**2. Methods**

#### **2.1 Field data collection**

The island of St. John is located in the Caribbean Sea between 18°18′ and 18°22′N latitude and 64°40′ and 64°48′W longitude (**Figure 1**). It is 11 km long and 5 km across at its widest point [15]. St. John is part of the US Virgin Islands which includes St. John, St. Thomas, St. Croix, and Water Island. Virgin Islands National Park (VINP) lies on the island of St. John and comprises 60% of the landmass of the island and protects one of the largest tracts of secondary dry forest in the eastern Caribbean [16]. Dryland plant communities on St. John include forests, shrublands, coastal hedges, and a rare cactus community [17, 18]. The forests and forest structure are largely shaped by hurricanes and drought [16]. The climate is relatively dry, with an average temperature of 27°C with 75% relative humidity. Water is in limited supply because of high temperatures, high evaporation rates, and run-off from steep slopes [15, 19]. There are no permanent streams or rivers on the island and only a few intermittent streams [20]. Precipitation is highest from May through November with a range of 890–1400 mm/yr [15]. The driest months are February and March [20]. The region is prone to cyclical patterns of drought and excessive moisture and structural damage from seasonal hurricanes.

**Figure 1.** *A map of the study location on the island of St. John, U.S. Virgin Islands in the Caribbean.*

This information builds upon a series of articles describing the natural history parameters of the deer of St. John. Despite their isolation, St. John deer demonstrated low inbreeding and average heterozygosity [21], were positive for antibodies for bluetongue and epizootic hemorrhagic disease but no clinical signs of the disease were observed during field observations [22]. A third study identified the ticks found on the St. John deer as a species of cattle tick and tropical horse tick [23].

Data were collected over three field seasons on St. John, USVI.; first during the onset of a drought (July 2015, henceforth "onset"); second at the peak of the drought (March 2016, henceforth "peak"); and third during recovery from the drought (July 2016; henceforth "recovery") (**Figure 2**). The drought occurred regionally throughout the Caribbean and was prevalent in 2015 on St. John [23].

Deer musculature, coat condition, body condition, and tick levels were recorded using two data collection methods: remote assessment and direct assessment of tranquilized deer. Remote assessments of nutritional conditions offer a noninvasive alternative when capture of deer is not possible, and can be used to evaluate changes at a population level [24]. Remote assessments were conducted at all three time points (onset, peak, and recovery of the drought), including collecting data on deer seen either on transects or opportunistically by trained technicians working in pairs and using binoculars at a distance of less than 25 m. Deer on the island are highly acclimatized to humans and did not move when observed by technicians, particularly near popular tourist trails or beaches. This allowed the research team to observe deer directly for accurate data collection. Technicians for

#### **Figure 2.**

*The 12-month rolling average precipitation (calculated using the previous 12 months) on St. John Island for the years 2015–2016, with annotations for the three data collection time points and field notes regarding observed conditions in a study of white-tailed deer (Odocoileus virginianus) health indicators during the onset, peak, and recovery phases of a 2015–2016 drought. Initially (December–April 2015) the rolling average is on par with average precipitation for the previous 10 years. Precipitation decreases beginning in April of 2015. Data collected in July 2015 ("Onset") occur when the rolling average is low but reserve supplies are still available. By October 2015 the rolling average precipitation drops below the lowest annual precipitation in the previous 10 years. Data collected in March 2016 ("Peak" of the drought) occurred when the monthly average precipitation for the past 12 months has remained lower than usual for several months. Precipitation returned shortly after, bringing the rolling average up to more favorable conditions ("Recovery" phase of the drought).*
