英语时差：独角鲸（上）

Narwhals aren’t fish, they’re mammals-a rare type of whale.And a spiraled tusk they famous for isn't a horn.But it is actually an elongated tooth. Only male narwhals grow tusks, which can be over six to almost ten feet long–more than half their body length. What Is The Function Of The Tusk? There is patchy evidence for several different hypotheses. Since only males grow tusks, it could have evolved in response to sexual selection. Like the tails of male peacocks, the tusk could be used to attract and impress females and intimidate rival males. Males may use their tusks in fights with other males to establish social hierarchies. Tusks could also be used for breaking through thin layers of ice in their frigid ocean habitat.