Divewing

In the picture below, you can see those slits.

Theory #1: They evolved gills. In the movie Waterworld, there is a character, Mariner, who is a human that evolved gills and webbed feet. I am sure this is science fiction.

Theory#2: Those are false gills. In real life, the Pygmy Sperm Whale has false gills, which are just folds. Like all mammals, they only breathe through air.

Which theory would you support?