Sei una tigre
If somebody would like to say to a male person: "You are a tiger" would you have to say "Sei una tigre" ?
Chyaarisan, I think you are quite right. In Spanish we have some examples: (una) tortuga=turtle, (una) gacela=gazelle, (una) jirafa=giraffe, (una) ballena=whale. We don't say for a male: (un) tortugo, (un) gacelo, (un) jirafo, (un) balleno. If we really want to specify whether it is a male or female animal, we would say: "una tortuga macho", "una tortuga hembra", "una gacela macho", "una gacela hembra". For some animals we DO have: (un) leon, (una) leona; (un) tigre, (una) tigresa, (un) perro, (una) perra.... Why? I don't know it is just the way it is.
If someone were to say "You are a tiger" while in Italy, you'd be laughed at at best. Really, "Tiger?" Tigre is one of the last words one needs to learn if one is attempting Italian. Same with Whale and Shark. I've been to Italy a dozen times and the animals you need to learn are: Polpo, gamberini, calamari and others you'll find commonly on menus (octopus, shrimp, and squid). Also, cane (dog) because you'll see a lot of them roaming around. L'ape (bee) is good because the Apian Way in Roma is named for bees (common to see emblazoned on architecture in Vatican too).