Why do Male Bears Kill Cubs?

Why Do Male Bears Kill Cubs?

Some things are tossed around as fact so often and for so long that they become generally accepted as truth. Bear hunters sit around campfires and discuss what they have seen or heard, and opinions are shared. Many times, the narrative does not meet with reality. The long-accepted answer to why male bears kill cubs is to put the cubs mother into estrus so the male can breed her. I began to question this a few years ago and after really diving into the biology and behavior of black bears and their breeding cycle, I have become convinced that this is nothing more than a myth.  

A little background in the black bear’s reproductive process needs to be understood to begin with. Across most of North America, black bears breed in June and July, and then the sow carries the fertilized eggs for a few months before she actually becomes pregnant. In the fall, the eggs implant in the uterine wall and start to grow as she enters the den for the winter. The cubs are born in mid-winter and nurse as they grow. When they emerge from the den in the spring, they are about the size of a house cat and totally reliant on her for protection and training. Plus they are still nursing, and even though they immediately add natural foods to their diet they will nurse for several months, often until they go back into the den with her in the fall.  

They will den with the sow the first winter, and then as spring comes around, she becomes ready to breed again. She then separates from the cubs as she comes into heat and the males start coming around. This is a complicated process that could be discussed at some length, but for brevity, she will become pregnant and then den alone that year. 

This time-honored cycle has served the bear well down through the eons. Those cubs are very vulnerable during their first few months of life. Adult bears, wolves, eagles, and other predators have been known to target these cubs as food. But if a cub is killed by a male bear, will the sow completely break this cycle and come back into heat the first year rather than the second? As far as I know there is no scientific evidence that she will, and that’s just one of many reasons why I believe this is a myth. Here are a few more.  

Sows rarely have one cub. Where habitat and food is good, they have two or three at a time and even four or more are not uncommon. In Minnesota, our bear biologist says their research shows that black bears average 2.7 cubs, so more have three cubs than have two. She’s nursing those cubs and will not come into heat as long as she is lactating, so killing a cub is not going to bring her back into heat. The male bear would have to kill all the cubs, not just one. I’ve never known that to happen, and I have not talked to anyone who has seen all the cubs killed at once.  

If the sow only had one cub and it was killed, or by some stroke of luck the male bear was able to kill all of her cubs, the sow couldn’t immediately come back into heat. It would take weeks at best as she stopped lactating and her body processes changed. Where is that boar going to be weeks from that point, and what are the odds that he would be the one to breed her if she did come back into heat at some point?  

Here’s another thing that doesn’t add up: from a survival of the fittest standpoint, why would a species trade living, viable members of the species for potential members? It doesn’t make sense to remove healthy members that can advance the species in exchange for the possibility of future members of the species. When presented with this problem, I have had some people respond that the male is trying to advance his own line by removing the DNA of other bears and spreading his own genetics. But my problem with that theory is how would he know that he is not killing his own cubs? 

I have observed a male bear killing a cub in person just one time in all my thousands of hours in the woods observing bears and sitting at bait sites watching interactions between bears. I have talked to a few others who have observed this behavior in person, and I have watched many videos of it taking place. All of these instances have one thing in common: the bear doing the killing ate the cub. There may be cases where the male did not eat the cub, but I do not personally know of any, and for certain, this would be a rare exception.  

This is the reason I believe male bears kill cubs. Black bears are not efficient predators, but they relish meat. They get some carrion, and they occasionally kill small mammals or happen upon an injured animal they can catch and eat. They catch a few deer fawns or moose and elk calves in the spring, but they don’t get meat in their diet all that much. Cubs are made out of meat. When they happen upon an opportunity to turn that cub into a meal, they take advantage of it.  

To support this theory, I will also add that there are some videos out there of sows killing cubs and eating them as well. I just saw one on YouTube the other day. Two sows, each with cubs, encountered each other and one sow killed a cub of the other sow and dragged it off, presumably to make a meal of it, although the actual eating of the cub did not take place on camera. But why else would she drag it off while being badgered by the dead cub’s mother if it wasn’t to eat it? I can’t think of any reason.  

So that’s my theory and you are free to disagree with me, of course. But I have become convinced that the standard belief that a boar kills a cub in order to bring the sow back into heat is a myth. Your mileage may vary.