18 Jul

The Science Behind the Moose Rut - Hunting Tips


Story by Vic Van Ballenberghe

Every autumn, people who observe moose ask me, “Is the rut early (or late) this year?”

Perhaps it’s a cold fall and people think that low temperatures will mean an early rut. Or maybe it’s warm and the rut will be delayed. People also wonder about the effects of the moon phases on the timing of the rut. As a moose biologist with over 40 years of field experience, I can answer these questions based on my own observations during the rutting season in addition to the work of other moose biologists across North America. We now know enough about the factors that govern the year-to-year timing of the rut to allow us to reliable predict whether or not in any given year the rut will be early or late. But before I reveal what we know, I’d like to describe the phases of the rut and the annual patterns in moose behavior and activity that characterize the rutting cycle of moose. These patterns apply all over North America whether it’s Quebec, Montana or Alaska.

From the time the rut is over and bulls lose their antlers in winter, through the spring and early summer, moose are totally unconcerned with reproduction and mating and display none of the behaviors that accompany the rut. They are mainly solitary and do not engage in social behavior unless several moose happen to occupy the same patch of habitat. During the late winter, moose are mainly trying to survive. Studies have shown that they are active only about half as much as during summer and they feed for only about half as much time. The twigs they eat are high in fiber and not nearly as digestible as green leaves. Moose chew their cuds and rest a lot in winter to allow their low-quality diet time to digest. And they avoid other moose in order to conserve wasted energy that might be required during social interactions.

During spring and summer, green leaves appear and suddenly there is abundant, nutritious food, and moose waste no time in consuming it. They may feed up to 13 hours per day and consume 45 or more pounds of leaves and twigs. Cows are rearing calves and need lots of high quality food to produce milk as well as to gain weight to replace that lost during winter. Bulls are also gaining weight-as much as 250 pounds by summer’s end- and are growing new antlers and new hair coats. They are focused on eating and resting and avoid interacting much with other moose.

During winter, the hormones that result in rutting behavior are shut down and the testes of bulls shrink greatly in size. But during spring and summer, alterations occur in moose that will ultimately lead to the restoration of hormone levels and changes in the reproductive organs. By late summer, we can observe the results of these changes that signal the approach of the rut.

In my field studies at Denali National Park in interior Alaska, I have observed changes in the social behavior of moose beginner in early August. Moose are still feeding intensively but are starting to interact much more with each other than earlier in the season. Cows that have lost their calves may form small, temporary groups. These may contain only two- to five moose and may only last several hours or a few days. Similarly, bulls may join other bulls in small groups. In radio-tracking individual moose in early August, it’s not uncommon to have a moose associate with 10 to 15 others during a two-week period by forming brief associations with different moose over time. And these associations are accompanied by social behavior, especially among bulls that may display their antlers to each other even though they are still growing.

The first real benchmark behavior of the rut, the one that begins phase one of the rut, is antler velvet shedding. Bulls actively remove antler velvet by thrashing antlers against shrubs or trees. It takes a mature bull just several hours to go from fully velvet covered antlers to largely velvet free antlers. The earliest I have seen a bull out of the velvet in August 18 and the latest date with velvet-covered antlers is September 10. The largest bulls shed first and the yearlings shed last. By September 1, most large bulls are out of the velvet. As soon as the velvet is shed (or rarely, as it is being shed) bulls are willing to display antlers or actually engage in sparing. Sparring is practice fighting with no real winners or losers and no attempts to injure opponents. Fights are much more violent and serious and are sometimes deadly, but they are very rare compared to sparring matches.

Bull moose begin frequently rubbing their antlers against shrubs as soon as the velvet is shed. “Bush thrashing” is the most common behavior displayed by bulls during the rut-a mature bull may bush thrash several times each hour during the entire rutting period. Early in the season, bush thrashing serves to advertise a bull’s presence to other bulls and bulls that hear it at a distance often investigate. Hunters use shed antlers, shoulder blades or even canoe paddles to imitate this sound with the hope of attracting a bull within shooting distance.

During the first week of September a very important rutting cycle benchmark occurs. As the reproductive systems of cows prepare for mating and conception, they undergo changes in early September that mimic estrus. No mating occurs at this time but cows change their behavior toward bulls and allow courtship. For the first time in a year, bulls and cows socialize for the purpose of reproduction. They may form small groups that last only a few days and the interactions among them may be serious enough to provoke fights among bulls that compete for the right to court cows.

During this early phase of the rutting cycle, bulls and cows begin to utter vocal sounds that are seldom heard at other times of the year. Bulls are normally quite silent, but as the rut begins, they advertise their presence with a low-frequency grunting or gulping sound that we call a croak. As they move across the hills and through the forests, bulls utter croaks several times per minute, varying their volume. At times, on still days, I’ve heard croaking bulls half a mile away. Other bulls often approach this sound hoping to evaluate a potential rival. As with bush thrashing, hunters may often imitate croaks in order to attract bulls.

