Captivity creates situations that would not naturally exist. We can put species together that would be separated geographically in nature. Abnormal behaviors are created in captivity where pacing, for example, or mounting anything becomes abnormal. Genetic compatibility is another issue to control if they can, in an abnormal situation, create a viable offspring. Many cases of different species could indeed produce offspring by today's definition of "species".
The common standard for describing a different species is the ability to produce fertile offspring. But that has been generally expanded into the understanding of the "likelihood" of breeding. Factors such as geographic separation, mating ritual differences that developed, "attractiveness" of the prospective mate (coloration, head striping, pheromone attraction) etc, etc. have all now become considerations.
Since in order for an organism to reproduce, the chromosomes must be of the same number and sequence to match up and provide the genetic map that creates a successful embryo and animal. Think of the paired chromosome as a zipper. An egg holds one half of the zipper. A sperm holds the other half. When fertilization occurs, the zipper halves have to match perfectly. The same number of chromosomes, and the exact same order. If the zipper cannot match and close completely, no embryo will develop. But in cases where the match is extremely close, the fertility is extremely low, but CAN produce and offspring.
As populations experience mutations to the chromosome (which is relatively common) those subtle mutations can exist in a population. This mutation would cause a mismatch in the gene/chromosome alignment and It becomes hard for the member that has that mutation to produce fertile offspring sometimes. But in those cases, it that individual mates with another member of the population that is also now carrying that mutation, there is again high fertility and the offspring will carry on that mutation. If that is occurring in an isolated population, this trait could eventually become the dominate and more successful breeders and we have a new species emerging.
So, now there are considerations of this that are not simply genetic, but also geographic isolation that makes mating impossible in nature. Or new behavioral mating rituals that makes mating not occur in nature any more. Those animals can still carry compatible chromosomes, yet do not breed in nature. Scientists may agree there is now enough of a total NATURAL breeding barrier to establish a new species. However, in captivity, if introduced 'artificially' they can breed successfully. In most cases there has been enough of a mutation in the order of chromosome alignment where fertility is greatly reduced, but it can happen. The resulting offspring would normally be infertile as they are genetically not compatible to another possible mate they could come in contact with. HOWEVER - if they happen to be introduced to another animal that has a similar genetic arrangement (similar mutation) they could, on rare instances, then produce a viable offspring. So we can have the very rare mule produce a viable offspring.
Chelonians that are in contact with other similar species have specific mating rituals. In fact, often those differences are what were used to justify naming a different species by scientists that decide those things. Those differences in mating rituals, pheromones, coloration, all have served to keep populations from selecting the "wrong" mate. In captivity, we disturb this preference. Where chelonians exist in areas where not other chelonian can ever be encountered, often there is a much more general ritual of acceptance. As in the sulcata that will breed with a leopard as it "looks close enough"!! Genetically there is enough of a difference to make fertility quite low, but there is enough of a similarity, that it CAN work. If the result itself is fertile is even more remote a possibility.
So today we have many "species" that indeed "CAN" create offspring. Since the acceptance of a new species is now defined by geographic and behavioral operations, we can indeed in captivity, alter those separations.
