Fawn Production: A South Texas Perspective
Article by Kory Gann, Wildlife Biologist, Texas Parks and Wildlife Department
Photos by Steve Bentsen, Larry Ditto, Russell Graves, David Hewitt, Butch Ramirez, Joseph Richards
Fawn recruitment, it can be argued, is the most important component of a successful South Texas deer management program. In this region, fawn recruitment, the rate of fawns surviving to one year of age, is highly variable and often low, with autumn helicopter surveys averaging 35 fawns per 100 does.
Deer populations here persist because survival of adult deer in the absence of hunting is around 90 percent. In fact, sampling of un-hunted deer populations on East Foundation properties shows that mature deer, 6 years or older, make up 30 percent to 40 percent of these herds. Such a high proportion of mature deer can only occur if adult survival is high.
For hunted populations where the harvest of mature bucks is the goal, consistent fawn production and recruitment of buck fawns into the population is key. Although it is not uncommon for fawns to get overlooked as an important component of the herd, managers must remember that all mature bucks begin their lives as buck fawns.
WHERE DO ALL THE FAWNS GO?
To understand the dynamics of fawn production and their recruitment into the deer herd, we first must understand how many fawns are actually being born in a given population. David Hewitt, Director of the Caesar Kleberg Wildlife Research Institute (CKWRI), recently presented data indicating that, during any given year, a typical population of 100 does in South Texas may give birth to as many as 136 fawns.
This number assumes 95 percent pregnancy rates and litter sizes of one fawn for juvenile does (2 years old) and 1.8 fawns for mature does. Of these 136 fawns, on average only 35 will be counted during the autumn deer survey (along with the 100 yearling and older does used to make the observed fawn:doe ratio), meaning that 74 percent of these fawns died before they were three months of age.
This estimate is supported by results of a long-term research project conducted on the Comanche and Faith Ranches near Carrizo Springs in South Texas. In that study, Asa Wilson, CKWRI graduate student, recorded fawn mortality of 65 percent to 75 percent from birth to three months of age.
But does the autumn deer survey tell the whole story? A fawn is not recruited into the population until it graduates to the yearling age class. Most deer managers recognize that mortality is greatest when fawns are small, but many managers assume high fawn survival during winter because of the relatively mild South Texas winters.
Researchers at CKWRI have shown that fawn mortality from 3 months to 8−12 months of age ranges from 35 percent to 50 percent in South Texas. Of those 35 percent surviving fawns seen on the survey, another 40 percent may die before they reach one year of age. This high mortality rate means that only 15 percent of fawns born in the summer will likely live to see their first birthday. So, what is happening to all the fawns?
South Texas fawns die for a variety of reasons. Predation by coyotes, bobcats and feral hogs is undoubtably an important mortality source; however, it is difficult to differentiate predation from scavenging in most situations.
Fawns that are weak from poor nutrition or disease may be more susceptible to predation. Disease, more than likely, also plays a role in fawn mortality but is difficult to assess in free-ranging fawns as disease may be caused by underlying factors like malnutrition, heat and other stressors.
Another factor that can potentially affect fawn growth and survival is the extreme heat common in South Texas during summer. Fawns in South Texas are born in July when high temperatures may average 100 degrees. Research on captive fawns conducted by CKWRI Graduate Student Nicole Alonso revealed that fawns raised in a hot environment drank less milk from birth to 4 weeks of age, weighed 8 percent less at 3 months and had greater stress hormones compared to fawns raised in a cooler setting.
Good nutrition for both does and fawns is extremely important to fawn survival. South Texas is a semi-arid environment where rainfall is highly variable, and drought is common. Rainfall variability affects both availability and nutrient content of forages and in turn is the single most important factor influencing the region’s deer productivity.
Spring and summer rainfall produce high-quality, nutritious forbs (weeds) that provide does with nutrition needed to produce and raise fawns. When rainfall is lacking, forbs dry up and disappear. The browse (leaves and stems of woody plants) that remains provides deer with a large amount of low-quality food that meets nutritional requirements for maintenance—and ultimately survival—but not fawn production. Dry summers and periods with low precipitation may not produce sufficient forage quality to support lactation.
Three key nutritional periods exist regarding fawn production and survival: gestation, pre-weaning and post-weaning.
