Abstract
BackgroundDirect observations of underwater behaviour of free-living marine mammals are rare. This is particularly true for large and potentially dangerous species such as the walrus (Odobenus rosmarus). Walruses are highly specialised predators on benthic invertebrates – especially bivalves. The unique feeding niche of walruses has led to speculations as to their underwater foraging behaviour. Based on observations of walruses in captivity and signs of predation left on the sea floor by free-living walruses, various types of feeding behaviour have been suggested in the literature. In this study, however, the underwater feeding behaviour of wild adult male Atlantic walruses (O. r. rosmarus) is documented for the first time in their natural habitat by scuba-divers. The video recordings indicated a predisposition for use of the right front flipper during feeding. This tendency towards dextrality was explored further by examining a museum collection of extremities of walrus skeletons.ResultsDuring July and August 2001, twelve video-recordings of foraging adult male walruses were made in Young Sound (74°18 N; 20°15 V), Northeast Greenland. The recordings did not allow for differentiation among animals, however based on notes by the photographer at least five different individuals were involved. The walruses showed four different foraging behaviours; removing sediment by beating the right flipper, removing sediment by beating the left flipper, removing sediment by use of a water-jet from the mouth and rooting through sediment with the muzzle. There was a significant preference for using right flipper over left flipper during foraging. Measurements of the dimensions of forelimbs from 23 walrus skeletons revealed that the length of the right scapula, humerus, and ulna was significantly greater than that of the left, supporting our field observations of walruses showing a tendency of dextrality in flipper use.ConclusionWe suggest that the four feeding behaviours observed are typical of walruses in general, although walruses in other parts of their range may have evolved other types of feeding behaviour. While based on small sample sizes both the underwater observations and skeletal measurements suggest lateralized limb use, which is the first time this has been reported in a pinniped.
Highlights
Direct observations of underwater behaviour of free-living marine mammals are rare
We suggest that the four feeding behaviours observed are typical of walruses in general, walruses in other parts of their range may have evolved other types of feeding behaviour
The 2.5 hours of digital videotapes of foraging walruses showed very uniform feeding behaviour. 31 dives were defined as bottom sequences; these did not necessarily correspond to complete feeding dives because of the practical limitations associated with video recording under water
Summary
Direct observations of underwater behaviour of free-living marine mammals are rare. This is true for large and potentially dangerous species such as the walrus (Odobenus rosmarus). Walruses (Odobenus rosmarus) are highly specialised benthic feeders feeding almost exclusively on bivalves, making them an important component of the benthic ecosystem [1,2,3,4] It is mainly the soft parts of the bivalves that are found in walrus stomachs, with pieces of shells seen only rarely [1,2]. Almost 6400 bivalve siphons have been reported in a single walrus stomach [2] This feeding pattern makes walruses unique among pinnipeds, and has led to different theories as to how the walruses are able to locate and excavate their benthic prey [5,6,7,8]. This paper is the first that gives details of the foraging behaviour of free-living Atlantic walrus (O. r. rosmarus) based on close-up underwater video recordings in Northeast Greenland
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