Two Species when brought together in a zoo are capable of mating and producing fertile offspring even though they are two distinct species because Zoos are not natural environments
Animals who can not mate with each other or won't produce offspring which is fertile are known as 'reproductively isolated', and are therefore said to be different species.
However, some animals, which we consider to be different species can interbreed and will produce fertile offspring. Examples include lions and tigers, who breed to create ligers.
However, under natural conditions they normally will not do so as they tend to 'hang around' with animals exactly the same as them.
In a zoo though, they don't exactly have a choice as to who they live with etc. so under these unnatural circumstances they can't be as choosy as they would in the wild with regards to who they mate with. All they know is that their instincts are telling them that it is time to mate.
And so, in this unnatural environment, they follow their instincts and mate with a different species, even though biologically this result in an infertile offspring: the exact opposite of what mating instincts are all about (continuation of the species).
Everyone knows what a zoo is but I didn't know until recently that it is an abbreviation. The full name is Zoological garden. The abbreviation was first used by London Zoo in 1828 when it opened for scientific study and was later opened to the public in 1857.