Cows utter long, wailing moans when pestered by bulls attempting unwelcome courtship. Each cow has her own individual voice that varies greatly from individual to individual, as does the volume and intensity of the moans. Loud moans can be heard half a mile away by humans and farther by moose. Some people think cow moose vocalizations are mating calls that solicit courtship by bulls. In fact, they are protest moans signaling that cows are not yet ready to mate, as well as attempts to attract the attention of larger bulls to displace smaller, younger ones that more aggressively court unreceptive cows.

By about September 7 mature bulls stop feeding and begin scent marking. They dig rutting pits and urinate in them, then splash the mud-urine mixture onto their antlers and neck. Mature bulls may produce four or five pits each day early in the rut. Metabolic changes induced by lack of feeding produce strong-smelling chemicals in the urine that interact with the reproductive systems of females and may help synchronize estrus. Cows are strongly attracted to rutting pits and vigorously wallow in them. Younger bulls that lack strong-smelling urine also wallow in pits, but dominant bulls may try to prevent this behavior. Despite all the attention that moose give to scent marking at pits, they very seldom reuse pits after a few hours as the chemicals in urine seem to quickly lose their potency. Hunters may gain information about the location of rutting bulls by locating rutting pits, but are unlikely to intercept bulls returning to pits that are more than one day old.

All of the frenzied activity around rutting pits early in the rut allows moose to interact with each other as they assess potential mates and evaluate possible rivals. The pits provide an arena in which rutting behavior can be performed, behavior that later in the rut contributes to an individual’s reproductive success.

By about September 10 after the first phase of the rut is complete, a 10-day second phase of the rut begins. Cows are not yet ready to mate and no longer solicit courtship; in fact, they discourage it by retreating from courting bulls while uttering protest moans. Bulls are fully ready to mate but must postpose their attempts until receptive cows first appear. Some bulls wander widely searching for cows in remote places, perhaps trying to avoid rival bulls that may have already staked out the best areas with many resident cows. Sparring matches among bulls are frequent at this time as younger bulls try to learn fighting strategies and techniques as well as assess the importance of antler- and body size of rivals. I have occasionally watched large, mature bulls spring with yearlings whose small antlers could only engage one large antler palm at a time.

The third phase of the rut begins about September 20 when the first cows enter estrus and the first mating occurs. In the far north, cows without calves form rutting groups that typically contain three to six females but at times can have as many as 25. Such groups can be quite stable or females can come and go over time. Each group has a dominant bull that maintains his status by intimidating or fighting challengers. Dominant bulls have mating rights to the cows in their groups unless they are displaced by challengers. I have seen large groups that had five different dominant bulls during the course of the mating period.

In places farther south, rutting groups are rare. Bulls seek out receptive cows and stay with them several days before moving on. Mating generally involves one bull tending one cow at a time.

During the third phase of the rut, bulls court cows by slowly approaching them, while softly croaking. They test the urine of the females by smelling it and drawing some of the scent into their mouths so as to expose it to a gland in their upper palate. They tilt their head back and curl back their upper lip. Bulls determine the estrus status of cows in this way. Receptive cows produce chemicals that signal their readiness to mate, chemicals that bulls can detect. If a cow is not receptive, she resists further courtship by moaning and moving away. Persistent bulls, especially younger inexperienced ones, may pursue her. At times, bulls chase cows long distances, trying to herd them and control their movements, but these efforts are mostly futile as cows can normally escape.

Receptive cows allow bulls to approach them, signaling their willingness to mate by letting a bull rest his chin on their rump. Bulls then mount the cow and mate. In my field studies over a 12-year period I found that the general rule is that one cow mates with one bull one time per year, and that the mature dominant bulls do most of the mating. But like all things related to wild animals, there are exceptions. Cows will mate with any bull when they become receptive. If a dominant bull is occupied with fighting or chasing off rivals or mating, a cow might allow a small bull to mate with her. And while 98 percent of the cows I observed mated only once per year, a small number mated two or more times.

By radio-collaring bulls and tracking their movements during the mating period I learned that some bulls remained within a relatively small area while others roamed widely. The dominant bulls that had cow groups moved little. Some of the non-dominant bulls also did not travel far but rather stayed near the cows hoping for a chance to mate. But some bulls spent the mating period searching for cows, at times moving 30 miles or more from where they started. These were often small bulls whose body- or antler size was too small to successfully fight larger rivals. Or, they were old bulls past their prime. As a result of their long distance movements, bulls like these were more likely to encounter hunters that were resident bulls if hunting seasons were open during the rut.

Fights between bulls are most common during the third phase of the rut, as bulls compete for mating right. While people may occasionally see sparring bulls, they rarely see fights. When they do, they remember the drama, power, and violence forever. Mature bulls are agile, fast and unbelievably strong and have great endurance. They display all these traits when fighting.

