Can we kill too many does?
Unless natural mortality is excessive, the cropping of female deer helps maintain a healthy deer population
January 01, 2008
The effects of hunting on deer populations remain a hotly debated issue. Although the detrimental effects of excessive buck harvesting are frequently addressed by those concerned with quality deer management, seldom do managers or hunters question the advisability of heavy doe exploitation.
An essential part of successful deer herd management is an adequate harvest of antlerless deer, particularly adult does. Unless natural mortality is excessive, the cropping of female deer helps maintain a healthy deer population, below range carrying capacity, in balance with available food and cover resources.
On the flip side, some scientists contend that killing too many mature does tends to remove the prime-age social leaders and disrupts the whitetail's adaptive social system.
Will a heavy harvest of these so-called "social governors" disrupt the whitetail's complex social organization, change deer behavior, and produce biological and genetic consequences, as some claim?
Can we really kill too many does?
Scientists have explored some of these questions, and new research techniques are now providing some interesting answers.
The basic social organization in female whitetails is matrilineal, consisting of a family group comprised of a matriarch doe, several generations of her daughters, and their fawns. Several such genetically related families share common range and associate seasonally as a cohesive clan.
In such societies, close associations between mothers, daughters, and other female relatives are maintained into adulthood. This assures that young individuals learn the location of food, water, cover, and potential dangers. For example, some deer have learned to migrate 50 miles or more between their summer and winter ranges, in order to survive harsh northern winters.
In natural (unhunted) populations, the young and very old females normally suffer the greatest mortality. Does that survive to maturity commonly exhibit high reproductive success until they are 10 or more years old and are the least likely to die from natural causes. Heavy hunting pressure tends to remove these prime-age group leaders that would normally show high survivorship and productivity, thereby disintegrating the whitetails adaptive social structure.
Allen Rutberg, a spokesman for the Humane Society of the United States, charges that "Rigorous scientific evaluation of the effects of sport hunting on deer behavior, population structure, and population genetics are astonishingly rare," and says "state wildlife agencies will not fund, and the cooperative research facilities will not sponsor, studies that may embarrass hunters or damage hunting interests."
Ironically, Rutberg conveniently fails to recognize the Quality Deer Management Association and its emphasis on employing hunting strategies to help maintain natural deer population social structure, as well as nutritional balance.
Rutberg also ignores the numerous published studies of whitetailed deer sociobiology — many of which involved the effects of deer-herd management strategies.
Granted, the investigation of deer sociobiology is difficult, primarily because few natural, undisturbed whitetail populations exist for study. However, Rutberg's allegation that governmental agencies control or limit the study of hunting effects on deer welfare is pure hog wash.
|An essential part of successful deer herd management is an adequate harvest of antlerless deer, particularly adult does.|
William Porter and his students at state University of New York have conducted extensive studies exploring the behavioral characteristics and dynamics of hunted and unhunted deer herds in northern New York.
Based on long-term research, Porter has proposed that locally abundant whitetails can be effectively managed by removing entire family groups of deer. Since a group of related does controls and shares an ancestral range, he contends, eliminating such a group would result in low deer density in that area for 10 years or more. That is, because surrounding doe groups are also attached to their established ranges, overflow of deer from surrounding doe groups would be gradual.
Subsequent studies have confirmed such "localized" deer removal might reduce deer numbers in highly fragmented habitat as occurs in residential areas. Likewise, investigators believe such a "surgical approach" might be feasible in the central Adirondacks.
Logically, unintentional overharvesting of female deer might produce similar results and, due to social disruption, cause unfavorably low deer numbers in local areas for a relatively long time. Likewise, excessive natural deer mortality, for whatever reason, in any given area, could theoretically produce similar localized consequences.
In most instances, however, it would be difficult to remove all members of a given kinship group; quite likely, even with intensive hunting effort, some females would evade harvest.
If so, the obvious question is this: how do these sole remaining (socially isolated) females fare, now that they are deprived of kinship associates?
We recognized the close kinship relationship of female whitetails back in the 1970s, when we studied herd density effects on deer reproductive performance in Northern Michigan's Cusino enclosure. In follow-up investigations, we sought to determine how removing doe family members influenced the behavior and productivity of those remaining.
These studies were possible because all deer in our square-mile enclosure were marked for individual recognition, and we knew female relationships from telemetry studies and intensive observations. We also live-trapped the entire population each winter, when we x-rayed does to determine the number of fawns they carried, hand-picked individual deer for return to the enclosure according to study design, and had an accurate fix on newborn fawn mortality. Hence, we compared the performance of isolated does versus members of intact social groups.
