Harvesting mature does may not always be a good idea!
September 01, 2011One objective of Quality Deer Management (QDM) is harvesting enough female deer to maintain population in balance with the habitat. QDM guidelines generally recommend harvesting the largest doe in a group because they are most likely adults. Such selective harvesting accomplishes two things: it usually removes the most productive individuals, thereby most effectively lowers deer density, and protects button bucks from harvest.
For good reasons, this practice of harvesting mature does may not necessarily be the best thing to do in areas with low deer populations.
In Michigan's Upper Peninsula, for example, deer populations have dropped sharply since the mid-1990s, when they admittedly were too high. Many areas now have fewer than 10 deer per square mile on summer range capable of supporting two or three times that number.
In this northern environment, winter weather, availability of quality winter habitat, and the forestry industry tend to control deer populations - more so than summer range limitations and hunters. On average, due largely to poor nutrition in winter, malnourished does lose about 50 percent of their newborn fawns. In addition, about 50 percent of the fawns fail to survive their first winter.
Given such circumstances, then, the general advice is to harvest does under 2.5 years old - supposedly those with the lowest reproductive potential. In fact, I've advocated selective harvesting of fawns in this northern region. Yes, even buck fawns, often to the dismay of QDM advocates, because fawns are least likely to survive (unpredictably) tough winters.
Most female whitetails reproduce at a young age and continue to do so throughout their lives. The logical question here is: does reproductive performance change appreciably with female age? Or, is there any advantage in maintaining populations of mature does for greater reproductive potential?
More specifically, how much do conception dates and fetal rates differ among doe age classes? Are some doe age classes more successful at raising fawns than others? Do sex ratios of progeny vary according to doe age? Do some doe age classes produce superior offspring? These questions are not easily answered when studying free-ranging deer herds.
Studies I conducted in the square-mile Cusino enclosure, located in Michigan's U.P., provided a special opportunity to follow the reproductive performance of individual does, and ponder these questions under highly controlled conditions.
The Cusino enclosure was built in 1952 to investigate how vegetative changes influenced deer population dynamics and, conversely, how deer affected vegetation.
In 1972, we initiated new studies that emphasized deer sociobiology. At first, we evaluated the pros and cons of supplemental feeding whitetails to achieve populations much higher than the habitat would support naturally. We also sought to more precisely define the whitetail's social organization, and to determine the consequences of density stress on deer, independent of nutrition.
In follow-up studies, we lowered deer density and manipulated the herd's sex-age composition to mimic various harvest strategies, and then monitored the population's physical and reproductive response. In effect, we shaped populations exactly as we needed to determine how changes in social behavior influenced deer biology.
Although deer foraged naturally in the enclosure, we provided a nutritious pelleted ration year-round to supplement their diet, and to assure that limited nutrition did not influence our results.
Each winter we conditioned the herd to feed from livetraps and then live-trapped all deer in March. Each deer was then weighed, measured and blood-sampled to assess its physical condition and blood-hormone levels. At that time, surplus deer were released outside the enclosure. At that time, surplus deer were released outside the enclosure. All those scheduled to be returned were tagged and marked for field recognition, and temporarily held in separate holding-pen until trapout census was complete. Depending on study objectives, some deer were equipped with radio transmitters for detailed behavioral studies.
Our most valuable data were gathered by x-raying does to determine how many fetuses each doe carried. We even developed techniques to age fetuses on radiograms. This allowed us to calculate breeding and fawning dates, and determine potential fawn production. Combined with tagging of newborn fawns and live-trapping and marking fawns in autumn, subsequent field observations of marked deer permitted us to assess fawn-rearing success on an individual basis.
As we watched the enclosure's population grow, it became obvious that whitetails have evolved a highly organized social system - one that permits them to rapidly exploit favorable patches of food and cover, and to live harmoniously even at fairly high densities.
We found that whitetails exhibit "sexual segregation" and "resource partitioning." That is, bucks and does live separately outside of the breeding season, and do not compete equally for the same space, food and cover resources.
The basic female social unit consists of a matriarchal doe, several generations of her daughters, and their fawns. Members of such cohesive groups share common ancestral range, and associate with each other except during the fawning season.
Based on their degree of physical and behavioral development, individual deer occur in distinct social classes that are hierarchical in nature. Among females, the oldest animals normally occupy the supper classes and suppress the aggressiveness and reproductive performance of younger individuals as means of maintaining social order.
I recorded 503 breeding dates for 179 does (1.5 years or older) over a continuous 19-year period (1974-1992). Although only four of the adults failed to breed, only three of the 274 doe fawns (6-7months old) bred.
The first enclosure mating occurred Oct. 25 to Nov. 9 annually, the average was Nov. 4 Although the bulk of the herd (90 percent) bred during November, for unknown reasons, a few does (less than one percent) bred in January and February. Typically, does 3.5 years of age and older bred earlier, as compared to younger does - especially at high herd density.
From 1972 to 1977, we allowed the enclosure herd to increase from 23 to 159 deer. Prior to 1975, peak breeding occurred Nov. 16-19.
Changes in the rut's timing were first noted in 1975 when the herd reached 116 deer, then the average breeding date for yearling (1.5 year-olds) does was Nov. 26, about one week later than normal. When the herd reached 159 in fall 1976, the average breeding date for yearlings was Nov. 23 and for 2.5 year-olds Nov. 29. However, the breeding dates for older does were influenced by herd density.
Pregnancy rates for adult does in the Cusino enclosure increased rather dramatically with supplemental nutrition. With improved diet, fetal rates doubled among yearlings, increased 50 percent among 2.5 year-olds and 21 percent among older does.
