Hybridization: A Unique Tail of Whitetails and Mule Deer

Whitetail-Mule deer hybrids are often distinguished by their tail, among other characteristics.

Whitetail-Mule deer hybrids are often distinguished by their tail, among other characteristics.

Throughout history, crossbreeds have fascinated us. Many of our monsters are a mixture of man and beast including werewolves, Dracula and Moth-man. Likewise, consider some of our heroes: Spiderman, Batman and Cat Woman. This trend is not a recent Hollywood construction; the “Minotaur” of Greek mythology was a creature that was half man and half bull.

Even the mule deer was described by John J. Audubon in 1846 as having fur like an elk, but hooves like a whitetail. The very scientific name of mule deer, Odocoileus hemionus, means “deer that is half mule.”

Different species of animals, even closely related ones, are normally kept from crossbreeding by being geographically isolated from one another, or by separating themselves into different types of habitat. If closely related species live together in the same habitat, then they generally have different courtship and breeding behaviors or breed at slightly different times to prevent hybridization. All of these are true for whitetails and mule deer, but their ranges overlap across large areas of North America and this leaves plenty of room for exceptions to the rule.

MEET THE PARENTS
Being able to accurately identify both species is especially important in areas where their ranges overlap, because hunt permits are generally prescribed separately for each deer species. Mule deer differ from whitetails in several recognizable characteristics, but there is so much variation in each species that we frequently see some specimens that cannot be quickly identified. Some characteristics, when used alone, can be confusing or yield an incorrect deer identification. It is important to use all of the information available when differentiating these deer species, and understanding these differences is crucial to recognizing hybrids.

Tails: Whitetails have a wide, flattened tail that is broad at the base and narrower at the tip. The pure white underside is contrasted by a darker brownish back side. In contrast, mule deer have large, obvious white rumps with rope-like tails that are usually white on the back side and have a distinctive black tip.

Whitetail-Mule deer hybrids may have antler characteristics of mule deer.

Whitetail-Mule deer hybrids may have antler characteristics of mule deer.

Antlers: Antlers are the least useful characteristic when trying to differentiate between these two deer species. This is because of the incredible variation in antler shape in both species. There are, however, differences that can be used in combination with other characteristics.

Mule deer antlers have small brow tines, if they have them at all. The antlers sweep out and upward, forking once and then each fork divides again in mature bucks. Mature bucks typically have 4 points on each side if you ignore the brow tines.

Typical whitetail antlers have several antler tines that each arise independently off a main beam that sweeps outward and forward from the bases. The brow tines are nearly always present and usually prominent.

Facial Markings: A whitetail’s forehead is usually about the same coloration as the rest of the face, or slightly darker. White eye rings and nose markings are prominent. A mule deer has a distinctive black forehead that contrasts sharply with a light gray face. The lighter facial coloration of mule deer results in less obvious white eye rings and muzzle.

Ears: Whitetail ears are relatively shorter than those of mule deer. The ears of a whitetail are generally two- thirds the overall length of the head (back of head to nose) while those of mule deer are three-quarters the length of the head (about 91⁄2 inches!).

Preorbital Glands: The preorbital gland is situated in the front corner of the eye socket and differs considerably between the two species. The preorbital gland of whitetails is very small, appearing as a small slit about 3/8-inch deep. In mule deer, however, this gland is much larger, forming a substantial pocket with a depth averaging 3⁄4-inch.

Metatarsal Glands: The only physical character that can be used to accurately diagnose a hybrid in the field is the metatarsal gland, which is located on the outside of the lower portion of the rear legs. The metatarsal glands on mule deer sit high on the lower leg and are 4 to 6 inches long and always surrounded by brown fur. The whitetail’s metatarsals are below the mid-point of the lower leg, less than 1 inch long, and surrounded by white hairs. A white- tail/mule deer hybrid has metatarsal glands that split the difference in location, appearance and length (between 2 to 4 inches).

