Deer osteophagia, although rarely occurring in Japan, was observed on Mageshima Island. We examined the relationships among bone density of sika deer, plant species composition, and Ca and P contents in plants of the island. Samples were collected during summer in 1994, 1995, 1997, and 1998 from 1 m2 quadrats placed in grasslands in northern, central, and southern areas of the island. Species constitution of the flora changed considerably among years but Gramineae plants comprised more than 89% of the flora in any year. Plant P contents in northern, central, and southern areas were as low as 0.08%–0.1%, 0.12%–0.23%, and 0.07%–0.13%, respectively. The Ca:P ratio mostly ranged from 1:1 to 5:1. The bone density tended to be reduced (p = 0.0542) between 1991 and 1993, and kept low from 1993 through 1998. During the same period, the Mageshima Island deer population was significantly (p < 0.05) lower in bone density than the Tanegashima Island deer population. As the period of low bone density coincides with 1992 when the incidence of osteophagia increased drastically, a relationship between bone density and osteophagia may exist. These results suggest P shortage could occur in the Mageshima Island sika deer population.
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