Cannibalism: A Perfectly Natural History(20)

All right now, what about those polar bears?

In 2009, mainstream media outlets began reporting that polar bears were undergoing a serious change in dietary habits. The take-home message was that global warming had reduced the Arctic sea ice, thus resulting in shorter hunting seasons for the bears and fewer seal kills. As a consequence, the stressed-out bears were starving and resorting to cannibalism in order to survive. The problem with most of these stories was that the authors left out a rather important fact—and it was one that researchers have known for decades.

According to wildlife biologist Mitchell Taylor, “Polar bears will readily eat other polar bears when they can do so without excessive risk of injury.” In fact, males of most North American bear species will kill and eat conspecific cubs pretty much whenever they can get their paws on them. Researchers believe that infanticide during the breeding season may provide males with “a reproductive opportunity as well as a nutritional reward” since like the previously described lionesses, female polar bears will come into estrus more quickly if their offspring have been killed. Because of this, cannibalism has been, and continues to be, one of the greatest contributors to bear cub mortality, especially just after leaving the maternity den. The threat from adult males is one of the key reasons that mother bears are so protective of their cubs and also explains why females give males such a wide berth when selecting maternity den sites.

Recently, another probable cause of polar bear cannibalism was added to the mix. Because of incomplete reporting by the media, though, and a tendency to stress sensationalism over detail, the result has been a cannibalism-themed fiasco.

The mess came about soon after the 2006 publication of a paper by Arctic researcher Stephen Amstrup. He and his coworkers were clearly alarmed by three incidents of cannibalism by polar bears in the southern Beaufort Sea, which occurred during a two-and-a-half-month period. Two of the incidents involved the death and partial consumption of adult female bears. In one, the female’s body was found inside a maternity den that collapsed during an attack by a predatory male bear. In the second case, the female polar bear was killed on the sea ice, presumably not long after emerging from its den with a cub. In the third case, a one-year-old male was killed and partially consumed by an adult male. According to Amstrup and his colleagues, these attacks were unique because they had taken place in areas not generally frequented by male polar bears. Each year, once the Arctic sea ice melts and polar bears are forced onto the land, males are usually found near the coast while females and their cubs venture farther inland, and away from the males.

In the cases documented by Amstrup, the researchers concluded that “the underlying causes for our cannibalism observations are not known.” They suggested that the incidents could have been “chance observations of previously unobserved rare events, or even a single rogue bear that adopted a [hunting] strategy including cannibalism.”

What got the media machine cranking, though, was the researchers’ hypothesis that these attacks and subsequent cannibalism might have resulted from male polar bears being “the first population segment to show adverse effects of the large ice retreats of recent years. . . . We hypothesize that nutritional stresses related to the longer ice-free seasons that have occurred in the Beaufort Sea in recent years may have led to the cannibalism incidents we observed in 2004.”

The problem was not in the presentation of Amstrup’s hypothesis, but the fact that many of the media reports that followed neglected to mention that cannibalism in polar bears was already known to be a naturally occurring event, with the first published report surfacing in 1897. By leaving out this vital fact, those working to publicize the effects of global climate change suddenly found themselves on the wrong end of some serious butt-kicking from climate change deniers. These zealots were quick to point out that cannibalism was quite common in polar bears and that the attempt to link polar bear cannibalism to what they referred to as the “Global Warming Hoax” was just another instance in which scientists were flat-out lying to the public. In reality, modern researchers have been reporting on non-climate-change-related infanticide and cannibalism in polar bears for decades, a point Amstrup and his coauthors also discussed their paper, and a point neglected in most of the media coverage.

Ultimately, though, the authors of the sensationalized headlines ignored that information. Instead they cobbled together their stories from non-scientific sources, including a short article by another non-scientist. This one warned of “GRAPHIC PHOTOS” and opened with the line, “Cannibalism is not part of the polar bears’ M.O.” As a result, a valid scientific hypothesis—Global climate change has led to a reduction in Arctic sea ice, and this may be causing increased incidences of cannibalism in polar bears—now takes a back seat to a distorted take on the subject as well as a deceptive but well executed argument by climate change deniers.

This would be my first experience with cannibalism-related sensationalism, but it would definitely not be the last.





6: Dinosaur Cannibals?


Personally, I suspect that a whole pack of full-grown T. rex would have a very hard time finding enough to eat.

— Paleontologist Nicholas Longrich, Discovery News, October 15, 2010

While we’re on the topic of large, meat-eating animals embroiled in cannibalism-related controversies, I thought this would be the perfect time to bring up the topic of cannibalism in dinosaurs—or the lack thereof.