Actually, tiger lion crosses can be fertile. Also bison and cattle (beefalo). I think they have changed the definition of species in recent years as information became known.
susie
Wikipedia....
The ligon is not really fertile......
“While male ligers are sterile, female ligers can usually reproduce[citation needed]. Because only female ligers and tigons are fertile, a liger cannot reproduce with another liger or with a tigon. The sterility in males is caused by the fact that because of the unstable genes, male ligers never reach puberty. While they continue to age and grow they do not become sexually mature”
Don’t know if your interested, but I found this and thought it interesting:
http://en.allexperts.com/q/Genetics-1795/hybrids-sterile.htm
susie