Despite its long, hot days, summer is a time of renewal within the whitetail world. Throughout North America, most does give birth in May or June, following a 195-day gestation period. At birth, whitetail fawns are hiders, and they don’t begin traveling with their mothers for about 30 days. This is why fawns begin to become visible in most areas in July or August. Most hunters and deer managers give little thought to this important time of year. Sure, many plant food plots for summer nutrition or create patches of dense, low-growing natural habitat for fawning cover, but few consider the sex ratio of the fawns being born. You may wonder why you should be concerned with this, and what you could do about it even if you were.
While the sex ratio of fawns at birth across a large region is roughly 50-50, research has identified factors which can skew this toward one sex or the other on a local basis. This is where Whitetail Science becomes both fascinating and useful. Knowing these factors can potentially increase the number of male fawns born on your property and, thus, the number of adult bucks you can pursue in future years.
Factors Affecting Fawn Sex Ratios
To date, research has identified two factors that influence the sex ratio of whitetail fawns, doe age and time of breeding relative to onset of estrus (heat). Research by DeGayner and Jordan in Minnesota revealed that does 4.5 years of age and younger produced proportionally more male fawns while those older than 4.5 produced more female fawns (see Figure 1). Comparing extremes, 1.5-year-old does produced approximately 70 percent male fawns while 9.5+-year-old does produced only 30 percent male fawns.
Let’s examine the prevailing theory regarding how doe age could impact fawn sex ratios. Social organization within the whitetail world is based on a matriarchal system of related females banded together in individual family units. In most cases, the oldest doe occupies the highest rank, and serves as group leader. This “alpha” doe also selects the highest quality fawning cover, with lower-ranking group members taking progressively lower quality fawning areas. This system allows the oldest does in each family group to occupy the best fawning areas. Under such conditions, older does invest in producing more female fawns because these offspring are likely to survive and join the family group, thus maintaining the group and its genetic basis. In contrast, younger does, which are relegated to lower-quality fawning areas, invest in more males. This counters the over-production of females by older, dominant does. Also, since most young bucks will disperse (leave) from their birth area and travel 1-4 miles before establishing a new home range, they will not be relegated to the poor quality habitat of their birth area.
Research suggests that time of breeding within the estrus or heat period also affects fawn sex ratios. A study by Verme and Ozoga in Michigan reported that does being bred relatively early in the estrus period (13-24 hours), produced only 14.3 percent males, while does bred later (49-95 hours) produced 80.8 percent males.
The theory behind these findings is based on impacts to deer herd health. Within a deer population, time of copulation within the estrus period is determined largely by the adult sex ratio (number of adult does to adult bucks) and the age structure of the buck population (older bucks are more experienced breeders than younger bucks). Thus, does in herds with balanced sex ratios and abundant adult bucks would tend to breed early in their estrus cycle and produce more female offspring. In contrast, does in populations with skewed sex ratios favoring does, or those with insufficient numbers of adult bucks, would tend to breed later and produce more male offspring. The production of males when they are in short supply could be nature’s way of restoring balance.
Putting it All Together
Collectively, research suggests that older does, and those which breed early in the estrus period, are more likely to produce female fawns, while younger does, and those breeding later in estrus, tend to produce more male fawns. From a deer management perspective, deciding which does to harvest depends on your objective. If your goal is to increase the size of your deer herd, an abundance of female fawns would be an advantage. They are more likely to remain on your property (as most young males disperse) and contribute to herd growth. As such, you should restrict harvest to younger does. You should also manage for an adult sex ratio of one buck for every three or fewer does to maximize the number of does that breed early in their estrus period.
However, if you have a desirable deer density and simply want to increase the percentage of buck fawns, you should target the oldest does. Some deer experts recommended “turning over” a doe herd about every four years. In other words, to maximize the percentage of buck fawns hitting the ground on your property, few does should be allowed to survive beyond 4.5 years of age. It could be argued that a population with a young doe age structure and an unbalanced sex ratio with few adult bucks would maximize the proportion of buck fawns. However, the disadvantages of having few adult bucks far outweigh any potential benefits. For example, with few adult bucks, breeding-related stress would increase on those present, resulting in additional loss of body weight and overall condition. This could result in increased post-rut mortality, further decreasing the number of adult bucks the following year. Also, an extreme lack of bucks often increases the number of does that are not bred on their first estrous cycle, thereby delaying the time of fawn birth by one month – a huge disadvantage regardless of whether it’s a buck or doe fawn. So, under most Quality Deer Management programs, manipulating doe age, not time of conception through a scarcity of adult bucks, is the logical approach.
While unrelated to fawn sex ratios, a study by Holzenbein and Marchinton in Virginia concluded that harvesting a doe with a buck fawn at her side can significantly decrease the odds the fawn will disperse when it reaches 12-18 months of age. They compared 34 buck fawns divided into two groups – 19 that were left with their mothers (non-orphans) and 15 whose mothers were harvested or removed (orphans). The results were surprising. By 30 months of age, 87 percent of the non-orphans had dispersed from their birth areas, but only nine percent of the orphans had left theirs. In other words, dispersal was greatly reduced if the young buck’s mother was removed prior to dispersal. So, harvesting a buck fawn’s mother before she forces it to leave is another potential strategy to increase the number of bucks on your property.
As the hot, hazy days of summer begin to give way to the promise of autumn and opening day of archery season, take advantage of this Whitetail Science and devise a doe harvest strategy that best meets your objectives. Who knows, you might just be able to increase the number of bucks on your favorite hunting area. At the least, I hope you have gained a new appreciation for the fascinating and complex world of the whitetail.
Brian Murphy is a wildlife biologist and CEO of the Quality Deer Management Association (www.QDMA.com). He also has been an avid bowhunter for the past 30 years.