Polar bears are powerful predators whose survival is intricately linked to sea ice. They rely on the frozen ocean for hunting, which provides essential sustenance. This dependency raises a fundamental question: what unique characteristic of water enables the formation and flotation of this ice, thereby supporting the polar bear’s existence? Water exhibits behavior that deviates from most other substances, with profound implications for life in cold regions.
Water’s Unusual Density Behavior
Water possesses a distinctive property concerning its density as temperature changes. Unlike most liquids, which become progressively denser as they cool, water reaches its maximum density at approximately 4 degrees Celsius (39.2 degrees Fahrenheit). As water cools further from this point down to 0 degrees Celsius and then freezes into ice, it expands in volume and consequently becomes less dense. This phenomenon is known as the anomalous expansion of water.
The molecular basis for this unusual behavior lies in water’s unique hydrogen bonding structure. In liquid water, molecules are constantly forming and breaking hydrogen bonds, allowing for relatively close packing. However, as water freezes, these hydrogen bonds arrange the water molecules into a more open, crystalline lattice structure. This hexagonal arrangement creates more space between the molecules than in the liquid state, leading to the observed expansion and decrease in density.
How Ice Floats
Building upon water’s unusual density behavior, the principle of buoyancy explains why ice floats. Buoyancy dictates that an object submerged in a fluid experiences an upward force equal to the weight of the fluid it displaces. Since ice is less dense than the liquid water it displaces, the buoyant force acting on the ice is sufficient to support its weight, causing it to float.
This means that to displace a weight of water equal to its own weight, ice needs to submerge only a portion of its total volume. For typical sea ice, approximately 90% of an ice floe remains submerged below the waterline, with only about 10% visible above the surface. This characteristic allows large expanses of ice to remain afloat, creating a dynamic frozen landscape.
Floating Ice as a Hunting Platform
Floating ice is directly connected to the polar bear’s survival strategy. Polar bears utilize these stable, mobile ice platforms to access their primary prey: seals. Without sea ice, polar bears would face significant challenges obtaining the high-fat diet necessary to survive Arctic conditions.
Their main hunting methods involve ambushing seals at breathing holes in the ice or stalking them as they rest on the ice surface. Polar bears often wait patiently near these holes for seals to surface for air, or they may use their keen sense of smell to locate seal dens hidden under snow and ice, breaking through to catch pups. Floating ice provides a crucial vantage point and solid surface from which these predators can effectively pursue and capture prey across vast stretches of the Arctic Ocean.