Lions, bears, monkeys, crocodiles, parrots and iguanas（美洲蜥蜴） may seem inoffensive at first glance（乍一看） when they're behind bars in zoos. But some exotic species can escape and become invasive species. This has been confirmed by a scientific team that has checked 1,568 animal houses in 63 Spanish zoos. Birds are the animals most likely to escape. "As zoos house a large number of exotic (non-indigenous) species, they could become an entry channel for these species if they escape, with the potential environmental risk that this implies", María C. Fàbregas, lead author of the study and a researcher at the Ethology and Animal Welfare Unit of the Cardenal-Herrera University (UCH) in Valencia, tells SINC.

The study, which has been published in the journal Biological Invasions, reviewed the security of animal housing against creatures escaping, and 75% of the zoos studied were found to be problematic, while 14% of the animal housing was evaluated as "insecure" against the possibility of escape.

"Species that could pose a danger to public health are usually housed in secure accommodation, but those that could represent a danger to the environment if they escaped (invasive species) tend to be in insecure housing", points out Fàbregas. According to the research, birds are the group most likely to be in insecure housing.

Based on a report produced in 2003 by the Ministry of the Environment, Rural and Marine Affairs (MARM), the research team studied 30 animal houses in each of the 63 zoos. The MARM is currently completing the first inventory（存货） of zoos and aquaria（水族馆） in Spain.

"Of the 1,568 animal houses studied, 221 were insecure against the threat of the species housed in them escaping, 167 housed non-indigenous species (potentially dangerous to the environment), and of these 21 housed invasive species", the scientist explains.

The animals could escape from 71% of the insecure housing by getting through or over the physical barrier around their accommodation. The rest could get out "because the public could release them or remove them from their cages or tanks".