Age structure and growth of the common slow worm (Anguis fragilis, Reptilia, Anguidae) in the eastern Baltic region (Kaliningrad Oblast, Russia)

The most common method for determining the age of reptiles is skele-tochronology. Hematoxylin-stained cross-sections of the tubular bones in the extremities are most frequently used as recording structures. Certain challenges arise when determining the age of legless species. One potential approach to ascertain the age of species capable of autotomy and tail regeneration is to examine the caudal vertebrae obtained from live specimens. Slow worms of the genus Anguis are widely distributed throughout the Western Palearctic and often exist in significant populations. The common slow worm (A. fragilis) is found in Western and Central Europe, and in Russia, it is known only from the western part of Kaliningrad Oblast. Among the representatives of the genus Anguis, age has been determined by skeletochronology only in A. veronensis and A. colchica. This study presents the findings on the age structure and growth characteristics of A. fragilis as determined through skeletochronology. The specimens were collected in the second decade of June 2023 near Zelenogradsk, Kaliningrad Oblast. The snout-vent length (SVL) of the captured lizards was measured, and a section of the tail was removed. All lizards, except for the females, were released at their capture sites. Additionally, the dentary bone was also removed from four individuals that were killed by vehicles. A total of 24 individuals were examined, comprising 10 males and 14 females. The females were kept in the laboratory until they gave birth to their offspring. After measurements were taken, both the females and juveniles were released back into their natural habitat. Lines of arrested growth were observed in the prepared cross-sections of the caudal vertebrae and dentary bones (see Fig. 1). The studied animals ranged in age from 1 to 7 years. Mature females were aged 3 to 6 years (average 4.15 years), while males ranged from 4 to 7 years (average 5.00 years). The modal age for females was three years, comprising 35.7% of the sample, whereas the largest proportion of males (50% of the sample) were four years old. The body length of females aged three years and older varied from 116.28 to 170.23 mm (average 137.99 mm), while males ranged from 110.16 to 163.51 mm (average 146.51 mm) (see Table 1). The calculated maximum body length (SVLmax) for females was 143.25 mm, and for males, it was 159.70 mm. There was no significant difference in body length between females and males across different age groups. The body length of adult slow worms did not correlate with the age of either sex (see Fig. 2). Among all captured females, only two individuals produced offspring in the laboratory. A three-year-old female, measuring 129.06 mm in body length, gave birth to five young lizards measuring between 42.70 and 47.72 mm (average 44.90 mm). Additionally, a six-year-old female, with a body length of 159.03 mm, gave birth to eight young, which measured between 31.85 and 39.51 mm (average 36.38 mm). The authors conclude that A. fragilis has an average lifespan (see Table 2). The growth of these lizards slows down at the age of 3 years and older, which is associated with the onset of puberty. This is further supported by the minimum age of one of the breeding females, which was 3 years old. Based on the results of the research conducted, the authors draw the following conclusions: 1. An equal number of lines of arrested growth are formed in the caudal vertebrae and dental bones of the common slow worm; 2. The common slow worm in the eastern Baltic region typically lives for 6 years (females) to 7 years (males); 3. Adult common slow worms of different ages do not differ in body length, indicating a high degree of individual variability in growth rates; 4. Females reach maturity after their third winter and do not reproduce annually; they retain their fertility until the end of their lives. The article contains 2 Figures, 2 Tables, 43 References. The Authors declare no conflict of interest.