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TEXAS A&M UNIVERSITY-KINGSVILLE

Recruitment of White-tailed Deer Fawns in Texas

Investing in Future Herds

Article and photo by MICHAEL J. CHERRY, Stuart W. Stedman Chair for White-tailed Deer Research

Doe on the East Foundation’s San Antonio Viejo ranch during late gestation in 2020.

Population dynamics of white-tailed deer are often characterized by stable adult survival rates, and variable fawn recruitment. Fawn recruitment can be influenced by many factors and is often much more sensitive than adult survival to environmental variability.

There are circumstances such as extreme drought, where all deer, independent of nutritional condition experience suppressed reproduction or fawn survival. However, such events are rare and generally deer in better nutritional condition have higher probability of survival and a reproductive potential that is more resilient to environmental variation.

The intuitive mechanism connecting nutritional condition to fawn recruitment is that fawns in poor condition are more susceptible to disease, starvation and exposure. However, there are likely more subtle linkages as well. For example, deer in poor condition may be more susceptible to predation because they engage in more risky behaviors to meet their energy requirements.

While there is abundant evidence that nutrition can influence population dynamics of deer, there is also emerging evidence that conditions experienced during early life may be important in determining the lifetime growth potential for deer. Luxury traits, or characteristics that do not affect survival but enhance reproductive potential such as antler size, may be especially sensitive to early life conditions.

It appears offspring resource allocation between efficiency traits, which enhance survival, and luxury traits may be a result of maternal effects. Maternal effects are the programming mothers provide to offspring to prepare them for local environmental conditions.

Maternal effects are thought to be driven by epigenetic mechanisms. Epigenetic change is the heritable change in gene expression that is independent of changes in DNA.

The heritable nature of material effects explains why it took multiple generations to overcome regional effects on body and antler sizes in captive experiments where deer were all provided equivalent high-quality diets. Therefore, a bad year may not only reduce the number of fawns recruited, but the quality of surviving individuals in terms of body and antler size for their entire life.

If the gene expression is also heritable these effects will also be seen in their offspring's characteristics. In wildlife, this phenomenon is referred to as a cohort effect, and in deer this effect has been shown to transcend generations, as fawns pass along epigenetic changes in the way offspring express genes. Ultimately, if managers seek to improve the quality or quantity of deer, they need to pay close attention to fawn and doe nutrition.

Resource availability is highly dynamic on many landscapes and is often driven by factors that are independent of deer density such as weather patterns. Annual variation in rainfall has profound effects on deer recruitment and likely results in cohort effects that influence adult body and antler sizes.

To reduce the influence of fluctuations in environmental conditions, managers often provide supplemental feed or conduct habitat improvements to maintain more stable per capita resource availability. Feed is not a replacement for rain, but it reduces the importance of rainfall in defining resource availability.

This can have a stabilizing effect on fawn nutrition and recruitment in highly dynamic environments. However, there are many open and important questions regarding resource availability and its effects on fawn nutrition, recruitment and epigenetic changes. For example, when in the reproductive cycle should managers target for enhanced nutrition? Knowing the relative importance of resource availability during gestation, lactation, or post-weaning has important implications for deer management.

When data do not exist for wild animals, wildlife biologists often borrow from related fields of research, particularly biomedical and animal science. These bodies of literature suggest that the condition of the mother during early gestation (or breeding condition in livestock) can strongly influence quality of placental connections to the fetus which has implications for the transfer of nutrients, oxygen and waste between the mother and fetus during gestation.

This suggests managers should attempt to enhance nutrition in late fall and winter to ensure does are in good condition during breeding and early gestation. However, the majority of fetal growth occurs during the final trimester of gestation and lactation is the most energetically demanding portion of reproduction for the doe. Therefore, spring and early summer may also be important periods to target.

Furthermore, early life conditions in humans are key determinants of lifetime growth potential and appear to be an important period for epigenetic changes. For example, children who were younger than 2 years old during the Dutch Famine experienced during World War II, exhibited reduced adult height despite returning to a highly nutritious diet following the famine.

People were not able to overcome the effects of poor nutrition during early life and were shorter in adulthood than other cohorts. While other age classes also experienced malnutrition, they were able to overcome the temporary effects of food shortage.

If similar processes occur across mammals, managers should target nutritional supplementation to summer, early fall and winter. While it is well recognized that nutritional condition of fawns and does drive recruitment and fawn growth, research is needed to identify when during the reproductive process managers should target supplemental nutrition to enhance resource availability.

In addition to the timing, the specific types of resources are also an important question. How variation in digestible energy, crude protein and macronutrients influence growth rate, body design and epigenetic changes are important questions. The answers hold the key to optimizing management that ensures deer are in good nutritional condition and have plenty of the right resources at the right time to express luxury traits such as antlers.

In February, Texas experienced an extreme weather event that dramatically altered resource availability for deer. The winter storm decreased forage availability by killing or knocking back forages. This occurred during the energetically demanding period of gestation and some areas rebounded very slowly due to a lack of rainfall. How this storm influences fawn growth and survival will be worth watching.

Sponsored by JOHN AND LAURIE SAUNDERS

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