MORPHOLOGICAL FEATURES OF MICE OF THE GENUS SYLVAEMUS UNDER CONDITIONS OF SYNTOPY IN THE EASTERN FOREST STEPPE OF UKRAINE

The opinion regarding morphological changes in S. sylvaticus depending on being in syntopy with the competing sibling species S. tauricus is ambiguous. Most authors claim that allotopic and syntopic populations do not differ. Some of them indicate a tendency that the size of S. sylvaticus increases in allotopic populations, while others claim that such increase occurs in syntopic populations. There is an assumption that the nature of morphological changes in S. sylvaticus undergoes geographic variability and manifests differently in different parts of the range. Overall, the authors suggest that character change is an adaptive response of a subordinate species to the presence of a dominant competitor, so character change as a result of competitive interaction is more likely to occur in S. sylvaticus than in S. tauricus . The research was conducted in the territory of Kharkiv Oblast, Ukraine. During the observation period, from spring 2017 to autumn 2022, 666 specimens of the genus Sylvaemus were caught in 10 selected biotopes. Different types of oak forests, dry and floodplain meadows, riparian vegetation, steppe areas, chalk slopes, field protection forest strips, fields, as well as various ecotones were studied. According to the results of the research, being in syntopy with S. tauricus , S. sylvaticus has statistically significantly higher average values of parameters of external characters, which largely complicates the identification of these two species in the field. For the most part, confusion in identification occurs in forest biotopes, namely in dry and fresh


Introduction
Mice of the genus Sylvaemus live in sympatry in most of their range.Due to their similar habitat requirements, they are not uncommon to be found in the same habitat.Being in syntopy, species compete for shelter and food resources.Thus, interspecific competition and various environmental factors can affect the morphology of wood mice, which is manifested in size variations and fur colouration [Panzironi et al. 1993].The authors also suggest that the combination of these factors may cause morphological convergence in sympatric species to the extent that it leads to mutual camouflage in the same size and general habitus, which will be more adapted to escape from predators.
The value of Hutchinson's index for the group of wood mice Sylvaemus reaches HK = 1.17 for five craniometric characters of differentiation: condylobasal length of the skull, length of the upper row of molar teeth, length of the auditory bulla, length of the nasal bones, and occipital width of the skull.The Hutchinson's coefficient is below the critical value, which indicates a significant overlap of niches and the presence of competitive relationships between species [Zagorodniuk 2007].
As noted by Barciova & Macholan [2006], among the three species of wood mice (Sylvaemus uralensis, Sylvaemus sylvaticus, and Sylvaemus tauricus), interspecific competition is most pronounced in the sylvaticus-tauricus pair.S. tauricus is less ecologically plastic and more habitatspecific [Montgomery 1981], behaves as a dominant species [Alcantara 1991] and prefers habitats with old forest stands and dense canopy.Whereas S. sylvaticus occurs mostly on forest edges and in open habitats, although under allopatric conditions it is found in large numbers in forests.
For wood mice, Alcantara [1991] and Mikulova & Frynta [2001] suggested that character variation is an adaptive response of a subordinate species to the presence of a dominant competitor, so character variation due to such competitive interaction is more likely to occur in S. sylvaticus than in S. tauricus.Changes in characters will primarily affect the general dimensions of the animal, so measurements should focus on the skull and body.In particular, it is worth paying attention to characters related to certain functions, for example, the length of the hind foot and tail in relation to movement and climbing, measurements of teeth in relation to chewing [Mikulova & Frynta 2001;Lashkova & Dzeverin 2004].Amori & Contoli [1986] for populations from southern Italy showed that S. sylvaticus demonstrates an adaptive response to the conditions of syntopy with its sibling species, which is manifested in morphological changes, so that S. sylvaticus is potentially larger and S. tauricus is smaller in the absence of its competitor.Although Niethammer [1969] reports that allopatric S. sylvaticus in Germany and France do not differ in hind foot length, tail length, condylobasal skull length, and upper molar row length from S. sylvaticus in sympatry with S. tauricus.He also argues that sympatric S. sylvaticus do not tend to shift in size toward S. tauricus.
A comparison of syntopic and allotopic wood mouse populations was also conducted by Barciova & Macholan [2006] in the Czech Republic.The difference between the four samples (syntopic S. tauricus, allotopic S. tauricus, syntopic S. sylvaticus, and allotopic S. sylvaticus) was significant.Out of the selected cranial measurements, only five (brain case height including bulla tympanica, upper molar row length, bulla tympanica length, lower molar row length, and upper incisor thickness) showed significant differences between syntopic and allotopic populations of S. tauricus, with syntopic specimens having higher values than allotopic ones.No significant differences were found between allotopic and syntopic populations in S. sylvaticus.The same is noted for S. sylvaticus by Alcantara [1991], there is no difference in body size (weight, condylobasal length) between sympatric and allopatric populations of S. sylvaticus, but larger sizes are observed mostly in allopatric populations than in sympatric ones.Mikulova & Frynta [2001] also report no difference.
According to the results of geometric morphometry, the average centroid size of individuals from syntopic populations of S. tauricus was significantly larger than that of allotopic populations.However, syntopic S. sylvaticus were on average also larger than allotopic populations.These differences were significant for dorsal and ventral landmarks [Barciova & Macholan 2006].
The aim of this research is to find out whether the mice of the genus Sylvaemus differ in external characters inhabiting different habitats and depending on the level of syntopy in the studied biotopes of the eastern forest-steppe of Ukraine.

