Do they challenge the definition of species? Not really. The key words are "interbreed successfully"; tigons and ligers generally are sterile and short-lived -- an evolutionary dead end.

while liguars result from a female jaguar mating with a male lion. A liger is the hybrid offspring of a male lion and a female tiger. Since these two species do not share the same natural habitat ...

For example, animals called ligers are produced when a male lion and a female tiger reproduce. Tigons are produced when a female lion and male tiger have cubs. Unlike many young from hybrid ...

Do they challenge the definition of species? Not really. The key words are "interbreed successfully"; tigons and ligers generally are sterile and short-lived -- an evolutionary dead end.