Sexual dimorphism in Pseudopus apodus (Reptilia: Sauria: Anguidae) from the Steppe Crimea

The current paper is focused on sexual dimorphism of a giant glass lizard, or sheltopusik, Pseudopus apodus apodus (Pallas, 1775) from its northernmost populations inhabiting the Crimea. In total, 72 P. apodus individuals (45 males and 27 females) were collected at the Kerch Peninsula during 2013–2017. To estimate the variability, 13 morphometric parameters and 18 indices characterizing the head and body proportions were used. It was found that males and females differed significantly by means of almost all parameters, except the body and tail sizes. Besides that, the differences by 10 ratios characterizing head proportions were revealed as well. However, a reliable determination of the lizard sex using linear sizes and/or ratios values seems to be impossible because of the strong overlap of the variability ranges in both sexes. At the same time, the use of the canonical discriminant analysis by the complex of morphometric parameters and by ratios has shown that the males and females in both datasets are classified correctly basing on the sex with an accuracy approximating 100%. The differences in the allometric growth of males and females partially define the sexual dimorphism of P. apodus on head size and shape. A sex-related differences in the development of at least one pair of parameters (head and snout lengths) were clearly evident, since isometry was established for males, while allometry ‒ for females. Moreover, the systems of correlation between the body and head parts differ in both sexes. Thus, male characterisitics correlate significanly, while the female ones were less toughly connected, and some pairs of parameters did not correlate at all.


Introduction
A giant glass lizard, or sheltopusik, Pseudopus apodus (Pallas, 1775) is the biggest representative of Anguidae family in the current fauna of the Western Eurasia with the vast distribution range (Sindaco & Jeremchenko 2008). Recent genetic study basing on the molecular markers allowed to establish that an Submediterranean forest and forest-steppe regions of the south-western part of the Crimean Mountains, while the second -to the north and eastern coasts of the Kerch Peninsula characterized by the continental dry-steppe climate (Kukushkin & Karmyshev 2008;Kukushkin 2015). In the focus of our paper is the second population at the northern limit of the species range. The Kerch Peninsula was chosen because of the highest density of P. apodus as compared to the majority of the Crimean mountain populationsup to 5-11 individuals per 1000 m 2 , sometimes up to 50-75 individuals per a hectare (Kotenko & Kukushkin 2010). It is evident that the catching of the large serpentiform lizard moving rapidly in steppe or semi-desert landscapes requires less efforts, then in the stony mountain terrains.
We have studied in vivo 72 P. apodus adults (62.5% males and 37.5% females), catched at the coastal strip ranging from the Kazantip Cape (on the Sea of Azov) to the foot of the Mountain Opuk (on the Black Sea) during 2013-2017. More than two thirds of the sample (76.4% of individuals) were the lizards from the Bulganak-Osovinskaya Steppe collected between the Tarkhan Cape (N45.4550, E36.4430) and the Khroni Cape (N45.4406, E36.5771) at two plots of 7-8 and 2-3 km long, respectively ( Fig. 1. and Fig. 2). The elevation range of P. apodus collection sites varies between 1-2 and 164 m a. s. l.

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The sample of females have been collected for 5 years during the expeditions. Such a long time was required because of the shift of the sex ratio in the regional P. apodus population, where the males prevail pronouncedly . The sex ratio in our sample (males (1.67) / females (1.0)) is far beyond the real state-of-art, because we strived to have the representative sample of females, while only some of the collected males were measured. In fact, the number of males exceeds the female's one at least in five times. For example, sometimes among 20-30 specimens collected within one day no females were found.
It is supposed that P. apodus in the Central Asia become fertile at 3-4 years and body length of 320 mm (Bogdanov 1965). Our observations in the Steppe Crimea revealed that some females of 310-315 mmlong are able to reproduce. Hence, P. apodus adults become fertile and mature, when their body length exceeds 310 mm.

