Chasing the Rut
The Science Behind the Maddnes
Written by Jared Lloyd, founder of PhotoWILD Magazine
Stepping out into the pre-dawn darkness, my breath hangs in the air momentarily frozen in time. There’s the sting of cold on my face. The burn of my hands. It’s not like Yellowstone in the winter, with minus 40-degree temperatures that reaches deep inside your chest and steals the oxygen from your lungs, but it’s enough to let you know that it’s here, a time of change. I was here for one reason, and one reason only: the rut.
The Canadian Rockies are different. They’re younger, sharper, steeper, more dramatic, more glaciated than their southern counterparts. This region is properly boreal, where aspen and paper birch mix with spruce and firs, where great gray owls and woodland caribou still haunt the shadows of the forest.
For many years before moving to Alaska, I would make an annual pilgrimage to Alberta, Canada. Packing my old Land Rover, I would shove camping gear next to pelican cases filled with camera equipment and pull out onto interstate. North was the new west.
My itinerary was always the same. I like my back roads, especially in Montana where speed limits on two lane roads are faster than four lane interstates in the east. You can drive for hours without passing another vehicle sometimes. The route for me was always the same: 287 north to 89 – that super scenic highway of the west. From there I skirted the eastern edge of Glacier National Park before jumping the border at the Chief Mountain Port of Entry. Then it was on to Calgary, Banff, the Icefields Parkway, Jasper.
This business of heading north might seem strange given that I was living in Montana and surrounded by the lion’s share of charismatic megafauna in North America. I had ready access to seven different species that descended into madness and would fight to the death for the right to breed. But the nature of making a living as a wildlife photographer is a peculiar thing.
This habit of traveling to Jasper National Park began with an email from a magazine editor. It was simple and to the point. “I need more photos of elk.”
A fair request, or demand. No problem for me, of course. I lived in a landscape largely denuded by elk after nearly a century of being overpopulated thanks to the absence of wolves on the scene. So, I replied with a link to a lightbox of curated photos of elk for her to choose from. The response? “No. I have these. I need new ones.”
It was with that email I realized it had been a few years since I had concentrated my efforts on the elk rut. Moose? Yes. Mule deer? Absolutely. But making a living as a wildlife photographer is all about being able to keep producing. Though my archives were full of sellable images of elk, I like to devote myself completely to a particular subject or species at any given time. While I had been investing myself in other animals the last few years, those photos of the elk rut had been published enough times on the magazine market that they had were of little interest outside of the wholesale stock agencies like Getty Images. So, I did what any self-respecting wildlife photographer who needed images of elk would do in a pinch: I drove to Rocky Mountain National Park in Colorado.
“For most species, if temperatures rise above 54° F (12° C), all activity shuts down; everything you hope to see and photograph will be confined to the dark of night when it’s the coolest.”
But upon arriving, it was immediately obvious this was not going to work. Unless you have been hiding under a rock, or living in a highly controlled urban environment where you never go outside, you may have picked up on the fact that things are quite like they used to be. Here it was the last week of September, the absolute statistical peak of the elk rut across North America, and temperatures were in the upper 80s in the mountains of Colorado.
While the timing of the rut occurs at the same time every year, temperature plays a big role in what you see. Bison and pronghorn are little different in this respect because they evolved in a landscape of tremendous temperature swings and have a hypothalamus that helps regulate body and brain temperature differently than other species. But for the rest of the big ungulates that experience the rut each year, the muskox, moose, elk, caribou, white-tail and mule deer, bighorns and the thinhorns, temperatures above the mid-50s shut down behavior. There are a few exceptions to this based on geography, but by and large if the temperatures are rising above 54° F (12° C), everything you hope to see and photograph will be confined to the dark of night when it’s the coolest.
For me, this meant the most I could hope for from Colorado was a bull elk laying in the shade trying not to succumb to heat exhaustion.
So, I drove north from one elk rut photography hotspot to the next. Jackson Hole, Wyoming, Yellowstone, Slippery Anne (Montana), the east side of Glacier National Park, Waterton National Park in Canada, and on to Banff. Still, the rut was being impacted by high temperatures that confined activity to night.
Upon reaching Jasper National Park, however, I immediately recognized things would be different. I had driven through snow flurries along the Icefields Parkway, and descending into the valley I noted the temperature gage on the dash of my vehicle read 34°F. The Fall colors were still at peak and the almost eerie sound of bugles, both battle cry and love song of bull elk, reverberated throughout the forest.
