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would do so is unclear. It seems unlikely it would be a purely predatory action: a newly born cub would not provide much energy, skewing the risk/ reward ratio heavily toward the risk, particularly given the certainty of attack from an angry, protective mother. Perhaps, it has been speculated, there is a cold calculus involved: killing the cubs of another male decreases the competition and increases the survival chances of a male's own progeny. But that supposes that the male, having walked away from the female upon insemination, is able to distinguish between his cubs and those of others. Maybe, then, it is a means of improving his mating opportunities: as long as a female has dependent cubs, she does not enter estrus and become sexually available; without them, she is soon sexually receptive. But while such an explanation may make sense for brown or black bears, which have relatively small territories and could be expected to keep track of a female over the few days after she loses her cubs and before she enters estrus, it does not so readily for polar bears, whose ranges are considerably larger. Indeed, Steven Amstrup notes that on the two instances of infanticide he has observed in the Beaufort Sea region, male and female were already dozens of miles apart the following day and traveling in opposite directions.
    Perhaps, speculates Amstrup, cannibalism of cubs doesn't actually serve any particular purpose at all. Perhaps it is a behavioral remnant from when polar bears were terrestrial, an atavistic anomaly that polar bears have not excised from their wiring. Ian Stirling, for one, thinks it happens only rarely and opportunistically. Yes, females do show a tendency to keep their cubs away from males, but they would be maternally remiss not to shelter them from hungry thousand-pound carnivores. And while it is not uncommon for males to move toward females with cubs, it is less common for them to be able to catch up to them, the cubs jostled along by their protective mother and all of them capable of maintaining a faster pace for longer than the heavyset males, which are large, well insulated, and quick to overheat when they run. When males do succeed in grabbing a cub, it is normally after surprising a family that is sleeping; if the mother is alert and able to respond, she will fight to protect them, even though the male is far larger. On occasion, it seems, she will do so even to the death: in two separate instances in 1984, researchers came across a male feasting on the carcass of a female, as her cubs cowered in the distance. In both cases, the scientists deduced that the male had killed the female; whether or not he went on to kill the cubs, they could not have survived for long.
    Some cubs do not even make it as far as the sea ice, and ironically their mothers may be responsible for more instances of infanticide than any males. The physical strains on a pregnant female are immense, sometimes too immense. If, while preparing to make a den or even after settling into it, a female is not strong enough to see a pregnancy through to its conclusion, her body simply reabsorbs or aborts the fetuses, and she emerges from her den ready to resume sexual activity with the arrival of spring.
    For some females, the physical limits are reached only when the cubs are born. By then it has been a long winter—for bears in Hudson Bay, eight months have already passed since their last meal—and the extra burden of rearing the newborns is a step too far. A 1985 paper in the journal
Arctic
cited seven reliably documented instances in which a mother was so malnourished that she killed and ate one or both of her cubs in order to survive. As Ian Stirling has written, "If there are seven documented instances of an event as difficult to observe as this, it may occur reasonably frequently."
    The urge to survive, the overpowering drive to perpetuate her genes, that perhaps appears so cold when the polar bear mother devours her own young