Morphometric characters
The body and tail lengths were estimated using the tape-measure with the precision of 1 mm. In turn, the head dimensions were evaluated using the caliper with the error of +0.1 mm.
The topography of the head scales in the studied P. apodus population is presented on Fig. 3 and Fig.  4. The measurements following the original scheme are listed below: L.body length from the tip of the snout to cloacal aperture, along the median line of the body (i.e., snout-vent length); L. cd.length of intact tail; TLtotal length (calculated for individuals with an intact tail by the summation of L. and L. cd. values); L. c. ot.head length from the anterior edge of the ear aperture to the tip of the snout; L. c. lr.head length from the posterior edge of the last supralabial scute (usually separated from the ear aperture by two small scales) to the tip of the snout; Pil.pileus length from the posterior margin of the occipital shield (rounded scute, bordering with interparietal shield by a short seam) to the tip of the snout; Lt. c. maxmaximum head width; Lt. c. or.head width at the level of the posterior edge of the eye orbits; Lt. c. oc.head width at the level of the seam between the third and fourth supraorbital scutes (i. e., approximately at the level of the posterior edge of the iris); measured perpendicularly to the body axis, because the supraorbitals position on the right and left sides of the head can be slightly assymetrical; L. on.distance from the anterior angle of the eye to the posterior edge of the nostril, was measured on the right side of the head; L. fr.length of the frontal shield; Lt. fr.maximum width of the frontal shield; L. r.length of the snout (rostrum) from the front edge of the frontal shield to the tip of the snout; Lt. r.width of the snout at the level of the nasal apertures (the distance between the inner edges of the nostrils).
These parameters selected for the sex identification can be easily measured in the field in vivo without any physical harm to animals. Some valuable parameters, though promising in relation to the studies of sexual dimorphism, were ommited because of the difficulties to measure them precisely: e. g., length of hind limbs rudiments, the maximum height and width of intermaxillar (rostral) scute, and some others.

Statistical analysis
The statistical analysis were performed in STATISTICA 6.0 and 10, as well as in PAST 3.11 software packages (Hammer et al. 2001). For each trait the basic statistical parameters were calculated. Since the data were evaluated as normal, the basic parametric statistical methods (correlation analysis and Student's test) were selected. The significance of the difference between the means was evaluated using of Student's ttest. The differences were supposed to be reliable at the confidence level of 5% (P < 0,05).

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For the large-scale comparison of the sex differences following the whole set of the morphological characters a canonical discriminant analysis has been used. Morphological measurements listed above as well as their ratios were applied separately. Characterisitcs L. cd. and TL were omitted here, since 38.9% of individuals in our sample had tails damaged by predators, namely 13 males (28.9%; n = 45) and 15 females (55.6%; n = 27).
For the analysis of the relative growth of the body and head the regression analysis was used. In turn, allometric equations were used for the approximation. The individuals of both sexes differ by the complex of both the linear sizes and indices. This can be explained by the fact that some organs/body parts develop in different pace regarding the others and the organism as a whole (allometric growth). The degree equation Y = b × X a (where Yorgan/body part size, Xthe other organs/body part or the organism size, a and bconstants) is used for allometry evaluation (Shebanin et al. 2008). The most important here is an allometry constant a. Тhus, if a > 1, then Y increases in size faster than X; if a < 1vice versa; if a = 1, then the body parts develop proportionally, i.e. isometrically.
Finally, the correlation analysis was used for the evaluation of the strength of linear relationship between the variables (morphometric parameters) (Lakin 1990).