I had driven over 1,500 miles in search of an elk rut to photograph. Sure, I had the directive from an editor and visions of cover shots dancing in my head (when an editor contacts you for a particular shot, it usually means they are looking for an important component of the magazine). But let’s face it, the time and money and effort to travel the spine of the Rocky Mountains across two countries in search of elk photos couldn’t be justified by the possibility of a single cover sale. No, there was only one word that could explain this: obsession.
What is the Rut
North America plays home to a suite of different species who enter the rut each year. Save for the wild horse, for whom the males fight year-round, every other species of hooved mammal experiences some window of time in which all niceties are set aside, and they prepare to do battle with any and every other male of their species. This is the rut, biologically defined as a hormonally driven breeding season in hooved mammals, during which males undergo a surge in testosterone and shift into reproductive overdrive. It is marked by aggressive behavior, combat, increased vocalizations, and a near-total cessation of feeding as males compete for access to females. The rut is not merely a time to mate — it is an evolutionary gauntlet through which only the fittest males emerge with the chance to pass on their genes.
But the rut doesn’t begin with the clash of antlers—it begins in the brain.
Bear with me here for a moment because things get a little nerdy as I explain this part.
As summer wanes and daylight hours decrease, ungulates respond to changes in photoperiod, the ratio of light to dark. This shift is detected by the pineal gland, which increases production of melatonin as darkness lengthens. That hormonal signal travels to the hypothalamus, triggering the release of gonadotropin-releasing hormone (GnRH). GnRH then prompts the pituitary gland to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH)—the same hormonal players found across nearly all vertebrate reproductive cycles.
The end result is a testosterone surge in males, which fuels muscle growth, thickens necks, initiates antler hardening, and rewires the brain for competition. Courtship, combat, and dominance behaviors follow in rapid succession. What looks like instinct in the field is, at its core, the result of a precise neuroendocrine cascade—refined over millions of years of evolution to time breeding with birth, and birth with survival.
Honest Signalling
In wildlife biology, we often say that there are two primary things that drive all life on Earth: food and sex. When it comes to the males of most animals on Earth, it starts to seem like food is just about the energy it requires for the sex. Those mammals of the world that descend into the hormone fueled madness we call the rut are no different than birds with their colorful plumages and intricate songs and dance. All of it comes at a cost. All of it makes you just as likely to wind up in the belly of a predator as it does copulating with a potential mate.
Consider the antlered members of this elite band of fighters as an example.
Antlers, which are different than horns, play only a minor role in a male’s ability to fend off predators and protect themselves. Their very presence comes with significant liabilities. For instance, a moose’s paddles (antlers) can weigh upwards of 75lbs. Imagine what it takes to support this extraordinary amount of extra weight on your head. Now imagine doing it in the winter, when food is scarce, temperatures are cold, and wolves are at your doorstep, and you’re navigating snow that is four feet deep. Antlers get caught in bushes and impede travel through forests. This is why antlers are shed each year. Once they fulfill their evolutionary duty in attracting mates and competing in tournaments, they are dropped, and the cycle begins again.
The fact of the matter is that antlers are what we call secondary sex organs. The bigger and more symmetrical they are, the better the genetics, the more desirable the bull or buck. It takes the same number of calories and requires the same excess of minerals to grow antlers as it does to grow a calf or a fawn. While a cow moose might not run the math to quantify such things, the quality of the antlers speaks volumes. Antlers advertise success. It’s no different from how one peculiar species of primate drives sports cars and give shiny rocks to the females of their species.
In evolutionary biology, traits like antlers fall under the theory of honest signaling. They’re costly to grow, cumbersome to carry, and impossible to fake – making them reliable indicators of a male’s fitness. Only individuals with superior nutrition, minerals, and genetic efficiency can afford to grow massive, symmetrical antlers. This concept is aligned with the handicap principle, which argues that extreme traits evolve not despite their cost but because of it. If you can survive with an evolutionary burden, you must be strong. And in the eyes of a discerning female, that strength is everything.
Natural Selection on Overdrive
The impact that the rut has on those individuals who participate cannot be overstated. Everything that makes a bull moose a bull moose is, in large part, thanks to the rut. This is both natural selection and sexual selection. If you want to breed, if you think you have what it takes to pass on your genetic legacy to posterity, then you must be willing to gamble with your life.
