Fertility
The fertility of hybrid big cat females is well documented across a number of different hybrids. This is in accordance with Haldane’s rule: in hybrids of animals whose sex is determined by sex chromosomes, if one sex is absent, rare or sterile, it is the heterogametic sex (the one with two different sex chromosomes e.g. X and Y).
According to Wild Cats of the World (1975) by C. A. W. Guggisberg, ligers and tigons were long thought to be sterile: in 1943, a fifteen-year-old hybrid between a lion and an ‘Island’ tiger was successfully mated with a lion at the Munich Hellabrunn Zoo. The female cub, though of delicate health, was raised to adulthood.[10]
In September 2012, the Russian Novosibirsk Zoo announced the birth of a “liliger”, which is the offspring of a liger mother and a lion father. The cub was named Kiara.[11]
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This only indicates the production of what I would call a third generation offspring, there have been isolated reports of a third generation mule but so far as I know there have been no reports of a fourth or fifth and so on. A third generation means nothing unless the third and succeeding generations are themselves fertile. Do you have any evidence of that? So far I am still convinced that true hybrids of mammals are sterile.