While the killer whale as a species has an extremely diverse diet, particular sets of animals are highly specialized (Baird 2000
). In the northeastern Pacific two sympatric forms specialize on fish and marine mammals, respectively, rarely if ever eating the food of the other form (Ford et al. 1998
). Similarly, three forms with distinct diets have been recognized from the Antarctic (Pitman & Ensor 2003
). Social groups of killer whales can sometimes adopt new foraging behaviour, such as depredation on long-lines (Yano & Dahlheim 1995
). These specializations are probably largely the result of social learning, i.e. they are cultural (Rendell & Whitehead 2001
Thus the scavenging killers were likely specialists to some degree. Scavenging on carcasses resulting from natural mortality, including possibly predation by other killer whales, may have been a part of the repertoire of some killer whales before whaling, but given the spatial and temporal dispersion of the carcasses, it would probably have been difficult to subsist completely on the carcasses of large whales that they had not killed. Scavenging likely became more important during the era of open-boat whaling (Dudley 1725
). We hypothesize that it was with modern whaling that scavenging became widespread and commonplace. Numbers of carcasses were being produced in reasonably small areas, with loud ‘dinner-bells’. The large, fast animals of the genus Balaenoptera
with their huge tongues and thin skin were suddenly available, already killed, inflated with air so that they did not sink, and often left floating unattended for several hours (Tønnessen & Johnsen 1982
). Those killer whales for which scavenging was already a part of the feeding repertoire would have had a bonanza, and possibly increased their reproductive rates, although mortality from armed, irate whalers (Robertson 1954
) would have had the opposite effect. Other groups of killer whales, probably naturally mammal-eaters (fish-eaters seem loathe to eat mammals; Ford et al. 1998
), were likely recruited to the scavenging way of life either through innovation, or by social learning from other killer whales already scavenging. During the carcass-rich decades of the mid twentieth century, as the scavenging culture spread, other ways of securing nutrients and energy could have receded from the behavioural repertoire, as well as perhaps the knowledge base, of these animals.
In this scenario, by about 1980 these scavenging killer whales faced a problem: the carcasses were almost gone. It seems reasonable that these animals would have turned to attacking pinnipeds, living large whales, small cetaceans and sea otters more frequently.
Killer whales have developed different feeding specializations in different parts of the world, and we should expect the significance of carcass scavenging, like natural predation, to have varied geographically. With variation in the cultures of the killer whales interacting with environmental differences, the switch from whale-carcass scavenging would have taken different paths in different areas, including, perhaps, significantly increased predation on sea otters and pinnipeds off Alaska (Springer et al. 2003
), pinnipeds and minke whales in the Southern Ocean (Branch & Williams in press
), and large whales off western North America (Reeves et al. in press
). The extent to which killer whale predation has contributed to apparent declines of some populations of pinnipeds and cetaceans is beyond the scope of this paper, and remains an open and controversial question.
Killer whales were not the only scavengers on whale carcasses. Flocks of seabirds scavenged from floating carcasses as well as the offal from whale processing (Dudley 1725
; Robertson 1954
). In the Arctic, polar bears (Ursus maritimus
) still feast on bowhead whale carcasses (Todd O'Hara, personal communication); their opportunities for scavenging during the peak era of commercial whaling must have had a dramatic effect on bear numbers and distribution. Scavenging sharks were major nuisances to whalers in tropical areas (Reeves et al. 2001
). Therefore, the ecological ramifications of whaling on predators cum scavengers, in addition to killer whales, likely had long-term consequences for at least some of their populations and prey.