Gestation is the period when does are pregnant. A doe’s nutritional requirements increase slightly above maintenance levels during the first and second trimesters of pregnancy and are highest during the last trimester. Although aborted fawns are rare, low nutrient intake by does during gestation, particularly the last trimester, can increase the instance of weak or stillborn fawns.
Pre-weaning occurs after the fawn is born. During this time, the fawn depends on its mother’s milk. Lactation is nutritionally expensive for the mother, requiring her to use her fat reserves and muscle tissue to produce enough milk to grow a big, healthy fawn.
A lack of high-quality forage to meet a doe’s needs for lactation has several implications. First, the more fat and muscle a doe carries entering the summer, the more milk she can produce. Second, because a doe’s body can only meet a portion of her energy needs during lactation, having access to high-quality foods during late summer and early autumn will increase milk production, helping her fawn grow fast.
Post-weaning, when the fawn is weaned from its mother’s milk, generally occurs in September or October. At this point the fawn is still growing but is a fully functioning small ruminant.
Small ruminants like fawns must consume high-quality forage because they have small stomachs that cannot process a high volume of poor-quality forage, and they must meet the high nutritional demands of body growth within the limits of their small digestive tract. Malnutrition from a poor-quality diet can result in reduced growth rates and low over-winter fawn survival.
Lactation’s cost is extremely high for does raising fawns and is by far the most energetically expensive life history stage. During lactation, digestible energy requirements may increase by 40 percent over maintenance levels. Even during wet periods, does may have a difficult time consuming enough forage to meet energy demands and will use fat and muscle tissue reserves within the body in an attempt to make up for the energy deficit. This energy deficit is even more pronounced during dry periods when high quality forage is not available.
As does use body tissue, their body condition begins to decline. Because they use body tissue to fuel lactation, does raising fawns will be in visibly poorer condition in autumn than those that do not. If resource availability is low or if a doe is in poor body condition when she gives birth, she may simply choose not to raise the fawn.
In this instance, the high energy cost associated with lactation may cause the doe to favor her own survival rather than invest resources toward current year reproduction. Because pregnancy rates of adult female white-tailed deer are around 95 percent, her survival ensures that she can attempt to reproduce in subsequent years.
WHICH DOES ARE RAISING FAWNS AND WHEN?
Nutrition is a limiting factor of fawn recruitment in South Texas, and rainfall changes the nutritional environment and age influences a deer’s nutritional requirements. So, how might rainfall and a deer’s age interact to influence patterns of fawn recruitment?
A long-term research project conducted by CKWRI as part of the East Foundation’s research program showed that both doe age and rainfall timing greatly influence fawn production in South Texas. Researchers evaluated the lactation status of does captured in October and November as an indicator of raising fawns to 3 to 4 months old and determined that, on average, 3 percent of yearling (1-year-old), 11 percent of juvenile (2-year-old), and 28 percent of mature (3-year-old or older) does showed signs of lactation at capture.
Furthermore, when March through July was wet, 40 percent of juvenile and 47 percent of mature does showed signs of lactation, but when spring and early summer were dry, 4 percent of juvenile and 22 percent of mature does were lactating. Although these captures only give us a brief snapshot in time, they show that a doe’s probability of lactation is strongly correlated with both spring rainfall and age.
Good spring rains increase forage quality allowing does to enter late gestation and lactation periods in good body condition. This improved body condition gives females an insurance policy against future nutritional deficits because they now have excess energy in the form of body tissue to offset lactation’s nutritional cost. Younger does may not have excess energy to devote to reproduction, particularly during a dry spring, because they require additional energy to reach physical maturity and cannot attain the same tissue surplus as mature does.
Even though younger does are often a prominent part of the population, mature does appear to be responsible for most fawn recruitment. CKWRI Research Scientists Aaron Foley and Randy DeYoung used DNA analysis to determine the parentage of fawns as part of the recently completed Comanche-Faith Research Project. Their results found that even with high pregnancy rates for 1-year-old and older does, mature does accounted for 80 percent of fawns recruited into the population regardless of the presence of supplemental feed.
Why? Mature does are finished growing. They tend to be in better body condition. And mature does tend to be more dominant which can give them better access to vital resources such as prime foraging sites and productive fawning cover.
However, Foley and DeYoung found 27 percent of mature does with access to supplemental feed and 44 percent of mature does without access to supplement did not raise a fawn for at least three consecutive years. This suggests that some does are unlucky or do not have what it takes to be a successful mother. This variable productivity among mature does is likely caused by a combination of maternal experience, competitiveness and dominance rank, all of which are indicators of individual quality.
