In cases where they have not returned, they have been found emaciated and traumatized by other dolphins. The only way to visually determine the sex of a dolphin is to get a good look at its underside. The small indentation in the center of the abdomen is the navel. Below the navel is a central slit called the genital slit. On either side of the genital slit, females have two smaller mammary slits.
Males have a central genital slit followed by a single smaller anal slit.
There are exceptions - a killer whale is the largest member of the dolphin family, but it has a blunt snout. Over twenty-three million.