Fights have a beginning, middle, and end and result in a winner and a loser. Bulls approach each other while displaying their antlers and body size by rocking their heads from side to side. They lower their ears as a threat, paw the ground and continue to display while nose-to-nose with their rival. If neither one backs down, antler clashes ensue as each tries to push the other back. Most fight end quickly but some continue for hours until one bulls turns and runs off while the other pursues him.

Many bulls are injured in fights, some fatally. Eyes, ears and foreheads are damaged and scarred. Serious puncture wounds may become infected and result in a delayed death. Muscles, tendons and ligaments may be torn. Despite this, most bulls survive the rut and live to fight another day during the following year.

The third state of the rut ends at mating ceases by about October 7. Rutting groups disband and individuals go their separate ways. Bulls that did not feed for about three weeks now feed intensively as they try to replenish some of the reserves they lost during the rut.

The post-rut phase begins during this period. Young bulls continue to engage in social behavior by frequently sparring with each other. Sparring in common as late as mid-December. Bulls also continue sign-posting behavior that began during the mating period. Sign post are rubbed trees that bulls create with their antlers by scraping bark off living trees, often small trees with trunks less than three inches in diameter. The tree often dies. In my study area, sign post from previous years are common in those areas that have rutting groups year after year. Whitetail deer bucks create rubs before the rut whereas moose bulls rub after the rut. Biologists think that moose rubs function as signals to other bulls that a rival is present in the area, or indicate to cows that a potential mate is available.

Around October , about three weeks after the main mating period ends, those cows that did not conceive (and those that are two years-old and mating for the first time), come into estrus. In most populations, this includes only a small percentage of cows—most conceive during the third phase of the rut in late September and early October. The mature bulls that participated fully in the early mating period forego late mating and continue to feed intensively. Much of the mating in late October is done by younger bulls. If a cow still fails to conceive at this time, she may continue to cycle at three-week intervals. Captive cows kept away from bulls have been observed to be in estrus as late as March.

Now that we know the sequences and timing of the different phases of the moose rut and the characteristic behaviors that occur during each phase, let’s return to the basic question posed at the beginning: during any given year is the timing of the rut affected by temperature or moose phases? For example, do cold temperatures signal an early rut? Put another way: do external factors drive the sequence of events that characterize the rut, or do moose have within them mechanisms that regulate their behavior such that the timing of the rut is consistent from year-to-year?

During my 37-year study of moose rutting behavior at Denali National Park I have seen the full spectrum of weather extremes that occur from mid-August to early October. Snow fell as early as August 19. One year it snowed three feet in mid-September and temperatures fell to 5 degrees Fahrenheit. Winter immediately set in. Other years has mild weather with shirtsleeve temperatures as late as early October. What effects did these weather extremes have on the timing of the rut?

I observed no significant differences in the year-to-year timing of the various phases of the rut (and no changes in when the benchmark behaviors began and ended) that were related to difference in weather events. All of the typical behaviors leading up to the mating period occurred during the time periods indicated above regardless of temperature and snowfall. The mating period consistently started about September 20 and ended about October 7 during cold years and during warm years. Early in the study I expected to see year-to-year differences because I heard hunters talk about them but no differences were observed in the field.

I did observe important differences in the daily activity of moose during the rut that were related to temperature. By late August, moose are fat and their new hair coasts provide good insulation but deter heat loss. During warm days, especially when sunlight is bright, moose are subject to overheating. As a result, they reduce their daily activity and rest in the shade for long periods. Warm weather can persist well into September, even in the far north, and when it does moose are much less active during daytime than during cool weather. People observe these differences and I believe this in the basis of the early rut/late rut theory. Cold weather in September results in more activity, which seems to indicate an early rut. But we know from field studies conducted over many years that this is not the case.

Similarly, phases of the moon do not affect the timing of the rut, but may affect daytime activity. In my research, I did not try to correlate moon phases with moose activity. I would not be surprised if moose were more active on bright moonlight nights.

If temperature, snowfall and moon phases do not affect the rutting cycle of moose, what primary factor regulates it? Biologists think that moose evolved responses to day-length changes (which do not vary from year-to-year) such that changes in the length of days influence internal factors that control hormonal changes that in turn regulate behavior. Naturally, year-to-year consistency in mating time (and hence time of conception) results in consistent dates of birth for calves the following spring. Timing of births to coincide with the green-up of plants so cows can produce abundant milk is likely the driving evolutionary factor for consistent year-to-year timing of the rut.

If you are a moose hunter, plan to see changes in daytime activity of moose that result from different temperature patterns, but don’t expect to see changes in the timing of the rut. My field studies and those of others in several other places in North America confirm this basic fact.

Vic Van Ballenberge is a wildlife biologist who has studied moose and wolves since 1967. His moose behavior study in Denali National Park is now in its 37th year and is one of the longest running moose studies in the world.