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The South American sea lion (SASL, Otaria flavescens) is an endemic pinniped native to South America. Its breeding period is notably seasonal and highly synchronized, occurring during the austral summer. Its breeding behavior has been extensively documented; therefore, any unusual observations warrant careful attention and monitoring. In late 2022, highly pathogenic avian influenza (HPAI) H5N1 emerged in South America, rapidly spreading among marine birds and mammals and causing mass mortality events in SASL. Between August and November 2023, mass mortalities and clinical signs consistent with HPAI were reported in several SASL colonies in Argentina. This study documents and quantifies an atypically high number of abortions and abnormal reproductive behaviors observed in SASL during the 2023 HPAI outbreak in Northern Patagonia. A total of 73 abortions events were recorded, some involving females exhibiting clinical signs of avian influenza. Additionally, unusual sexual behaviors were observed, including males retaining and mounting females outside the breeding season and instances of necrophilia documented for the first time in this species. We discuss potential factors associated with reproductive failure and abnormal behaviors in pinnipeds and suggest a high likelihood that these patterns could be related to the HPAI/H5N1 outbreak in the Atlantic. These findings underscore the need for continued research to understand the impacts of emerging diseases on marine mammal health and behavior, emphasizing the importance of sustained population monitoring.
