Common vole (Microtus arvalis) population sex ratio: biases and process variation

Abstract

Vole population sex ratio varies seasonally. However, population sex ratios have usually been estimated using naïve estimators that do not allow for biases owing to the sex difference in capture probabilities and movement distances (i.e., effective areas sampled). Here we aimed to advance the methodological approach, recognizing that there are two different classes of contributing mechanisms to the pattern which are best addressed separately: (1) those mechan isms imposing a systematic error (bias) in population estimates and (2) those generating the true process variation. Analyzing 7-year capture–recapture data in the common vole (Microtus arvalis (Pallas, 1778)), we quantified both types of biases and revealed that the bias owing to differential capture rates was often severe and less predictable, whereas that owing to differential effective areas was smaller and overestimated male numbers for most of the year. We demonstrated unambiguously that the unbiased population sex ratio indeed varies seasonally, with the males usually being more numerous over winter and spring. By testing predictions from two mechanistic hypotheses to explain the process variability, we found evidence for both the differential recruitment hypothesis and the differential survival hypothesis. From April–May to August, it was the females that were recruited more to the population and that had higher survival rates than males. We suggest that the seasonal variation in the population sex ratio is not merely a result of biasing mechanisms but an important population property driven by the joint effect of differential recruitment and differential survival between sexes.

Temporal Variation in Individual Factors Associated with Hantavirus Infection in Bank Voles During an Epizootic: Implications for Puumala Virus Transmission Dynamics

“…2), supposedly as a result of high resource availability in the form of tree seed production in our study region as explained in Tersago et al (2009). Prolonged breeding activity might also explain the lower sex ratio's as a similar effect was observed in common voles (Bryja et al 2005). In contrast, PUUV IgG prevalence significantly decreased in autumn 2005, even though infection was still present in eight study sites and bank vole numbers were still relatively abundant (Fig.…”

Sex Ratio of Rodents as Barn Owl (Tyto alba) Prey

“…We initially suspected females would be more abundant than males in the general populations of both taxa as a result of agricultural tillage destroying nests, cover, and burrow systems (e.g., Witmer et al, 2007) and surviving females adjusting their reproduction to favor females in order for the population to increase (or at least remain static). Previous research showed that microtines tend to produce more females than males when population density is low (Lambin, 1994;Bond et al, 2003;Bryja et al, 2005). After tillage was reduced by converting some land to soil bank, and population density increased, we suspected the sex ratio would become more even.…”

“…Observations on reproduction in the wild of several vole species indicate greater numbers of females born in the spring and early summer and greater numbers of males born in the late summer and autumn. Researchers suggest this pattern results because it allows females to breed prior to the coming winter and males to breed the following spring (e.g., Jannett, 1981;Lambin, 1994;Bond et al, 2003;Bryja et al, 2005). Whether the females .…”

“…Whether the females . males ratio is the result of female manipulation of the sex ratio of their offspring (Bond et al, 2003), sex-based variability in recruitment and survival (Bryja et al, 2005), population density and the nature of interaction between philopatric females (Lambin, 1994), or dominant males disallowing subordinate males to breed (Jannett, 1981) is less clear. Reproduction in the wild among deer mice tends to produce either equal numbers of males and females (Kaufman and Kaufman 1982), more males than females (Terman and Sassaman, 1967), or to fluctuate somewhat randomly (Havelka and Millar, 1997;Stewart et al, 2014).…”

Population Ecology of Wild Mongolian Gerbils Meriones Unguiculatus

“…Survival, reproduction, emigration, and immigration influence the dynamics and structure of populations (Bryja et al 2005;Clutton-Brock and Coulson 2003;Krebs 2003). For instance, sexually differential survival, recruitment, and dispersal alter the sex ratio of populations.…”