MANAGEMENT ACTIONS TO INCREASE FAWN RECRUITMENT
Most conversations about fawn survival include a discussion on predators and predator removal. Research from predator removal studies has shown mixed results when it comes to increasing fawn survival. We know that predator exclusion increased fawn survival along the Texas Gulf Coast, but this region receives more rainfall than other parts of South Texas and nutrition is a less limiting factor.
Predator removal efforts, if desired, should occur the month before fawns arrive. This will create a temporary void in the predator population that may give fawns a head start before the void is filled by dispersing young predators. Because most coyotes are territorial, the removal of dominant individuals may lead to a net increase in overall coyote numbers as more, young coyotes move in seeking their own territory.
To reduce the risk of predation, managers should manage grazing in a way that leaves plenty of grass to provide hiding cover for fawns. However, grass by itself does nothing to protect fawns from the summer heat. In this case brush is extremely important to provide thermal cover for does and fawns helping them regulate their body temperatures and reducing overall energy expenditures.
In the summer, shade from woody canopies, like mature mesquite trees, can reduce soil temperatures by 23 degrees compared to open, grassy areas. If brush management is being conducted on your property, seek out areas with overall low diversity to manipulate, leaving thick, tall, diverse mixed brush and riparian areas undisturbed. Mechanical brush management should be conducted in a mosaic or motte pattern. In the summer deer will utilize the edge of these brush stands taking advantage of shade they offer in conjunction with the prevailing south or southeast wind.
We know that malnutrition of both does and fawns is the primary limiting factor for fawn survival in South Texas, so what can we do to insulate deer against poor nutrition resulting from drought? Management decisions that promote diversity over preferred plant species are key.
Habitat management practices that promote forb production such as discing and other ground disturbance techniques are okay in moderation. Spring forb production is key to increasing doe body condition as they prepare for lactation.
Because forb production can be limited by rainfall, it is important that we do not put all our eggs in one basket. Open areas managed for forb production should be adjacent to stands of diverse mixed brush. This mixed brush provides deer with more foraging choices during dry periods.
Even plants species that do not get much credit for their forage value, such as prickly pear and mesquite, are important. When available, mesquite and prickly pear fruits may compose up to 80 percent of a doe’s summer diet. Why? These fruits are primarily composed of energy rich carbohydrates including sugar. Because energy is usually a limiting factor for lactating does, these fruits are extremely important.
Supplemental feed is a common practice on South Texas ranches. Feed is usually provided to increase to the size of bucks, but it is also a way to increase fawn survival and overall herd health.
Supplemental feeding has been shown to increase fawn survival to 3 months by 15–30 fawns per 100 does over un-supplemented herds. Fawn survival to eight months may be as high as 40 percent−60 percent compared to 10 percent to 15 percent for un-supplemented herds. However, social displacement and fences around feeders may limit fawn access to feed sites. Fences 33 inches and lower are necessary to facilitate fawn access to feed sites. Gaps in fencing 8 inches wide and beginning 10–15 inches above the ground will keep most pigs from entering feed pens while allowing fawns access.
Which does we harvest and when they are harvested affect fawn recruitment. Fawns orphaned in early November have smaller home ranges and a 21 percent decline in survival compared to un-orphaned fawns. These results support waiting until Thanksgiving to begin harvesting does.
What about that “old, skinny doe?” Should she be harvested? We know that pregnancy rates in adult females are very close to 100 percent, so labeling these does “barren” is largely a misnomer.
Doe body condition in the autumn is also highly correlated with whether that doe raised a fawn to the post-weaning period, with lactating does generally having poorer body condition than non-lactating does. Thus, poor body condition during autumn is generally a sign the doe was successful raising a fawn and is worth keeping.
If doe harvest is necessary to meet management objectives, the harvest strategy should be to retain good mothers. Because there are does that rarely raise fawns, does with fawns should not be harvested. These does have shown they have the capability of raising fawns on your ranch or lease.
Although big, mature does may be great for filling the freezer, mature does may be responsible for around 80 percent of the fawns recruited into the population. In this instance, favoring young does for harvest would make sense while leaving mature does because of their immediate reproductive capabilities. Having a large pool of mature does in the population may help increase fawn recruitment, thus ensuring mature bucks are available for harvest in the future.