In a previous post (“Polar bears in winter: insights from Behouden Huys, 1596-1597”) I discussed the chronicle of Gerrit De Veer, who documented polar bear activity during the winter that William Barents’ crew spent on northern Novaya Zemlya more than 400 years ago. De Veer noted that the crew did not see or hear polar bears during the time that the sun was below the horizon, a period that did not correlate with the period of most intense storms and cold (De Veer 1609). However, the bears were active (and often causing trouble!) before and after that time.
De Veer’s account suggests the possibility that polar bears spend the darkest part of the winter curled up in a sheltered spot regardless of whether this is the coldest or stormiest period or not.
However, the experience of Barents and his men occurred over a single season and may not be representative of polar bear winter activity in general. As promised, in this post I’ll discuss the evidence collected by polar bear biologists and Inuit hunters relevant to the question of what polar bears do during the cold and dark of an Arctic winter.
Let’s start with evidence collected from Canadian Inuit hunters, who before 1968 often hunted polar bears in dens over the winter (this was legal then, not legal now). A seminal paper on this topic was written by Ian Stirling and Evan Richardson using data collected in the Canadian Arctic by Father Franz Van de Velde, who died in 2002 at the age of 92 (Brandson 2002). Stirling and Richardson say this about the data (Van de Velde et al. 2003:196):
In the field of reporting traditional knowledge, the data recorded by Van de Velde are of unusual quality because so much of the information he received was carefully written down at the time and reviewed with individual hunters to ensure accuracy while their memories were fresh. With hindsight, as this paper was being written, he regretted not seeking more specific information that hunters might have remembered about some of the dens they plotted on his map from memory.
Even so, his records are sufficiently quantitative to provide unique insights into pregnancy rates, litter sizes of cubs prior to leaving the den, and winter denning in the region hunted by Inuit from Pelly Bay. It is no longer legal to hunt or harass polar bears in dens, so the collection of similar data in the future, anywhere in the Arctic, is unlikely. Although we have previously heard hunters comment that they sometimes see bears other than pregnant females in dens in winter, so far as we are aware, only Van de Velde’s data give any quantitative insight into the extent and ecological importance of this phenomenon. [my bold]
So what did Van de Velde do? He interviewed Inuit hunters of Pelly Bay, Nunavut (see Fig. 2 below), in their native language, after they returned from hunting trips. The hunters operated throughout a region that ranged from about 680N latitude to 69 045’N or so (a similar latitude to Murmansk, Russia, which I showed previously has a “winters night” that lasts from early December to early January, see Fig. 2 below). Van de Velde’s interviews pertaining to winter polar bear den hunting were conducted between 1952 and 1955 and again in 1968-69. Both recent trips and those that had taken place many years before were recorded, since “many hunters have accurate recall of the details of every individual bear they have killed” (Van de Velde et al. 2003:192).
The hunters interviewed by Van de Velde often found bears other than pregnant females in winter shelter dens on numerous occasions. Over the period of detailed records, 33 females with fetuses or newborn cubs were recorded in dens but over the same period, the following bears were found in shelters in mid-winter: two lone females who may have lost their litters, six lone females who had not given birth, seven females with older cubs (yearlings or two year olds), and three adult males (16 total shelter dens or 31% of the 51 dens recorded).
Two of the shelters with lone females were found in December, five were found in January and one was found in February (exact dates not given). The temperature at the time the January dens with lone females were found ranged from -280 C to -410 C. Seven females with yearly or two year old cubs were found in shelters: two in November, two in January and three in February. The temperature at the time the two January shelter dens with females and older cubs were found was -570 C and -320 C. One adult male was found in a shelter in November (temperature -250 C) and another two were found in January (temperature -250 C and -410 C). The exact times and temperatures for the other lone males was not recorded.
Similarly, over the entire period (in living memory) for records that had less detail, when the sex was recorded for bears found in winter shelters dens, 32 out of 180 (18%) were male. Stirling and Richardson conclude (Van de Velde et al. 2003:195): “From these data, it is clear that the number of bears other than just pregnant females that use dens regularly during winter to conserve energy during cold weather is much larger than is generally apparent from the literature.”
What can we deduce from these observations, relative to those made by De Veer (1609)? While it appears from the Canadian Inuit data that bears sought refuge because of the cold (as Stirling and Richardson conclude), this is clearly an assumption. There is no indication of how long the bears had been in the shelters before they were found (i.e. when they entered), nor how long they might have stayed, had they not been so rudely awakened and killed.
