Mammal Study

We reviewed the history of studies and conservation efforts related to terrestrial mammals in the central and southern Ryukyu Archipelago in Japan, covering six orders, 11 families, 20 species, and six subspecies, with 77% endemic and 62% of species classified as threatened. The most significant feature is that phylogenetically ancient species have survived alongside humans, who have inhabited the islands for approximately 30 000 years. This coexistence is unique to this region and not found in other parts of Japan, making it a globally rare phenomenon. In 2021, four islands in the region were designated as Natural World Heritage Sites, marking a conservation milestone, although challenges remain. Island biodiversity is crucial to global biodiversity because of its high endemism. Island biology provides an essential biogeographical perspective to understand evolution and shape conservation strategies. The region has become a focal point of interest due to its unique biota and distinctive island ecosystems. We concluded that a comprehensive re-evaluation of the islands is essential, focusing on biogeography, ecology, and species interactions. Conservation measures should be tailored to each island by incorporating insights from ethnography, sociology, and economics. It is our responsibility to protect the region's endangered species and the biodiversity for future generations.

Published online 2 April, 2025; Print publication 30 April, 2025

The Ryukyu Islands have experienced a complex history of island connection and separation since the Late Miocene. Questions persist regarding how organisms have evolved through changes in island configurations over millions of years particularly in the Central Ryukyus, which is characterized by high species endemism. We conducted comparative phylogenetic and phylogeographic analyses among island populations targeting five mammalian species (Pentalagus furnessi, Diplothrix legata, and three Tokudaia species), all of which belong to genera endemic to the Central Ryukyus, employing genome-wide SNP and mitochondrial DNA variation analyses. The SNP and mitochondrial DNA analyses across these genera revealed distinct lineages on each island (Amami-Ohshima, Tokunoshima, and Okinawajima, except for P. furnessi absent from Okinawajima), with the Amami-Ohshima population showing the highest genetic diversity. Divergence times between island populations were estimated to be in the Middle Pleistocene for P. furnessi and D. legata and ranged from the Late Miocene to the Early Pleistocene for Tokudaia. These findings, along with those of previous studies on other terrestrial animals, emphasize the significant impact of pre-Pleistocene island isolation on genetic divergence in species with limited dispersal abilities, while some species exhibited inter-island migration by the Middle Pleistocene.

Published online 20 January, 2025; Print publication 30 April, 2025

後期中新世以降の島嶼集団ダイナミクス:中琉球に固有の哺乳類 3 属（Pentalagus，Diplothrix，および Tokudaia）における比較系統地理学的研究．琉球列島は後期中新世以降，島間の連結と分断を繰り返してきた複雑な歴史を有している．特に多くの固有種が生息する中琉球では，過去数百万年にわたる島嶼構造の変化の下，生物がどのように進化してきたかについて，未だ多くの疑問が残されている．本研究では，中琉球に属レベルで固有の 5 種の哺乳類（アマミノクロウサギ Pentalagus furnessi，ケナガネズミ Diplothrix legata，およびトゲネズミ類 Tokudaia 属の 3 種）を対象に，ゲノムワイドな一塩基多型（SNP）およびミトコンドリア DNA 変異に基づいた比較系統学的・系統地理学的解析を行った．SNP およびミトコンドリア DNA の双方の解析結果から，上記の 3 属いずれでも，各島集団（奄美大島，徳之島，および沖縄島．ただしアマミノクロウサギは沖縄島に生息しない）はそれぞれ独立した系統であることが確認され，奄美大島の集団の遺伝的多様性が最も高いことが明らかとなった．島集団間の分岐時期は，アマミノクロウサギとケナガネズミでは中期更新世と推定されたのに対し，トゲネズミ類では後期中新世から前期更新世に遡ると推定された．以上の結果は，他の陸生動物の先行研究の結果も合わせて考えると，分散能力が限定されている種については更新世以前に生じた島嶼の隔離が遺伝的分岐に重要な役割を果たした一方で，一部の種においては中期更新世まで島間の移動が生じていたことを示している．

