Deer Options Enterprise
White-Tailed Deer Biology and Ecology
White-Tailed Deer Biology and Population Dynamics
by Larry Marchinton*
White-tailed deer may be the world’s most thoroughly studied wild animal. There have been many books and literally tens of thousands of papers written on them. Like any other scientific focus, however, when one question is answered that answer creates 10 new questions. We are to the point with white-tailed deer where some very intriguing questions are now being asked. In fact, sometimes I think we are almost getting into the animal’s "head."
The white-tailed deer’s scientific name is Odocoileus virginianus. It ranges from southern Canada to below the equator in South America. There have been 38 subspecies recognized, 18 of these existing in the continental United States. The validity of some of the subspecific designations is now open to question because deer have been moved around so much in restocking programs.
Like most mammals, the white-tailed deer illustrates Bergmann and Allen’s rule, which in essence says that as you follow the geographic range of a species north, individuals get relatively larger and their appendages more compact. This is an energy- saving mechanism. As the volume of an animal’s body increases, the relative amount of surface area decreases and heat is conserved. Therefore, deer in South America are likely to be smaller and slimmer than those in Maryland, which, in turn, tend to be smaller than those in Canada. There are exceptions to these rules, of course.
CALENDAR OF ACTIVITY
I am going to discuss the annual calendar of a white-tailed deer’s activities. The particular model that we will use applies to much of the whitetail’s range in the United States, especially the central and northern parts. Some deer populations, particularly in the southern portions of the species’ range, have very different schedules.
During the warmer months of the year, starting in spring and continuing through summer, does are raising fawns. They stay in these family groups throughout most of the year. Bucks are in bachelor groups during the warm months. This is the period of time when they grow their antlers. This process starts about the beginning of May and finishes at the first of September, when the velvet begins to come off the antlers.
The physiology of seasonal reproduction is tied to photoperiod, e.g., the length of daylight relative to the length of darkness. White-tailed deer are considered short-day breeders because as the day length decreases in late summer and early fall, a neural impulse to the pineal gland is triggered which stimulates the reproductive organs by production of melatonin. In the bucks, testosterone increases, and as it rises the antlers harden and the velvet is shed. The period of hard antler lasts from early September to early February, although there certainly is variation on both ends of this period.
When the antlers become hard, bucks begin to rub saplings and smaller trees, removing the outer bark and exposing the cambium underneath. Rubbing peaks shortly after velvet removal but continues throughout the breeding season. Rubs are one of a number of signposts whitetails make. Signposts in the animals’ world are very similar to signs in our world. They convey information to other members of the population. Bucks rub the exposed cambium with their faces and foreheads. The forehead area has been classified as a scent gland because of the glandular tissue underlying this part of the face and head. Rubs by alpha bucks—i.e., bucks at the top of the social hierarchy—may express social or physical superiority while advertising availability to does. Large mature bucks sometimes rub large trees, i.e., greater than 6 inches in diameter, but immature ones almost never do this. These large rubs seem to be special signposts and are returned to year after year.
Scraping is another form of signpost behavior. It generally peaks just before or during the period of actual breeding and involves a combination of overhead branch marking, scraping, and urination. In overhead branch marking, the buck will mouth a limb that he can reach, even by standing on his hind legs, if necessary. He may rub the preorbital gland just in front of the eye and the forehead glands on the branch. Next he uses one foot and then the other to paw away litter exposing mineral soil underneath the marked branch. He then steps forward and urinates in the pawed area. Urination may be normal urination or it may be a kind of urination we call "scenting," where the hind legs are held together and the urine passes over the tarsal glands on the inside of the hind legs. Usually the dominant buck in the area is the one that will do the full scrape sequence, although subordinate bucks also will come to a scrape and mark the overhanging branch and/or to taste and smell it.
Pheromones and Hormones
The physiology of semiochemical reception is of two kinds. Odors or chemicals come through the main olfactory system and enter the prefrontal area of the brain’s neocortex. This is the normal way an animal smells. It is aware of the smell because it is stimulating the conscious part of the brain. The other receptor is called the vomeronasal system. Chemicals enter it through a pit in the roof of the mouth by means of flehmen—a noticeable behavior where the buck rolls the front lip up (most of us have seen horses do this). Eventually the stimulus reaches the hypothalamus, which is a subconscious part of the brain. This is where intraspecific communication chemicals called pheromones are interphased with chemical messengers within the body called hormones.
Bucks spar by pushing antlers together in mock fights. These are not extremely vigorous or violent behaviors. Sparring usually begins right after velvet is removed and basically seems to be a form of testing. The bucks, you remember, do not have mirrors to see what their new head adornments look like each year, so one way to find out is to push them against other bucks and find out how they compare. This is the period when they are establishing their dominance hierarchy, and we have noticed that often after sparring the smaller buck will lick the face and forehead region of the larger buck. One can hypothesize that this is a mechanism to learn the scent and taste of that particular buck so that later, when the smaller buck encounters a rub or other signpost of the larger buck, he will know who it was made by and how he would stack up to that individual in a confrontation.
Dominance fights are more likely to occur during the peak of breeding season between two large individuals. Typically they may be caused by a dispute over a doe. It is not uncommon for a doe beginning her estrus or heat period to be followed by one dominant buck and enter into an area where another buck is dominant, resulting in a dispute over the doe. These are violent occurrences, sometimes causing injury or even death for one or both bucks.