Materials and Methods
Micromammals were captured using the trap-line method [Kucheruk 1952;Numerov et al. 2010], using Gero traps equipped with standard bait (rye bread crust with unrefined sunflower oil).The traps were placed 5 metres apart in a line.In each habitat, 25, 50, or 100 traps were placed, and the trapping was carried out during one night.Whenever possible, the trapping was carried out three times a year: in spring, summer, and autumn.
Three metric external characters were selected for analysis: hind foot length (P), body length (L), and tail length (C) [Zagorodniuk 2002].Ear length (Au) was not taken into account in the study as a measurement with a large measurement error and therefore not recommended by the authors for use in the analysis [Barciova & Macholan 2006].
Species diagnostics of the captured specimens was carried out using metric methods with the measurement of a complex of external and craniological characters.Initial identification was based on visual assessment of fur colour, presence and shape of the chest spot, eye diameter, and measurements of body, tail, and hind foot length.The next step was to measure the length of the upper molar row and the greatest length of the skull [Markovska 2023].The selected set of characters is usually sufficient for field identification of wood mice [Barkaszi 2018].The specimens that were questionable were checked in more detail according to a set of significant odontometric and craniological characters [Markovska 2023].

The features of external characters of species in different habitats
The first task was to find out whether mice of the genus Sylvaemus that inhabit different biotopes (Table 1) differ in external characters: hind foot length (P), body length (L), and tail length (C) (Table 3).According to the results of the discriminant analysis, the external characters of each species contributed to the distribution in different ways (Table 4): The correctness of the classification of wood mice by external characters in the studied biotopes in general is 42% for S. uralensis (the highest for DM 93%), 37% for S. sylvaticus (the highest for FMLF 79%, DM 61%, RV 60%), and 45% for S. tauricus (the highest for DMLF 93%, MMLF 50%).
The least variable external characters were: in the RV/FM, RF in S. uralensis, in the RV in S. sylvaticus, in the MMLF in S. tauricus.The morphospaces of the samples in the studied habitats, according to the distribution of external characters, overlap in all three species (Figs.1-3).

Variability of external characters of species under conditions of syntopy
The next task was to find out whether the external characters of mice of the genus Sylvaemus differ depending on the level of syntopy in the studied habitats, that is, depending on the number of species found in the same habitat (one, two, or all three) (Table 5).The external characters of each species contributed to the distribution in the same way as in the previous analysis, with the hind foot length and tail length contributing the most (Table 6).The correctness of the classification of individuals by external characters depending on the level of syntopy in the studied habitats is 46% for S. uralensis (the highest for US 87%), 55% for S. sylvaticus (the highest for SU 73%, ST 69%), 46% for S. tauricus (the highest for UST 97%).The least variable external characters were in the UT sample of S. uralensis and in the TU sample of S. tauricus.
Morphospaces of the samples depending on the level of syntopy in the studied habitats, according to the distribution of external characters, overlap in all three species (Figs 4-5), but in S. sylvaticus the ST sample stands out (Fig. 6), (Table 7).
It should be noted that the greatest confusion in identification occurs in forest habitats, namely in dry and fresh maple-linden-oak forests.In the studied oak forests in Kharkiv Oblast, only two species are found-S.sylvaticus and S. tauricus, whereas S. uralensis occurs only on the edges.
And it is in the oak forests that large specimens of S. sylvaticus are found, which are very similar in habitus to S. tauricus.Although the figures do not fully reflect it, in open habitats S. sylvaticus is closer to S. uralensis in absolute size, often young or moulting individuals are very similar even in fur colour.In the steppe regions of Kharkiv Oblast, S. sylvaticus individuals with a weakly expressed-and sometimes barely noticeable-chest spot are found.Although S. sylvaticus shows such trends in size changes towards S. tauricus and S. uralensis, the difference between the syntopic samples with these species is statistically significant (Table 8).The opinion on morphological changes in S. sylvaticus depending on syntopy with the competing sibling species S. tauricus is ambiguous.Most authors argue that allotopic and syntopic populations do not differ [Niethammer 1969;Alcantara 1991;Mikulova & Frynta 2001;Barciova & Macholan 2006].Some point to a tendency to increased size in allotopic populations [Amori & Contoli 1986], others to increased size in syntopic populations.There is an assumption that the nature of morphological changes in S. sylvaticus is subject to geographical variability and manifests itself differently in different parts of the range.It is also worth remembering that the pair of sylvaticustauricus is characterised by opposite clinal size variability, which also leaves its mark on the nature of morphological changes [Filippucci et al. 1989].
In this study, S. sylvaticus tended to increase in size in syntopic populations with S. tauricus, as observed in oak forests.The ST samples were statistically significantly different from the others in terms of the mean values of all external characters, especially in the hind foot length (Table 7).In terms of body and tail length, only the difference with the allotopic sample was not statistically significant.In European syntopic populations of S. sylvaticus, changes also occur in body and skull measurements, especially hind foot length, tail length, and tooth length [Mikulova & Frynta 2001].

Conclusions
1.No differences in external characters were found in mice of the genus Sylvaemus inhabiting different habitats; the morphospaces of the samples overlap in all three species.
2. Depending on the level of syntopy in the studied biotopes, according to the distribution of external characters, the sample of S. sylvaticus in syntopy with S. tauricus stands out noticeably.
3. Being in syntopy with S. tauricus, S. sylvaticus has statistically significantly higher mean values of body measurements.