The body and tail lengths
In the studied sample the specimens with the body length from 361 to 420 mm (66.7% males, 81.5% females, 72.2% in total) prevail among the representatives of both sexes. However, 51.1% of males are 381-420 mm-long, while 59.3% of females -361-400 mm. It can be supposed that these differences represent the real peculiarities of the size and age structure of males and females, though they can be explained by the random factors. The average body length (L.) of males and females in our sample were almost identical, while the maximum length (L. max) were somewhat higher in males (Table 1).
No significant sex differences in the absolute tail length were revealed, although the maximum values of this parameter in females were significanly lower than in males, while the mean values were just slightly lower (Table 1). At the same time, the relative tail length (L./ L. cd.) in both sexes is de facto equal.
Among the individuals with the intact tail the maximim total length of 1230 mm was recorded in male (L. 465 mm, L. cd. 765 mm), while the largest female in our sample was 1085 mm-long (L. 429 mm, L. cd. 656 mm).
The relative size, proportions and shape of the head The significant differences between the males and females at the absolute values of the head mesurements were found for all characters (Table 1). For Lt. fr. character the differences are reliable at P < 0.01; the differences for other characters are supposed to be highly reliable (P < 0.001).
The comparison using indices (Table 2) revealed the highly reliable differences at the parameters characterizing the head length in regards of the body size: L./ L. c. ot., L./ L. c. lr. and L./ Pil. The reliable differences at P < 0.01 were found also at three more indices (Pil./ Lt. c. oc., L. fr./ Lt. fr., Lt. c. oc./ Lt. r.), while at P < 0.05at four (Pil./ Lt. c. or., Pil./ L. r., L. r./ Lt. r., Lt. c. or./ Lt. r.). Furthermore, the differences at other six ratios are below the reliability level, although in all cases their values are slightly higher in females. Finally, the value of L. c. ot./ Lt. c. max ratio has no sex difference at all.
Basing on these data we can conclude that P. apodus females, in comparison to males, have almost the same body length, though shorter head and pileus, relatively wider head on the eye level (Pil./ Lt. c. or. and Pil./ Lt. c. oс.), wider frontal shield as well as relatively shorter and narrow snout. In turn, males have relatively bigger (but not wider) head as compared to females; elongated rostrum with the slighlty pronounced thickening in nostrils area, which is almost unexpressed in females. The upper surface of male snout frontwards the frontal shield has well-pronounced hump-like "crooky" profile. In general, the male head is more angular than the female one, and is visually more separated from the body. Females have more "compact" heads, evenly convex in the nasal area lacking evident "crook" and unobvious cervical interception. Besides that, males, particularly large ones, have more pronounced supraocular visors. The typical sex differences in relative head sizes and shape of P. apodus males and females are presented on Fig. 3 and Fig. 4. Definitely, the listed details favour to some variability and might be expressed in various individuals to different extent ( Fig. 5 and Fig. 6).

The analysis of intraspecific allometry
The allometry was assesed on the basis of the analysis of four pairs of characters, at first, the body length as well as head length from the ear to the snout tip (L. and L. c. ot.), what is expressed as the equation In frames of the current research the allometry was analyzed in the individuals of both sexes, and the resulted equations were L = 39.89 × L. c. ot 0,61±0.06 (males) and L = 67.79 × L. c. ot 0.48±0.11 (females). Basing on these equations, the constant a in both cases differed significanly from 1, what is an evidence for unproportional (allometric) growth of the head and body in males and females. However, the allometry constant a (0.61 in males, 0.48 in females) had no significant differences in both sexes (t = 1.04; P > 0.05). The same results were obtained for another pairs of characters. Тhus, the allomety constant was 0.68 for the ratio Lt. c. or. = b × Lt. c. max a (head width on the level of the posterior orbital margin and maximum head width) without the differentiation of sexes (following the common matrix), what indicates the allometry. In the same time, the similar values were obtained also for males (a = 0.68) and females (a = 0.62). The difference between these indices is insignificant (t = 0.64; P > 0.05). The male and female allometric constants for the equation Lt. c. oc = b × Lt. c. max a (head width at the level of the posterior edge of the iris and maximum head width) were practically identical to the constant values of the previous pair of characters (0.62 in males, 0.61 in females; t = 0.55; P > 0.05). Therefore, as a result of three equation analyses mentioned above, the allometric constant essentially differs from 1 in all cases, what proves that allometric development of the body and head is typical both for males and females.
However, other data were obtained as a result of the analysis of the relation between L. c. lr. и L. r. parameters (head length from the snout tip to the last supralabial and snout length). In the allometric function L. c. lr = b × L. r a the allometry constant corresponded to the isometric growth (a = 0.98) in the analysis of both sexes altogeather. The same results were for males (a = 0.97), although for the females it was different (а = 0.55), what corresponded an allometric growth. It is noteworthy that the differences between the constants in males and females were statistically reliable (t = 2.21; P < 0.05). Therefore, P. apodus has evident sex-related features of the relative development of at least one pair of studied parameters, which indicates the isometry in males and allometry in females. This feature might be probably expressed at the remarkable sex differences in the relative length of the rostral part of the head and the snout outlines ( Fig. 3-6).

