Polar bears and walruses: The evolutionary paths of two Arctic giants

Polar bears and walruses: The evolutionary paths of two Arctic giants

18 May 2015

The precise relationship between polar bears and walruses remained a mystery to scientists for a long time. Until the early 1990s they did not know whether polar bears attacked and killed walruses or just ate the remains of dead ones. Observations were sporadic and did not answer many questions, such as whether polar bears hunt walruses and how, the vulnerability of walruses of different ages, how dangerous it was for polar bears to attack larger and heavier walruses that had tusks, as well as the place of walruses in the polar bear’s diet. Researchers became able to study these issues systematically when the Arctic environment started to change.


This happened in the early 1990s when large numbers of polar bears and walruses began to regularly come ashore due to the melting of ice on the continental shelf and direct observations became possible. In the autumn of 1990 I started a long-term study of the relationship between polar bears and walruses around Wrangle Island, and I have taken every opportunity to observe them elsewhere in the Arctic. Gradually the picture began to clear up.


Evolutionarily, walruses are much older than polar bears. As a distinct group of marine mammals, walruses appeared on Earth about 20 million years ago and at one time were the dominant group of web-footed mammals. However, only one species has survived to the present – the walrus with the Latin name of Odobenus rosmarus that literally means “sea horse that walks on tusks.” As a distinct type of Arctic predator, the polar bear appeared on Earth much later. Scientists today believe they emerged somewhere between 300,000 and 900,000 years ago, with 600,000 being the average estimate. Walruses had already travelled a long evolutionary road before polar bears showed up in the Arctic. What happened when large four-footed predators capable of hunting walruses made their appearance? How do changes in the Arctic affect these species and the relationship between them?


At any rate, the evolutionary history of their co-existence has been long enough for them to get know each other, establish a certain relationship and adapt to the environment.


The autumn of 1999 was a unique season in the Arctic. This was the first time when sea ice completely disappeared on the continental shelf between Taimyr and Point Barrow, back when the polar bear and walrus populations were still fairly large. The continued extermination of polar bears by humans aggravated by global warming had not yet produced the consequences that we are observing now. The autumn of 1999 was the first time researchers managed to observe dozens of interactions between polar bears and walruses. During that year hundreds of polar bears had to survive on Wrangel Island the disappearance of ice. At the same time, tens of thousands of walruses formed huge coastal rookeries near the island, with up to 60,000 congregating in a single place.


The environmental situation and the conditions of walrus and polar bear populations changed in step with climatic changes and the melting of the drifting Arctic ice during the following 25 years. The polar bear’s diet was subject to change as well. One fact was beyond doubt – climatic changes and anthropogenic factors were increasingly damaging the Arctic habitat of both species.


Long-term continuous observations of polar bears and their prey, year in and year out at a single Arctic nature reserve, made it possible to monitor the evolution of their relationship and learn many new facts. It is critical that any study of the processes at play in wild animal populations be long-term and continuous.


According to confirmed observations alone, from 1990 to 2014, polar bears ate 396 walruses on Wrangel Island, including both dead walruses that washed ashore and those hunted by polar bears. They devoured the walruses’ entire bodies, leaving only the inside-out skin licked clean of fat, as well as parts of the spine and skull. Out of these 396 walruses 46 percent were adults, 10 percent were young – from two to four years – and 44 percent were calves born that year. It would take a scientific monograph or a popular science book to capture all the nuances of this drama. This brief article contains just the most important conclusions.


Polar bears were observed to hunt walruses whenever the latter came ashore to rest. In many cases polar bears approached the walruses but did not dare attack them. This behaviour is typical of young polar bears. When approached, the walruses reacted to them as a threat from a predator and began to retreat into the water. This behaviour was typical for polar bears of all demographic categories, males and females, adults and calves. The constant approaches did not allow walruses to rest and revealed weak or sick animals. Walruses were easier to run down and kill during panics, which is why polar bears constantly harried the walruses – a tactic that was sometimes successful.


However, polar bears are also serious hunters that attack walruses directly. Large polar bears, both males and females, are the best hunters. Young bears are less successful even in hunting ringed seals. Usually, young polar bears do not dare attack walruses. Apart from experience, seasoned adult bears have the power and weight required to successfully kill walruses.