Mating peaks in the middle to latter part of November in many areas, with fawns being born mainly in June. However, the sex ratio and condition of adults can affect breeding. For example, if food supplies are very low or the sex ratio is out of balance in favor of females, and there are few or no adult males in the population, the breeding season may be very long. This results in small fawns on the ground at the beginning of the colder season. These fawns are likely to have a harder time making it through the winter. We have found that when this situation is corrected through removal of more does and selectively hunting bucks so that many of them are allowed to grow to older age, the breeding season becomes shorter and moves forward.
QUALITY DEER MANAGEMENT
There is a movement led by the Quality Deer Management Association that is catching on around the country. It addresses the problems of out-of-balance sex and age structures. Basically, it involves improving the nutrition of the herd, restricting harvest of young bucks, and harvesting does to decrease density and increase the age and proportion of bucks. This type of management is believed to result in a herd that is more like it was when predators and Native Americans "managed" the herds—that is, in pre-Columbian times.
Hunters sometimes are concerned that harvesting does results in orphaned fawns that will disperse from the property or face a higher risk of death. A study we did in Virginia indicated this is not true, at least for buck fawns. Buck fawns were captured and radio-collared before their first fall when they were a few months old. They were radio-tracked until they died or reached two and one-half years of age. Half of the fawns were orphaned, that is, their mothers were accidentally killed by cars or were trapped and removed from the area. The results were surprising. The ones whose mothers were present tended to leave the area sometime during the two and one-half year period. In fact, 9 out of 10 of the fawns with mothers dispersed from the area. Of the fawns that were orphaned, only 1 out of 10 dispersed during the study. So the probability of leaving the area was greatly reduced in the orphaned fawns.
What about mortality? Interestingly enough, it was found that 87.5 percent of the fawns which dispersed died before the study was over. Only 62.5 percent of the fawns that stayed in their birth area died during the same period. In other words, there was a higher chance of survival in the orphaned bucks than those with mothers.
Does only the biggest buck breed? Not necessarily, but probably in most cases this is what happens. The courting behavior of the doe more or less ensures that all the bucks in the area know she is in heat. Typically the estrus doe makes a "spectacle" of herself by running all over the area, apparently encouraging bucks to chase her. Most bucks in the vicinity will be enticed into the chase. When the time comes and she is ready to stand, the dominant buck usually moves to the front of the group and is the one to breed with her.
When do bucks become dominant? This depends on the age structure among the bucks. With a full complement of age classes ranging all the way up to old age, bucks will likely become dominant at about 5 years old and may lose their dominance somewhere around 10—but this is highly variable.
How many does can a buck breed? This undoubtedly varies in the wild. There is one recorded instance where 21 antlerless deer were kept together in a pen. They were thought to be all does but 20 of the deer became pregnant and the other deer was belatedly found to be a very small buck with just nubbins for antlers.
What is the length of the heat period? We have found that the typical heat period for a doe is 24-36 hours, and the heat or estrus cycle is about 26 days on an average, ranging from 21-30. If a doe is not initially bred, she may cycle many times. Although does normally are bred on their first or second cycle, seven or more cycles are possible if there is no buck to breed with them.
What about old, barren does? Basically, does that are too old to have fawns don’t seem to exist. There is one instance where a doe in captivity died at 21 years old and had twins the year she died. This is one of the oldest whitetails I have heard of.
Both bucks and does that are born early and have plenty of high quality food to eat become fertile in their first fall.
What is the length of pregnancy? There is a slight range, but about 200 days or just a little under 7 months is the norm.
What is the sex ratio at birth? Out of every 100 fawns born, how many are males and how many are females? The answer is usually, but not always, slightly more males than females. We could discuss the theories about why this occurs for hours, but the bottom line is I don’t think anyone really knows.
How many fawns do whitetails have? Normally, a healthy adult doe will have two fawns every year. Does that breed while still fawns are likely to have only one offspring. Remember, does breed during their first year only under the best of conditions—they must have had good growth, plenty to eat, etc. The fact is that as a deer population increases and food supplies become lower in quantity or quality, there is a tendency for the number of fawns produced by all does to decrease.
Are fawns hiders or followers? White-tailed fawns are what we call hiders. As soon as they are born, they nurse and then go off into the bushes, curl up, and hide. The mother comes only about three times a day. She usually does not know exactly where the fawn is but approaches the general area and calls. The fawn comes to her and after nursing hides again. This is an adaptive mechanism to take advantage of the fact that fawns have relatively little scent as compared to mature animals. By moving away from the doe, the fawn is less likely to be found by a predator following the doe’s scent.
When do fawns start following their mother? Usually they are following some by the end of a month, and by the end of two months they are with the mother everywhere she goes.
How long do fawns nurse? By the end of two weeks, rumination begins. The doe starts to wean the fawn at about a month, and fawns are usually functionally weaned by 10 weeks, although they may occasionally nurse on through the summer and fall.
How fast can a white-tailed deer population grow? The answer is, Darn fast! Dale McCullough (1979) found that over a 6-year period, two bucks and four does produced 160-plus deer on Michigan’s George Reserve. Theoretically, two bucks and four does could produce more than 300 deer in six years. That is without any "environmental resistance." Environmental resistance is anything that will kill a deer or reduce reproduction, and includes predators, cars, hunters, parasites, diseases, and so forth.
Deer population growth follows what we call a sigmoid curve. McCullough suggested two kinds of carrying capacity. One being the density which, if exceeded, will result in habitat destruction or damage. This level of maximum sustainable density is referred to as "K." Another form of carrying capacity was identified as the "I" level, which is the inflection point on the Sigmoid growth curve. This level is roughly 60 percent of K and is the level at which populations produce the most new individuals each year. Managing for maximum sustained yield results in healthy animals and little habitat damage.