Linking Variation in Mule Deer Nutritional Condition and Age to Population Performance in a Temperate Environment

The amount of body fat animals have as they enter and exit winter can have major impacts on how well they survive and reproduce. Mule deer are widely distributed herbivores whose survival and reproduction are strongly tied to weather patterns and the quality of vegetation they consume, which in turn impacts nutritional condition. The objective of this study was to identify and quantify the factors (animal specific, climatic, or habitat) influencing mule deer body fat during both summer and winter in Utah and examine how nutritional condition influences survival. We found December body fat was most affected by December lactation status and duration, monsoon (July-September) precipitation, April precipitation, October temperature, previous March body fat, and habitat treatment use during summer/fall. Body fat exiting winter (March) was most influenced by starting body fat in December, age, February snow depth and temperature, and habitat treatment selection. Given the relationship between lactation and December body fat, we further explored lactation duration by assessing how it affected fawn weights entering winter, overwinter survival of adults and fawns, and why we observed differences in lactation duration. We found does that produced milk longer had lower body fat and lower overwinter survival. However, longer milk production resulted in increased fawn weight in December, which led to greater overwinter fawn survival. Lactation duration appeared to be influenced by precipitation with drier units and years having lower lactation, but individual and behavioral differences may also be a factor. Lastly, we explored the relationship between body fat and the leading causes of winter mortality in mule deer. Different sources of mortality disproportionately affected various segments of the population. Mule deer dying from malnutrition or being killed by coyotes tended to young (fawns) or old (8+) and had lower body fat. Conversely, mule deer dying from mountain lion predation and vehicle strikes represented all age classes and tended to be healthier animals. As such, these mountain lion and vehicle strike mortalities likely had greater influence on mule deer populations. Collectively, our results demonstrate the underlying factors influencing nutritional condition, and the importance of nutritional condition in driving mule deer population dynamics.