Such treatment had no effect on the productivity of yearling does (1-1/2 years old) or those 3-1/2 years of age and older. However, socially isolated 2-1/2-year-old does outperformed their social counterparts by breeding earlier, conceiving larger litters, and rearing a greater proportion of their progeny.
Therefore, heavy harvesting of doe groups did not impair reproductive performance of the survivors. In fact, we saw improved productivity among isolate does in at least one age class. As a result, population recovery would probably proceed faster than expected if not all members of kinship groups were removed.
Whether the same outcome would prevail for deer subjected to heavy predation, acute winter malnutrition, and lengthy seasonal migration is unknown.
Young Does Seek
In this study, doe groups typically consisted of genetically related animals, but even young individuals deprived of close kin seemed compelled to band together. All females younger than 2 years of age, for example, sought adult doe leadership and readily accepted subordinate roles within female groups by October. Social animals reunited with their respective families, whereas isolates linked with an older tolerant animal and shared its range.
In contrast, isolated 2.5-year-old does commonly became group leaders, whereas socials of the same age normally reunited with their matriarch and assumed subordinate family roles.
Stress of Isolation
Despite these findings, blood tests taken from deer each March revealed certain physiological differences related to doe treatment. Compared to social does, isolated individuals exhibited higher plasma progesterone levels indicative of stress. This trend was consistent within an age class.
During other studies, we learned that the adrenal gland can secrete significant amounts of progesterone when deer are stressed, due to handling, for example.
As a result, we theorized high progesterone among isolate does quite likely reflected difficulty in achieving compatible associations with other deer during winter, when sociability among whitetails reaches its peak and can be vital to an individual's well-being and ultimate survival.
Surprisingly, although the herd was supplementally fed, male fawns raised by social does exhibited superior growth rates as compared to those raised by isolate does. In March, when we live-trapped the herd, social male fawns weighed, on average, 91.3 pounds compared to 84.4 pounds for isolate male fawns. But we saw no such difference among social versus isolate female fawns, 71.1 pounds and 71.7 pounds, respectively.
The most plausible explanation for the difference noted in growth rate is that male fawns experienced certain nutritional benefits related to family living, a situation scientists refer to as "social facilitation."
Presumably, the compatible association demonstrated by members of a cohesive clan enable male fawns to obtain more (or perhaps better quality) forage in grazing over a larger ancestral range. Because males also are more independently active than females, male fawns more frequently accompanied close kin to feeders independent of the mother's schedule. By comparison, males reared by isolate does were entirely dependent upon maternal guidance.
Effects of Orphaning
Given the opportunity — and an antlerless permit — most hunters will choose to shoot a large-bodied doe rather than a small fawn, which often results in orphaned fawns.
A review of the literature concerning the effects of orphaning reveals contradictory findings. Some studies show orphaning to be detrimental. Others show it to be beneficial. Still others show no effects at all.
Based on studies he conducted in South Texas, wildlife biologist Bob Zaiglin warned, "There is the potential drawback of prematurely removing does that have fawns."
According to Zaiglin, "Early orphaning can reduce a fawn's home range. Although fawn movements were unaffected by orphaning, orphaned fawns occupied smaller home ranges (383 acres) than did unorphaned fawns (713 acres). More importantly, without a doe, the orphaned fawns could have experienced a disadvantage, particularly in search for food, water and, more critically, escape cover from predators.''
Probably as a result, in South Texas, orphaned doe fawns weighed an average of 9.1 pounds less than unorphaned doe fawns when killed about a year later. The orphans were also more likely to die from natural causes.
On the positive side, two of four ophaned female fawns bred, whereas none of the five unorphaned females bred. This suggests that orphaning might increase breeding rates in young does, at least in mild environments.
Although researchers in Virginia determined that orphaning had no influence on fawn survival, they found considerable differences in orphan behavior, depending on whether they had a sibling companion. Orphaned fawns without siblings more readily associated with adult deer and learned the location of seasonal food and shelter. By comparison, orphans with siblings occupied smaller ranges and seldom associated with adult deer.
As a result, the Virginia researchers warned that if some orphaned fawns on Northern range fail to achieve compatible adult associations, they might not learn lengthy migratory routes to winter habitat.
Effects on Dispersal
Social pressure from older female relatives — especially the mother — is believed to be the primary stimulus prompting yearling bucks to leave their birth range.
Studies conducted in Virginia by Stefan Holzenbein and Larry Marchinton demonstrated the importance of maternal domination in prompting dispersal among male whitetails. When comparing the movements of young males raised with their mothers versus those of males orphaned several months after being weaned, the investigators found few orphaned males dispersed from their natal home range as yearlings. Meanwhile, most doe-raised males dispersed as expected.
