**4. Discussion**

Positive pairwise correlations were present between all four measures of individual deer: tick prevalence, musculature, coat condition, and body condition. That is, deer that had one health issue were more likely to also have another. This could simply reflect that some measures may be related (e.g., coat condition and overall body

#### *Changes to Health Parameters of White-Tailed Deer during a Drought in the US Virgin Islands DOI: http://dx.doi.org/10.5772/intechopen.108270*

condition have some common sources of overlap). However, other measures (e.g., tick prevalence and musculature) do not naturally overlap. Therefore, our results suggest health factors are indeed associated perhaps because the underlying stress of deer represents a common etiology. The population of deer on St. John are largely diseasefree [22], and demonstrate relatively high allelic diversity for an isolated island population [21] which may act as a mitigating factor for stress. However, this study demonstrates a strong response during peak drought for all of the variables.

Our observations suggest that deer on St. John were highly nutritionally deficient, particularly within the drought peak in July 2015, when deer faced severely limited feeding within a denuded landscape. During the peak drought period, deer appeared to be eating atypical forage, often called famine foods, in the absence of typical and available forage. Famine foods are eaten by animals when all known food sources have been exhausted. They often contain low levels of protein, are low in calories provided [31], and can be very energetically expensive to metabolize. Observations during the drought peak included a fawn eating dead leaves and adult deer spending considerable time and energy digging up tubers and roots. Dependable sources of water on St. John were also severely reduced in both quantity and quality, and deer were often seen near anthropogenic water sources such as pipes, foot baths, and shower areas. Their body condition during the height of the drought indicated both severe water and nutritional stress [23]. Below we analyze multiple indicators of deer health to show individual responses to drought onset, peak, and recovery.

#### **4.1 Indicators of deer health**

### *4.1.1 Body condition and coat condition*

Results for body condition followed the High-High-Low pattern, demonstrating a higher prevalence of poor health during both the drought onset and peak. This indicates that the deer were in a state of stress at the drought's onset and were without the reserves needed from the lack of food and water availability during the drought peak. Body condition scoring represents an amalgam of multiple health parameters representing external markers of internal health. Many of the factors that were evaluated with body condition, including coat condition and musculature, are complementary data for assessing the health and nutritional state of an animal. These factors are influenced by both current and past food availability, and can therefore indicate health in both the present and recent past. Prolonged nutritional challenges such as drought can cause individuals to exhaust stored fat reserves which can result in a deterioration in body condition [6]. In contrast, coat condition demonstrated a Low-High-Low pattern during the drought. This suggests that coat condition is not as sensitive to the onset stage of the drought as body condition, and coat condition is able to return to original condition during drought onset within the drought recovery period. Coat condition represents a sub-category of the body composition assessment, and one that can further indicate nutritional health. Essential vitamins, as well as fats and oils in the diet, are needed to provide coat and hair luster [32, 33].

#### *4.1.2 Muscle and muscle atrophy*

Results for musculature followed the High-High-Low pattern, demonstrating a greater prevalence of poor health both during the drought onset and peak. The response of musculature is similar to body condition in that the pattern indicates that individual deer were stressed at drought onset, indicating a deficiency in both food availability

and in high quality foods that contain adequate protein reserves to support muscle development and growth. Muscle is an energetically expensive tissue to build and maintain [34]. Muscle presence and appearance can be an indicator of overall health and an excellent external indicator of both health and protein reserves. However, with chronic and insufficient protein consumption, muscle catabolism can occur, resulting in muscle atrophy [33]. In addition, because more than 80% of protein in the animal body is dedicated to maintaining proper functioning of the immune system, disease state can indirectly result in muscle loss and atrophy. As a result, there can be a strong correlation between muscle appearance and the disease state of a deer. If muscle atrophy is present, this indicates that the animal body is using muscle as an energy source in the absence of any remaining fat reserves. More energy is released when a unit of fat is metabolized as compared to a unit of protein. Therefore, when a deer has metabolized most of its fat and is using muscle for energy, it will lose weight very quickly as subcutaneous fat is already depleted [35] and will appear gaunt and emaciated in appearance as a result [32]. In contrast, excellent musculature indicates that the deer is eating sufficient protein to both maintain muscle and to support the high protein demands of the immune system [36]. These changes in subcutaneous fat can be evident via visual assessment [24]. The results of this study add to evidence that both musculature and fat reservoirs are sensitive to drought even in the early onset stages within island populations.

