Emergency security measures are needed at zoos and wildlife parks across Europe, animal rights campaigners have said, amid fears of an increase in so-called 'zoo poaching'.
It follows the slaughter of four-year-old rhinoceros Vince, who was shot and killed at Thoiry Zoo near Paris before having his horn sawn off.
Those responsible have not yet been caught.
Humane Society International has now called on the European Association of Zoos and Aquaria to carry out an urgent review of security to protect endangered and vulnerable animals.
While Vince's death was the first known time that poachers have killed an animal in captivity in Europe, activists have voiced fears it could mark the beginning of a disturbing new trend.
Joanna Swabe, executive director of HSI Europe, said poachers were becoming increasingly desperate as species such as rhino and elephant disappear from the wild, and their native countries clamp down on hunting.
Two other rhino at the zoo - Bruno and Gracie - were unharmed in the attack.
Global trade in rhino horn is banned by a UN convention and its sale is illegal in France.
But demand for the horn is strong in Asian countries such as Vietnam, where it is prized as an ingredient for traditional medicines.
A kilo of rhino horn fetched over £44,000 on the black market in 2015, according to Thoiry.
A coalition of wildlife groups including HSI Europe recently sent an open letter to all EU member states, urging them to make wildlife trafficking a priority issue.