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During the breeding season from January to mid-April, adult male Atlantic walruses (Odobenus rosmarus rosmarus) dive repeatedly for an average duration of 4-6 min and give stereotyped underwater vocal displays. Between dives, they surface for 1-2 min, take 4-6 breaths, and give stereotyped vocalizations between breaths. Male walruses vocalizein the presence of groups of females and calves, young adult males, or by themselves as lone singers. This pattern is repeated throughoutthe breeding season and can be maintained for extended periods, sometimes exceeding 48 h. The prolonged underwater vocal displays of malewalruses seem possible because the animals do not exceed the aerobicdive limit (ADL), estimated to be 9.8 min for a 1 100-kg animal, nordo they exceed the behavioral ADL of 7.9 min determined from the histogram of dives for males singing alone. The number of breaths taken after dives and the post-dive surface times remained fairly constant despite dive duration, suggesting that the walruses remained within their aerobic dive limits. The duration of most dives made by displaying males vocalizing alone during the breeding season, and dive duration of walruses feeding for protracted periods outside the breeding season, are both roughly half the estimated ADL. [ABSTRACT FROM AUTHOR]