While there are a couple outliers here, most species experience the rut in the autumn, when temperatures are dropping, and winter is on the horizon. During the rut, males all but cease to eat. On average, a bull elk will lose 20% of his body weight in just four short weeks. This means a thousand-pound animal will lose two hundred pounds in that short window of time. That’s just an average. More dominant males, with larger harems of cows to protect, lose more body mass because they engage in more fights than other males.
But not every male engages in full-contact combat. In many ungulate species, smaller or subordinate males adopt alternative reproductive tactics. These so-called “satellite” males linger at the edges of harems or wait for a dominant bull to be distracted before sneaking in for, well, a “quick one.” While they may sire fewer offspring overall, they avoid the injuries, energy depletion, and high mortality risks that come with being top dog.
“On average, males will lose up to 20% or more of their body weight during the rut - and all of this happens right before winter sets in.”
Then, of course, there are the inevitable consequences of all those battles. Males sustain countless wounds during the rut. Sometimes it’s in the form of a broken rib. Sometimes it’s the loss of an eye (I can’t begin to count the number of elk, moose, and whitetail deer I have photographed who were blind in one eye from previous fights). Other times it’s being peeled open along their flanks from the tine of an antler and brute strength of an opposing male as was the case with one of my favorite bull moose in Grand Teton National Park.
And even if a male survives the rut, there is no guarantee they won’t succumb to those wounds over the coming weeks or months. Right when every other animal in the ecosystem is desperately trying to prepare for winter, for the inevitable cold and lack of food and the many unknowns during the frozen months, those species that engage in the rut stop eating entirely, they begin fighting, fretting over mates, lose significant amounts of body fat that would otherwise protect them through the lean times of winter, and begin sustaining wounds that may prove fatal.
If you have ever wondered why there are more female white-tailed deer, elk, moose, etc., this is it, this is why. Fawns and calves are born at equal ratios, 50% males and 50% females. However, by the time those animals reach sexual maturity, the population becomes dramatically skewed. And on average, by sexual maturity, 70% of a population of these species tends to be females and only 30% (or less) are male.
Likewise, if you have ever wondered why so many of the photos you see of wolves on an elk kill in the winter are bulls with massive antlers sticking up from the snow, it’s for this reason. In fact, the wolves of Jasper National Park and Yellowstone National Park specifically target bulls with their winter hunting strategies simply because of generations worth of accumulated knowledge that large bulls are the ones who are the weakest and hobbled by injuries come winter.
And it’s all because of the rut.
The rut is like natural selection in overdrive. In any herd of American bison, for instance, genetic studies show that 50% of the calves each year are sired by one single bull. Many bison will never breed, will never sire offspring, and their genetic line will die with them. Only the biggest, the strongest, the most cunning, the most genetically fit will leave their legacy upon the next generation. We use terms like survival of the fittest when we speak of evolution and the rut puts all of that thinking on display.
“If you’ve ever wondered why you find more females than males in the wild, this is the reason. Though males and females tend to be born at a 50 : 50 ration, by the time they reach sexual maturity populations often skew 70% females to 30% males thanks to the blood sport of the rut.”
The Predator - Prey Connection
For hooved mammals across the globe, death is always at the heel. All ungulates are prey. And the evolution of large predators happened in lockstep with these animals.
Consider the gray wolf for a moment.
To understand how the rut is important to the survival of North American elk, for instance, we must first toss out all the old lies, mythologies, and fables concocted about wolves. When we peel back the layers of culturally imposed perceptions of these animals, and we look only at the science of wolf predation, the elegance of the rut begins to take shape.
First there is the strategy of the hunt. A bull elk who is willing to stand their ground is unlikely to become wolf food. I have watched elk and moose quite easily fight off entire packs. I have watched lone cow bison kick and stomp and jump and buck and spin only to send 19 wolves trotting away empty handed. But if they run, it’s a completely different story.
Wolf packs want their prey up and moving. Their strategy is to sew chaos, to intimidate and pressure a herd to run. It’s when an elk is running that their biography is on display for a pack of wolves. Who’s fast and who’s not? Who’s weak, old, injured, infirm, or young? A herd on the move allows a pack to scrutinize every individual and make calculated risk-reward decisions.
An eighty-pound wolf, the average size for Yellowstone National Park, is no match for a thousand-pound bull elk in his prime. A well-placed kick results in broken limbs or ribs for a wolf. These, in turn, are a death sentence to a predator in the wild. And it’s for this reason that the hunting strategy for a pack of wolves is to push a herd of elk to run, to flee, because it’s then that the low-hanging fruit can be singled out.
