Establishment of populations and live-trapping
A total of 288 voles of each sex, 6 months of age or older, were separately used to establish the enclosure populations in 2012 and 2015. They were either F2 generations born in the laboratory or captured as juveniles in the previous year. All individuals were tagged in the ear with identifying metal tags. The populations were introduced into the enclosures in April 2012 and May 2015 at two density conditions. The low-density condition consisted of six adults per sex in each of the four enclosures, and the high-density condition consisted of 30 adults per sex in each of the other four enclosures in 2012 and 2015. A high-density condition (400 vole ha-1) was observed in the habitat with dense vegetation where grazing activities were limited (Sun et al., 2002). The initial body weights did not differ among the voles in the different enclosures (F 7, 280 = 1.72, P = 0.103 in 2012, F 7, 280 = 0.192, and P = 0.987 in 2015). Live-trapping started after allowing the animals to acclimate to their new environments for two weeks and lasted until late July 2012 and August 2015, respectively. Standard capture–record–recapture methods were used throughout the study. Six trapping sessions were conducted in 2012 and seven in 2015; each consisted of three trapping days. The time interval between any two trapping sessions was one week. The traps were set between 7 AM and 7 PM, baited with a bit of carrot, checked every 2 h, and locked closed when trapping did not occur. Following each capture, we recorded animal identification, sex, and body mass. Females were considered reproductive if they had enlarged nipples and teats barren of hair. Males were considered in breeding conditions if their testes were scrotal rather than abdominal. The animal was, then, released at the point of capture after handling. The F1 offspring born in the enclosures were captured at 20–30 days of age and moved to the laboratory for use in subsequent experiments (Bian et al., 2015; Yang et al., 2018).