Introduction
Ringed seals (Pusa hispida), and to a lesser degree bearded seals (Erignathus barbatus) constitute a staple food of polar bears (Ursus maritimus) (Smith Reference Smith1980; Stirling and McEwan Reference Stirling and McEwan1975). Decreased sea ice reduces the ability of polar bears to obtain food, as they depend on sea ice as a platform from which they hunt for prey (Stirling and others Reference Stirling, Lunn and Iacozza1999; Amstrup Reference Amstrup, Feldhamer, Thompson and Chapman2003). Hunting in the water by polar bears is observed sporadically but is considered ineffective (Furnell and Oolooyuk Reference Furnell and Oolooyuk1980; Stirling Reference Stirling1974; Stempniewicz Reference Stempniewicz2006). During summer in several regions of the Arctic polar bears are forced onto land due to complete or partial melting of the sea ice. While polar bears are well adapted to fast through these periods (Watts and Hansen Reference Watts and Hansen1987), there are limitations to their capacity to remain on land without suffering declines in body condition (Molnár and others Reference Molnár, Derocher, Klanjscek and Lewis2010). Terrestrial food sources are a minor component of their diet (Lunn and Stirling Reference Lunn and Stirling1985; Ramsay and Hobson Reference Ramsay and Hobson1991). However, there are a few records of polar bear predation on reindeer (Rangifer tarandus) and muskoxen (Ovibos moschatus) (Ovsyanikov Reference Ovsyanikov1996; Derocher and others Reference Derocher, Wiig and Bangjord2000; Brook and Richardson Reference Brook and Richardson2002) and of cannibalism (Stone and Derocher Reference Stone and Derocher2007). Polar bears have been also observed to feed opportunistically on geese, ptarmigans (Lagopus spp.) and seabirds (Russell Reference Russell1975; Abraham and others Reference Abraham, Mineau and Cooke1977; Stempniewicz Reference Stempniewicz1993, Reference Stempniewicz2006; Donaldson and others Reference Donaldson, Chapdelaine and Andrews1995; Smith and Hill Reference Smith and Hill1996).
In this paper we describe our observation of a polar bear chasing reindeer on land and in the sea, hunting black guillemot (Cepphus grylle) in Magdalenefjorden, northwest Spitsbergen and feeding on the arctic tern chicks (Sterna paradisea) at a nesting colony in Hornsund, southwest Spitsbergen.
Description of observations
Reindeer pursuit
At 11:00 pm local time on 25 July 2011 we noticed an adult female reindeer with a yearling calf being chased by a subadult polar bear (3–4 years old). They crossed a moraine, below the site from which we were observing the chase, and ran through the camp located on the beach (Fig. 1). The calf triggered a flare when it jumped over the tripwire warning system that surrounded our camp. The detonation apparently caused the bear to stop, while the reindeer ran away. The bear returned within 400–500 m to the small patch of tundra, from whence it had started chasing the two reindeer. A few minutes later we noticed the same bear stalking and chasing an adult male reindeer. This reindeer alternated between running up and down the mountain slope for approximately 10 minutes to escape the bear (point 1, Fig. 1). It eventually ran down the slope to a seaside bluff that was 2–3 m high, and then jumped into the water and landed several meters from the coast (point 2).
Map of the observation area in Magdalenefjorden. Dashed line with numbers (1–6) shows successive stages of the terrestrial and marine parts of the pursuit path.
The bear followed the reindeer into the water and the two animals swam toward the opposite shore of the fjord. After ~ 700 m the reindeer turned right and swam parallel to the shore. The entire swim portion of the chase lasted ~25 minutes and covered ~1.6 km (point 3). During the swim the distance between the reindeer and polar bear remained 30–50 m (Fig. 2). The path of the reindeer and polar bear brought both animals to shore about 800 m from where they had entered the water (point 4). The reindeer continued fleeing on the stony beach. Several seconds later the bear leapt out from the sea and, not bothering to shake off the water in its fur, followed the reindeer for ~ 400 m (1.5 minutes). The reindeer continued across the next moraine, turned up the valley and disappeared from our sight. When the bear lost sight of the reindeer it slowed its pursuit and started to track it by sniffing the ground in the direction that the reindeer had run. While crossing a snow field the bear wallowed in the snow to remove the water in its fur (point 5). Then, it continued tracking the reindeer up the valley. At 0:20 am on 26 July 2011 it disappeared from our sight (point 6), and at 2:40 am we observed the bear passing the camp again.
Polar bear chasing adult male reindeer in the Magdalenefjorden (photo by Dorota Kidawa).
On 26 July at 10:20 pm we observed another chase, this one entirely on land. We observed an adult male reindeer crossing the moraine and running quickly along the seacoast. After several seconds a polar bear, apparently chasing the reindeer, appeared. The reindeer ran up a mountain slope, and the bear, on reaching this steep slope, slowed down, stopped chasing the reindeer, and then returned to the edge of the water and paced along the seacoast. The chase covered ~ 700 m.
