A common cause of sexual dimorphism in mammals is the selective choice of those traits augmenting the ability of males to compete for females. Besides body weight, such traits include adaptations of specific components of the musculoskeletal system that enhance strength, stability and agility. The results of our study show sexual dimorphism evident in the cranial characteristics of both adult (four observed) and subadult common vole individuals in western Slovakia, specifically from the Danubian Lowlands. These findings are consistent with those of Csanády and Mošanský (2021a), whose results in eastern Slovakia likewise recorded sexual dimorphism in common voles with higher values found among the females. In eastern Slovakia, however, they also observed sexual dimorphism on different traits than what we did outside of "ALLM”. Unlike us, they did not assess dimorphism separately for adults and subadults.
Rodents are characterised by growth that, instead of slowing down as their bodies age, lasts almost to the end of their lives. The cause is the short life of rodents, with not enough time to reach a period of stabilisation (Haitlinger 1962).
As a species with cyclical dynamics, common voles additionally undergo faster sexual maturation during periods of population growth (Oli et al. 1999, Zapletal et al. 2001). For these reasons, cranial characteristics were measured for adults and subadults as separate categories. A single case was recorded (“WO”) of higher average lengths in adult males, which conflicted with studies of sexual dimorphism in common voles published to date. Stein (1957), Bauer (2001), Morris et al. (2020), and Csanády, Mošanský (2021b) have described from several areas within their range of distribution females exhibiting higher values of sexual dimorphism for cranial discrete traits. The larger skulls found in female common voles and other small mammal species are probably related to selection for more efficient processing of food during gestation and lactation, the generally lower quality of food available to voles, and the increased ability of a female to burrow or defend her offspring (Morris et al. 2020). Overall, however, no clear statement can be made about sexual dimorphism in the species based on cranial characteristics because some past studies have not confirmed it Balčiauskienė et al. (2004). They noticed a larger tooth row length among females only in root voles without finding any differences between the sexes in common voles and field voles.
Several studies have mentioned a correlation of some craniological characteristics in either the body weight or other somatic traits of small mammals (Pagels and Blem 1984; Dickman et al. 1991; Goszczyński 1977; Blem et al. 1993; Trejo and Guthmann 2003; Borowski et al. 2008; Balčiauskas and Balčiauskienė 2011). Our results showed a demonstrable relationship in 24 of the 29 observed discrete traits in both the maxilla and mandible. Nonetheless, except for “LN” and “LD”, the dependency of cranial trait length on body weight was low (R2 < 0.18). For example, the R2 value for the Pannonian root vole (Alexandromys oeconomus meheli) at the three highest dependencies was greater than 0.59 (Baláž et al. 2021). Balčiauskas and Balčiauskienė (2011) noted dependency values of cranial and pelvic discrete traits in the root vole A. o. stimmingi (Nehring, 1899) where R2 was greater than 0.7.
Balčiauskienė (2007) likewise documented a relationship between body weight and mandibular traits in bank voles. Cranial growth ex situ among bank vole individuals between the age of twelve and 724 days demonstrated dependence in all 17 cranial discrete traits including sex. Stable growth among the bank voles continued until ten months along with body weight and length, which reached 45 grams and 12 centimetres, respectively. Growth in a majority of cranial discrete traits lasted 12 to 14 months while some dental features, such as the length of the first upper molar and the width of the incisors, continue growing throughout its life. Kapischke (2017) found the length of the upper row of teeth to be growing consistently with the age of the individual, where the older they were, the longer their dentition. Another study conducted by Balčiauskienė et al. (2004) confirmed the effect of age and body weight on the length of the tooth row on the upper jaw and palate in bank voles, common voles and field voles.
Population fluctuations among voles are characterised by a relatively repeatable pattern of changes in body weight and growth, age at sexual maturity and length of the breeding season (Boonstra and Krebs 1979; Krebs and Myers 1974; Boonstra and Boag 1987). Chitty (1967) postulated his own genetic behavioural hypothesis to explain it, where reciprocal interference is minimal at low and increasing densities with the selection of those non-aggressive genotypes that have high reproductive effort. At high population densities, reciprocal interference is intensive and aggressive genotypes are selected for their ability to resist intraspecific competition. The presumption in Chitty’s hypothesis is genetics as the basis for many density-related changes (Boonstra and Boag 1987). The Chitty effect is clearly noticeable in small mammal populations with cyclical dynamics, with the animals’ average body weight and length greater when the population is either growing or at its peak than when it is declining (Kshnyasev and Davydova 2021; Yakushov and Sheftel 2020). The findings from our study demonstrated the impact of the growth in abundance in 24 of the common vole’s 29 cranial discrete traits. Higher values of the coefficient of determination (R2) were recorded only in the cranial traits “LN” (length of the nasals) and “LDl (length of the diastema - left), bound to the common vole’s nasal part. In three traits, interaction between abundance and age was documented. However, growth in cranial discrete traits in tandem with rising abundance was confirmed in these cases only among adult individuals. In subadult individuals, on the other hand, the length of traits fell with growing abundance. Balčiauskienė et al. (2004) reported smaller subadults in harsh winters than when the season was mild, while juveniles were larger. They further added that growth was markedly more inhibited and lasted longer in cranial characteristics than in body weight. Different patterns were visible in the growth of the skull itself. Some traits stopped growing over time and there were no changes in mean mandible length over a three-month period from December to February, whilst the average width at the widest part of the brain declined in January and February. The coronoid height of the mandible (AMd) grew in the period from November to December before its mean length fell in February. In both subadult and juvenile individuals, growth slowed dramatically. These findings could explain why the size of cranial discrete traits would decrease during the winter, as more than 70% of subadult individuals originated during the winter or autumn.
To conclude, great variability can be seen in the common vole’s cranial discrete traits, particularly in WMTRL, WMTRR, FI and WO. Differences in some traits were also found between male and female common voles as an expression of sexual dimorphism (WMTRR, AMdm and AMd in adults; LOSD, WMTRR, LoM, and ALLM in subadults). Based on the size of cranial discrete traits (ML, LDl, LDr), it is possible to predict the size of individual common voles, which may be useful in food ecology for determining the biomass of food consumed by raptors and owls from an analysis of their pellets. Our study confirms the positive impact of common vole abundance on the size of cranial characteristics from the so-called Chitty effect. The findings from it could also be practically applied in agriculture as the larger average size of individuals would signal the onset of overbreeding as a characteristic sign of the common vole’s gradation.