin.…”

“…Among the main causes of fractures described in cetaceans are collision with vessels and attacks of predators (Groch et al, 2012;Hellier et al, 2011). Previous studies reported fractures in skulls and mandibles of cetaceans, such as Globicephala macrorhynchus, D. capensis, L. obscurus (Montes et al, 2004;Oremland et al, 2010), S. plumbea (Frainer et al, 2023) and Sotalia fluviatilis (Loch et al, 2009). However, the fact that Amazon river dolphins get close to vessels and propellers in search for food or out of curiosity may generate conflicts with humans, leading to physical aggression.…”