As a group, the orphans also exhibited a higher 2-year survival rate (46.2 percent) compared to nonorphans (6.7 percent), which reflects the dangers involved in trying to establish a new home range.
Despite these findings, they warned, "Orphaning may be harmful in migratory deer populations, because fawns must learn traditional migration patterns between summer ranges and winter ranges from their dams."
Other investigators have reported contrasting results. In a 2000 acre Virginia enclosure, for example, researchers found a higher dispersal rate among orphans, 24 percent, compared to nonorphans, 6 percent.
Researchers led by Charles Nixon also produced results different from Holzenbein and Marchinton. They found no significant difference in dispersal behavior between orphans and nonorphans on the intensively farmed lands of Illinois.
In another study, however, the Illinois researchers reported higher dispersal rates for orphan females (69 percent) versus nonorphan females (37 percent). Survival rates were high for orphans and nonorphans, as only eight of 107 female fawns died from natural causes.
Clearly, the response of fawns to orphaning is highly variable, depending upon their sex, age, and a host of environmental factors.
Few failings are as unforgiving — or as abruptly terminal — as a deer's failure to avoid a predator. Logically, then predation has been a strong selective force in the evolution of the whitetail's behavioral traits — especially during the precarious fawning season and during harsh winter weather on Northern range.
The tendency for female whitetails to form cohesive social groups of related individuals is probably an adaptation to exploit patches of ideal food and cover, and to maximize offspring survival. The close alignment of fawn-rearing territories, as occurs among related does, assures the harmonious use of available space and provides an effective defense against marauding predators.
Mature, maternally experienced does exhibit superior predator-avoidance skills and invariably rear a greater percentage of their offspring, as compared to younger ones. This is especially evident when faced with effective predators, such as the coyote, black bear, and gray wolf.
Young does fawning for their first time tend to set up and defend fawning territories border-to-border with their mothers' territories. As a result, the young inexperienced mother can reap certain benefits. Since a doe cannot distinguish the calls of her own fawns from those of strange fawns, it's not unusual to see two or more does rush to defend a bawling fawn. Such behavior likely proves especially beneficial to the inexperienced young mother and her offspring, as the matriarch sometimes inadvertently lends defense against predators.
On Northern range, associating with an older deer helps the young ones to learn lengthy migratory routes, to locate favorable wintering areas, and survive the threat from predators. Also, there is definite safety in numbers when deer are subjected to harsh winter conditions and potential threat from predators. Multiple deer also do a better job of maintaining packed trails, which are critical to the whitetail's mobility and escape from predators.
Since older does are better able to protect their newborn fawns from predators, and provide critical guidance for younger deer during harsh winter weather, this adaptive advantage is lost when the bulk of the mature does are removed from the
Hunting-induced mortality ranks as one of the potentially most important factors that might disrupt whitetail social organization and impact herd genetics, because it can be highly selective.
Employing DNA analysis, in South Carolina, Chris Comer and
other researchers demonstrated that many related does established home ranges farther apart than expected on a Savannah River study site where antlerless deer were intensively harvested. Many unrelated does also had overlapping home ranges.
Overall, their results indicated a low degree of genetic structuring, suggesting young female deer on the Savannah River area were inclined to disperse from their natal range. As a result, does were less likely to form cohesive social groups of related individuals.
Studies in Minnesota and Illinois have also demonstrated unnaturally high dispersal rates among young does in areas where mature does experience high mortality.
In Mississippi, DNA studies conducted by Randy DeYoung also showed that intensive harvesting of bucks and does can disrupt normal social behavior patterns and alter herd genetics. He postulates that harvesting of females fragments female kinship groups and leads to greater spacing of related females. In addition, orphaning of male fawns tends to reduce male dispersal and contributes to genetic substructuring.
Young does (fawns and yearlings) seem to have a strong drive to achieve compatible associations with other mature does. If their mothers or other close female relatives are not available, then they may disperse considerable distances to find these associates. On the other hand, does 2 years and older tend to remain on their established home ranges.
Harvesting some adult does is generally desirable. In fact, killing a large percentage of the prime-age individuals can produce certain short-term biological benefits — at least in mild environments.
However, overwhelming evidence indicates that excessive doe harvests can disrupt adaptive female organization, fragment cohesive social groups of related does, and cause adverse biological consequences in harsh Northern environments.
Findings also indicate that such social disruption can alter herd genetics. However, the exact long-term genetic consequences, if any, are still obscure and will likely require long-term research to clarify.
Meanwhile, at least one researcher advises that we manage deer hunting "so that the age-specific survival pattern (and thus age-specific structure) emulates that occurring in the absence of hunting." In other words, attempt to inflict natural-type mortality — by killing the young and very old — in an effort to maintain natural population social structure.