We observed that a captive melon-headed whale, Peponocephala electra, maintained close proximity to the abdomen of another member of Delphinidae. Therefore, we aimed to investigate whether this behavior was a “sleep” behavior by observing indicators, such as eye states and responses to external stimuli. The whale had at least one eye closed for more than 60 s and demonstrated fewer responses to external stimulation. These results suggest that the type of behavior exhibited by the whale was sleeping behavior. To the best of our knowledge, no prior studies have reported this behavior in cetaceans. We propose to coin the term “tandem sleep” to describe this behavior.

Published online 28 November, 2023; Print publication 31 January, 2024