Social interactions of polar bears: Causes and consequences

Social interactions of polar bears: Causes and consequences

19 October 2017

A large congregation of polar bears was discovered in mid-September at the bottom of Thomas Mountain on the western coast of Wrangel Island. About 200 predators gathered near a dead whale’s carcass. How the endangered animals share their food and build relationships, and what exactly polar bear congregations are is all disclosed in an article by co-authors Nikita Ovsyanikov, a doctor of biological sciences, polar bear specialist and member of the polar bear specialist group of the International Union for Conservation of Nature and Natural Resources, and Steffen Graupner, a German scientist, journalist and photographer.


The Professor Shokalsky ship crew spotted a large congregation of polar bears at the bottom of Thomas Mountain on the western coast of Wrangel Island on 19 September 2017.


The information about the congregation was immediately posted online and attracted attention because of the unusually large number of polar bears gathered on such a small piece of land. About 200 predators gathered near the carcass of a Greenland whale that had washed ashore. 


Such massive congregations of polar bears near a large food source are neither unique nor common.


Since 1990, the first year when sea ice totally disappeared from the continental shelf of the Eurasian Arctic, polar bear congregations on Wrangel Island took place nearly every year. Occasionally, the animals could be seen gathering near sources of food, if any.


Long-time monitoring of polar bears in various environments has given us an insight into why and how polar bear congregations are formed, and how polar bears of different genders, age and weight regulate their relationships within such crowded gatherings.


Steffen Graupner watched the assembly of the animals closely this year and made a detailed photographic account of this particular polar bear congregation. High-resolution photographs of the event and monitoring from boats allow us to estimate demographic aspects and distribution specifics of the bears while they stand around the whale’s carcass. The photos very clearly illustrate the situation and many aspects of the animals’ social behaviour in this untypical situation.


The bowhead whale’s carcass is a real treat for polar bears that are going through the glacier melting season on the coast of Wrangel Island. This amount of food will not only last the bears until the ice comes back and they can resume hunting without any losses, but also will support their physical health ahead of the Arctic winter. Such a dense concentration of polar bears on a small area is an exclusive gift to those who study polar bears and assess their population’s demographic composition in this geographic region.


This congregation of polar bears near Thomas Mountain is unique in terms of their number and concentration. This is the second time in polar bear research history that a congregation of about 200 or more bears has gathered in a relatively small area. The first incident took place also on Wrangel Island near Cape Blossom in 1990 next to huge (several tens of thousands) congregations of walruses. However, at the time, the bears were spread across a much larger area along the coastline (10 or more kilometres), as walruses near Cape Blossom would emerge from many parts of the coast, often simultaneously.


The local concentration of polar bears in 1990 on Cape Blossom reached 40 animals per hectare but most of the bears were scattered across an area where the walruses were coming out of the water near the cape. Scientists at one observation post counted up to 144 bears, but many more dozens of bears were either spread along the coast, were in the neighbouring tundra or were simply out of sight.


At the same time as the Cape Blossom polar bear assembly, significant congregations gathered in other areas of the coastline, including on the northern side where walruses were also getting together.


This year, all the polar bears have assembled at one spot – around the huge carcass of the deceased bowhead whale. Why and how did this congregation take place? What kind of bears are the ones to ultimately survive the thaw season on Wrangel Island?


First of all, what is a polar bear congregation? It is not some amorphous gathering of animals in one place. A congregation can be defined as a relatively stable temporary community of polar bears where animals meet regularly, recognise each other, interact and regulate their relationships and social distance, including the establishment of personal relations. In other words, a congregation is a behaviorally structured community where individual relations between species are relatively stable (a friend remains a friend, and a dangerous peer stays a dangerous peer).


Evidently, if polar bears corresponded to their traditional description, even in scientific literature – a lone predator living on his own even on floating ice – there would not have been so many polar bears of both genders and all age groups uniting into such a massive community, living and feeding off the whale carcass, side by side.


Many years of studies have revealed that out of the Ursidae bear family, polar bears are the most socially advanced. Even their closest relative – the brown bear – does not form such congregations and is unable to be so socially tolerant of its peers!


Polar bears do not hunt in packs like, for example, wolves, hyena dogs, lions and many other large predators. But they do have a rule of sharing their prey (a sea mammal’s body, be it a hunting trophy or something found on the shore). Here they demonstrate their special social skills – making a request to approach the carcass, already being eaten by the others – which can be described as “a polite approach.”


Collectively consuming prey is an important characteristic of polar bears that is useful for the population (and if so, then for all the species in the population without exception, natural selection works at an individual level but supports those qualities that are useful for the entire species of the habitat).


Polar bear congregations are normally formed near large sources of food. The whale’s carcass is the most generous and nutritious food supply for polar bears. As soon as it is discovered, all the bears in the area that can sense the smell (and polar bears’ sense of smell is the strongest among mammals!), rush towards the food source. It is hardly surprising that so many polar bears have gathered near the whale’s carcass at Thomas Mountain. The wind bearing from the west, quite common here in autumn, carries the smell all across the 8,000 sq km island. During the melting of sea ice, Wrangel Island becomes a veritable Noah’s Ark for the entire Chukotka and Alaska polar bear population in the Bering area.


