Discussion
In the present study, high-density populations had lower recruitment and
proportion of reproductive conditions than low-density populations, and
the number of founders was negatively associated with recruitment rates
and the proportion of reproductive voles. Because offspring born in
enclosures were moved to the laboratory to use to examine the effects of
density-induced maternal stress on offspring phenotype (Bian et al.,
2015; Yang et al., 2018), the negative relationship between founder
numbers and reproductive traits was only due to the suppressive effects
of founder numbers on reproduction. In addition, because the enclosures
were isolated from common vole predators, predator-induced density
dependence on demographic processes was excluded from our study.
Therefore, our results concluded that high-density can induce the effect
of density dependent reproduction, which corroborates other studies on
vole populations as described in the introduction section, indicating a
universal of density-dependent reproduction in the population of
microtine rodents.
In our previous papers we have reported that high-density population in
both years had higher FCM level than low-density population (Bian et
al., 2015; Yang et al., 2018). In the present study, we also revealed
that FCM levels were positively associated with founder numbers, which
also corroborates recent studies on other mammalian species (Boonstra &
Boag, 1992; Novikov & Moshkin, 1998; Viblanc et al., 2014). However,
Harper and Austad (2004) and Charbonnel et al. (2008) did not find a
positive correlation between density and FCM levels in water voles and
red-backed voles (Clethrionomys gapperi ). In those studies, the
faeces were sampled in both breeding and non-breeding season (Harper &
Austad, 2004; Charbonnel et al., 2008) and some samples were from
individuals of different ages and reproduction conditions (Harper &
Austad, 2004). However, our experiment was performed during the breeding
season and the faecal samples of pregnant individuals were not
collected, due to pregnancy naturally raising glucocorticoid levels in
most mammals, not necessarily as a result of stress but for
developmental and energetic reasons (Boonstra & Boag, 1992; Edwards &
Boonstra, 2018; Edwards et al., 2019). In addition, our previous study
has validated the effectiveness of detecting corticosterone levels in
the faeces of root voles (He et al., 2013). Therefore, our experiment
excluded the confounding effects of reproductive condition, seasonality,
and the effects of trapping/handling stress on FCM levels and the
differential between our and those results may be due to these
confounding factors.
In addition, for unknown reasons, when founder voles were introduced
into enclosures to establish an enclosed population, a few voles died
during the two-week period of acclimation to their new environments. At
the first trapping session in 2012 and 2015, the mean number of founders
in high-density treatments was 54.5 and 53.8 voles per enclosure (363
and 358 vole ha-1), respectively. In this study area,
Sun et al. (2002) reported that population density of root vole was 356
vole ha-1 in October in the habitat with mainlyE. nutan s where grazing activities were limited. Moreover, Rodd
and Boonstra (1984) reported that the high density of meadow voles
(M. pennsylvanicus ) population in abandoned farmland reached 469
vole ha-1 in October. Although we realise that the
high-density treatments (400 vole ha-1) were the
approximate peak density of natural vole populations in this study area,
it is not an unseen natural extreme level.
Thus,
our findings of correlation between FCM levels and density can represent
what happens in a natural vole population.
In semelparous and partially semelparous species (i.e., Australian
semelparous marsupial, the arctic ground squirrel Spermophilus
parryii plesius ) and dominant individuals in social species and
cooperative breeders (i.e., the African wild dog Lycaon pictus ,
dwarf mongoose Helogale parvula , and grey wolf Canis
lupus ), dominance was strongly correlated with reproductive success.
However, dominance was also associated with heightened glucocorticoid
levels, which reflected either a classical trade-off of reproductive
success for potential survival during short mating periods (2–3 weeks),
only one breeding period followed by programmed death (see review by
Boonstra, 2005), or an adaptive stress response for competing intensely
for access to females and maintaining dominant status by frequent
physical aggression and challenges (see review by Sapolsky, 2005).
Conversely, iteroparous voles breed continuously throughout the breeding
season, and antagonistic interaction is not particularly severe. For
example, breeding male meadow voles do not appear to be engaging in
costly aggressive acts to assert dominance or access females (Edwards et
al., 2019). Dominant status is maintained by cues rather than physical
combat; subordinates tend to have the highest indices of stress. Thus,
for these iteroparous species, elevated corticosterone induced by high
population density can suppress reproduction (Wingfield & Sapolsky,
2003; Boonstra, 2005). In fact, antagonistic behaviour is not the only
factor that induces stress responses; increased population or breeding
density can also lead to an increase in parasite load, attraction of
predators, and food shortage. These factors can induce stress responses
in individuals (Creel et al., 2013). Thus, the density-induced stress
responses are an additive stress effect of intrinsic and extrinsic
factors, reflecting the biological cost of cumulative stress responses
(Goymann & Wingfield, 2004) rather than a single factor effect (i.e.,
antagonistic behaviour).
In the present study, we found that the influence of female- and
male-founder vole numbers on the proportion of reproductive condition
was mediated through FCM levels in 2012, and that female-founder number
indirectly negatively affected recruitment through FCM levels in 2015.
Although the influence of FCM levels on reproductive traits had
different pathways in both years, we still conclude that density-induced
stress participates in density-dependent reproduction; also,
density-induced stress is one of
the factors generating density-dependent reproduction effects.
In conclusion, in the present study, high density generated
density-dependent reproduction and increased FCM levels of founders. The
negative effect of high density on reproduction was partly through its
positive effects on FCM levels of founder voles. Thus our results
provide the first evidence that density-induced stress is one ecological
factor generating density-dependent reproduction effects in vole
populations.
Author contribution : Jianghui Bian and Yan Wu conceived and
designed the experiments. Shouyang Du, Yanbin Yang and Yifan Cao
performed the experiments. Jianghui Bian and Guozhen Shang analyzed the
data. Guozhen Shang and Jianghui Bian wrote the manuscript. Yan Wu
provided editorial advice.
Acknowledgements: We thank Hui He, Xuheng Nie, and Xin Zhang
for their assistance in the field. This work was funded by the National
Natural Science Foundation of China (Grant Numbers 31870397, and
31570421), the Natural Science Foundation of Qinghai Province (Grant
Number 2018-ZJ-906), and the
Strategic Priority Research Program of the Chinese Academy of Sciences
(Grant/Award Number XDA2005010406). The use of animals in this study was
in accordance with the guidelines of the regulations of experiments on
animals and was approved by the Animal Ethics and Welfare Committee of
the Northwest Institute of Plateau Biology, Chinese Academy of Science.