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Spermophilus citellus (Linnaeus, 1766) is a medium-sized sciurid commonly called the European ground squirrel. The species inhabits grasslands in central and southeastern Europe on a range from short-grass steppes to meadow orchards, and from sea level to an elevation of 2,500 m. These squirrels live in colonies and construct burrows used for hibernation, reproduction, refuge, and resting. Due to serious declines in many portions of its range, the species is listed as “Vulnerable” by the International Union for Conservation of Nature and Natural Resources. It is also protected by the Bern Convention (Appendix II), and the European Union Habitats and Species Directive (Annexes II and IV). CONTEXT AND CONTENT. Order Rodentia, suborder Sciuromorpha, family Sciuridae, subfamily Xerinae, tribe Marmotini, genus Spermophilus (Helgen et al. 2017). Designation of subspecies for S. citellus has been contentious. Eight subspecies have been described since 1929 based on slight differences in color, size, and body proportions (Ružić 1978), but with no proof of discontinuity (Corbet 1978). Ružić (1978) tentatively recognized 7 subspecies (all, except balcanicus, which she synonymized with martinoi), whereas other authors have recognized only 4 subspecies (Thorington and Hoffmann 2017; Thorington et al. 2017): citellus, gradojevici (karamani is a synonym), istricus (laskarevi is a synonym), and martinoi (balcanicus and thracius are synonyms). Skull characters, ratios, and coloration are of low diagnostic value in S. citellus and the above subspecies were not consistent with the interlocality variation observed in an analysis of 13 cranial measurements of ground squirrels from throughout the range (Kryštufek 1996), suggesting that further revision of the subspecies is needed. NOMENCLATURAL NOTES. The name Citellus (Oken, 1816) was in routine use for the genus until Hershkovitz (1949) argued that Oken's work was invalid, and that Spermophilus (Cuvier, 1825) was the oldest available name for the genus. The opinion was supported by the International Commission on Zoological Nomenclature (1956; Opinion 417), and has been adopted by American authors. However, some European and Russian authors continued to use Citellus as recently as 1995, although others adopted Spermophilus (Harrison et al. 2017). Until recently, Spermophilus contained all Holarctic ground squirrels, except those in the genera Marmota, Cynomys, and Ammospermophilus and was further divided into 6 subgenera. Presently, Spermophilus contains 14 species (sensu lato), all restricted to Eurasia (Helgen et al. 2017). Spermophilus is derived from the Greek spermatos for seed and phileo for love (Helgen et al. 2017), whereas citellus is from the Latin ziesel (German for ground squirrel or souslik—Palmer 1904). S. citellus also has been referred to as the European souslik (Coroiu et al. 2017). DIAGNOSIS Spermophilus citellus together with S. xanthoprymnus (Asia Minor ground squirrel), S. dauricus (Daurian ground squirrel), S. suslicus (speckled ground squirrel), S. alashanicus (Alashan ground squirrel), and S. taurensis (Taurus ground squirrel) form a group of closely related species, so that single specimens are sometimes indistinguishable; however, their distributions are different (Vinogradov and Argiropulo 1941; Herron et al. 2017; Gündüz et al. 2017; Helgen et al. 2017). Compared to S. taurensis, diagnostic characters for S. citellus are as follows: less reddish dorsal pelage, underside more yellow, smaller body size (S. taurensis, range: 194–201 mm; S. citellus, range: 174–228 mm), tail slightly longer (S. taurensis, range: 62–65 mm; S. citellus, range: 31–90 mm) with a dark stripe along its dorsal side, and hind-foot length shorter (S. taurensis, range: 39–44 mm; S. citellus, range: 30–43 mm—Kryštufek and Vohralík 2017, 2017; Gündüz et al. 2017; Özkurt et al. 2017). Compared to S. xanthoprymnus (see Gür and Gür 2017), diagnostic characters for S. citellus are as follows: the skull is less angular, back indistinctly spotted, and tail longer (S. xanthoprymnus, range: 30–72 mm; S. citellus, range: 31–90 mm) with black hairs dorsally (Kryštufek and Vohralík 2017, 2017). S. citellus, S. taurensis, and S. xanthoprymnus can be clearly distinguished on the basis of their alarm calls (Schneiderová and Policht 2017a, 2017b). GERAL CHARACTERS Spermophilus citellus is a medium-sized ground squirrel, with a round