The discriminant canonical and correlation analyses
The distribution of individuals within the two canonical axes according to the absolute values of the dimensional features is shown on Fig. 7. The matrix of sex-based classifications suggests that 42 out of 45 males (93%) fit their group, while the rest 3 individuals better fit to females in the set of characters. In turn, 100% of the females fit their own group. A sex-based discrimination is provided by the first canonical axis by all the measurements (Table 3). Thus, the whole set of measurement characters can be used in the studies of sexual dimorphism in P. apodus populations. However, the overlap of the variability ranges makes the sex identification using linear sizes and their ratios impossible.   The similar results were obtained using indices (Fig. 8). The matrix of classifications suggests that both sexes reveal almost absolute (100%) sex-based discrimination: 95% of males and 96% of females were identified correctly. In this case, the best discrimination was also observed at the first canonical axis, and the correlation coefficients between the indices and the canonical axes were statistically significant for all indices (Table 4). The variability range of the both sexes overlaps substantially. Hence, there is no reliable value for a single index, which allows to identify correctly the sex of each individual. At the same time, the discriminant analysis revealed a relatively significant differences in the complex of characters and indices. Undoubtedly, the sheltopusik body parameters correlate between themselves. It is assumed that the correlation systems of males and females may differ. To test this hypothesis, the correlation analysis of all measurements of the lizard body was performed for the whole sample and also for males and females separately. As a result, the reliable correlations between all measurements were obtained for the whole sample (Table 5), in which the sex differences were neglected. All male characters correlates significantly as well ( Table 6). Despite that, the results of the correlation analysis of the female group were diverse, since the strength of the relationship between the characters were somewhat lower than in males, and totally absent between some pairs of characters (Table 7). Therefore, the representatives of both sexes have certain differences in the correlation system between the individual body parts, what presumably allows to discriminate sexes qualitatively.

Other features and body coloration
The head relative size and shape are not the only sex-specific habitual characters. For instance, the presence of hemipenises makes the tail base to be more massive as compared to females. Moreover, in males the transition between the body and the tail base is unpronounced, while females have noticable tail narrowing right after the cloaca ( Fig. 9 and Fig. 10). Besides these characters, some other features typical for males and females were registered, and they will be only shortly mentioned here, though require more detailed analysis. Thus, the rudiments of the hind limbs in males in average are somewhat longer (more than 4 mm) as compared to females (less than 4 mm). Intermaxillar shield is much narrower, more convex and slightly protruding in females than in males. Furthermore, the sex-dependent differences in shape of pupil of the eye is also observed: in males, especially big ones, its margin is mostly sharply scalloped, while in femalesmore roundish (Fig. 6).

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The differences between P. apodus males and females in body coloration is not so evident as in other representatives of Anguidae family (Fitch 1989;Sos 2011) and, especially, Gerrhonotinae (Dashevsky et al. 2012). Nevertheless, it is possible to distinguish some diversifications in P. apodus body coloration in the studied population related with the specificity of the phenocomplex of each sex. At late Mayearly June, the gray-olive or yellowish dorsal body surface in males is noticeably lighter than in females, which is reddish-brown. Moreover, females have pronounced white spots and patchy plots on the back forming recognizable "speckled" type of coloration ( Fig. 9-12). Hence, it is possible to differentiate rather reliably the representatives of both sexes at the peak of their seasonal activity in late spring -early summer on the basis of different coloration patterns.
The observed sex-dependent differences in coloration of the background might be defined mainly by the moulting stage, i. e. features of lizard phenology. For example, at the Kerch Peninsula, an overall moulting of males was observed at early-mid-May before the mating period, while non-gravid females usually prevailing in the population moult from late-July to mid-August . According to our observations, the moulting of pregnant lizards takes place at late Juneearly July approximately 2-3 weeks before the egg laying, which lasts the whole July in the Kerch Peninsula populations (Szczerbak 1966;our unpublished data).