In the history of systematic polar bear studies, whale bodies have rarely washed ashore on Wrangel Island, and these were usually entire bodies or parts of grey whales. A bowhead whale was discovered here for the first time.


In 1985 and 1993, mature grey whales were washed ashore on the northern coast of the island. Both times the bodies attracted hundreds of polar bears but the whales were found in late autumn, deeply frozen in the ice and after the majority of polar bears had left the coastline and headed out to the ice. In the past few years body parts of grey whales were discovered more often but these were mostly small and very decayed remains that did not attract any substantial congregations – only 20-40 polar bears at a time.


The polar bears that had gathered around the bowhead whale, when it was discovered by people, included only a few fat bears. Most of them were in normal physical shape, neither skinny nor overweight. Under favourable circumstances, these bears will feed on the whale’s carcass to become even healthier – polar bears gain weight quickly with a good diet.


Family groups in the congregation included two females with four bear cubs born this year. Females can have up to three cubs in one litter. Four bear cubs in a litter have always been a rare occurrence, and not only on this island. But if they do occur it is usually the case that one of the cubs has been adopted.


It is common practice among female polar bears to adopt cubs. For instance, in many years of monitoring the social behaviour of polar bears on Wrangel Island, scientists have identified family groups with two bear cubs with an age difference of only one year. This is physiologically impossible as one female only becomes fertile again after its previous litter is weaned. Also, adopted cubs in a litter behave differently: they are normally less confident and more nervous in response to their mother’s strictness. Scientists recorded one bear cub behaving this way in a family group around the bowhead whale.


The bowhead whale is one of the largest whale species. An adult whale weighs between 70 and 140 tonnes. Judging by the top part of the whale visible above water near Thomas Mountain, this is was a mature whale, but not the largest. Anyway, if it is not washed out to sea by possible storms, it will be good food supply for all the polar bears gathered on the island this summer. What is this congregation like?


In a time of global warming and the subsequent complete melting of the ice on the continental shelf, Wrangel Island attracts polar bears that are the core (the main reproductive) part of the entire Chukotka and Alaska polar bear population. The reason bears come to the island is due to the dynamics of its ice surface in the summer and autumn period when the ice melts.


The key and most prolific feeding site for polar bears is drifting ice on the continental shelf. Polar bears of the Chukotka and Alaska population that inhabit the entire water area of the Chukchi Sea, the eastern part of the East Siberian Sea and the western part of the Beaufort Sea, are concentrated on the ice of the so-called Wrangel ice sheet – vast ice fields in the water area around the entire island.  Even the polar bears that our American colleagues supplied with tracking microchips in the southeast of the Chukchi Sea near Alaska finish their summer season every spring on Wrangel Island or on the ice to the north of it. When this ice finally melts, most of the population has no choice but to move to the island.


In 2017, the dynamics of the melting ice (analysed using online free-access ice maps of the Arctic and Antarctic Research Institute of the Federal Service for Hydrometeorology and Environmental Monitoring) were interesting. The edge of the drifting ice was quickly receding northwards beyond the continental shelf, with the Wrangel ice sheet (the main habitat of the whole bear population in the second half of summer) nearly disappearing by the end of the first ten days in August. This not only accelerated the relocation of the polar bears to the island’s shore but also left enough time for storms to bring the dead whale largesse to the polar bears.


The large congregation of polar bears around the whale’s carcass is due to two factors: the size of the single food source and the coastline terrain around it. The coastline here is quite steep, precipitous with a compact, nearly vertical grassy brook valley leading straight to the whale’s carcass. The bears in the valley that could not get a place closer to the whale have a good view of the site and can monitor the situation, while waiting for their chance to join the feast. Moving further away would make no sense for them as they are attracted to the food and might risk missing their opportunity if they leave. Hence the dense concentration of animals.


About 200 or more polar bears on the hillside near the whale’s carcass – is that a lot? It depends on how you look at it. It really is quite a lot in terms of local concentration. It is truly impressive! Seeing such a giant congregation is a stroke of luck. In terms of a concentrated core part of the entire population – this is far from many! The critically important question is how many more polar bears could be in other parts of the island at the same time.


According to the most optimistic estimates based on the number and demographic composition of the core group staying on the island, the current Chukotka and Alaska population of polar bears stands at around 1,200-1,500 bears.


These figures are true even if the core group on the island comprises no more than a third of the whole regional polar bear population, and the other two thirds remain on the bulk of the drifting ice and move to the central Arctic area as the Wrangel ice keeps receding.


Estimates also take it into consideration that polar bears migrate in winter and spring to the Chukchi Sea area, which is rich in food. All the data indicate that most polar bears of this population are concentrated on the Wrangel Island ice in the second half of summer where they stay on the continental shelf until the ice melt has finished. About a half or most of the population settles on the island during this period. Judging by the current ice situation, no ice is left in the Chukotka-Alaska areal beginning from the latter half of August. Accordingly, the polar bears that gathered on the island are the complete summer residential geographic population within the areal and are the most conservative core part of the winter population in its attachment to the areal.


Hence, the impressively large congregation of polar bears around the bowhead whale’s carcass is not so numerous compared to what it could have been in more favourable ice conditions for polar bears in this part of the Arctic. 


(Photo © Steffen Graupner)