body, short tail (20–40% of length of head and body), and reduced outer ears (Fig. 1). Head is convex in profile with large eyes (about 8 mm in diameter) and short vibrissae (25–30 mm—Ružić 1978; Kryštufek and Vohralík 2017). The eyes, situated wide apart on the upper side of head, present an ill-defined eye-ring, whitish or yellowish according to general coloring of body (Miller 1912; Ružić 1978). The tail is cylindrical at the base and inconspicuously flattened beyond the middle, where hairs are about 15 mm long. The tail lacks a definite color pattern, with the upper surface grizzled, essentially like the back, somewhat darker at tip, and the pencil with light margin; the undersurface is a nearly clear dull buff. The ears are low, obscurely pointed above, and densely covered with fine, short hairs on both surfaces but without tufts (Miller 1912; Kryštufek and Vohralík 2017). The back and sides of the body are usually cream-buff but occasionally more yellowish, with sides inconspicuously lined with black, the entire dorsal surface from nape to rump vermiculated with black, and the dark and light areas along the middle of the back usually well enough defined to produce an effect of obscure light mottling, with spots about 5 mm in diameter. The belly is washed with a buff usually somewhat more yellow than that of sides. The crown and upper half of cheeks are grizzled, with the light element paler than on the body and the dark relatively more evident, the muzzle sometimes with a rusty tinge, and the sides of muzzle and lower half of cheeks clear whitish or buffy, continuous with the similarly colored pale area covering throat and forepart of chest and inner side of foreleg (Miller 1912). Feet are yellowish, with longest claws up to 8 mm in length, and 4 fairly large and naked pads on palms and soles; rudimentary thumbs still bear a nail (Kryštufek and Vohralík 2017). Soles are covered with short silvery hairs from heel to pads (Calinescu 1934; Kryštufek and Vohralík 2017). In 2017, a light-colored morph was detected in Vienna, Austria, with white tail and belly, and interspersed white blotches on head and dorsum (n > 5—I. E. Hoffmann, in litt.). Size and body mass of S. citellus vary with age, sex, and activity cycle, but also with locality and environmental conditions (Ružić 1978; Hoffmann et al. 2017; Millesi and Hoffmann 2017). Male-biased sexual dimorphism develops during the juvenile summer (Millesi et al. 1999b) and increases with age (Ružić 1978: table 15; Spitzenberger and Bauer 2017). Mean body mass of Pannonian juveniles at 30 days of age was 61.4 g (Ružić 1965). Data from an Austrian population ranged between < 50 g (juveniles at emergence from their natal burrow) and > 450 g (≥ 1-year-old males prior to hibernation). At vernal emergence, males were heavier than females in the same age class, and adult animals were heavier than yearlings. Adult males attained a minimum annual mass during the mating phase (Millesi et al. 1999b), whereas yearling males and females weighed least at emergence from hibernation (Millesi et al. 1998; Millesi and Hoffmann 2017). Growth of hind feet may terminate in juveniles, whereas head length still increases during the 2nd year (Ružić 1978; Millesi et al. 1999a). FORM AND FUNCTION Form The skull of Spermophilus citellus is similar to that of Sciurus but smaller (Kryštufek and Vohralík 2017). The braincase is broadly ovate when viewed from above, with greatest width equal to length, and its posterior width exceeding that at the postorbital constriction (Miller 1912). Dental formula is: i 1/1, c 0/0, p 2/1, m 3/3, total 22. P1 has 1 root; P2, M1, M2, and M3 have 3 roots; p1 has 2 roots; and m1, m2, and m3 have 4 roots (Özkurt et al. 2017). Age of individuals can be assessed through enamel abrasion of dental material (Ružić 1966) and on the basis of layers of appositional bone in the mandible (Hrabě and Zejda 1981). Kryštufek (1998) found differences in 15 pelvic measurements between localities and sexes. The characters most affected by interlocality variation were those describing general length and pelvis breadth, whereas males attained higher average sample means in all characters except breadth at pubic symphysis and pubis width at symphysis. The number of mammae is: 1-1 p, 2-2 a, 2-2 i, total 10 (Miller 1912). Spermophilus citellus has internal cheek pouches. The paired retractor muscles attach dorsally to scapular metacromion and ventrally to sternum. These muscles