Discussion
The body sizes Among the various representatives of Anguidae the males regarding the body length can be significantly smaller (Sos & Herczeg 2009) or bigger than females (García-Bastida et al. 2013), while sometimes both sexes are equal-sized (Dashevsky et al. 2013). Usually, males of anguid lizards seem to be bigger (Cox et al. 2007). In our P. apodus sample males and females have almost the same mean body lengths, but the differences in the mean tail lengths are insignificant (P > 0.05).
The data characterising the dimensions of P. apodus from the Kerch Penunsula (Kazantip Cape and Karalarskaya Steppe) at late 1990's were obtained: L. is 392+11.13 mm (limits are 320-450 mm; n = 33) in males, and 360+30.82 mm (limits are 310-450 mm; n = 4) in females; TL is 1011+14.22 (limits are 910-1140; n = 19) in males, and 933+77.62 (limits are 850-1150; n = 4) in females; L. cd. is 621+9.01 (lim. 560-700 mm; n = 19) in males and 573+42.70 (520-700 mm; n = 4) females (Yu. Karmyshev, personal communication). This dataset has higher maximum values of all size characters in females than in our sample (Table 1), while the TL and L. mean values are substantially lower. Undoubtedly, these results supplement the morphological descriptions of P. apodus from the Kerch population, however, the small number of females in the sample do not allow to evaluate the sex differences adequately.
In a very large P. apodus sample from Turkmenistan (225 males and 115 females), the maximum body length was 460 mm for both sexes (Bogdanov 1962). At the same time, in the sample of 28 males and 22 females from the Northern Caucasus, the maximum and average body lengths in females were somewhat higher than in males: 444 mm (373.5 mm in average) and 418 mm (367.1 mm in average) respectively (Siroitchkovsky 1958). This might be related to the reproductive function of females and "the necessity to bear numerous eggs" (Siroitchkovsky 1958, P. 43). On the contrary, other authors indicated larger male sizes (L. and/or TL) for the Central Asian and Pre-Caucasian populations (Bogdanov 1960;Yakovleva 1964;Skrynnikova 1977), although in all cases there were a few females in the samples, what might affect the reliability of the conclusions. It seems to be correct that P. a. apodus males and females have no significant differences in the body length (Yakovleva 1964).
It is known that the body size variability in males and females within the same Squamata species may be complex depending on the geographical location and habitat conditions of a particular population: in one part of the range the larger males can prevail significantly, while in another -the larger females (Cox et al. 2007;Maliuk & Peskov 2011). In regards of the aforementioned data, it can be added that the sizes of P. a. thracius males from two Croatian populations in Cres Island and Split environs varied. Thus, they had larger maximum sizes in both populations, while their average sizes were slightly larger than in females in the first locality, though smaller in the second one (Lovrić 2012).

The relative tail length
Some authors studying sheltopusiks noticed that the mean tail length in males is somewhat bigger than in females at relatively equal maximum indices (Siroitchkovsky 1958;Lovrić 2012). At the Crimea L./ L. cd. ratio comprised 0.55-0.64 in males and 0.57-0.67 in females (Szczerbak 1966). In Kyrgyzstan P. apodus males have 1.52-1.72-fold longer tails as compared to the body, while in females -only 1.36-1.49fold; hence, there was no overlap in the variability ranges (Yakovleva 1964).
Our results clearly confirm that both maximum and mean tail lengths in males is bigger than in females, although the sex differences on this parameter are not reliable (Table 1). At the same time, no sex differenses basing on L./ L. cd. were found (Table 2), what corroborates the data obtained in 1990's: L./ L.cd. index value in males соmprised 0.63+0.01 (limits are 0.59-0.69; n = 19), in females -0.63+0.02 (limits are 0.57-0.65; n = 4) (Yu. Karmyshev, personal communication). It is noteworthy that in the sample collected at the end of 1990's, 36.8% of individuals have the damaged tailthis finding is very close to the recent ones (see above).

