In Australia about 3,000 females in a colony of koalas are to be administered contraceptive drugs after eating most of their habitat.
Park rangers in the southern state of Victoria are worried the animals could face starvation if their population is not controlled.
The koalas are the victims of their own success
There are now an estimated 10,000 koalas in Mount Eccles National Park in western Victoria - a huge increase in population after just 77 were introduced to the area about 25 years ago.
But the thriving colony has created a problem. The koalas have eaten about 70% of their favourite food - leaves from the manna gum tree.
Without drastic action, park rangers fear the trees could die back altogether and then the koala population would quickly starve to death.
Previous efforts at sterilising the koalas have failed.
So now, rangers plan to capture 3,000 females and implant a small tube under their skin which will release the same hormone that women take in the contraceptive pill.
It is a project that carries some risk. The koalas could develop tumours, and the Australian Koala Foundation has criticised the scheme as unnecessary and cruel.
But it is a method that has been used successfully with other animals, including marsupials, primates and big cats.