Deer Biology…

Prey species, from mice to moose, constantly run the risk of being killed by a predator. Understandably, few failings are as unforgiving — or as abruptly terminal — as a deer’s failure to avoid a predator. Obviously, the best way to avoid predation is to avoid predators. As a result, prey species such as white-tailed deer have developed specific behaviors that help them recognize, avoid and defend against predators. Such anti-predator systems are fundamental to survival.

In order to detect predators, such as mountain lions, wolves, bears, coyotes, bobcats, and even human hunters, healthy whitetails typically remain “vigilant.” Sick, malnourished, or injured animals are less likely to do so, hence are more likely to fall victim to predation.

Deer living in open habitat may rely heavily upon vision for predator detection and flee to avoid them. In dense cover, auditory signals, such as predator sounds or alarm cries from other deer, may signal danger. For example, researcher Aaron Moen demonstrated that young fawns display an innate freezing response referred to as “alarm bradycardia” in response to recorded wolf howls. Likewise, a young fawn’s bawling will not only attract its mother but sometimes other nearby deer as well to defend them.

Detecting danger by the odor predators emit is another important way many prey species, including deer, can detect, identify and avoid predators. While much research concerning prey response to predator odors has been conducted on small mammals, such research concerning deer has been more limited. But remember, we human hunters also are predators, and deer readily respond to our odor as they do to that of other predators.

Most deer hunters waiting in ambush have experienced those frustrating moments, when a kill seemed a certainty, only to be foiled by a mere puff of air due to an unexpected directional change in air currents. And those same hunters will readily acknowledge that much of what has been learned from other mammalian research concerning prey response to predator odors probably applies to whitetails as well. With that said, here’s my take on this intriguing subject of deer response to predator odors.

Predator Odor Basics

A large body of literature clearly shows that small mammalian prey species exposed to predator odors often change their foraging and resting location, feeding, and general activity schedules, and sometimes even show poor reproductive performance. While novel odors may elicit similar responses, prey habituate more quickly, in contrast to slower or no habituation in response to predator odors.

Odors from predators sharing the same range (sympatric) tend to be more effective in prompting a prey response, suggesting that evolutionary (innate) adaptive processes are involved. Likewise, prey animals with previous experience (learning) of a predator tend to show more avoidance of that particular predator or its odors. Keep these basic principles in mind, as they definitely apply to whitetails and their management.

Effective predator odors come in many forms, including fur and skin-derived odors, fecal and urine odors, and odors from scent glands. There is good evidence that fur-derived odors have superior potency and longer-lasting effects, but the range of potential chemicals manufactured by scent glands appears to be almost limitless.

One way to avoid predators is being active when predators are not. Typically, active prey species are more vulnerable to predation than inactive ones. Even so, a prey species must allocate sufficient time to feeding.

Nocturnal prey species frequently alter their foraging schedules and feed more during daytime when threatened by nocturnal active predators. On the other hand, heavily hunted whitetails tend to reduce their daytime activity in response to daytime active human hunters. Investigations conducted with other prey species have shown that predator odor alone can produce similar effects. When the predator and its odor is removed, species are more likely return to their normal activity schedules.

Another way for a prey species to avoid predators is to shift away from risky habitat — especially one that is saturated with predator odor. Some species show a tendency to use heavier vegetation, whereas others concentrate their activities in portions of their home range having minimal predator odor. In either case, those animals having previous predator experience more likely shift habitat and as a result demonstrate better survival.

Limited literature suggests that predator odors have powerful, stress-like effects on the endocrine system of some mammals.These effects may help to explain the frequent findings of weight loss in predator odor-exposed individuals of prey species, and breeding suppression in females, as well as some reports of reduced aggression in males.

A wide range of predator odors have a proven repellent effect on a variety of prey species. While some effort has been made to chemically identify the components or the mixture of components involved, research results have not been especially encouraging.

There is evidence that sulfurous metabolites of meat digestion may be important for the repellent effects of predator odors to potential prey. For example, mountain beavers were found to react more adversely to urine from meat-fed coyotes as compared to that from coyotes on a vegetative diet — a finding bow hunters might consider.

In general, prey species try to avoid predators and their odors. This finding has stimulated laboratory as well as field studies designed to develop repellents based on predator odors. Such repellents, if powerful and sustained in their effects, would have obvious protection benefits against unwanted deer browsing. While some studies have produced promising results, others have not, indicating that far more research is necessary.

Deer Response

To Predator Odors

Clearly, in some prey species, odors of predators elicit the same response as actual sightings of the predator. Furthermore, there is good research evidence that black-tailed and white-tailed deer also show aversion behavior when exposed to urine or fecal odors of familiar predators.

Most deer hunters have experienced deer response to human (predator) odor. I know I have; one such event comes to mind. In the late 1970s, I was perched on a knoll overlooking a sizeable snow-covered aspen clearcut. It was a good spot, I had taken a nice 11-pointer there the year before.

Toward dusk, I saw deer movement 500 yards or so to the north. A quick check through the scope identified a doe leading 2 fawns, their movements clear against the snowy background. Given the northwest wind, I was confident they’d pass just below me, cross over my fresh boot prints, and continue their undisturbed migratory trip to the southwest. However, as the doe came to my tracks, now a couple of hours old, she stopped abruptly, nosed the prints briefly, turned and casually retraced her steps back across the clearcut. She had obviously detected a predator’s presence and potential danger — me!

