Whitetail NEWS

Studies have shown that, contrary to popular belief, mature bucks often breed a small percentage of the doe population. Young bucks appear to participate in the rut more than once thought. Photo by Whitetail Institute

The breeding system of whitetails involves formation of a tending bond, where a buck guards, tends and courts an individual estrous female. A buck’s body size, antler size, fighting ability, sign-posting skills, courtship finesse and dominance rank increase with age. Typically, these traits and skills peak when the buck is 4 1/2 to 8 1/2 years old. Hence, these prime-age individuals would be expected to do the bulk of the breeding; whereas smaller, younger and less skilled bucks would have little opportunity to secure mates.

Traditionally, we believed that a strict breeding hierarchy results, wherein relatively few of the most dominant males monopolized and bred nearly all of the estrous females. However, new research findings indicate this may not be the case. Studies employing new genetic techniques now challenge this line of thinking, even when whitetail populations are unhunted and large prime-age bucks are well represented in the herd.

Presumably, such a breeding strategy, wherein male mating success is determined by individual rank on the dominance hierarchy, assures genetic fitness of the offspring conceived, as only the physically superior males would reproduce. For example, this has been proven to be the case for some ungulates, such as reindeer. And, based on observational studies, we also thought whitetails employed this type of breeding strategy.

While all this makes good adaptive sense, and tends to be supported by observational data, new evidence based on highly sophisticated DNA testing techniques indicates we may have to adjust our thinking for whitetails. It appears that a large proportion of the whitetail bucks 2 1/2 years and older in any given deer population actively participate in the rut’s frenzy and do some breeding. And even some yearlings may not be excluded from mating, despite the presence of mature bucks.

New highly sophisticated DNA microsatellite techniques now allow researchers to test many hypotheses pertaining to mammalian reproduction. These new techniques enable researchers to identify the mothers and fathers of offspring and to accurately determine relationships among individuals, whereas in the past, such things were determined strictly by observations.

Genetic methods are now providing important breakthroughs in the study of animal mating systems. In fact, a number of studies – including several on whitetails – have revealed patterns of male mating success that are quite different from those derived from behavioral observations. Furthermore, there is evidence that individual male reproductive success may change depending on sex ratio, resource availability, herd density and other factors that can influence patterns of deer social behavior.

Donnie Frels of the Texas Parks and Wildlife Department and Jim Ott of Texas State University conducted one of the earliest investigations of whitetail buck mating success in 1998. Their study, reported in the Quality Deer Management Association’s journal, Quality Whitetails, was carried out in two, high-fenced enclosures of 608 and 499 acres located in central Texas.

First, Frels and Ott removed all resident deer from the enclosures and replaced them with wild-caught deer. The experimental herd in one enclosure held 16 bucks and 28 does, the other held 22 bucks and 47 does – a relatively high density. However, only bucks 1 1/2 (yearlings) and 4 1/2 years and older were introduced, in order to compare the breeding success of young versus older bucks.

The objective of the study was to determine how buck age and antler size influenced a buck’s breeding success.

All deer were harvested from both enclosures after one breeding season. Then, DNA determinations were made from the hair of all bucks and compared to that of the DNA of fetuses collected from pregnant does. This allowed the investigators to determine each buck’s breeding success.

All does in both enclosures were pregnant; 27 of 48 does in one enclosure carried twins, and 18 of 28 in the other carried twins. One doe in each enclosure carried triplets. Some of the twin and triplet litters had more than one sire, a phenomenon referred to as multiple paternity.

Generally, yearling bucks sired few fawns. Although they comprised 32 percent and 45 percent of the bucks in the two enclosures, they bred only two does in each enclosure.

According to the article published in Quality Whitetails, “In one enclosure, the ‘high-antler-quality’ mature bucks dominated the show when compared to the mating success of the ‘low-quality’ mature bucks. While high-quality males made up 27 percent of all bucks, they accomplished 81 percent of the breeding. The other enclosure revealed a slightly different story: high-quality bucks made up 33 percent of the buck population, but there was no significant difference in the proportion of does bred by high-quality and low-quality mature bucks.”

Interestingly, the buck with the largest antlers in each enclosure did not breed the most does. In fact, in one enclosure the largest-antlered buck (146 5/8 Boone and Crocket score) bred only one doe, whereas a smaller-antlered buck (126 B&C score) of the same age (5 1/2 years old) bred a total of 16 does. Likewise, in the other enclosure, a low-quality (48 B&C score) 5 1/2-year-old buck bred six does, while a large-racked (134 3/8 BC score) 7 1/2-year-old buck bred only two does.

