Age structure and growth of Dryophytes japonicus (Amphibia: Anura: Hylidae) on Sakhalin and Kunashir Islands (Russian Far East)

The article presents the results of studying the age and growth in island populations of the Japanese tree frog, Dryophytes japonicus (Günther, 1859), by the skeletochronology method. The material was collected in the last ten days of July 2023 on Sakhalin Island (Pionery village, Kholmsky District) and on Kunashir Island (between the Yuzhno-Kurilsk settlement, Lagunnoe village, and Otrada village). In total, preparations (sections of the third phalanx from a hind-limb finger) from 91 individuals were studied: 43 from Sakhalin (25 immature, 1 female, and 17 males) and 48 from Kunashir (3 immature, 6 females, and 39 males). The average age of sexually mature males from Sakhalin was 1–5 years (on average 3.2±1.60) and from Kunashir 1–8 years (on average 2.6±1.61). The estimated life expectancy of males after puberty on Sakhalin was 3.88 years (S = 0.704) and on Kunashir 3.13 years (S = 0.620). The only adult female captured on Sakhalin was 2 years old. The age of adult females on Kunashir was 2–6 years (on average 3.0±1.55). The estimated life expectancy of females on Kunashir after reaching puberty was 2.70 years (S = 0.545). The body length of males on Sakhalin was 36.17–41.07 mm (on average 38.50±1.66) and on Kunashir 35.17–43.13 mm (on average 39.41±2.07). The body length of females on Kunashir was 39.81–45.18 mm (on average 42.17±1.93). The aggregate of females of all ages on Kunashir was statistically significantly larger than aggregates of males from each locality. Body lengths of males of all ages from Sakhalin and Kunashir did not differ between them. Body lengths of sexually mature hylids of different ages and sexes from Sakhalin and Kunashir differed significantly. Dryophytes japonicus in both localities grow intensively until puberty, after which their growth rate slows down. The body length of males increased statistically significantly with age. It is noted that tree frogs on Sakhalin and Kunashir reach puberty (males after the first wintering and females after the second) earlier than individuals from mainland populations, with island D. japonicus being characterised by similar longevity to mainland populations. It is assumed that this phenomenon is due to the mild climate of this island and the almost complete absence of predators.