The actual hunt always starts with the bear approaching a walrus rookery, first walking and then running the last several dozen metres. From this point, different bears will chooses different tactics. Out of 75 polar bear hunting episodes on coastal walrus rookeries that were observed from start to finish, about 9 percent were successful. In all cases, young walruses under 12 months did not survive. Only large male bears risked directly attacking adult walruses on the beach, and often received bloody tusk wounds. Female bears were more cautious hunters than males, who were more vicious in their attacks even though it didn’t improve their chances of success. In all successful hunts, bears raided a rookery, running along the perimetre to cause the walruses to flee for safety. They would then snatch an unprotected walrus calf from the crowd of panic-stricken fleeing walruses.


Over the entire period of my observations, I never witnessed an adult male bear stop and kill a healthy adult walrus on a beach. I once observed and videotaped an adult male bear successfully hunt an adult walrus on an ice floe, but on the beach adult walruses always had enough strength to make it to the water, even with a bear riding on their back. Most likely, the slippery surface of an ice floe doesn’t provide enough traction for the walrus to make it to the edge and escape into the water. On the beach, the bears could successfully hunt only young walruses or adult walruses that were sick or dying from exhaustion.


Hunting adult walruses is undoubtedly dangerous for polar bears. On Wrangel Island, Irina Menyushina (PhD in Biology) and I found two dead polar bears with wounds caused by walrus tusks. On many occasions, we saw live bears with tusk wounds of varying severity.


In all their encounters and interactions with polar bears, walruses invariably responded as prey to a predator: they became anxious, frightened, and fled into the sea. If they couldn’t escape, the walruses defended themselves by using their tusks to threaten or strike the attacking bear. In most cases, showing the tusks was enough to stop the bear in his tracks. Bears are well aware of the threat and stop attacking when faced with active defence.


In the water, walruses are out of reach for polar bears. In fact, an aggressive walrus can be dangerous for a polar bear in the water. There have been isolated observations of walruses attacking polar bears in the water, though it is unusual. In the water, too, walruses generally react to polar bears as to predators – they fear and avoid them. Only rarely do they attack.


According to our observations, wherever walruses were intensely hunted by polar bears, they tended to change their routes onto the beach and switch from more comfortable but less protected beaches, such as open pebble or sand bars, to less comfortable but better protected areas with large rocks and steep banks or isolated coastal rocks. Thus, polar bears’ hunting of walruses affects not only their death rate, but also the distribution of walrus rookeries.


A detailed study of the relationship between polar bears and walruses showed that polar bears are not only able to successfully hunt walruses, but also that walruses are a major food source for polar bears living in the Bering Sea’s sector of the Arctic. It is a typical predator-prey relationship, which manifests itself in evolutionary, population and behavioural aspects. The adaptations of both species in the course of evolution include, among other things, the qualities that are necessary for the predator to hunt this kind of prey, and the prey to defend itself against this kind of predator. From a population perspective, we can see that the death rate and the distribution of both species are related and depend on the conditions of interactions and the intensity of predation. The behaviour of both species leaves no doubt about the fact that their relationship is that of predator and prey.


What about other regions of the Arctic? There’s not much data on the relationship between polar bears and walruses in other regions of the Arctic. Our data and observations from the New Siberian Islands during the RGS expeditions of 2012 and 2013 indicate that polar bears on the New Siberian Islands concentrate near walrus rookeries and feed on them. We didn’t have the chance to observe the hunting, since such observations require time and closer monitoring. Recent data indicate that polar bears residing in the Atlantic sector of the Arctic can hunt walruses and use them as a food source more often than previously thought. Further research is needed to clarify the situation.


The evolutionary paths of polar bears and walruses have progressed side by side for hundreds of thousands of years now. Currently, both species face new challenges posed by global climate changes and ensuing changes in their habitat. The disappearance of ice floes negatively affects both species in a major way, since the well-being of both the polar bears and the walruses directly depends on the availability of such ice off the continental shelf. Whenever the ice disappears, the intensity of polar bear hunting of walruses increases, since walruses become the only readily available food source in such times. The physical condition of polar bears during the iceless periods directly depends on the availability of coastal walrus rookeries, the possibility of hunting walruses at rookeries and the availability of walrus carcasses. For both species, global warming means greater susceptibility to diseases. Pollution in the Arctic and the surrounding oceans is bad for their health. The well-being of both species is threatened by the destruction of their habitats and the risk of man-made disasters in the Arctic. Both species continue to be killed by humans. Both species need better protection and greater care to guarantee their future.


Nikita Ovsyanikov, Doctor of Biology, independent expert, polar bear expert, Distinguished Polar Explorer of Russia


(Photo © Nikita Ovsyanikov)