Bone histology is a valuable tool for studying growth patterns and life history traits in animals. This study examines four Japanese sika deer (Cervus nippon) populations—mainland (Hokkaido and Chiba) and insular (Yakushima and Kerama Islands)—using femoral and tibial histology to investigate growth patterns and sexual dimorphism. Growth curves based on lines of arrested growth (LAGs) revealed slower growth rates in insular populations, supported by extensive parallel-fibered bone (PFB) development and reduced vascularity. Earlier external fundamental system (EFS) formation and epiphyseal fusion in mainland populations (three to four years) compared to insular populations (5.5 to 7.5 years) further support this finding. Sexual dimorphism was pronounced in mainland populations, with males exhibiting delayed EFS formation, epiphyseal fusion, and extensive bone remodeling than females. In contrast, the Kerama population showed no significant sex-related differences in remodeling, which may influence unique growth patterns. These findings highlight the influence of insularity, such as resource limitation and predator-free environments on life history traits, which could associate slower growth rates and delayed maturity in insular deer. This study underscores the utility of bone histology in understanding adaptive strategies of endemic and endangered mammals.

Published online 2 April, 2025; Print publication 30 April, 2025

The Amami rabbit (Pentalagus furnessi), endemic to Amami-Oshima and Tokunoshima Islands, exhibits distinct life history traits compared to mainland lagomorphs, yet its growth patterns remain largely unexplored. This study conducted bone histological analysis of Amami rabbit limb bones to uncover its growth and ecological adaptations. Results indicate rapid growth until about two years old, followed by gradual growth until approximately five years old, when growth ceases with epiphyseal closure. This delayed maturation contrasts sharply with mainland species like the Japanese hare (Lepus brachyurus), which matures within a year. This slower growth and delayed maturation in the Amami rabbit aligns with previous observations suggesting a shift toward a K-strategy in its reproductive traits. Additionally, the Amami rabbit's dense limb bones with thick cortices and a small medullary cavity, provide bone robustness suited for its steep, rugged habitat—contrary to other lagomorphs which adapted to leaping locomotion. These findings suggest that the Amami rabbit's growth and skeletal structure respond to unique island ecological conditions, with limited predation and resources, highlighting adaptive strategies in insular mammals and emphasizing the Amami rabbit's unique ecological adaptations.

Published online 2 April, 2025; Print publication 30 April, 2025

The Ryukyu long-furred rat, Diplothrix legata (Muridae) is an endangered large arboreal species endemic to the central Ryukyus, Japan. Previous studies have recorded the food habits of this species through direct observations. However, its observation records are limited. Here, we aimed to clarify the food habits of D. legata using DNA metabarcoding of its stomach contents. The ITS2 intergenic regions in plant nuclear genomes and the COI gene regions in animal mitochondrial genomes were used as DNA markers to identify the prey species of D. legata based on its stomach contents. We successfully identified 63 plant and 36 animal species as the prey species of D. legata. Finally, 84 plant and 46 animal species are listed as food menu in total. Thus, a greater number of its animal prey species were identified in this study than in previous observation-based studies. As this species is omnivorous, the conservation of a wide array of ecological habitats is necessary for maintaining its population. Regardless of the limited sample size and unknown parts consumed, slight differences in food items were observed between different sex- and age-based groups. Relatively more detailed DNA reference databases for the local fauna and flora are required for further analysis.

Published online 16 October, 2024; Print publication 30 April, 2025

The Amami rabbit (Pentalagus furnessi) and the Ryukyu long-furred rat (Diplothrix legata) are rare, endemic mammals of the Ryukyu Islands. So far, knowledge about their basic ecology is limited. This study aims to expand data on their feeding ecology by dental microwear texture (DMT) analysis using skull specimens (n = 20 for each species) collected as roadkill. DMT data were compared with those from laboratory-reared rodents in controlled feeding experiments, to infer the physical properties of their diets. The results indicated no sexual difference in DMT for either species, suggesting similar diets between males and females. However, a significant seasonal variation was observed in the Ryukyu long-furred rat, with increased DMT roughness in winter, suggesting a dietary shift from more abundant and less mechanically demanding summer foods like soft-bodied invertebrates to harder foods such as acorns. In contrast, the Amami rabbit showed consistently less rough DMT throughout the year. Microwear patterns suggested that the Amami rabbit consumes plants with toughness comparable to bamboo and dry grass. These findings highlight the importance of seasonal dietary shifts in the Ryukyu long-furred rat and the consistent diet of the Amami rabbit, providing insights into their ecological adaptations.