In addition, because the period of darkness in this region is relatively short (1st December or so through the first week of January), bears might have sought shelter during the darkness (explaining late November, December and early January finds) and emerged to hunt or wander about, only to seek shelter later in the winter during storms or periods of intense cold (explaining late January and February finds). In other words, both reasons for polar bears curling up in a shelter den might explain the data presented by Van de Velde and colleagues – if “November” den occupants were actually found in late November.
OK, but what about studies by modern polar bear biologists – what do these add to the picture? A bit more but only for females because the data all come from radio collars that cannot be used on males (since the necks of males are bigger than their heads, collars just fall off).
One useful study by Francois Messier and colleagues (1994) found that shelters occupied by non-pregnant females in the Canadian Arctic Archipelago (1989-1992) were entered primarily in mid-December (mean date entered, 18 December (± 7 days). The average stay in shelters for the 20 bears studied was 53 days (±9, range 8-150 days), although two females of these (accompanied by young of the year) stayed much longer than average (150 and 102 days, respectively). The average date that shelters were abandoned was “late January.”
These authors suggest that this mid-December to late January period corresponded to the “coldest part of winter” for their area, which ranged from 72-760 30′ N latitude (see Fig.3 below). The “winter’s night” at this latitude is about one and a half to two months long, from early-to-mid November through late January-early February.
The authors say (Messier et al. 1994:429):
“In contrast to dens, some shelters occurred on the sea ice. Because shelters were used most often in January and were occupied for an average of only 53 days (compared to 186 days for maternity dens), their ecological function appears different from that of maternity dens. We hypothesize that the use of shelters is related strictly to energy conservation during the coldest period of the winter when conditions for hunting seals generally are unfavorable.”
So, these authors are clear: the explanation that polar bears use shelters to avoid the coldest part of winter is a hypothesis. That is, it’s a plausible explanation that explains a pattern in their data – but new information might blow it out of the water.
A second relevant study comes from researchers working in the Canadian Arctic from 1991-1997 (see Fig. 4 below), again with radio-collared females. Steven Ferguson and colleagues (2000:1123) found that out of a sample of 28 winter shelter dens that ranged from about 6o to 800 N:
“Female polar bears entered winter shelter dens in late December (median date, 24 December; range, 12 December-14 January; n=28) and exited near the beginning of March (median date, 2 March; range, 7 February-8 March; n=27). Average time spent in shelter dens in winter was 65 days (range 35-86 days, n=21).”
Later (pg 1125), the authors state:
“Use of shelter dens in winter may have occurred because of the overall low seal density and the greater proportion of adult seals, which are more difficult to hunt. Polar bears also may use shelter dens in winter to take refuge from storms, although females with cubs were the least likely to use winter shelters.”
The data collected by Ferguson and colleagues is less useful for addressing the question of whether polar bears use shelter dens during the darkest part of the winter because of the enormous range in latitude of their sample. However, they do confirm there were no winter dens used below 700 N and therefore, it appears that where the sun did not stay below the horizon for at least part of the winter, bears did not use winter shelter dens, regardless of storms or extreme cold.
I suggest, based on the limited modern and historical data, that the impression Gerrit De Veer had more than 400 years ago may be correct – that polar bears become inactive in the dark that affects regions above the Arctic Circle, even when the weather is not especially cold or stormy. Polar bears also spend time in shelter dens later in the winter in order to stay warm and save energy in foul weather.
So, while we certainly need more data on the activity of both sexes during the winter, it appears that De Veer’s observations provide an invaluable clue about what polar bears do over the long dark winter – his journal is more than an amazing story of Arctic survival. Since it is now illegal to disturb polar bears in their winter dens – even to study them – De Veer’s observations on polar bear activity over the Novaya Zemlya winter are all the more valuable.
De Veer, Gerrit. 1609. The Three Voyages of William Barentsz to the Arctic Regions (English trans.). http://archive.org/details/cihm_18652 [downloaded Dec. 19, 2012]
Ferguson, S. H., Taylor, M. K., Rosing-Asvid, A., Born, E.W. and F. Messier 2000. Relationships between denning of polar bears and conditions of sea ice. Journal of Mammalogy 81:1118-1127.
Messier, F., Taylor, M.K. and Ramsay, M.A. 1994. Denning ecology of polar bears in the Canadian Arctic Archipelago. Journal of Mammalogy 75:420-430.
Van de Velde (OMI), F., Stirling, I. and Richardson, E. 2003. Polar bear (Ursus maritimus) denning in the area of the Simpson Peninsula, Nunavut. Arctic 56:191-197. http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/615
Brandson, L.E. 2002. Obitutary: Franz Van de Velde, O.M.I. (1909-2002). Arctic 55:407-408. http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/725