Being hybrids, Savannahs typically exhibit some characteristics of hybrid inviability. Because the male Savannah is the heterogametic sex, they are most commonly affected, in accordance with Haldane's rule. Male Savannahs are typically larger in size and sterile until the F5 generation or so, although the females are fertile from the F1 generation. As of 2011, breeders were noticing a resurgence in sterility in males at the F5 and F6 generations. Presumably, this is due to the higher serval percentage in C and SBT cats. The problem may also be compounded by the secondary nondomestic genes coming from the Asian leopard cat in the Bengal outcrosses that were used heavily in the foundation of the breed.