Source characteristics of the underwater knocking displays of a male Pacific walrus (Odobenus rosmarus divergens).

Walruses breed in winter at high latitudes in conditions that make close‐range observations difficult. Males are known to produce complex underwater songs that can extend over multiple days and propagate over several kilometers. These acoustic displays are comprised of highly rhythmic sharp “knocks” punctuated by occasional metallic “bells.” The source characteristics of the knocking sounds that were regularly emitted by a male walrus raised in captivity were examined. Knocks were produced as single 20 ms pulses, or as doublets and triplets, and were typically repeated at rates of 0.8/s to 1.2/s. These were loud sounds with greater bandwidth than previously reported: mean source levels were 186 dB pk‐pk re 1 μPa at 1 m (range 161–196) with maximum frequency >24 kHz. Production of each knock was associated with visible impulsive movement of the forehead. During rut, this walrus had difficulty inhibiting sound production and would often continue to emit knocks in air during haul‐out and even while eating, suggesting an endogenous component to this behavior. A strong correlation between his seasonal testosterone levels and the persistence of knocking displays was confirmed. Captive research provides unique access to acoustic and reproductive behavior that is presently impossible to study in wild walruses.