HYBRIDIZATION HAPPENS
In some cases, the many barriers to crossbreeding fail and whitetails successfully mate with mule deer. For the most part, the resulting offspring show characteristics that are intermediate between the two species. The tail of a hybrid looks very much like a typical whitetail, but is usually longer and dark brown or black on the back surface. The preorbital gland in front of the eye is intermediate in depth or might be deep like a mule deer’s. Antlers are typically more whitetail-like, but might fork in older hybrids. I’ve noticed that many hybrid antlers have “wavy” tines, as if the antlers were receiving mixed signals about which way to grow.

Hybrids have been reported in captivity as early as 1898, when a whitetail/mule deer cross was produced at the Cincinnati Zoo. Occurrences were later reported from the zoo in Minot, N.D., deer pens in Alberta, and others. Researchers in Tennessee also produced whitetail/blacktail hybrids in captivity. The male hybrids are almost always sterile, as is the case in other mammals; however, female hybrids are fertile and can breed back to either parent species.

During the 1930s, and again in the 1970s, biologists in Arizona produced hybrids in captivity. These matings resulted in 19 hybrid fawns, of which only eight survived the first six months. This showed that survival of hybrid fawns is very low even when pampered in a captive facility. Survival in the wild is even more difficult when food doesn’t come from a feed trough and there’s no fence between them and animals with sharp teeth.

To complicate matters, hybrids inherit predator avoidance strategies from both types of parents; the problem is whitetails and mule deer have drastically different techniques for escaping predators. The whitetail’s key to escaping is speed. They try to put as much distance between them- selves and the predator as fast as possible. Mule deer, on the other hand, have developed a pogo stick-like bounding called stotting. Research by

Susan Lingle using captive hybrids in Alberta, has shown that stotting is so specialized that hybrids couldn’t do it. She found the hybrid’s escape behavior was confused because they approached the threat and jumped around in confusion. Such behavior is not likely to be passed on to the next generation when there are predators around!

Whitetail/mule deer hybrids in the wild have been reported in most areas of range overlap. Despite these widespread occurrences, true hybrids are actually very uncommon. The relative scarcity of confirmed hybrids among the thousands of deer seen each year throughout the area of range overlap illustrates how rare they are. Every year numerous reports are received of hybrid deer from hunters, but most aren’t. Some of these hybrid reports come from hunters who have placed their whitetail tag on the leg of a mule deer and are trying to convince the game warden they are at least half right.

Hybrids raised in captivity typically don't fare well. Those in the wild usually fare worse.

Hybrids raised in captivity typically don’t fare well. Those in the wild usually fare worse.

MIXED RELATIONSHIPS
Although the adaptable whitetailed deer has made steady gains in much of the country, mule deer populations rise and fall from one decade to the next. Although some mule deer populations are improving now, the past decade has seen mostly lower mule deer abundance through- out their range. As the distribution of these species changes, it alters the dynamic relationship between them. The reasons for these changes in distribution are not always known and sometimes happen so slowly they are hardly noticed. Whitetails in the Rocky Mountain states are expanding to the point that agencies are changing regulations to take advantage of the more abundant game species in their midst. With this expansion in many places, hybridization seems to be either on the rise or at least noticed more.

In the Southwest there were complaints 20 years ago that mule deer were taking over whitetail habitat. Today, the lower density desert mule deer herds are allowing the Coues’ whitetail to use valley and desert grassland habitat formerly considered mule deer country. Along with this shift has come an increased occurrence of hybrids reported and verified. It is very hard to pinpoint the cause of changes to deer distribution in most cases — it is a complex and dynamic situation and varies on a case-by-case basis, not allowing us to generalize.

DECIPHERING DEER DNA
Recent advances in DNA analysis technology has allowed us to look at more definitive things than ears or antlers. Although we know a lot about the physical features of hybrids, there are cases where the whole animal or the diagnostic parts are not saved. Also, a first generation (50:50) female hybrid might breed with a mule deer buck and the offspring will be three-quarters mule deer. These offspring might breed with mule deer, resulting in deer that are seven-eights mule deer and are probably not distinguishable from pure mule deer just by looking at them. This scenario could also occur with back-crosses to whitetails.

