question: How developed are other animals' sense of taste? Is it just primal notions like bitter=bad and sweet=good or similarly as complex as ours?
comment: Canines have an evolved sense of taste. Put a piece of raw beef liver next to a piece of raw beef steak on a plate, a dog will have a preference.
passages: Section::::"TAS2R".
  
  The bitter taste receptor family, "T2R" ("TAS2R"), is encoded on chromosome 7 and chromosome 12. Genes on the same chromosome have shown remarkable similarity with each other, suggesting that the primary mutagenic forces in evolution of "TAS2R" are duplication events. These events have occurred in at least seven primate species: chimpanzee, bonobo, human, gorilla, orangutan, rhesus macaque and baboon. The high variety among primate and rodent populations additionally suggests that, while selective constraint on these genes certainly exists, its effect is rather slight.
   Section::::Relaxed constraint.
  
  Neutral evolution in the bitter taste trait in humans is well documented by evolutionary biologists. In all human populations there have been high rates of synonymous and non-synonymous substitutions that cause pseudogenization. These events cause alleles that are present to this day because of relaxed selective constraint by the environment. The genes under neutral evolution in humans are very similar to several genes in chimpanzees in both their synonymous and non-synonymous mutation rates, suggesting that relaxed selective constraint started before the divergence of the two species.
   The pseudogenes mentioned earlier are produced by a number of gene silencing events, the rate of which is constant throughout primate species. Several of these pseudogenes maintain a role in modulating taste response, however. By studying the silencing events in humans, it is possible to theorize the selective pressures on humans throughout their evolutionary history. As is the case with the usual distribution of human genetic variation, the highest rate of diversity in "TAS2R" pseudogenes was often found in African populations. This was not the case with two pseudogene loci: "TAS2R6P" and "TAS2R18P", where the highest diversity was found in non-African populations. This suggests that the functional versions of these genes arose before the human migration out of Africa into an area where selective constraint did not remove non-functional versions of these gene loci. This allowed the pseudogene frequency to increase, creating genetic variance at those loci. This is an example of relaxed environmental constraint allowing silencing mutations to lead to pseudogenization of once important loci.
   The cause of this relaxed constraint was primarily in lifestyle changes in hominids. Roughly two million years ago, the hominid diet shifted from a primarily vegetarian diet to an increasingly meat-based diet. This led to a reduction in the amount of toxic foods regularly encountered by humanity's early ancestors. Additionally, the use of fire began around 800,000 years ago, which further detoxified food and led to a decreased dependence on "TAS2R" to detect poisonous food. Evolutionary biologists have theorized how, with fire being an exclusively human tool, relaxed selective constraint has been found in chimpanzees as well. Meat does account for about 15% of the chimpanzee diet, with much of the other 85% being made up of ripe fruits, which very rarely contains toxins. This comes in contrast to other primates whose diets are entirely composed of leaves, unripe fruits, and bark, which have comparatively high levels of toxins. The differences in diets between chimpanzees and other primates accounts for the different levels of selective constraint.
   Section::::"TAS2R" development in human history.
   The other protein, TAS1R3, is present and identical to that of other animals, and the relevant taste buds are still present but inactive. Such a genetic marker found in the entire family and not other animals must be the result of a mutation in an early ancestor; as a deletion mutation it could not revert, and thus would be inherited by all descendants, as the evolutionary tree branched out. Some scientists now believe this is the root of the cat family's extremely specialized evolutionary niche as a hunter and carnivore. Their modified sense of taste would cause them to some degree to ignore plants, a large part of whose taste appeal derives from their high sugar content, in favor of a high-protein carnivorous diet, which would still stimulate their remaining taste receptors.
   The ability to taste sweetness often atrophies genetically in species of carnivores who do not eat sweet foods like fruits, including bottlenose dolphins, sea lions, spotted hyenas and fossas.
  
  Section::::Sweet receptor pathway.
   The evolution of bitter taste receptors has been one of the most dynamic evolutionary adaptations to arise in multiple species. This phenomenon has been widely studied in the field of evolutionary biology because of its role in the identification of toxins often found on the leaves of inedible plants. A palate more sensitive to these bitter tastes would, theoretically, have an advantage over members of the population less sensitive to these poisonous substances because they would be much less likely to ingest toxic plants. Bitter-taste genes have been found in a variety of species, and the same genes have been well characterized in several common laboratory animals such as primates and mice, as well as in humans. The primary gene responsible for encoding this ability in humans is the "TAS2R" gene family which contains 25 functional loci as well as 11 pseudogenes. The development of this gene has been well characterized, with proof that the ability evolved before the human migration out of Africa. The gene continues to evolve in the present day.
   BULLET::::- Presynaptic parasympathetic fibers to the submandibular ganglion, providing secretomotor innervation to two salivary glands: the submandibular gland and sublingual gland and to the vessels of the tongue, which when stimulated, cause a dilation of blood vessels of the tongue.
  