Published online 2 April, 2025; Print publication 30 April, 2025

Age estimation of the Ryukyu long-furred rat Diplothrix legata, which inhabits only three islands in Central Ryukyus, was conducted by analyzing molar wear. We measured the skulls and examined the molar wear condition of 87 and 19 individuals from Okinawajima and Amami-Oshima Islands, respectively. The molar wear was divided into seven stages. Stage I individuals whose third molars were in the process of eruption were considered juveniles emerging from nests and their skull lengths were < 50 mm. On Okinawajima, Stage I individuals were found only from November to January, and Stage II individuals (yearings) were found only from March to August. In contrast, the relationship between skull growth and molar wear was unclear on Amami-Oshima because individuals with skull lengths < 50 mm were included in Stage II and later. The skull length of Stage VII was significantly larger on Okinawajima than on Amami-Oshima. Furthermore, growth stage discrimination based on coat appearance did not coincide with the molar wear on Amami-Oshima. These results indicate that estimating the age of this species on Amami-Oshima using molar wear as on Okinawajima is difficult and that the relationship between age and molar wear should be examined on each island.

Published online 2 April, 2025; Print publication 30 April, 2025

歯の摩耗度分析を用いたケナガネズミ（Diplothrix legata) の齢推定の試み:沖縄島と奄美大島における臼歯摩耗パターンの違いに関する初報告．中琉球の 3つの島嶼にのみ生息するケナガネズミについて，臼歯摩耗度を用いて齢推定を試みた．沖縄島産 87 個体と奄美大島産 19 個体の頭骨を計測し，上顎左側臼歯の摩耗状態を観察した．上顎左側臼歯の摩耗は 7つのステージに分けることができた．最も臼歯の摩耗が進んでいないステージ I の個体は沖縄島のみで確認された．第 3 臼歯が萌出過程にあるステージ I の個体は出巣後間もない個体であると考えられ，頭骨長は 50 mm 未満であった．沖縄島ではステージ I の個体は 11〜1 月にのみ，ステージ II の個体は 3 月〜8 月にのみ確認されたことから，これらの個体は当歳仔であることが明らかになった．一方，奄美大島ではステージ II 以降にも頭蓋長 50 mm 未満の個体が含まれており，頭骨成長と臼歯摩耗度の関係が不明瞭であった．最も高齢と考えられるステージ VII の個体の頭骨長は沖縄島産が奄美大島産より有意に大きかった．さらに，奄美大島では被毛による成長ステージ判別と摩耗度が一致しなかった．これからのことから，奄美大島では沖縄島と同様の臼歯摩耗度を用いた齢推定は難しい可能性が高く，齢と摩耗度の関係性は島ごとに検討する必要があることが明らかになった．

In the northern part of Okinawajima Island, a large nest box equipped with an automatic camera trap was set up at a height of 5.2 m above the ground from April 2022 to March 2023 to study the reproductive ecology of the Ryukyu long-furred rat Diplothrix legata. This species is endemic to the central Ryukyus and is the largest arboreal murid species in Japan. Videos captured between October and February showed that D. legata was carrying branches and leaves into the nest box and nesting. Calls of newborn rats, sometimes overlapped between different individuals, were recorded from late February to early March, indicating that multiple rats were born in the nest box. These observations suggest that large nest boxes are useful for studying the nesting behavior of D. legata. However, habu snakes Protobothrops flavoviridis were observed entering the nest box and preying on newborns, resulting in no newborns being observed leaving the nests. Although previous studies have recorded the predation of D. legata by habu snakes, this study is the first to provide direct evidence of habu snakes attacking the nesting site and preying on newborns of D. legata.

Published online 20 January, 2025; Print publication 30 April, 2025

ケナガネズミの営巣過程とハブによる新生児の捕食事例．中琉球の固有種で，日本で最も大きな樹上性ネズミ類であるケナガネズミの繁殖生態を調査するために，沖縄島北部において，2022 年 4 月から 2023 年 3 月に自動撮影カメラ付きの大型巣箱を高さ 5.2 m の位置に架設した．10 月から 2 月に，本種が枝や葉を巣箱に運び入れ，営巣する動画が撮影された．その後，2 月下旬から 3 月上旬に複数の新生児の鳴き声が記録された．この結果は，本種が巣箱で複数の新生児を出産したことを示している．本研究により，大型巣箱がケナガネズミの営巣行動調査に有用であることが示唆された．しかし，3 月にハブが巣箱内に侵入し，新生児を捕食した．そのため，巣立ちは確認できなかった．ハブによるケナガネズミの捕食記録はこれまでにもあったが，本研究によってハブが営巣場所を襲撃し，新生児を捕食する直接的な証拠が初めて得られた．