It’s important to recognize, however, that this particular study did not represent a free-ranging deer herd, because there were no 2 1/2- or 3 1/2-year-old bucks present. Also, such things as a buck’s body size and behavioral aggressiveness were not considered in the data analysis. Not to mention the fact that this population was enclosed, which eliminated any deer movement into or out of the population.

Clearly, yearling bucks did minimal breeding, despite their fairly high numbers. However, because low-quality versus high-quality-buck breeding success varied between the two enclosures, the investigators could not evaluate the importance of a buck’s antler size in relation to his breeding success.

Anna Sorin also conducted a study to determine buck-breeding success. Her investigation was carried out in the University of Michigan’s 1.8 square-mile Edwin S. George Reserve, located in southeast Michigan.

Sorin investigated reproductive success of whitetail bucks using a microsatellite paternity analysis, and found that adult males from all age classes fathered offspring. That is, the oldest males did not monopolize matings.

In March 2001, every deer possible (103 total) was harvested from a known population of 127 deer (34 bucks, 67 does, and 26 fawns). In addition, 20 deer were found dead, presumably as the result of winter stress. Many of these animals had been captured and marked previously and, therefore, were of known age. Tissue samples were collected and examined for DNA from all harvested deer and from fetuses carried by pregnant does in order to identify the sires.

The fathers of 67 fetuses (including 20 sets of twins) from 47 does could be assigned to 17 different males. The bucks included three of 16 yearlings, two of four 2 1/2-year-olds and 12 of 14 males aged 3 1/2 years and older.

As demonstrated in the accompanying table, the number of females mated and the number of offspring fathered per buck were highly variable. While five of the 17 bucks only mated with one doe each, the others each mated with two, to as many as seven, different does. Likewise, each buck fathered from one to as many as nine offspring.

Multiple paternity (the fathering of individuals within a single litter by more than one male) was found in 22 percent of twins. In each case, the males that jointly sired twins were at least one year apart in age.

It’s also noteworthy that while three yearling bucks mated and produced offspring, they only mated with 1 1/2- and 2 1/2-year-old does. By comparison, all other age classes of males mated with females from a range of age classes.

Given the available evidence, it appears multiple paternity is fairly common among whitetails. It was first documented by Randy DeYoung in 25 percent of twin and triplet litters produced by deer confined to small research pens at Mississippi State University. Since multiple paternity was detected in 22 percent of the twin litters from the George Reserve, it appears the phenomenon may be fairly common among free-ranging whitetails.

There are several hypotheses that might explain multiple paternity in whitetails. When one male challenges a tending male, a third male could sneak in and mate with the female. Or, a buck might actively tend and protect a doe but not mate with her. Admittedly, neither scenario seems quite likely with whitetails.

Researchers generally agree that the best explanation for multiple paternity in whitetails is the displacement hypothesis. In such a case, a young male is the first to find and mate with an estrous doe. Subsequently, an older dominant male comes along and easily replaces the young buck. The older male then also mates with the female and tends her until the end of her receptive period. In this way, both males could copulate with the female, and each could father one of her twin offspring.

The George Reserve deer population studied in 2001 closely resembled a natural (unhunted) deer population, in that bucks of all ages to 6 1/2 years old were present and 41 percent of the adult bucks were 3 1/2 years or older. Also, the adult sex ratio was one adult buck per two adult does, which can be considered natural.

Most yearling bucks were excluded from mating, but one half of the 2 1/2-year-old bucks fathered offspring. Once males reached the age of three years almost all of them successfully reproduced. Therefore, the presence of males from a range of age classes did not result in a strict breeding hierarchy and the very oldest males did not monopolize all the estrous does.

Sorin concluded the following: “Because male whitetail deer can tend individual females for as long as 24 hours and because all females ovulate within weeks of each other (on northern range), the oldest males appear incapable of finding, defending, and mating with all receptive females. This situation indicates that when single-mate defense is the dominant male strategy and females have synchronized estrus, limited numbers of males are unable to monopolize the majority of females. These findings demonstrate that, contrary to previous thought, reproductive success of male white-tail deer is unlike other polygamous species, in which a few dominant males monopolize access to estrous females.”

