With skyrocketing Asian demand for rhinoceros horn to use in "medicine" or as a display of wealth, Europe's museums, auction houses, antique dealers and taxidermist shops have long been targets as traditional sources dry up.
In many museum exhibits, stuffed rhinos already sport fake horns to discourage thieves.
But in 2011, police agency Europol warned that zoos too, could fall prey.
Monday's killing of Vince the white rhino marked the first time a European zoo had been breached in this way.
Why rhino horn?
Despite a dearth of scientific evidence that it has any curative powers, rhino horn commands astronomical prices of about $60,000 (57,000 euros) per kilo (2.2 pounds) -- more than gold or cocaine.
The highest price ever recorded by French conservation NGO Robin des Bois (Robin Hood) was 100,000 euros for a kilo of powdered horn in a private sale in China.
One horn can weigh four kilos -- comprised exclusively of keratin, the same substance in human hair and fingernails.
Demand for the commodity is soaring in Vietnam and China, where it is thought to cure anything from hangovers to cancer.
Why a zoo, and why in Europe?
Wild rhino numbers are plummeting. About 1,400 are killed every year, out of an estimated population of 25,000 -- mainly in South Africa but also in Asia and India.
In the last eight years alone, roughly a quarter of the world population has been massacred in South Africa, home to 80 percent of surviving rhinos.
Today, it may be easier to poach in a European zoo than an African game park, where just about every rhino has its own guard.
There are about 160 rhinos in European zoos -- a potential goldmine for horn smugglers.
What can zoos do to protect live animals?
According to Europol, zoos and other public places with rhino horns on display or in storage, must remain on alert for "possible 'visits' from persons likely to defraud or attack them to obtain specimens."
The NGO Robin des Bois recommends ramping up zoo patrols and giving guards the right to fire warning shots.
It also wants to boost customs procedures and surveillance of postal services to stop the horns, whose sale is illegal everywhere, from ever reaching the Asian market.
Education is also needed to convince possible consumers that rhino horn does not have any of the healing powers it is credited with.
Another worrying development is a rise in the theft of live animals from European zoos in the last 15 years -- anything from monkeys and flamingos and penguins, according to Robin des Bois spokeswoman Charlotte Nithart.
"If this first blow (for zoo rhinos) is not followed with rigorous security measures, it is certain to be repeated in another zoo in France or in Europe," she told AFP.
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