Primatological researchers concerned with the increasing population of monkeys at a natural zoological park in Oita Prefecture are continuing with experimental procedures to curb their numbers, despite controversy over the propriety of such operations.

Last month, the Kyoto University Primatological Research Institute, based in Inuyama, Aichi Prefecture, implanted contraceptive devices into 15 female monkeys at the Takasakiyama Natural Zoological Park in Oita Prefecture. Some argue that such surgery is necessary to prevent wild monkeys, which have been rapidly increasing in number, from causing damage to crops. Others doubt the wisdom of artificially limiting their propagation.

About 1,700 wild monkeys live in the park and have reportedly been causing damage to fruit and crops around it. Experts estimate the park can adequately accommodate between 1,200 and 1,300 monkeys.

In the trial operations, the second of their kind following another series in September last year, pieces of silicon containing hormones to restrain ovulation were implanted in the monkeys’ backs. This method of contraception is being used for people in developing countries, and is expected to be effective for about three years.

The institute intends to continue to carry out similar operations on monkeys in parks in various parts of the country where their numbers have been rising due primarily to feeding.

Keiko Shimizu, an assistant at the institute, said she would like to observe the monkeys for at least three years after their operations to see how long the effectiveness of the hormone agent will last and to determine if their fertility can be restored.

At a subcommittee meeting of the Japan Mammalogy Society held recently in Tomioka, Gunma Prefecture, Shinichi Hayama, an instructor at Nippon Veterinary and Zootechnical College, said that monkeys have been living longer due to improved nutrition and global warming, and that such contraceptive measures would further distort ecology.

Hayama also told the session that merely controlling fertility without eliminating the increase factors will not resolve the problem.

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