In the absence of older males, our studies in Upper Michigan’s Cusino enclosure revealed that yearling males were quite capable sires. Although they lacked the sign-posting and courtship finesse of mature bucks, fought a lot amongst themselves and with does, and failed to form a strict breeding dominance hierarchy, yearling bucks bred the enclosure does on schedule and produced just as many offspring as when mature bucks were present.

Given their low position on the dominance hierarchy, it’s not surprising that most yearling bucks are prevented from mating in the presence of older bucks. I find it more surprising that some of them were able to mate in these three enclosure studies when plenty of older bucks were present. Even when dominated by older bucks, these young bucks appear to be extremely alert to, and take advantage of, every possible mating opportunity.

In the George Reserve study, yearling bucks mated almost exclusively with yearling does, whereas older bucks mated with does of all ages. Since these young males were unable to compete with older, more dominant males, Sorin speculates that differing social behavior patterns among yearling versus mature bucks may have accounted for this apparent mate selection by yearlings.

Among whitetails, only yearling groups include both adult sexes. That is, there are matriarchal groups (related females and their young), fraternal groups (adult males), and yearling groups (that may include females and males, generally ranging in age from 12 to 16 months old).

Yearlings usually band together in small groups when driven away by their mothers with new fawns, generally in June on northern range. These young males and females associate closely until early autumn. It’s this close association that may condition the young doe to the young buck, whereas young does sometimes appear intimidated by the advances of much larger mature bucks.

In our penned-deer studies at Cusino, for example, we only used mature sires in breeding trials. In doing so, we found that young does, mating for the first time, were apprehensive and delayed mating until late in their estrous cycle. By comparison, older does usually conceived much earlier in their cycle.

One could argue that mating by yearling bucks is not adaptive. On the other hand, it might be better for a yearling doe to mate with a young buck, and produce fawns on schedule in spring, rather than delay mating, recycle and have late-born fawns. Also, don’t forget, yearling does tend to conceive several days later on average than mature does, which would increase their chances of mating with a mature buck.

While a buck’s dominance status is an important factor in determining his mating success, other factors must also be operating.

The doe obviously informs the buck that she is in estrus and ready to breed through chemical signals and her general behavior. When males can only mate with a limited number of females and females exhibit reliable cues regarding their reproductive condition, males would be expected to be selective. Also, research with penned deer has shown that some bucks are much more adept at detecting these estrus cues than are other bucks.

It appears that yearling females may respond more favorably toward the mating attempts of yearling bucks. Likewise, older adult does might respond more favorably to the highly ritualized courtship style of mature bucks and not tolerate the advances of young males. Mature bucks also might more actively seek mature does in estrus rather than chase shy, yearlings does.

Some researchers suggest that estrus among all but the youngest of related reproducing females should be synchronous, because estrus can be induced by male-produced pheromones. Therefore, if a dominant female and a subordinate female come into estrus at the same time, the dominant doe might displace the subordinate and copulate first. If so, subordinate does are more likely to delay mating or will more readily mate with a subordinate male.

On the other hand, if adult females of a clan come into estrus only a few days apart, a mature buck might remain with the clan for several days and breed a number does within a relatively short period of time. This might explain how some bucks in the above studies managed to breed as many as six to 16 individual does in a season.

Application of new genetic techniques and employing DNA analysis now permits researchers to determine individual buck breeding success with a high degree of accuracy.

Contrary to previous thought, whitetail bucks apparently do not form a strict breeding hierarchy wherein only a few of the most dominant and physically superior males sire most of the progeny. Instead, most 2 1/2 year old and older bucks share the job of siring offspring. And, even in the presence of mature bucks, some yearling bucks are not excluded from mating and fathering offspring.

Therefore, the oldest bucks with the best antlers may not necessarily father the most offspring. If this is the case, we’ll have to re-evaluate some of our ideas concerning deer herd management. Genetic engineering of free-ranging deer herds, as sometimes proposed, for example, to produce large-antlered bucks, may be tough to accomplish.

Table l. Reproductive success for whitetail bucks in the Edwin S. George Reserve, Michigan, during the 2000 rut. Reproductive success depicted by number of females mated and offspring produced for individual bucks as determined by DNA analysis (from Sorin 2004).

DNA Studies Challenge Traditional Thinking

By John J. Ozoga