The head relative sizes and proportions
According to Yu. Karmyshev, the comparative analysis of both sexes established sexual dimorphism at two indices -"a relative head width" (the definition is not described in details) and "a relative height of intermaxilar shield" (the shield is relatively narrower in females) (Karmishev 2002). Two indices, (1) the ratio of the maximum head width to the head length from the mouth corner to the snout tip, and (2) the ratio of the head length to the body length, revealed the highly reliable differences (at P < 0.001) at the comparison of 24 males and 5 females in the jointed sample from the Mountainous and Plain Crimea (Yu. Karmyshev, personal communication). As far as we know, there are no any other data characterizing head proportions of P. a. apodus of both sexes.
In males and females of P. a. thracius from the Northern Adriatic (Cres Island) population the sexual dimorphism has been revealed on the snout width (internostril distance), head length (from the snout tip to the cervical interception) and some others size parameters (Lovrić 2012). Unfortunately, the measurements and ratios used by this researcher distinguish from ours, what excludes the possibility of thorough comparison of two northern populations of different P. apodus subspecies.
As it was mentioned above, the observed sex differences in the relative head length of P. apodus from the Kerch Peninsula population are not related to the allometric growth specificity in males and females. At the same time, our data suggests that such a specificity defines the sex differences in the ratio between the different head parts and, to some extent, its outlines.
The putative issue of allometric growth specificity has been discussed earlier for the representatives of the sister genus Anguis Linnaeus, 1758: "Slow warms exhibit a distinct sexual dimorphism in body proportions. In equal-sized individuals, always the males have a bigger and wider heads. Relative to the body, their head growth is much more positively allometric than that of the females…" (Böhme 2006, P. 243). It is noteworthy that the developmental patterns are even more complex for other anguids. Тhus, both Gerrhonotus infernalis Baird, 1859 and P. apodus have significant sex-dependent differences in head size. G. infernalis females reveal negative allometric growth pattern for the head length, and isometric growth for the parameters, characterizing head width and height. In turn, males of this lizard show isometric growth for the head length character, while for the head width and heightpositive allometric one (García-Bastida et al. 2013).
As it was mentioned above, P. apodus males and females have differences in correlations between the body parts. It has to be taken into account that the correlations systems are dependent on the environmental conditions. It is assumed that in other P. apodus populations the sex differences might be expressed in similar manner, though the regional peculiarities may not be exluded as well because of some indirect evidence. Тhus, in P. a. thracius sample from Cres Island comrising 21 males and 14 females, males showed higher absolute values at the majority of the parameters than females, what indicates the strong expression of sex dimorphism in this population. In turn, in the coastal Croatian sample consisting of 16 males and 11 females sex dimorphism is less pronounced (Lisičić et al. 2012). Hence, it is the issue of a high interest to study P. apodus sexual dimorphism in biogeographical aspect.
Among the possible reasons of the head size increase in males of anguid lizards as compared to females, at first, the sexual selection has to be mentioned, what might occur due to malemale combats and mating rituals common among the anguids, while the head elongation and widening are related with the development of strong jaw muscles and the increase of the biting force (Fitch 1989;Böhme 2006;Sos & Herczeg 2009;Dashevsky et al. 2013;García-Bastida et al. 2013;Sharf & Meiri 2013). The same tendency was registered to other Sauria systematic groups: males with evidently bigger heads might have advantages in agressive interactions and mating as compared to males with the smaller heads (Kratochvíl & Frynta 2002;Maliuk & Peskov 2011). It has to be noticed in this regards that the brutal and prolonged combats of P. apodus males are quite common during the reproductive season (Jablonski, 2018), while males tightly hold the females with the jaws during the copulation .
Among the other reasons for the sexual dimorphism in head shape and size, the nutritional preferences of males and females can be mentioned. In the diet of this big lizard the invertebrates with the hard integuments prevail: mollusks, big Orthoptera and Coleoptera, fresh-water crabs, etc. (Siroitchkovsky 1958;Szczerbak 1966;Rifai et al. 2005;Tuniyev 2007; own observations). The data on the diet of P.apodus representatives of both sexes is still scarce (Çiçek et al. 2014), and, as far as the evidence will be accumulated, the sex differences in the intensity and the nutritional spectrum might be found in future. To strengthen this assumption, the observations that P. apodus females have more secretive lifestyle and shorter distance of activity than males have to be mentioned. Probably, some differences in daily activity of P. apodus males and females also exist. Altogeather, the aforementioned differences might affect the spectrum of lizard feeding.

