Abstract: Daily rhythms enable organisms to adapt to the daily fluctuations of environmental factors. There is a debate on whether organisms would exhibit 24h rhythms when living in habitats without obvious daily cycles of external signals. To answer this question, we investigated the daily vocal pattern of Adélie penguins (Pygoscelis adeliae) breeding on Inexpressible Island (74°54′S, 163°39′E). Adélie penguins use calls to contact with their chicks during the brooding period. Recordings were made using five passive acoustic recorders during January 24th to February 1st, 2018. Calls were detected through automatic recognition. The daily rhythm and the relationship between the intensity of calls and environmental factors were studied. There was no prevailing daily vocal pattern. Vocal activity was positively related with environmental humidity and temperature. Our results would help understand the behavior rhythm in the polar day.

Key words: Inexpressible Island, passive acoustic recording, sound detection, environmental factor.