kitten, eating and watching
Credit: FHgitarre on flickr / Licensed under Creative Commons Attribution 2.0 Generic

Ready to EatRandom Retail on flickr / CC-by-2.0A few days ago, I caught a post in ScienceDaily that shed some light on why cats are often fussy eaters.

I've been baffled by some of my rescue cats who I assumed would be grateful for any type of cat food. (And I bought the healthy expensive brands that my veterinarian recommended too).

Well, a couple of my cats actually flick or shake a paw in the air after sampling some food as if to say, "Take it away, it's substandard."

So I was fascinated by what the PLOS ONE study Functional Analyses of Bitter Taste Receptors in Domestic Cats (Felis catus) published on October 21st, 2015 revealed.

Here's some of what is known and what researchers questioned:

  • Bitter food sources are generally avoided by the vast majority of animals and humans. The belief is that this protects animals from eating toxic compounds in plants or creatures (such as amphibians, invertebrates, and reptiles). Likewise, toxic-tasting creatures and plants are provided with some protection from predators.
  • Well, it turns out that species differ greatly in the number of bitter taste receptors. And cats have (at least) seven functional bitter taste receptors.
  • Since carnivores (such as cats) lost the ability to detect sweet substances, scientists have felt "there must be a selective reason for the existence of multiple bitter receptors." So, they studied the domestic cat since there is a good body of research already about their sweet taste receptors and because of the interest in what factors influence "feeding this companion animal."

How Bitter Taste Receptors Were Tested in Cats

The researchers used 25 commercially available compounds to simulate bitter taste stimuli. Eight of these were from plants that humans find bitter.

Scientist found (via nucleotide sequence searches) that cats have 12 intact Tas2r receptor genes. However, two of the 12 receptors "could not be amplified from feline DNA" using the current technology to amplify these regions, identified as Tas2r1 and Tas2r43.

So, they tested the 10 bitter taste receptors "with a coupling chimeric G protein (Gα16-gust44) in a heterologous system." The page Materials and Methods describes their process in-depth.

The Results

Some receptors seemed to be broadly tuned, some were narrowly tuned, and some (as you'd expect) were in-between.

Three of the bitter receptors elicited no response to any of the 25 compounds. However, this doesn't mean they were not functional – it could mean that the sample size (of 25 compounds) just did not represent them.

Compared to Other Species

This was interesting. The scientists compared bitter receptor gene counts with other species such as: dogs, giant pandas, ferrets, seals, polar bears, and walruses. What they found was that there were less taste receptors in aquatic mammals (as was previously believed). 

Notably, the Giant Panda (an exclusive plant-eating animal) has 16 intact Tas2rs. That is 1 to 4 more intact bitter taste receptors than these meat-eating species: cats have 12, polar bears have 13, ferrets have 14, and dogs have 15. Walruses have 5 and seals have only 4 intact bitter receptor genes.

The Outcome and Discussion

Cats have multiple fully functional bitter receptors which had similarities and some differences compared to those in humans. 

A wider variety of bitter compounds, particularly from animal products that cats eat, would most likely uncover even more interesting properties of these receptors.

The belief that "bitter receptor function is strongly influenced by whether a species consumes plants" means that obligate carnivore (like cats) would have fewer functional bitter receptors than herbivores. This is similar thinking to why cats may have lost their need for sweet receptors.

Instead, the scientists discovered that the numbers "among terrestrial Carnivora were roughly the same" regardless of the amount of meat they ate (although the giant panda did have 1 to 4 more intact bitter receptors than meat-eating carnivores).

So, they concluded that this study does not support the idea that a carnivorous diet would result in fewer bitter receptors or in how well those receptors function. But, the researchers do note that this was a relatively small comparison, so they cannot disregard this belief entirely. More studies are needed to flesh out these findings.

Notable Difference in Aquatic Mammals

The researchers state they "cannot conclude with confidence that mammals that have become sea living consistently or universally lose bitter receptor number and/or function. Nevertheless the consistency is striking."

Dolphins, whales, manatees, walruses, and seals appear to have few functional bitter taste receptors. The study stated: "Future work with more species is warranted to further clarify the role of ecological niches (e.g., water-dwelling vs land-dwelling) in taste receptor evolution."

Possible Reasons for Bitter Taste Receptors in Cats

The researchers speculated why cats have so many bitter taste receptors. These theories include:

  • Cats do consume plant material when they eat prey (it's in the guts of the animals they eat). Wolves appear to avoid plant material in their prey's viscera.
  • There are many non-plant bitter compounds (such as bile acids or skin secretions in prey) that could be toxic to cats if consumed.
  • The bitter receptors could have other non-oral functions. Amazingly, bitter receptors are also found in other cells (not just on the tongue). And scientists report that "neither the natural ligands nor the functions of these receptors are fully known."

So avoidance of plant or animal-based toxins might be just one purpose of bitter taste receptors. The study states "respiratory-expressed bitter receptors are important for innate defense against bacterial infections."

Study Might Help Us Make Tastier Cat Food and Medicines

Sure, cats have a reputation as being fussy, finicky, or picky eaters. But perhaps now we can appreciate the reasons why. And the value of this study is that researchers can predict a cat's response to certain foods, without subjecting him or her to any health risks.

Since cat food and veterinary medicines often contain bitter-tasting ingredients, now a cell-based system can predict (to some degree) whether a cat will consume it. Lead study author and Monell molecular biologist, Peihua Jiang, PhD., explained: 

"Cats are known as picky eaters. Now that we know that they can taste different bitters, our work may lead to better formulations of cat food that eliminate the bitter off-taste associated with certain flavors and nutrients."

I have a feeling that most of these researchers are owned by a cat.

Up next is a short video by Alex Farnham that also explains more about why cats are picky eaters. Their heightened sense of smell is an important factor and what they are fed as kittens. After that video, is an excellent one by Dr. Marty Becker. It's serious when a cat stops eating and I felt it important to include this information.

Another Reason Your Cat is a Picky Eater

Published on March 24th, 2014 by Animalist

Tips for Feeding Finicky Cats by Dr. Marty Becker

Published on May 1st, 2013

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