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1 December 2017 Age and sex skull variation in a model population of the common shrew (Sorex araneus)
Lucie Nováková, Vladimír Vohralík
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Abstract

Sexual and age variation of the common shrew was assessed in 173 individuals captured in the Novohradské hory mountain range in South Bohemia, Czech Republic. Age variation was found in seven of the fourteen measurements examined. In six of them (height of mandible, height of mandible measured below the first molar, length of mandible, cranial width, condylobasal length, length of neurocranium), values in adults were higher than those in juveniles, while the opposite was found for the length of the lower incisor. Evidence of sex differences was found only in three measurements: height of mandible measured below the first molar, cranial width and length of the lower incisor. Our results suggest the need for separating age and sex groups in studies of skull variation in Sorex shrews.

Introduction

There are many studies concerned with morphological variation of the common shrew (Sorex araneus). Most of them focused on the Dehnel's phenomenon, i.e. the winter reduction of some internal organs and several body and skull measurements (e.g. Dehnel 1949, Pucek 1955, 1963, 1970), or morphological differences between chromosomal races (e.g. Wójcik et al. 2000, Stefen 2013). Relatively detailed information is available about skull development during nidal life of the common shrew (Vogel 1973). Unfortunately, much less attention has been paid to skull variation in shrews during the spring-autumn period. In general, it was assumed that there are no or only negligible sex differences in skull measurements (e.g. Schubarth 1958, Homolka 1980, Poroshin et al. 2010). As to the differences in skull measurements between young in the calendar year of their birth and overwintered individuals, previous studies dealt mostly with changes in braincase height or condylobasal length (e.g. Kubik 1951, Pucek 1955, 1970), with a few exceptions (Homolka 1980, Spitzenberger & Bauer 2001). In fact, detailed analyses of sex and age differences in skull measurements within a common shrew population, based on a large enough sample, are very scarce. We used classic morphometry (linear measurements), as we wanted to retain information about size variation in the population. In addition, this method gives values that are commonly used in the mammalogical literature. It should be stressed that without such analysis it is impossible to distinguish geographical variation due to environmental variables from intrapopulation variation due to sex and age differences. Therefore, the aim of our study was to fill this gap in the literature and make a detailed comparison of skull measurements in two age groups of shrews with the aim of determining if there is sexual dimorphism, especially in adults, which have been rarely studied to date.

Material and Methods

Material

The material used in this study consisted of skulls of common shrews snap-trapped between 1972 and 1976 in the locality Žofín situated in the Novohradské hory mountain range, South Bohemia, Czech Republic. Shrews were collected mostly along streams flowing through a wet meadow, while a smaller part of the material was collected in a nearby beech-spruce forest, at an elevation of ca 750 m. For further details about the locality, see Vohralík et al. (1972).

All captured animals were processed by standard mammalogical methods, i.e. measured, dissected and conserved in 4% formaldehyde. Later, skulls were extracted and cleaned by Dermestes maculatus beetles. We divided the animals (n = 173) into four groups — juvenile males (52 specimens), juvenile females (47 specimens), adult males (53 specimens), and adult females (21 specimens). The juvenile categories include only immature individuals trapped between September and November of the year they were born in. They were identified based on the size of testes in males and absence of embryos and signs of previous parturition in females. In addition, juveniles exhibited visibly more prominent hairs on the tip of their tail and different pelage colour. Teeth of juveniles clearly differ from those of adults by less abrasion. Adults include overwintered individuals trapped between April and November.

Fig. 1.

Mode of taking of the skull measurements. Mandible: a) buccal view, b) lingual view; cranium: c) dorsal view, d) ventral view.

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Fig. 2.

Variation of the cranial width (CW). Abbreviations: ad m (adult males), ad f (adult females), juv m (juvenile males), juv f (juvenile females). Boxplots show the interquartile range with median for each group. Dots represent individual values, outliers are shown as circles.

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Fig. 3.

Variation of the length of the lower first incisor (LI). For description see Fig. 2 legend.

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Fig. 4.

Variation of the height of mandible measured below the first molar (Hm1). For description see Fig. 2 legend.

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Measuring and statistics

Skulls were magnified under an Olympus SZX 12 stereomicroscope and high-resolution photos were taken with an Olympus DP70 camera. Pictures of crania from dorsal and ventral views and left mandibles from buccal and lingual views were taken after placing them on a horizontal surface without any correction of their position. All measurements were recorded from the images in the tpsDig 2 software (Rohlf 2016) to the nearest 0.01 mm. We took seven cranial and seven mandibular measurements mostly according Vesmanis (1976), see Fig. 1. On the buccal side of the mandible, we measured height of mandible (HM) and postcoronoid height (HP), both taken at the least vertical distance, height of mandible below the first molar (Hm1), measured at the aboral margin of foramen mentale, and length of incisor (LI), measured at the greatest length of the visible part of the incisor, without the root. On the lingual side, we measured length of mandible (LM), length of tooth row (Lc1-m3), and length of molar row (Lm1-m3). From the dorsal view of the skull, we measured cranial width (CW), zygomatic width (ZB), and interorbital width (10). Condylobasal length (CB), length of rostrum (LR), length of neurocranium (LN), and length of upper molariform tooth row (LP4-M3) were measured from the ventral view of the skull. All teeth measurements were taken across crowns, with the exception of LI. One person took all the pictures and performed the measurements.

Fig. 5.

Variation of the condylobasal length (CB). For description see Fig. 2 legend.

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All variables were normally distributed (Shapiro-Wilk test). The effect of age and sex was tested by two-way ANOVA. Mutual differences between all four groups (juvenile males, juvenile females, adult males, adult females) were tested by independent samples t-test. Using a general linear m