Taxonomical aspect of morphometric studies
At the conclusive part, the taxonomical aspect of P. apodus morphological studies has to be discussed. F.J. Obst (1978) had subdivided sheltopusik on 2 subspecies: a nominative one, widespread mainly in Asiatic part of the species range, and a Balkan's one (P. a. thracius), which inhabits South-Eastern Europe and Western Anatolia. Both subspecies were identified on the basis of the geographical pattern of the external morphology characters variability including one mophometric parameter, namely "head length to width ratio". In the original paper it was defined as "...Kopflänge von der Schnauzenspitze bis einschließlich Occipitalschild im Verhältnis zur Kopfbreite in Höhe des hinteren Augenrandes..." ("...the ratio of the head length from the snout tip to the occipital shield to the head width at the level of the posterior eye edge...") (Obst 1978, S. 130), though it is not clear from the context, whether the eye itself or the orbital fossa has been mentioned. The average value of this ratio was 1.93 (variation range is 1.72-2.12; n = 33) in P. a. thracius, while in P. a. apodus -2.16 (variation range is 1.88-2.33; n = 38) disregarding the lizard sex.
The Pil./ Lt. c. or. ratio used in our work is supposed to be equivalent to "head length to width ratio", which has the diagnostic significance for P. apodus subspecies distinguishing. It was mentioned before that sheltopusiks from both Crimean populations are fully corresponding the description of a nominative subspecies by the pholidosis characters, while the coloration specificity and the head ratio mentioned above have intermediate character and is more in accordance with P. a. thracius description (Kukushkin & Sviridenko 2005;Kukushkin & Karmyshev 2008). In the sample from the Kerch Peninsula (n = 72), what is the basis for the current research, value of the ratio Pil./ Lt. c. or. comprised 1.97+0.01 (limits are 1.75-2.21).
Recently, the division of the species into eastern and western subspecies was confirmed by the molecular and genetics methods (Jandzik et al. 2018). Nevertheless, the morphological differences between the subspecies and population still remain unclear, and the taxonomical revision of P. apodus intraspecific systematics on the basis of morphological data is strongly required. Moreover, the sexual dimorphism might be considered as well. At the initial stages of the research, the unification of the P. apodus measurement scheme will be rather important because of the incomparability of the published data on species morphology. Furthermore, a geometric morphometry approach can be also helpful at the comparative studies of sexes, populations and/or subspecies of P. apodus (Kaliontzopoulou 2011).

Conclusions
No significant sex differences in P. a. apodus body and tail sizes were found in the population from the Kerch Peninsula, although the values of the maximum body lengths as well as the maximum and average tail lengths in males were somewhat higher than in females. At the same time, the relative size and shape of the head were considerably different in both sexes. However, it seems impossible to identify correctly the sheltopusik sex basing only at the linear head sizes and/or ratios characterizing the head proportions of both sexes because of the significant overlap of their variability ranges of the morphometric parameters and the derived ratios. Nevertheless, the use of the canonical discriminant analysis with the complex of absolute values of the morphometric parameters allowed to identify reliably 93% of males and almost 100% of females, while the same analysis using ratios -95% males and 96% females. Thus, the proposed set of measurements can be used for the sexual dimorphism assessment in P. apodus populations.
The observed sex differences in head size and shape might be partially explained by the peculiarities of its growth in general as well as its parts. It is noteworthy that the sex differences in P. apodus relative development of L. c. lr. and L. r. characters (isometric growth in males and allomertic -in females) might be partially responsible for an easy distinguishing between males and females in the head outlines. Besides that, the representatives of both sexes show some differences in the correlation systems between the morphometric parameters. However, the obtained results could not be directly extrapolated to other populations, since the absolute and relative size characters are highly dependent on local habitat conditions such as the duration of the activity period, growth rate, nutritional spectrum, etc.
Sex differences of the body coloration are not much pronounced, and are defined mostly by the time of moulting of males and females. The background color of the dorsal body side is recommended to be used only as an extra "field" character for the rapid preliminary sex identification.
In P. apodus from the Steppe Crimea the diagnostic character for the subspecies identification, so called "head length-width ratio", is intermediate between the values known for the nominative subspecies and P. a. thracius, according to F.J. Obst (1978). In general, P. apodus intraspecific taxonomical revision basing both on recent molecular data and external morphology characters considering sexual dimorphism is highly required. Furthermore, the development of a unified system of measurements is desirable for this purpose.