London is infested by urban foxes. Delhi is besieged by urban monkeys. Addis Ababa is plagued by urban hyenas that have moved in to the city from the surrounding hills and taken up residence in the capital. In some ways they perform a useful service, keeping the city's growing population of stray dogs and feral cats under control and consuming the carcasses of dead horses and other animals. But there are now reckoned to be somewhere between 300 and 1,000 of them living in the city and they are dangerous.