However, even with supplemental nutrition, yearlings and 2.5 year-old does, on average, carried fewer fetuses per doe, 1.5 and 1.8 respectively, as compared to older doe age classes (1.8 to 2.2 fetuses per doe).
Typically, a doe is presumed to be past her prime by age 8, and her productivity is expected to decline. However, that was not the case for well-fed Cusino does. As a group, does 9 years and older often conceived triplets and had the highest fetal rate: 2.2 fetuses per doe.
Fawn Sex Ratio
Overall, the enclosure doe herd produced a slight preponderance (51 percent) of buck fawns, but progeny sex-ratios varied greatly among doe age classes.
Yearling does produced proportionately more males, as did does 6 years or more in age. Conversely, 3, 4 and 5 year-olds produce more females.
In the whitetail's matriarchal society, 2 year old does fawning for their first time establish fawn-rearing territories bordering their mother's. The tendency to produce bucks - which ultimately disperse - helps minimize mother/daughter competition for fawn-rearing space.
Second-time mothers - generally 3 year-olds, disperse varying distances and establish new fawning territories. They also expand the ancestral range and establish new family groups. Hence, their production of more does than bucks for several years quickens the establishment of new group consisting of daughters independent of the matriarch.
Conversely, older does more likely have already formed complex matriarchal groups, and benefit by producing dispersing males.
Before the enclosure herd was supplementally-fed, pregnant does lost about one-third of their fawns soon after birth, largely due to malnutrition during winter and resultant poor fetal development. Newborn fawn mortality declined after diet supplementation, but still averaged 18 percent overall.
Total fawn mortality was greatest among 2 year-old and 3 year-old does - 27 percent and 26 percent, respectively. Older does only lost about 11 percent of their fawns. Fawn losses were especially high in times of high deer density, and from predation when black bears entered the enclosure.
Despite unlimited nutrition, once deer density surpassed 100 deer, 2 year-old does lost 63 percent of their fawns, whereas 3 year-olds lost 24 percent. Meanwhile, older does only lost 6 percent of their fawns.
Indirect evidence suggested that most fawns died shortly after birth and that losses were related to territorial behavior associated with fawn rearing. That is, because does with newborn fawns are extremely antagonistic and defend a territory (10-20 acres) for about one month, crowding at peak density limited fawn-rearing space and disrupted maternal behavior. We concluded that heavy fawn mortality resulted either because of imprinting failure or outright abandonment of otherwise healthy offspring by socially stressed young, inexperienced mothers.
A 6-year telemetry study revealed maternal experience is also an important factor in determining whether a newborn fawn survives when subject to predation.
Black bears entered the enclosure during three of the six fawning seasons studied. Newborn fawn losses averaged 32 percent for the years when bears were present, compared to only 10 percent otherwise. However, first and second time mothers (2 and 3 year-olds) suffered the greatest losses when bears were present, 32 percent and 58 percent respectively; as compared to when bears were absent, 13 percent and 4 percent, respectively.
By comparison, older does lost only slightly more fawns when bears were present, 17 percent, as compared to when bears were absent, 11 percent.
Our findings indicate that young fawn's bed-site habitat, movement patterns, social and special relationships, and evasive tactics when threatened by bears are largely maternally controlled. Apparently, learning experience and outright defense of their offspring largely accounted for older does' improved fawn-rearing success - a phenomenon worth considering when managing whitetail populations plagued with heavy predation.
From a recruitment standpoint, older does far surpassed younger does, on average, raising 1.8 fawns per doe, as compared to 1.1 and 1.3 fawns per doe, respectively, for 2 and 3 year-old mothers.
Although we handled fawns at various ages, all of them were captured and examined annually in March, when 9 to 10 months old. However, small-bodied fawns usually gained some weight over winter while consuming the nutritious ration. At the same time, large fawns lost weight. So, differences narrowed by the time we handled all of them in March.
We were able to determine the mothers of 264 enclosure fawns. Mature does consistently produced the largest buck fawns, whereas difference in body size among female fawns differed little with regard to the mother's age.
Among doe fawns, early-born individuals raised by mature mothers, on average, were the largest (80 pounds), but even the smallest, which were born to late-breeding 3 year-old does, were respectable (72 pounds).
Difference in buck fawn weights, relative to their mothers' age and time of conception were much greater. For example, the largest males, weighing an average of 92 pounds, were born to early-breeding mature does. The smallest buck fawns, averaging 77 pounds were to born to late-breeding 2 year-old does. Interestingly, even late-born buck fawns raised by mature does were as large (83 pounds), on average, as early-born males raised by 2 year-old mothers and almost as large as early-born males raised by 3 year-old mothers (85) pounds.
Litter size was also an important factor determining buck fawn size. Within a doe age-class, single males were usually larger than males raised in twin or triplet litters. The largest buck fawns, averaging 94 pounds, were singletons raised by mature does.
Even with supplemental nutrition, yearling bucks varied greatly in their physical and antler development. On average, if a buck fawn did not weigh more than 85 pounds by late fall, it was also under-sized as a yearling and usually grew only spike antlers. These poor-quality individuals also weighed less and carried smaller-than average antlers when 2 years-old, which runs countered to studies, conducted in small pens.
So, it appears early-born single males, raised by older does have a definite advantage. This also suggests there is adaptive logic involved in the tendency for older does to produce a preponderance of buck fawns. That is, these large buck fawns stand the best chance of becoming large dominant bucks that sire many offspring and perpetuate their mothers' genes.
Populations comprised chiefly of mature does (4 years and older) will not solve the long-term winter habitat/winter weather problems whitetails face on northern range. But such populations have the highest reproductive potential. They exhibit the best fawn-rearing success, produce physically superior offspring, and more likely yield a huntable surplus of deer during times when more favorable conditions (i.e., mild winters and/or winter habitat improvements) prevail.