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In cases such as this, we have to stop using our low-tech ruler to measure relatively big things and turn to high-tech scientific methods to measure very tiny things such as DNA molecules. The DNA molecule holds a tremendous amount of information and just during the past few decades have computers, software and genetic analysis techniques allowed scientists to begin to unravel the data that is twisted up in that double helix molecule. Researchers have been able to identify some genetic markers that differ between these two species and use these genetic differences to look for evidence of hybridization in the field.

Research has revealed that hybridization occurs in both directions — that is, mule deer bucks mating white-tailed does and white-tailed bucks mating mule deer does. The most common “direction” of hybridization seems to be between a white- tailed buck and mule deer doe. This is because whitetails are much more aggressive in their breeding behavior and any mule deer doe that doesn’t run fast enough is going to be bred pretty quick. The fact that most hybrids seem to be found in mule deer groups supports this.

The original genetic tests were primitive by today’s standards and relied mostly on fresh tissue, which was not always available. At that time there was no proven genetic test to determine if a suspicious-looking animal was a hybrid using dried skin, bone or antler. Over the past years, I orchestrated a collaborative research project to develop a better genetic test with David Paetkau and Renee Prive (Wildlife Genetics International) and Dr. Irv Kornfield (University of Maine), funded primarily by the Boone and Crockett Club, Pope and Young Club and Camp Fire Conservation Fund, Inc. We developed a group of genetic markers to analyze pure mule deer, pure whitetails and some known hybrids of various fractions (from captivity) to develop a test that can be used for any deer suspected to be a hybrid.

The results were stunning, with known pure mule deer, whitetails and 50:50 hybrids separating out nicely in three discrete clusters. When we added some suspected hybrids from the wild (parentage unknown), some were shown to be pure white-tailed or mule deer, some 50:50 hybrids, and others looked like second generation back-crosses (one-quarter and three-quarters) because they were between the clusters of pure parents and 50:50 hybrids in the middle.

This urged us to obtain hybrid samples from University of Alberta’s captive deer facility with known combinations of hybridization other than 50:50 (five-eights, three-quarters, seven-eighths, etc.). Analyzing these hybrid samples of varying fractions allowed us to see if we could identify second (one-quarter, three- quarters) or third (one-eight, three- eights, five-eights, seven-eights) generation hybrids.

When we compared the known percent mule deer for each sample (we knew who their parents and grandparents were) to what the new genetic test estimated, there was a remarkable similarity (although not perfect). This showed that we now have a genetic test to identify all first- and almost all second-generation hybrids. We can also offer a reasonable opinion about hybridization events three or more (deer) generations ago.

THE FUTURE OF OUR DEER
So does this hybridization mean our two main deer species are doomed to merge into a hybrid mess? Not hardly. The limited amount of hybridization that occurs throughout their ranges is not great enough to affect either species significantly. Add to that rarity the fact that hybrid fawns have a low survival rate and we should not expect this issue to have much of an impact in the long run.

The mule deer will continue to dominate the open spaces we leave them in the West and the adaptable whitetail seems to just keep prospering amid advancing urban encroachment, habitat alteration, weather fluctuations, predation, disease and heavy doe harvest. Our charge is to make sure we remain good stewards of the habitat and work to maintain the historical ecological balance between these two species.

— Jim Heffelfinger is a certified wildlife biologist who has authored or co-authored more than 200 magazine articles, scientific papers and book chapters in national, international and regional publications. He is an adjunct professor at the University of Arizona, Professional Member of the Boone & Crockett Club, and currently works as Regional Game Specialist for the Arizona Game and Fish Department. For more information from Jim, or to order his book “Deer of the Southwest,” visit www.DeerNut.com.

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