A feeding strategy used by many animals from different groups, necrophagy — the act of consuming the carrion of dead animals — places top predators, even unintentionally, as suppliers of a large amount of energy.
In the Arctic, where the polar bear (Sea bear) is the main predator of marine mammals on the ice, this means that these white giants are the largest food providers for a vast network of scavenging speciessuch as arctic foxes, crows and several species of gulls.
By quantifying and systematizing this hitherto little-understood function of Arctic ecology and carrion itself, it is estimated that polar bears leave around 7.6 million kilograms of seal carcasses on sea ice annually, the equivalent of 39.3 million megajoules of energy.
Led by experts from the Universities of Manitoba and Alberta, and Environment and Climate Change Canada, the research also included the participation of the San Diego Zoo Wildlife Alliance, in the USA, and identified at least 11 species of vertebrates that directly benefit from this carrion.
In a statement, the authors highlight the important role of polar bears as “a crucial link between marine and terrestrial ecosystems.” This means that they “capture” energy from the marine ecosystem (seals) and “transfer” the rest of this energy, in the form of carrion, to the terrestrial environment, over the ice.
Carrion Mathematics Reveals the Real Value of Leftovers
Contrary to what it may seem, it is an evolutionary strategy, as seal blubber is five times more calorie-dense than muscle, in addition to reducing the bears’ drinking water needs.
As available freshwater in the Arctic is basically in the form of snow and ice, this resource becomes critical for animals, as body heat must be expended to melt snow or ice and convert it into liquid water. Furthermore, the catabolism (breakdown) of fat generates metabolic water, useful for hydration in polar bears.
To calculate the biomass of carrion left by the bears, the research developed a sophisticated mathematical model: based on predation rates observed in the field and the body composition of the seals, the scientists estimated that Each polar bear kills around 1,001 kg of marine mammal biomass per year.
This sum of the mass of all captured prey is not what these predators eat, but the total biomass removed from the marine ecosystem, of which approximately 50% is left as carrion available in the ice, to sustain scavengers.
Working from an estimate of a global population of 26,000 polar bears in the Arctic, this totals approximately 7.6 million kilograms of carrion biomass available annually. When bone mass is subtracted, there are still about 4.6 million kilograms of seal muscle available.
A natural refrigerator at the service of ecological balance
The unique conditions of the Arctic make this ecological system highly efficient, which, adapted to extreme cold, becomes a unique natural energy transfer mechanism. In practice, extremely low temperatures act as a natural refrigerator.
This makes seal carcasses abandoned by polar bears a long-lasting food resource for carrion-eating species. This would not be possible in hot climates, where bacteria and insects would decompose one of these carcasses in less than 24 hours.
Furthermore, the fact that polar bears feed heavily in the spring (before hibernating) — exactly when ringed seal pups are born and are vulnerable — makes a large number of carcasses available in a short period of time, allowing other animals to obtain energy without expending calories.
As hunting in the Arctic is an activity that consumes a lot of calories, in addition to being risky, when the bear leaves a carcass, another animal gets a very high quality meal (fat and protein) without having to hunt it. This is essential for survival in an environment of harsh winters and scarcity of prey.
On the other hand, researchers warn, it is reducing polar bear populations. Therefore, the research “highlights the urgency of conservation efforts to protect them, not only for themselves, but also for the species that depend on them”.
