Adaptation (Survival Value)

How does it increase fitness?

Adaptation of the infanticide behavior in Polar bears refers to the benefits of this behavior relating to a Polar bear's fitness.

Polar bears, and males of other ursids, are known to kill cubs that are still dependent on their mothers. By killing the cubs, the male is allowing for a survival advantage to his own cubs, by eliminating potential, future competitors. In addition to this, the female bears go through a lactational anestrus (i.e. not receptive to male advances), when they have dependent cubs (Amstrup et al., 2006). In this sense, infanticide is a possible mechanism by which males increase their relative fitness and that of their offspring. While infanticide does not represent a significant portion of mortality, it is believed to be increasing due to the longer ice-free seasons and increase in population density as a cause of this increase (global warming).

In a nutshell, sea ice loss potentially means (Amstrup 2011):

• Reduced access to food

• Drop in body condition

• Lower cub survival rates

• Increase in drowning

• Increase in cannibalism

• Loss of access to denning areas

• Declining population size (of a species that is already at risk of extinction)

So not only does the population density increase (even with the decreasing population size), increasing a male's need to kill cubs to end female anestrus to facilliate mating, but it is evident that other environmental factors may drive the behavior as well.

Figure 1. Male polar bear with remains of young cub carcass.

The competing hypothesis of the adaptive value of the infanticide behavior is that relating to decreasing food supply. Normally polar bears will feed on ringed and bearded seals, while occasionally feeding on larger animals like walruses. Due to longer ice-free seasons, polar bears are limited by their available resources. This change either leaves the polar bears to swim across great distances to the ice over deep waters of the polar basin, where oceanic productivity is poor, or move to the land where foraging is just as poor. This was the case for three reported incidents of infanticide in 2004, for the Southern Beaufort Sea polar bears (Amstrup et al., 2006). The combination of longer ice-free seasons and the decrease in resources/the inefficiency of going the distance to acquire adequate food, is a possible cause for observed cases of infanticide. While we mentioned in our Mechanism section that this was not readily evident and focussed on the population density dependent causes, it is not impossible that this is a survival linked cause of the infanticide behavior. Cubs may not be as nutritive as seals, but a desperate Polar bear may kill one anyway out of desperation and may then, after some investigation, become disinterested with the carcass. It could also be that there have simply been few observed cases of the adult male consuming the carcass, though they may actually exist. It is a plausible hypothesis for the occurence of the infanticide behavior in Polar bears nonetheless.

Figure 2. Ringed seal- makes up the greatest portion of the polar bear's diet.