  Section::::Function.:Taste.
  
  There are similarities between the tastes the chorda tympani picks up in sweeteners between mice and primates, but not rats. Relating research results to humans is therefore not always consistent.
   Section::::Reception and legacy.
   Since 2001, a non-charitable organisation called the Waltham Foundation has funded a number of humane scientific research projects dedicated to furthering the health and nutrition of companion animals.
  
  Section::::Research.
  
  BULLET::::- In July 2005, in collaboration with the Monell Chemical Senses Center in Pennsylvania, Waltham discovered that cats cannot taste sweet food because they have a faulty gene. Lions, tigers, leopards and pumas (Felidae family) have the same defective gene.
   Studies on rats to determine how they react to different tasting liquids and injections indicate this difference. Scientists measured the facial and somatic reactions of rats after exposure to a flavored solution (sucrose or salt) which do not induce abnormal feelings. However, immediately after the rat ingests the solution, the rat is injected with a drug that induces nausea. The rat subsequently expresses a disgust reaction towards the solution, seen by mouth gaping. This is a Pavlovian conditioned response as the rat is associating the disgust with the solution that it drank immediately before the injection. The rat experiences taste "aversion". This is similar to when a human, for example, eats a steak that is perfectly safe and edible and coincidentally contracts a stomach bug and starts vomiting within a few hours of eating the steak. Although the human may know that the vomiting was due to a virus and not from eating the steak, the conditioned response in the brain associates the steak with vomiting due to the timing and the human may avoid steak because he has developed a learned taste aversion to the steak.
   Cattle have a well-developed sense of taste and can distinguish the four primary tastes (sweet, salty, bitter and sour). They possess around 20,000 taste buds. The strength of taste perception depends on the individual's current food requirements. They avoid bitter-tasting foods (potentially toxic) and have a marked preference for sweet (high calorific value) and salty foods (electrolyte balance). Their sensitivity to sour-tasting foods helps them to maintain optimal ruminal pH.
   Taste aversions have been developed in wolves, coyotes, and other canids to protect livestock and vulnerable wildlife. In 1974, it was reported that coyotes ("Canis latrans") learned to avoid hamburger after eating hamburger treated with lithium chloride and could transfer such a drug-induced aversion of LiCl-tainted sheep or rabbit ("Sylvilagus" sp.) flesh to the corresponding live prey. Olfactory (cologne) and visual (a red collar on the sheep) cues increased the suppression of predation through conditioned learned aversion in coyotes, although this was for a limited duration.
   Taste (or, the more formal term, "gustation") is the ability to perceive the flavor of substances including, but not limited to, food. Humans receive tastes through sensory organs called "taste buds", or "gustatory calyculi", concentrated on the upper surface of the tongue. The human tongue has 100 to 150 taste receptor cells on each of its roughly ten thousand taste buds. There are five primary tastes: sweetness, bitterness, sourness, saltiness, and umami. Other tastes can be mimicked by combining these basic tastes. The recognition and awareness of umami is a relatively recent development in Western cuisine. The basic tastes contribute only partially to the sensation and flavor of food in the mouth — other factors include smell, detected by the olfactory epithelium of the nose; texture, detected through a variety of mechanoreceptors, muscle nerves, etc.; and temperature, detected by thermoreceptors. All basic tastes are classified as either "appetitive" or "aversive", depending upon whether the things they sense are harmful or beneficial.
   Conditioned taste aversion occurs when an animal associates the taste of a certain food with symptoms caused by a toxic, spoiled, or poisonous substance. Generally, taste aversion is developed after ingestion of food that causes nausea, sickness, or vomiting. The ability to develop a taste aversion is considered an adaptive trait or survival mechanism that trains the body to avoid poisonous substances (e.g., poisonous berries) before they can cause harm. The association reduces the probability of consuming the same substance (or something that tastes similar) in the future, thus avoiding further poisoning. It is an example of classical or "Pavlovian" conditioning.
   Cats have relatively few taste buds compared to humans (470 or so versus more than 9,000 on the human tongue). Domestic and wild cats share a gene mutation that keeps their sweet taste buds from binding to sugary molecules, leaving them with no ability to taste sweetness. Their taste buds instead respond to acids, amino acids like protein, and bitter tastes. Cats also have a distinct temperature preference for their food, preferring food with a temperature around which is similar to that of a fresh kill and routinely rejecting food presented cold or refrigerated (which would signal to the cat that the "prey" item is long dead and therefore possibly toxic or decomposing).
   Common vampire bats ("Desmodus rotundus") do not learn taste aversions despite being closely related to other species of bats that do. The diet of common vampire bats only consists of vertebrate blood and therefore it is hypothesized that the development of a taste aversion to their only food source would not be advantageous for these animals.
  
