Migratory Behavior Between Land-based Denning Areas and Ice-based Hunting Ranges for Female Polar Bears (Ursus maritimus) in the Hudson Bay Region.
The annual migratory cycle of female polar bears (Ursus maritimus) in the Hudson Bay region of North America, is driven by stimuli that trigger the motivations to relocate from land based dens to ice-based hunting regions and vice versa.
The motivation that triggers the change from hibernating to migrating might be called "hunger" because one of the major internal factors contributing to this is the reserve fat level of the animal. Polar bears during the period when there is no ice covering the waters of the Hudson Bay are sustained by their fat reserves (Dyck & Baydack, 2002). As fat reserves decline, the need to replenish them increases. In addition to this direct link, reserve fat levels also have a determining impact on pregnancy.
Mature female polar bears without accompanying cubs mate in April/May (Macdonald, 2001). Implantation of fertilized ova is delayed until mid-September/mid-October (Macdonald, 2001); the ova stop developing and float free in the uterus (Hedberg, 2002). If there are insufficient fat reserves the embryo will not implant but be reabsorbed instead (Hedberg, 2002).
If the pregnancy occurs, then this has an extremely strong inhibitory effect on the motivation to migrate towards hunting regions. The female polar bear will not leave the den until the cubs have been born and nursed to sufficient size. Polar bear young are born two to three months after implantation weighing only 0.6kg on average (Macdonald, 2001). They are nursed up to a weight of 10 to 12kg by late March/early April, at which point they are capable of traveling with their mother (Macdonald, 2001).
The last of the internal stimuli impacting migration is the level of hibernation inducing trigger (HIT) in the blood of the animal. This has only a minimal effect, as the level rises and lowers depending on circumstances. HIT level increases when conditions do not favor activity, for example when the ice has melted and hunting is not possible or during inclement weather, giving polar bears the ability to hibernate at any time of year (Bruce et al., 1989), thus lowering their maintenance energy expenditure. As such, if other stimuli favored activity, e.g. migrating to hunting regions, the HIT level would lower naturally.
The external stimuli include the ability of newborns to travel, which has been covered above, because the cubs once born are external. The other external stimuli are air temperature and the angle of the sunlight as the polar bear uses these to determine when the ice will start forming on the Hudson Bay. The skin of the polar bear is a very effective temperature-sensing sensory organ (Tributsch et al., 1990).
Polar bear cubs stay with their mothers for the first two and a half years of their lives (Macdonald, 2001), giving them the time to learn what external stimuli signal when it is appropriate to leave the den for the hunting range. This is clearly a learned response rather than instinctive because polar bears living in different areas behave differently. For example, polar bears hunting on land-locked ice have an average hunting range of "only" 50,000 square kilometers while those hunting on drift ice have an average hunting range of 250,000 square kilometers (Mauritzen et al., 2002).
The various internal and external stimuli combine and once a certain threshold has been reached, motivate the polar bear to migrate towards the coastline. If she successfully gave birth this season and is nursing young, this will override all other stimuli and migration will not occur until the young are ready to travel. If not, migratory behavior will be triggered by a combination of lowered fat reserves and favorable environmental indicators. Fat reserves is probably the more significant based on the early arrival of polar bears at the coastline, prior to ice forming, as shown by the popularity of a polar bear viewing ecotourism industry in places such as Churchill in Manitoba, Canada (Dyck & Baydack, 2002).
Towards the end of the hunting season, high levels of body fat combined with external stimuli will trigger a learned response motivating a migration back to the denning region on terra firma. Even in bad weather, the energy transparent pelt of the polar bear and the temperature sensitivity of the skin let the animal determine the sun's position in the sky (Tributsch et al., 1990). They also work together to allow her to detect exposed, ice-free water (Tributsch et al., 1990). Warming air temperatures, the sun higher in the sky and a decrease in ice-covered water combined with a satiated appetite from high levels of fat motivate the polar bear to perform the behavior learned at her mother's side - to head for land to build a den and hibernate.
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