are presumably derived only from the facial musculature (platysma myoides p. branchialis and musculus sphincter colli profundus p. intermedia ventralis), rather than from musculus trapezius p. auricularis, as is the case in other rodents (Zherebtsova 2017). The eye lens is yellow and does not transmit ultraviolet radiation (50% cutoff around 493 nm—Hut et al. 2017). The retinal photoreceptor mosaic is composed mainly of cones and < 3% of the visual cells show rod-like characteristics (Szél and Röhlich 1988). Half of the rod-like cells are recognized solely by a rhodopsin antibody, whereas the other half also are recognized by a blue opsin antibody (Schantz et al. 1994). The species is dichromat having only blue (440 nm) and green (520–525 nm) photopic spectral sensitivities, together with 1 scotopic component with an absorption peak at about 500 nm (Szél and Röhlich 1988). Similar to other burrowing mammals (Kley and Kearney 2017), the forelimb of S. citellus is characterized by well-developed scapular retractors, arm retractors with distal insertions upon a robust humerus, elbow extensors and flexors, forearm pronators with distal insertions, and carpal and digital flexors with extensive origins from a prominent medial humeral epicondyle (Lagaria and Youlatos 2017). Three anal glands secrete an odorous scent that is perceivable near burrow entrances, especially during the mating season. Subcutaneous fat layer ranges from 2 to 5 mm in width (Ružić 1978). Function During hibernation, Spermophilus citellus exhibits a regular pattern of torpor interrupted by euthermic arousal phases (Strijkstra 1999). Evidence suggests that animals interrupt torpor states regularly during hibernation to protect the brain against the effects of prolonged hypothermia (Ruediger et al. 2017). Memory retention in captive S. citellus was negatively affected by hibernation. The resulting memory loss is apparently related to the physiological processes occurring during torpor and the short euthermic phases (Millesi et al. 2017). Protein and RNA contents in liver were higher during arousals than during torpor. However, adrenal ascorbic acid concentration was less in aroused squirrels than in hibernating or euthermic controls (Petrović et al. 1985). Average euthermic body temperatures are highest after hibernation and lowest during hibernation. Lowest body temperatures during torpor may reach < 0°C (Hut et al. 2017a). S. citellus shows reduction or absence of circadian rhythmicity in body temperature for several days posthibernation, which may be due to the effects of prolonged periods of extreme low body temperatures on the suprachiasmatic nuclei (Hut et al. 2017b). Blood glucose concentrations in captive S. citellus decreased substantially during hibernation, as in other hibernators, but hemoglobin levels remained the same. Despite decreased liver metabolic activity in captive hibernating animals, antioxidant defense was maintained at a level comparable to that of animals in the spring, or actually increased (Buzadžić et al. 1990). Levels of the glucocorticoid hormone cortisol in captive individuals varied seasonally in euthermic animals, being higher from July to August than from April to May, and most elevated from late October to December. In torpid animals, blood cortisol levels also were low from October to December and higher from January to March (Shivatcheva et al. 1988). The levels of 17-hydroxycorticosteroids and free amino acids in the plasma of aroused animals were higher than those in hibernation and euthermic controls in captive individuals (Petrović et al. 1985). Estradiol levels in free-living S. citellus increase from proestrus to estrus among females (Millesi et al. 2017). During late lactation, estradiol levels also are considerably elevated, indicating initiation of follicular maturation processes during that period (Huber et al. 1999). After the termination of lactation, both estradiol and progesterone secretion peak and decrease before hibernation (Millesi et al. 2017). Females with high estrogen titers produce larger litters the following year (Huber et al. 1999). Testosterone levels in mature males are high at the beginning of aboveground activity and during mating, and decrease thereafter. In nonreproductive yearling males, androgen levels remain baseline throughout the active season (Millesi et al. 1998; Strauss et al. 2017b). 网站原创范文除特殊说明外一切图文作品权归所有；未经官方授权谢绝任何用途转载或刊发于媒体。如发生侵犯作品权现象，英语论文，保留一切法学追诉权。()，英语论文 |
