These observations provide the first detailed description of a complete and successful polar bear hunt of a fully functional, adult Svalbard reindeer. Polar bears can travel very quickly over short distances (Derocher 2012), so presumably most reindeer taken by polar bears are killed because the attacking bear gets close up to the reindeer before the latter can react. The most interesting aspect of this particular observation is that the bear drove the reindeer male into the water and was then able to catch it because it could swim faster than the reindeer, although an earlier observation (Stempniewicz et al. 2014) showed that a reindeer was capable of escaping from a younger polar bear in open water. The differences in predation success may depend on both the distance between hunter and prey at the beginning of the pursuit, and individual factors relating to the two animals, such as age, size and physical condition. The second reindeer, killed only two days later by the same bear seemingly using much the same method, may indicate that this particular bear has become specialized in hunting reindeer, and possibly, that reindeer may be an important prey to that bear, at least in summer, when the lack of sea ice may prevent it from taking seals.
In general, reindeer run faster and have greater stamina than polar bears over longer distances (Brook and Richardson 2002). This also applies to the Svalbard reindeer, despite its shorter legs, compact body, and therefore, slower speed. A bear is a short-distance runner and can overheat if it has to run fast for a longer time (Hurst et al. 1982; Pagano et al. 2018), although chasing prey in cold water minimizes this risk. Whenever possible, a polar bear uses the terrain to creep up to within a few metres of its prey before the attack. This method of hunting is possible thanks to its ability to smell potential prey from a distance. When chasing prey, the bear was observed accelerating rapidly. It performed such a charge right at the beginning of the attack, but might also have repeated it later. This behaviour of polar bears (three instances) has been observed both on land and in water when they hunted caribou/reindeer (Brook and Richardson 2002; Stempniewicz et al. 2014) and flightless geese (Stempniewicz 2006).
Polar bears may succeed in driving the deer into the water, especially where the water is shallow and the bottom is rocky and uneven. They can also gain an advantage in areas where the reindeer are hindered by the terrain. Reindeer often feed on steep mountain slopes. There have been many observations of crippled reindeer found in such rubble-covered areas in Svalbard, and these animals may be easy prey for bears (Derocher et al. 2000; Stempniewicz et al. 2014). Reindeer can break their legs when trying to escape a bear on steep scree, but also when they feed on steep icy slopes in winter and lose their footing. In addition, to the reindeer they actually kill, other carcasses may be available to bears and contribute to their diet. The high frequency of reindeer remains found in polar bear scats from Svalbard, reaching 27.3% in summer, suggests that reindeer may be a significant part of the polar bear’s diet in that area, whether from carcasses or predation (Iversen et al. 2013).
The level of alertness observed among Svalbard reindeer is very low. When selecting feeding and resting sites, they appear to underestimate the risk of a bear attack. In Hornsund they often graze and lie among the rocks, which limit their field of view; this makes it even easier for a predator to hide and catch a reindeer by surprise. In addition, the relatively long time it takes for a reindeer to get to its feet also reduces its chances of escaping when ambushed by a bear. Our own observations of the reactions of reindeer to people walking on the tundra seem to confirm this low level of vigilance. This modest reaction of Svalbard reindeer to an approaching bear was known to trappers (Lønø 1970). Part of the reason may be the absence of other large terrestrial predators in Svalbard. This behaviour may also be a pointer to the past, perhaps a period when, due to over-hunting by humans, both bears and reindeer became rare in Svalbard and the frequency of their contacts was very low. At the same time, the cold climate continued to favour the availability of seals to bears. Under such conditions, polar bears may have had no interest in hunting reindeer. Reimers et al. (2011), however, showed that the flight distance of reindeer in different parts of Svalbard was the greatest on Edgeøya, where the probability of interaction with polar bears was high. Elsewhere in the polar bear’s distribution, they coexist on shores with brown bears Ursus arctos, which hunt caribou. Thus, the greater use of reindeer in Svalbard relative to that of caribou by polar bears elsewhere in the Arctic could be a result both of competition with brown bears and wolves Canis lupus, and the heightened vigilance of caribou in areas where these predators occur, making reindeer a more challenging prey for polar bears. As the frequency of bears hunting for reindeer increases, strong selection pressure may come into play in favour of increased vigilance and other adaptations against predators in Svalbard reindeer.
The number of direct observations of polar bears hunting Svalbard reindeer appear to have been increasing in recent years. In addition, there are now a significant number of cases where predation can be assumed (Table 1). Although bears in the area are most likely to be fully dependent on the much more energy-rich seals hunted on sea ice, at least from spring to early summer, the rather frequent reports of the successful hunting of reindeer by Svalbard polar bears may mean that the importance of such prey in their diet has either been underestimated, or more likely, that this phenomenon has become more common in recent years. It is hard to believe that such behaviour was ubiquitous before the 1970s, when Lønø (1970) stated that bears did not hunt reindeer, given all the extensive knowledge he had accumulated as a trapper and scientist himself, and from other trappers that had hunted in the area for decades.
Land-based food sources cannot provide bears with sufficient energy, and bears in the southern Beaufort Sea and western Hudson Bay lose weight when on land (Stirling et al. 1999; Rode et al. 2010). In other areas (e.g., the Chukchi Sea), increased land use has occurred but without weight loss among bears in the population (Rode et al. 2015a, 2015b). In Svalbard, reindeer are numerous and easily available. As prey, they may offer bears a considerable energy return. In late summer, the Svalbard reindeer are in prime condition. The average body weight of adult males shot in August was ca 118 kg, including lean body weight (37 kg), digestive tract (30 kg), skin, head, hooves (23 kg) and fat (28 kg) (Reimers 1984). The bear observed in Hornsund ate ca 80% of the soft body parts of the first reindeer it killed and ca 40% of the second one, an estimated consumption of ca 70–80 kg of meat, innards and fat. This is a large amount of food, equivalent in weight to one adult or two young ringed seals, even though reindeer carcasses may provide only about half the energy per weight unit (Best 1977). Even before the hunt, the bear we observed was in excellent condition. Moreover, the ease with which it hunted two adult reindeer within a short time strongly suggests that it had few problems maintaining an energy balance.
Currently, nearly 300 polar bears live in Svalbard all the year round (Aars et al. 2017), and the number of local bears in Spitsbergen (W Svalbard) has increased significantly in recent decades (Prop et al. 2015). They have been shown to be highly philopatric, using the same area year after year, and also over generations. If polar bear mothers learn to hunt reindeer efficiently, their offspring, which stay with their mothers for over two years, may also become reindeer hunters. Reindeer numbers in the area may continue to increase, given the increased productivity of the tundra as the climate warms. The local bears, numbering about a tenth of the Barents Sea population where most hunt along the ice edge and visit the islands more sporadically (Aars et al. 2017), may become an increasingly important component of Svalbard’s terrestrial ecosystem. Their impact on colonial birds, not only on the reindeer population, is expected to increase, although one can assume that predation on sea ice of the associated seal species in spring and early summer will still provide the major item in their diet.