Prediction of the body mass of the bank voleMyodes glareolusfrom skull measurements

Abstract

INTRODUCTION Small mammals are commonly preyed by various avian predators, and the bank vole (Myodes glareolus) is among the main prey species for owls (Balciauskiene, 2006). Indigestible bones and teeth give a possibility for estimating prey composition. Also, it is possible to estimate the preyed small mammal body mass and age classes from cranial or mandible dimensions (Blem et al., 1993; Zalewski, 1996). The topic was explored several decades ago (Pagels & Blem, 1984; Stewart & Barss, 1985; Dickman et al., 1991), and recently it has come back onto the stage again (Balciauskiene, 2006, 2007a, b, c; Borowski et al., 2008). In this paper we will discuss possibilities of estimating the body mass of bank voles from several skull measures, with respect to the character preservation rates in owl pellets and prey remains. MATERIAL AND METHODS The skulls of 376 bank vole individuals trapped in seven sites of Lithuania from July to October 1999-2005 were used for our calculations. After dissection, based on their reproductive status, the trapped voles were divided into three age categories: juveniles, subadults, and adults. All overwintered and breeding individuals, i.e. males with scrotal testes and lactating or pregnant females, were defined as adults. All individuals that stayed non-breeding during the year of birth (reproductive organs developed, but inactive--small nipples and closed vagina in females, abdominal testes in males) fell into the category of subadults. All individuals without expressed sex attributes (reproductive organs still developing--threadlike vagina or hardly visible testes) were treated as juveniles. The presence and the status of glandula thymus (gl. thymus involuted in adults, gl. thymus disappearing in subadults, gl. thymus functioning in juveniles) as well as animal mass were taken into account. The skulls of trapped voles were cleaned by Dermestes larvae. We used 17 skull characters (Balciauskiene, 2006, 2007a) measured under a binocular with a micrometric eyepiece with an accuracy of up to 0.1 mm. We measured the characters of the right set of the skull, the left set was measured only in the cases the skull was damaged. The following skull (8 mandibular and 9 cranial) characters were measured (Fig. 1): [X.sub.1]--total length of mandible at processus articularis excluding incisors; [X.sub.2]--length of mandible excluding incisors; [X.sub.3]--height of mandible at, and including, first molar; [X.sub.4]--maximum height of mandible excluding coronoid process; [X.sub.5]--coronoid height of mandible; [X.sub.6]--length of mandibular diastema; [X.sub.7]--length of mandibular tooth row; [X.sub.8]--length of molar [M.sub.1]; [X.sup.9]--length of nasalia; [X.sup.10]--breadth of braincase measured in the widest part; [X.sup.11]--zygomatic skull width; [X.sup.12]--length of cranial (upper) diastema; [X.sup.13]--zygomatic arc length; [X.sup.14]--length of foramen incisivum; [X.sup.15]--length of maxillary toothrow; [X.sup.16]--length of molar [M.sup.1]; [X.sup.17]--incisor width across both upper incisors (Balciauskiene, 2006, 2007a). [FIGURE 1 OMITTED] The rate of preservation of skull characters in bank voles was evaluated from the pellets and food remains of the Tawny Owl (Strix aluco) collected in Lithuania in 1997-2005 (Balciauskiene, 2006). Regression equations describing the relation of skull characters with body mass were calculated using the skull characters that correlated best with body dimensions. Multiple regressions were combined not only from the best-working characters, but also depending on character availability in food remains or pellets. Regressions were calculated separately for mandibular and maxillary characters. We used the stepwise forward regression method (StatSoft, 2004). The number of included characters was minimized deliberately, including only (i) characters with p-level