How does loss of social group impact behavior of remnant white-tailed deer?
Spread of infectious diseases throughout a population requires the disease to spread from infected animals to susceptible animals. In gregarious animals, transmission rates of diseases are typically high within social groups but low between social groups. Modeling transmission rates can aid in understanding disease ecology, but transmission rates and impacts of wildlife diseases can be altered if social structure is disrupted by within-group disease outbreaks or by lethal management. Specifically, if remnant animals join or increase contact with neighboring groups after their own groups are depopulated, between-group transmission may increase, even as density decreases. To test whether this phenomenon could apply to diseases of white-tailed deer (Odocoileus virginianus), we constructed proximity logger (PL) and global positioning system (GPS) collars to monitor space-use and contacts among 48 adult female and juvenile deer in southern Illinois during winter-spring for 2011-2014. From 10 centrally located groups, we removed all members but 1 collared animal (the "remnant") during March of 2012-2014, leaving control groups (n = 38) intact. Using a Before-After-Impact-Control design, we compared remnant animal behavior to control animal behavior. Specifically, we examined changes in indirect contact using volume of intersection (VOI) and changes in weekly direct contact rates recorded by PLs.
We found that remnant adult females were similar to control animals in shifts (from pre- to post-removal periods) in space-use (VOI=0.65±0.03 vs. 0.63±0.02; p=0.67), home range overlap with neighboring groups (VOI=0.07±0.03 vs. 0.04±0.01; p=36), and intergroup contact rates (F1,93=0.59, p=0.44). Remnant juveniles had similar intergroup contact rates with control animals (p=0.13), but remnant juveniles had greater post-removal shifts in space-use (VOI= 0.15 vs. 0.35±0.06; p=0.002), and greater increase in home range overlap with neighboring groups (VOI= 0.04 vs. 0.01±0.01; p=0.03). Our results suggest that mature female deer that experience social disruption are not likely to greatly increase direct or indirect disease transmission with neighboring groups. In addition, our results suggest that remnant juveniles are not likely to increase direct transmission with neighboring groups, but may increase indirect disease transmission with neighboring groups. This pattern may be problematic for slow-acting diseases such as chronic wasting disease and bovine tuberculosis, which are more likely to leave remnant juveniles after group depopulation.