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).