Due to varaitions, one can always find fertile hyrbrids. In some cases (with very closely related "species") it is quite common, while in others, such as the mule, it ought to be very rare.
Don't know how the case is with the roan/sable. Any litterature or other references anyone?
Fertile hybrids seem to be more common among bird species than mammals?? (and no!, I'm not bringing up the Mallard subject again! )
I found one other rather technical reference. The term "anecdotal" in the article clearly emphasises the isolated character of this particular "roable"'s occurrence, but obviously it is possible.
Absence of geographic chromosomal variation in the roan and sable antelope and the cytogenetics of a naturally occurring hybrid.
Robinson TJ, Harley EH.
Department of Zoology and Entomology, University of Pretoria, South Africa.
The determination of geographic chromosomal variation in rare or endangered species, or those of special management concern, is important, since geographically defined cytotypes can negatively influence breeding programs involving founders drawn from widely divergent localities. We cytogenetically analyzed specimens of the roan (Hippotragus equinus) and sable antelope (H. niger) collected from widely divergent localities throughout their respective ranges. Each species was characterized by a diploid number of 60 and an invariant karyotype. In contrast to the absence of intraspecific variation, however, the two species differ with respect to centromeric constitutive heterochromatin and numbers of nucleolar organizer regions. These cytogenetic landmarks were subsequently used to verify an anecdotal account of a naturally occurring roan x sable hybrid. The data show that, despite their markedly distinct phenotypes, the roan and sable antelope are nonetheless sufficiently similar genetically to produce viable offspring. Hybridization, although a rare event between these species, is probably partly promoted by behavioral differences which are not always sufficient to prevent mating between them.