Studies of wolf kills in Yellowstone National Park during the winter, the season of the wolf, reveal trends. For years, Doug Smith, the head researcher who oversaw the reintroduction of wolves in the park, would cut open the bones of elk killed by wolves to assess the physical fitness of the individual. While something of a gruesome task, this research quickly revealed that most wolf killed elk had no marrow left inside their bones. Bone marrow contains large reserves of fat. The absence of marrow in the bones of an elk meant that the individual was starving to death; the elk had already consumed all the reserves of fat on their body and were literally digesting the very marrow of their bones to stay alive.
Look at it this way: while both winter and wolves work in tandem to cull herds of elk, it’s the rut, that self-imposed selective pressure that ensures only the best, the strongest, the genetically fittest are breeding to ensure that elk as a species are always one step ahead.
Ladies’ Choice
For all the hype about big males locked in dubious battle during the rut, however, at the end of the day it’s still the females who choose their mates. With all this focus on brute strength and the blood sport of males, it’s still the females who ultimately decide. Across most ungulates, females come into estrus within a narrow window – often just a few days, which forces males to compete in a tightly compressed timeframe. This estrus synchrony drives the intensity of the rut. Females assess males by antler symmetry, vocalization, body condition, and behavior. The strongest bull might win the fights, but he still must convince each female to mate with him. Sexual selection doesn’t end with victory: it ends with her choice.
Females of all ungulates undergo their own trial by fire through the process of bearing and raising offspring. Their bodies must be in peak physical condition to carry a calf or a fawn through winter; they must have the ability to find and store enough fat for themselves but also enough to produce milk with a caloric content that makes our own species’ milk look like water. Then they must protect that calf in a landscape filled with predators for whom young animals are at the very top of the menu. This is no easy task. And the simple act of bringing the next generation into this world is enough to ensure only the genetically fittest will succeed.
The rut crams all the physical demands that a cow or a doe must go through for many months into just a few short weeks for males. Those who come out on top will be selected by females to mate with. Those who survive the months of deprivation and predation afterward will have the opportunity to try to do it all over again. Whether it’s a harem of cow elk or a pronghorn doe who orchestrates tests of speed between males that are equally matched in their fighting prowess, at the end of the day it’s the females who choose those who will have the right to sire offspring.
Timing is Everything
Despite popular misconceptions, the rut doesn’t change timing. Just because the availability of food may change year to year, or the fact that one year is dryer or wetter, or colder or warmer, the timing stays the same. While the timing of the rut for an individual species is predicated upon their length of gestation, the trigger that creates that hormonal switch in males is the ratio of daylight to dark. For those ungulates who experience the rut, bison have the longest gestation period in North America while bighorn sheep have the shortest. Therefore, it’s no coincidence that bison begin rutting in late summer while bighorns start banging heads on the doorstep of winter.
Whether bison or bighorn, everything is timed perfectly so that in the spring all these different species in the ecosystem give birth at roughly the same time. While there is a measure of getting the timing right so offspring can prepare for the inevitable winter ahead in places like Wyoming and Montana, this behavior is seen across the planet where multiple species of ungulates live side by side even in the absence of what we might consider “real” winter. Instead, this synchronous calving season, so the leading hypothesis goes, is all about flooding the landscape with babies all at once to dilute the impact of predation. The carrying capacity of an ecosystem for large predators is finite and X number of predators can only eat X number of babies in the two to four weeks it takes for those babies to be strong enough and fast enough to keep pace with mom and outrun wolves and bears and such.
This is six months of action for the ambitious wildlife photographer, six months of non-stop opportunities to photograph the charismatic megafauna of the continent in their prime, when males are the biggest, the most photogenic, and so completely drunk off their testosterone so as to drop their guard around humans and other predators.
For all wildlife photographers around the world, the breeding season presents opportunities not found at any other time. Whether you photograph birds in Australia or fireflies in Asia, elephants in Amboseli or moose in Denali, that time of year when the males of the species are giving it everything they have to look their best, stand out from the crowed, and do what it takes to win the hearts and minds of the ladies is when memory cards are easily filled. And for the big ungulates of North America, much like Eurasia and Africa, the rut is the universe’s gift to wildlife photographers.
To Be Continued. . .
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