#### *4.1.3 Ticks*

The dynamics of infection often depend on the host's vulnerability, as poor body condition is likely to predispose individuals to infectious and parasitic diseases [6, 7]. Tick prevalence in this study followed a High-High-Low pattern, indicating that ticks were already both prevalent during both the onset and peak of the drought. The additive effect of this continued high level of parasitism has negative consequences for individual deer health for blood loss and the potential for disease development. A common parasite found on the deer of St. John included ticks. The two tick species found on St. John include the southern cattle tick (*Rhinocephalus* (*Boophilus*) *microplus* (Canestrini) and the tropical horse tick, *Dermacentor* (*Anocenter*) *nitens* Neumann [23]. High tick densities could result in associated health problems, including pruritis, alopecia, anemia, and low weight gain. In addition, ticks can deplete the iron resources of the deer through each blood meal taken, which can result in the development of iron-deficiency anemia and deprive tissues of necessary oxygen [37, 38]. There can be a considerable energy requirement to replace daily blood loss which could result in further accelerated nutritional decline and weight loss. Additionally, the relative energetic cost associated with compensating for blood loss is higher for animals in poor condition who can experience more pronounced energy and protein deficits compared to healthier animals [37].

#### *4.1.4 Stress hormones*

Stress hormones help the body handle adverse conditions, and levels can vary by situation and species [39]. Cortisol is released by the body when there is either an acute or chronic stressor. Cortisol can suppress the conversion of T4 thyroid hormone into T3, and lower circulating T3 levels can be an indicator that cortisol levels are high. Together, both T3 and cortisol levels create a complementary data set that demonstrates stress levels in individual animals [28].

Cortisol was the only measure from this study to display a "Medium-High-Low" pattern. This measure had statistically discernible (significant) differences when comparing *Changes to Health Parameters of White-Tailed Deer during a Drought in the US Virgin Islands DOI: http://dx.doi.org/10.5772/intechopen.108270*

each of the three time points with each other. Our data for all three phases of the drought show that the deer were already stressed in July 2015 at the start of the drought, and to a degree that shows widespread stress across the population. This was further exacerbated at the height of the drought, in March 2016, when the deer were in acute stress. The stress hormone data ranged from 24.3 to 110.1 ng/g for T3 and 26.5 to 258.5 ng/g for cortisol.

The cortisol values for St. John deer at the onset and recovery of the drought were within published values for animals within ongoing stressful situations, but cortisol values for deer during the peak of the drought were more consistent with wildlife undergoing acute trauma. For example, values ranged from 23.9 to 114.9 ug/g for musk deer in crowded conditions [40], and deer undergoing high parasite burdens had cortisol values that reached 93–144 ng/g [39] and could result in diminished body condition for deer over time [25]. These published values for deer with ongoing stress are consistent with the values found in this study for St. John deer during drought onset and recovery. However, the highest values found in our study at the peak of the drought (258.5 ng/g) were instead consistent with values that more reflected acute and traumatic events for animals. For example, koalas who had been in vehicle collisions (202 ng/g), were burn victims (200 ng/g), or were in an area during land clearance (669 ng/g) showed cortisol values similar to the deer of St. John during the peak of the drought [41]. High fecal cortisol levels can often be used as a predictor of mortality. Ring-tailed lemurs that died following traumatic events showed average fecal values of 51.1 ng/g [42]. The deer of St. John both endured sustained cortisol levels above that value for the duration of the drought (2015–2016), and also at values five times that amount at peak drought. This highlights the noteworthy resiliency of this population to continue to live with this ongoing stressor present on island for over 2 years.

Stress hormones are also important regulators of energy balance. When food availability diminishes to the point of starvation, cortisol remains chronically elevated, body condition declines, and fat stores are used for energy [43, 44]. Prolonged elevation of glucocorticoids can result in the suppression of reproduction, growth, immune function, and responses to pathogens and parasites [45].

### **5. Conclusions**

This study was able to describe the resilience of an isolated population of whitetailed deer on St. John, US Virgin Islands during a severe drought in 2015–2016. Health observations collected through three successive field seasons representing drought onset, peak, and recovery of the drought enabled novel work documenting physiological changes of deer under the stress of climate change For all six health measures in this study (musculature, coat condition, body condition, the presence of ticks, cortisol, and T-3 levels), there is an association between deer health and time relative to drought (onset, peak, or recovery phase). All measures indicate a rapid return to health following the drought peak.

This study provides a baseline foundation for future research needed to inform the extent to which isolated populations of herbivores cope with increasing climate variability. Further work is needed to explore health differences between males and females during various stages of drought. Future studies would also benefit from using tagging methods for a study design that enables direct observations to individual deer and clarifies how the population maintains resilience in extreme weather. In addition, future work may include a closer analysis of stress hormone levels related to reproduction and survival of individuals. Periodic re-evaluation of the St. John

deer population and health will add additional data to population densities that can be supported by the island ecosystem. This work provides a critical baseline to document physiological changes to an isolated Caribbean deer population, and the results of this work can be extended regionally throughout the Caribbean and to analogous species.
