Living animals have five primary tastes: sweet, savory, sour, bitter, and salty. But not all animals are able to taste all five. Cats can't taste sweets, pandas can't taste savory, insects can't taste sour. Taste receptors can evolve if a species changes their diet or learns that certain receptors indicate danger. For example, salty and bitter are the best indicators of danger, so most species held onto those. But it seems whales and dolphins have evolved too far to the extreme. A new study has found some whales and dolphins are only able to taste salty.
As io9 explains, researchers in Britain and China studied the genetic material of twelve whale and dolphin species, including bottlenose dolphins and sperm whales (those are the ones you are probably most used to seeing.) In the DNA scientists studied, they found the taste for sweet, savory, sour, and bitter had all devolved to the point of being unusable. That left just salty tastes. So everything these dolphins and whales have ever eaten and will ever eat tastes like salt.
The team of scientists traced the genetic change back about 53 million years ago. The common ancestor of whales and dolphins likely lost four out of five tastes after they branched off from artiodactyls.
As for why it happened, researchers are still determining this. Their main theory is that the salt-only receptors were caused by an all-meat diet, or perhaps by swallowing food whole instead of breaking it into pieces then chewing it. Also, there is the whole living in saltwater thing.
So next time you see a whale or dolphin eating a fish and looking sad, it's because they're basically consuming a giant piece of salt.