Taste

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Hunger and a sense of taste drive animals to search for food.

In cats, a sense of taste has been documented as early as 5 days prior to birth. With the exception that cats are unable to taste sweet stimuli due to the absence of a gene encoding for sweet receptors (Tas 1r2), their motivation for food is not unlike that of other mammals. Although domestic cats possess an otherwise functional sense of taste, they, unlike most mammals, do not prefer and may be unable to detect the sweetness of sugars.

This behavior toward sweet stimuli is in marked contrast to the avidity for sweets shown by most omnivores and herbivores and even some other carnivores such as the dog[1]. The indifference that cats display toward sweet-tasting compounds contrasts with their otherwise normal taste behavior toward stimuli of other taste modalities. For example, they show preference for selected amino acids and generally avoid stimuli that to humans taste either bitter or very sour[2]. Congruent with these behavioral responses to taste stimuli, recordings from cat taste nerve fibers, and from units of the geniculate ganglion innervating taste cells, demonstrate responses to salty, sour, and bitter stimuli as well as to amino acids and nucleotides, but do not show neural responses to sucrose and several other sugars. The sense of taste in the cat, in general, is therefore similar to that of other mammals, with the exception of an inability to taste sweet stimuli[3].

The primary reason cats cannot tast sugar is due to the Tas1r2 gene, which, while retaining structure similar with that of the human TAS1R2 gene, is an unexpressed pseudogene. The likely important molecular event that resulted in cat Tas1r2 becoming a pseudogene is the 247-bp deletion in exon 3. This deletion results in a frame shift that brings about a premature stop codon in exon 4 (Figure 2B). An additional stop codon can be found in exon 4, with three more in exon 6 (Figure 2B). This apparent accumulation of mutations suggests that there is no pressure from natural selection on the cat Tas1r2 gene[4].

Cats have approximately 475 taste buds in their mouth, much less than dogs (1700) and humans (9000), however their sensitivity to bitter tastes is 400 times more than dogs, perhaps explaining why they are less prone to garbage eating and poisoning compared to their canine relatives.

Cats are also strict carnivores (whereas dogs are omnivores), and eat small meals often (up to 10 times a day). Since cats are also asocial by nature, there is no social bonding involved during feeding as is seen in dogs.

References

  1. Bradshaw JW. (1991) Sensory and experiential factors in the design of foods for domestic dogs and cats. Proc Nutr Soc 50:99–106
  2. White TD, Boudreau JC (1975) Taste preferences of the cat for neurophysiologically active compounds. Physiol Psychol 3:405–410
  3. Boudreau JC, Bradley BE, Bierer PR, Kruger S, Tsuchitani C. (1971) Single unit recordings from the geniculate ganglion of the facial nerve of the cat. Exp Brain Res 13:461–488
  4. Li, X et al (2005) Pseudogenization of a Sweet-Receptor Gene Accounts for Cats' Indifference toward Sugar. PLoS Genet 1(1): e3