An Attempt to Clarify the Causes of Osteophagia among Sika Deer on Mageshima Island

Abstract

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.