  Section::::In humans.
   Adaptive memory behaviours have been observed in animal species, as well as humans. Dr. John Garcia discovered that taste aversion conditioning in rats results in a lasting association between sickness and an ingested substance, and that aversion can be established after only one trial. This rapid learning led others to the idea that rats have biological dispositions for learning to associate sickness with a taste memory as a result of its evolutionary history. The results of a study that compared rats and quail in the acquisition of taste aversion suggested that rats rely on their memory of taste to avoid nausea while quail relied on their visual memory. Since the rat is a nocturnal feeder and has poor vision, it was suggested that they rely on taste cues to learn to avoid toxic substances, leading to a highly developed chemical sense system.
   Among humans, taste perception begins to fade around 50 years of age because of loss of tongue papillae and a general decrease in saliva production. Humans can also have distortion of tastes through dysgeusia. Not all mammals share the same taste senses: some rodents can taste starch (which humans cannot), cats cannot taste sweetness, and several other carnivores including hyenas, dolphins, and sea lions, have lost the ability to sense up to four of their ancestral five taste senses.
  
  Section::::Basic tastes.
   researcher at Hebrew University in Jerusalem, almost all babies, when tasting
  
  sweet stimuli, show a ’’marked relaxation of the face, resembling an expression of ’satisfaction.’ ’’ 
  
  This expression is often followed by a slight smile and even ’’eager licking of the upper lip.’’ 
  
  When the babies were fed a bitter solution, on the other hand, their facial expressions turned to 
  
  ’’dislike and disgust or rejection.’’" 
  
  New York Times April 16.1989. Can Society Tell You What Tastes Good?
   This phenomenon is confirmed with genetic analysis. One measure of positive selection is "K"/"K", the ratio of synonymous to non-synonymous mutations. If the rate of synonymous mutation is higher than the rate of non-synonymous mutation, then the trait created by the non-synonymous mutation is being selected for relative to the neutral synonymous mutations. For the bitter taste gene family, "TAS2R", this ratio is over one in the loci responsible for the extracellular binding domains of the receptors. This indicates that the part of the receptor responsible for binding the bitter ligands is under positive selective pressure.
   Schizophrenia is associated with deficits in motivation, commonly grouped under other negative symptoms such as reduced spontaneous speech. The experience of “liking” is frequently reported to be intact, both behaviorally and neurally, although results may be specific to certain stimuli, such as monetary rewards. Furthermore, implicit learning and simple reward related tasks are also intact in schizophrenia. Rather, deficits in the reward system present during reward related tasks that are cognitively complex. These deficits are associated with both abnormal striatal and OFC activity, as well as abnormalities in regions associated with cognitive functions such as the dorsolateral prefrontal cortex (dlPFC).
   The oral cavity of the fetal pig begins developing before birth. The tongue's taste buds, located in the enlarged papillae, facilitate food handling after birth. These taste buds develop during fetal development. Adult pigs have up to 15,000 taste buds, a much larger number than the average human tongue, which has 9,000.
   Studies indicate that responsiveness to sugars and sweetness has very ancient evolutionary beginnings, being manifest as chemotaxis even in motile bacteria such as "E. coli". Newborn human infants also demonstrate preferences for high sugar concentrations and prefer solutions that are sweeter than lactose, the sugar found in breast milk. Sweetness appears to have the highest taste recognition threshold, being detectable at around 1 part in 200 of sucrose in solution. By comparison, bitterness appears to have the lowest detection threshold, at about 1 part in 2 million for quinine in solution. In the natural settings that human primate ancestors evolved in, sweetness intensity should indicate energy density, while bitterness tends to indicate toxicity. The high sweetness detection threshold and low bitterness detection threshold would have predisposed our primate ancestors to seek out sweet-tasting (and energy-dense) foods and avoid bitter-tasting foods. Even amongst leaf-eating primates, there is a tendency to prefer immature leaves, which tend to be higher in protein and lower in fibre and poisons than mature leaves. The 'sweet tooth' thus has an ancient evolutionary heritage, and while food processing has changed consumption patterns, human physiology remains largely unchanged.
  