The Evolution of Sweet Tooth

We love eating sweet food because, errr.. they taste good. Chocolate anyone?

But do you know that cats are oblivious to the taste of sweet food?

Other than your pet feline, Asian otters, spotted hyenas, sea lions and dolphins are also oblivious to the heavenly taste. I'm starting to feel really bad for these animals.

Image: punjabigraphics.com

Peihua Jiang, a researcher from the University of Zurich has discovered that a plethora of carnivores can’t taste sugars--spotted linsang, fossa, fur seal, Pacific harbour seal.
But why carnivores?

Two genes are largely responsible for our sweet tooth – Tas1r2 and Tas1r3. They encode two proteins that combine to form a sensor, which dots our tastebuds and detects sugar molecules. In 2005, scientists showed that all cats, from house tabbies to wild tigers, carry a useless version of Tas1r2. Over time, it has picked up so many mutations that it has become genetic scrap.--http://blogs.discovermagazine.com/notrocketscience

Image: thetwentyfirstfloor.com

The researchers revealed that all the exclusive meat-eaters had disabled their copies of Tas1r2. There are, however, some carnivores that have working copies of sweet-tooth gene: dog, ferret, giant panda, aardwolf, Canadian otter, raccoon, etc. Unlike their feline counterparts, these species sometimes stray from a meat-only menu.

Jiang discovered that Asian small-clawed otter and cats, which had a broken sweet-tooth gene will drink from a bowl of sugary water as often as it will from a bowl of plain water, but the spectacled bear (with its working copy) clearly prefers the sweet drink.

Spectacled Bear. Image: animals.nationalgeographic.com

It's possible that the ancestor of these carnivores had a working Tas1r2, which eventually deteriorated in different ways as time passed: some survived while some became pseudogenes--dysfunctional relatives of genes that have lost their protein-coding ability. The researchers think that exclusive meat-eaters, especially  sea mammals like sea lions and bottlenose dolphins do not need to taste sweet things--they swallow their food whole, and the fact that they constantly dine in a salty environment makes it unnecessary to taste their morsel--everything is salty anyway.

Image: en.wikipedia.org

The study shows that evolution is quick to jettison abilities which living things no longer need. Just like astronauts who spend months in space would lose their muscle due to the zero-gravity environment.
It also reminds us that we're not as good as we think we are. Though some predators can't detect taste which we can, they could hear or see things which we can't. It's up to nature to decide which gene to discard to meet the demands of our lifestyle.


Malcolm
info: Jiang, Josue, Li, Glaser, Weihua, Brand, Margolskee, Reed & Beauchamp. 2012. Major taste loss in carnivorous mammals. PNAS DOI: 10.1073/pnas.1118360109