In a life-or-death struggle, does it pay off to give up your weapons?
Yes, if you’re the biggest elk in the woods. In what seems a suicidal move, some Yellowstone National Park elk make themselves more vulnerable to winter wolf attacks so they have a better chance of breeding cows the following fall.
University of Montana Ph.D candidate Matt Metz was digging through a decade of wolf research when he found something that didn’t add up. Wolves typically target the youngest, oldest or sickest animals in a herd. But in late winter, they start preying on bull elk right after they drop their antlers. The weird thing Metz noticed was that the biggest, most dominant bulls dropped their racks — their best defense against wolves — up to two weeks sooner than the other males. Yet they somehow survived to become kings of the cow harems during the following rut.
In a study published this week in the journal “Nature: Ecology and Evolution,” Metz and co-authors Mark Hebblewhite, Douglas Emlen, Daniel Stahler, Daniel MacNulty and Douglas Smith figured out an answer. Those early-antler-droppers got several more days to grow a new rack, and the bigger racks translated into bigger breeding success with cow elk.
“These males that shed their antlers first are more vulnerable to being killed by wolves despite being in better nutritional condition,” Metz said. “The individuals who are in the best condition are the first to drop their antlers, to get a leg up on growing larger antlers for the next season and therefore gain the greatest reproductive success. Wolves are coursing predators, mostly targeting individuals who are in the poorest nutritional condition, or something else, like their age, that makes them vulnerable. Here we identified a new vulnerability.”
Compared to other antlered ungulates, elk have a tough time defending themselves in winter. Whitetail and mule deer can usually outrun wolves in snow. Moose can out-fight them, even without antlers. Male elk depend on their antlers for defense, and as a result keep them for months after the deer and moose have shed theirs.
“Antlers are a very expensive, big, bulky nuisance,” Hebblewhite said. “It’s a Catch-22. On one hand, why keep them on your head any longer than you have to? But as soon as big bulls drop their antlers, they’re really picked on by wolves. So why drop early if you face risk of being killed by wolves? It must be worth the risk because benefits of breeding are so high. None of this matters unless you have tons of babies.”
So elk split the difference. They keep their racks until March or April, while deer and moose usually drop theirs in January or February. But then the most dominant ones take an evolutionary gamble, and it appears to pay off.
“Wolves aren’t selecting for big or small antlers, just for antlers that drop too early,” Emlen said. “That seems to favor animals who delay dropping their antlers. But the only currency that matters is reproduction. With elk, 90 percent of males fail to pass on their genes. They’re a genetic disaster. Only a very small fraction get to succeed, and they succeed spectacularly well. So the whole game is managing to be one of those super-successful males, and the obvious choice is not to protect yourself.”
Bowhunters who traveled into the woods on Saturday with the start of Montana’s archery season saw those gambles pay off. The bulls with the biggest racks win the most battles, breed the most cows and sire the most offspring. Hebblewhite said the study depended on that kind of patient observation in the field.
“This is the core of science, making observations in nature,” Hebblewhite said. “This was one of our very best students, doing his own observation, and collecting data for years to test it. Sometimes we don’t know what we can answer until we look.”
The study also indicates that careful timing of human hunting seasons can affect antler size. Archers and rifle hunters usually go for the animal with the biggest antlers or horns, and other research has shown that can depress the rack size of future generations. Timing the heavier rifle-hunting pressure to late fall after breeding season helps keep those successful breeders in the mix.
“Uncoupling the hunting season from the rut really helps do that,” Emlen said. “The winners have already fertilized the cows.”