In northeastern Minnesota, noted researcher Dave Mech observed that during declining deer densities surviving deer were almost exclusively found along edges of wolf pack territories, in buffer zones avoided by wolves. He speculated that deer inhabiting those areas with minimal wolf activity tend to survive longer and form a reservoir for maintaining and recovering deer populations in wolf country. Whether predator odors play a role in this space-use relationship is unknown.

Fecal Odors

To my knowledge, Utah State University researcher Dietland Muller-Schwarze was one of the first to test the response of deer (blacktails) to predator fecal odors. Using 2-month old captive deer, he exposed each test animal to bowls of food, some of which contained a vial of predator feces. In all, he tested the effects of 5 predator species, including the coyote, mountain lion, African lion, snow leopard, and Bengal tiger. He then monitored the behavior of deer relative to the various bowls of food. The most consistent avoidance response was elicited by odors of the coyote and mountain lion.

Given these findings, Muller-Schwarze concluded the following: “It is likely, therefore, that black-tailed deer and perhaps other ungulates possess a largely genetically determined, negative response to odors of predators. How later learning processes refine this spontaneous reaction is unknown.”

In 1978, Anthony Melchiors and Charles Leslie, researchers from the Western Forestry Research Center, of Weyerhaeuser Company, in Washington, conducted a more elaborate series of tests to evaluate the effectiveness of predator fecal odors as deer repellents. They tested deer response to bobcat, mountain lion, wolf, and coyote fecal odors.

Using freshly collected predator scats mixed with water, Melchiors and Leslie first homogenized the material and formulated concentrations ranging from 5 to 30 percent of the scat material. The solutions were then sprayed on test branches of browse in an enclosure to determine their repellent effects.

For comparative purposes, the researchers also sprayed some browse with solutions containing extracts of egg solids that reportedly have some repellent effects on deer.

These tests showed that fecal odors of predators were as effective as or better than commercial egg-based repellents in reducing deer browsing by captive deer. Of the predator treatments examined, bobcat odors were most effective, reducing browsing by 51 percent. Although mountain lion odors reduced browsing by 27 percent, those of wolves and coyotes had less effect (17 and 8 percent, respectively). Increasing the fecal concentrations from 20 to 30 percent did not increase effectiveness.

As encouraging as these tests were, Melchoirs and Leslie indicate additional laboratory and fieldwork will be required to identify more effective concentrations, species, or formulations to reduce deer browsing in the wild.

Urine Odors

Field experiments, designed to test the effectiveness of predator urine odors in reducing white-tailed deer browsing on woody plants, were conducted by Connecticut Agricultural Experiment Station researchers led by Robert Swihart.

During the winter of 1989-90, Swihart and his co-workers tested whether urine from coyotes, bobcats, and humans sprayed on Japanese yew and eastern hemlock would reduce browsing by deer. Urine of cottontail rabbits and distilled water served as controls.

The researchers found that bobcat urine was significantly more repellent than coyote and human odor, while coyote urine was more repellent than human odor. Urine from cottontail rabbits and humans did not reduce damage.

In addition, Swihart and his group identified more than 25 volatile constituents of bobcat urine. At least one of which was found to be just as effective as fresh bobcat urine in reducing deer browsing.

It’s interesting to note this and several other studies found deer more responsive to bobcat odor than they were to coyote odor — a finding I wouldn’t have predicted. Some investigators suggest this difference is due to different hunting styles of the two predators. That is, canines in general are more likely to chase deer, whereas cats tend to wait in ambush. Hence, deer never can be certain of the whereabouts of a cat.

Swihart and his fellow researchers concluded the following: “Deer do not respond aversively to odors of nonpredatory mammals or occasional predators [i.e., humans) with which they lack a long evolutionary association.” In other words, deer can more easily be habituated to the presence of humans, and must “learn” they are potential predators.

In the early 1980s, studies I conducted in the Cusino square-mile deer enclosure revealed that does also were responsive to coyote urine when it was presented as a potential predator threat to their young fawns. Coyote urine or water (a control) was alternately sprayed at 2 locations within 10-15 feet of bedded radio-collared newborn fawns during the evening. The fawns were then relocated the next morning to compare movement distance with treatment status and mother’s age.

Only the fawns from prime-age does (4 or more years old) moved much farther away in response to coyote urine versus the water control deposited near their bedding sites. Normally, mothers moved their fawns about 400 feet overnight. However, when exposed to coyote urine the more experienced mothers moved their fawns twice as far. This suggests, of course, although whitetails may have an innate fear of coyotes, learning plays an important role in their ability to protect their defenseless fawns from predation.

Conclusions

All prey species show specific adaptations involving inherited as well as learned traits that allow recognition, avoidance and defense against predators. For deer, this includes being especially sensitive towards volatile odors from the feces and urine of familiar predators such as bobcats, coyotes, wolves, and mountain lions.

Since deer and humans have existed together for a relatively brief period, in evolutionary terms, deer apparently have not developed an innate fear of humans. Humans are only occasional predators of deer. Nonetheless, given certain life threatening experiences involving humans, deer soon “learn” to recognize and avoid humans — often employing their fascinating sense of smell.