Sweet, savory (umami), sour, bitter, and salty . . . the ability to detect these five primary tastes is shared by nearly all living animals, passed down to us from some ancestral gustatory prodigy over 900 million years ago. Fish, amphibians, birds, reptiles, mammals, and even insects share this tightly-conserved taste palate.
Not every animal can detect every taste, of course. Invertebrates like insects taste "carbonation" instead of "sour". Cats can't taste sweets. Giant pandas cannot taste savory/meaty foods, something they probably lost when their ancestors switched from meat to bamboo. Animal groups are reluctant to let go of "salty" and "bitter", because "bitter" basically means "poison", and balancing salt levels is pretty important for animals trying to not become dead animals.
Enter the whales. Like cats, whales have lost their taste for sweets. They've also given up savory-tasting like the giant panda. Surprisingly, they've taken taste-loss further still, dumping their awareness of sour and bitter too. In what seems like a first for the animal kingdom, the whales have lost four out of five primary tastes.
Researchers from the UK and China examined the DNA of 12 different whale and dolphin species (seven toothed and five baleen). Across all 12 species, they found that genes encoding for sweet, umami, sour, and bitter taste receptors were all degraded to the point of uselessness, no longer coding for anything. Only their salt receptors were conserved, which for ocean-feeding animals means that every single meal tastes like nothing but a mouthful of sea salt.
Further genetic analysis indicates that the common ancestor to all whales probably lost its sense of taste shortly after they branched off from the rest of the artiodactyls sometime prior to 53 million years ago.
Why would whales lose so much of their sense of taste, when it is so tightly conserved across the rest of the animal kingdom? The researchers have educated guesses, involving an ancestral all-meat diet combined with a tendency to swallow food whole rather than bite or chew it . . . plus the omnipresent taste of seawater diluting and overpowering any other possible tastes.
The paper (Massive losses of taste receptor genes in toothed and baleen whales. Feng et al. 2014) is open access, so you can download and read the PDF whenever you please. Image is in the public domain. Thanks to artiofab for correcting several of the doubtless dozens of errors that originally appeared in this post.