Bone Histology Suggests Insularity and Sex Differences in Japanese Sika Deer Growth (Cervus nippon)

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.