Predation on birds
On 1 and 4 August 2009 a subadult polar bear was noticed in the vicinity of the Polish Polar Station in Hornsund, southwest Spitsbergen. The bear was observed feeding for 1–2 h in a small arctic tern colony. The bear walked within the colony and caught nestlings (2–3 weeks old), which either attempted to hide or escape. Despite the parents’ persistently attacking the predator to defend the chicks, the polar bear caught and consumed at least 12 nestlings. Another time, on 19 July 2011 a polar bear walking along the coast in Magdalenefjorden was observed from ~500 m away. The bear entered the water and swam ~ 50 m towards a flock of ~ 10 black guillemots. On approaching the flock the bear dived several times covering ~10 m long distances under the water. After ~ 10 minutes of stalking, the bear caught one guillemot and was observed by telescope at the water's surface holding it in its paws while eating it.
Discussion
Polar bears use different hunting techniques. The most common involve waiting at a seal breathing hole, breaking into subnivian lairs to catch adult seals and pups inside, and stalking a seal from a distance. Polar bears can also approach prey using an underwater stalk to approach as close as possible, making use of wind direction and configuration of the terrain. If the bear can approach close enough it will attempt to seize the prey. This is a common technique used for hunting seals basking on ice floes (Stirling Reference Stirling1974; Smith Reference Smith1980). Hunting for resting/sleeping reindeer seems to be a slightly modified version of the stalking behaviour described above (Derocher and others Reference Derocher, Lunn and Stirling2004). If the reindeer attempts to flee, the bear charges and then gives up if unsuccessful (Brook and Richardson Reference Brook and Richardson2002). The long–lasting pursuit of a healthy adult reindeer that we describe is a novel polar bear hunting behaviour.
Polar bears prefer seals to other prey (Stirling and McEwan Reference Stirling and McEwan1975; Best Reference Best1982). Despite this, preying on reindeer during the summer could be beneficial to a polar bear stranded on land. However, polar bears are relatively inefficient walkers (Hurst and others Reference Hurst, Øritsland and Watts1982), hence, we can expect that there is a significant energy cost involved in chasing reindeers, which are adapted to move at high speeds to outrun predators. Because of the high cost of locomotion, chasing reindeer should be beneficial only over short distances (Lunn and Stirling Reference Lunn and Stirling1985; Derocher and others Reference Derocher, Wiig and Bangjord2000; Brook and Richardson Reference Brook and Richardson2002). Moreover, polar bears are susceptible to hyperthermia during any longer effort, especially in warm weather (Best Reference Best1982; Hurst and others Reference Hurst, Øritsland and Watts1982). Healthy adult reindeer, based on our observations, also appear to swim at least as fast as polar bears. Thus, the likelihood of a polar bear catching a healthy, adult reindeer seems to be low. However, a recent study by Iversen (Reference Iversen2011) found a frequency of 9.2% of reindeer remnants in polar bear excrement on Spitsbergen and suggests that reindeer is a regular food item from either actively hunted animals or from scavenged remains. The observed hunting attempts involved immature individuals that may have higher energy demands than adults during the summer to reduce weight loss because they are less successful at hunting and protecting their kills, and are actively growing. Therefore, subadults may be more eager to explore new food sources and try different hunting techniques than adults (Derocher and others Reference Derocher, Andriashek and Stirling1993).
Exploring the arctic tern colony by polar bears and feeding on nestlings does not require any special skills because tern colonies are accessible and eggs and chicks easy to catch. It generally does not differ from the more frequently observed feeding in goose colonies (for example Abraham and others Reference Abraham, Mineau and Cooke1977; Smith and Hill Reference Smith and Hill1996), except for the energetic returns of different size preys. Hunting in auk (Alcidae) colonies requires either risky climbing onto the cliff shelves in case of guillemots (Donaldson and others Reference Donaldson, Chapdelaine and Andrews1995) or time and energy consuming removal of boulders covering the nests in case of little auks (Alle alle) (Stempniewicz Reference Stempniewicz1993). Polar bears also prey on birds on the water by swimming toward them, diving and catching them from below (Russell Reference Russell1975). Our observation of successful catching of black guillemot confirms such abilities of polar bear. This hunting behaviour could be a slightly modified technique of underwater stalking seals resting on the ice floes.
Our observations of polar bears suggest increasing exploration of terrestrial food sources. In the 1970s, reindeer carcasses were observed uneaten for extended periods (L. Stempniewicz, personal observation, July, 1974) but today they are usually detected and consumed by bears quickly. This may be interpreted as a response to reduced seal availability as a result of shrinking of the sea–ice range in the Arctic (Smith and Lydersen Reference Smith and Lydersen1992; Stirling and Smith Reference Stirling and Smith2004). Furthermore, the increased number of the polar bear population, since their complete protection from harvesting in the early 1970s in Svalbard (Derocher Reference Derocher2005), may also force the predator to use different terrestrial food sources.
Acknowledgments
These observations were made while conducting research project ALKEKONGE (PNRF–234–AI–1/07).
