Male walruses grab the attention of ice-lying females with loud vocalizations, including underwater bell-like sounds, clicks and pulses, as well as teeth-clacking and whistles above the surface. In this polygynous society, males defend a large harem of females that join him underwater for copulation. To keep up with so much water romping, the bulky bulls are equipped with a penis bone called a baculum, which extends up to 30 inches--the longest of any living mammal.