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Why do cats love tuna so much? Scientists may finally know
Apart from Garfield’s legendary love of lasagna, perhaps no food is more associated with cats than tuna. The dish is a staple of everything from The New Yorker cartoons to Meow Mix jingles—and more than 6% of all wild-caught fish goes into cat food. Yet tuna (or any seafood for that matter) is an odd favorite for an animal that evolved in the desert. Now, researchers say they have found a biological explanation for this curious craving.
In a study published this month in Chemical Senses, scientists report that cat taste buds contain the receptors needed to detect umami—the savory, deep flavor of various meats, and one of the five basic tastes in addition to sweet, sour, salty, and bitter. Indeed, umami appears to be the primary flavor cats seek out. That’s no surprise for an obligate carnivore. But the team also found these cat receptors are uniquely tuned to molecules found at high concentrations in tuna, revealing why our feline friends seem to prefer this delicacy over all others.
“This is an important study that will help us better understand the preferences of our familiar pets,” says Yasuka Toda, a molecular biologist at Meiji University and a leader in studying the evolution of umami taste in mammals and birds. The work could help pet food companies develop healthier diets and more palatable medications for cats, says Toda, who was not involved with the industry-funded study.
Cats have a unique palate. They can’t taste sugar because they lack a key protein for sensing it. That’s probably because there’s no sugar in meat, says Scott McGrane, a flavor scientist and research manager for the sensory science team at the Waltham Petcare Science Institute, which is owned by pet food–maker Mars Petcare UK. There’s a saying in evolution, he says: “If you don’t use it, you lose it.” Cats also have fewer bitter taste receptors than humans do—a common trait in uber-carnivores.
But cats must taste something, McGrane reasoned, and that something is likely the savory flavor of meat. In humans and many other animals, two genes—Tas1r1 and Tas1r3—encode proteins that join together in taste buds to form a receptor that detects umami. Previous work had shown that cats express the Tas1r3 gene in their taste buds, but it was unclear whether they had the other critical puzzle piece.
So McGrane and colleagues biopsied the tongue of a 6-year-old male cat that had been euthanized for health reasons unrelated to the study. Genetic sequencing revealed his taste buds expressed both the Tas1r1 and Tas1r3 genes—the first time scientists showed that cats have all the molecular machinery needed to detect umami.
When the researchers compared the protein sequences encoded by these genes with those of humans, however, they found a striking difference: Two critical sites that allow the human receptor to bind to glutamic and aspartic acid—the main amino acids that activate umami taste in people—were mutated in cats. “So I began thinking, maybe cats can’t taste umami,” McGrane says.
To double check, he and his team engineered cells to produce the cat umami receptor on their surface. They then exposed the cells to a variety of amino acids and nucleotides. The cells did respond to umami—but with a twist. In people, the amino acids bind first and the nucleotides amplify the response. But in cats, the nucleotides activated the receptor, and the amino acids further boosted it, McGrane says. “That’s the exact opposite of what we see in people.”
In the last part of the experiment, McGrane and colleagues gave 25 cats a taste test. In a series of trials, they presented the felines with two bowls of water, each with various combinations of amino acids and nucleotides, or just water alone. The cats showed a strong preference for bowls that contained molecules found in umami-rich foods, suggesting this flavor—above all others—is the primary motivator for cats.
“I think umami is as important for cats as sweet is for humans,” Toda says. Dogs, she notes, can taste both sweet and umami, which may explain why they’re not such fussy eaters.
But it wasn’t just umami in general the cats craved. The felines showed a particular preference for bowls containing histidine and inosine monophosphate—compounds found at particularly high levels in tuna. “It was one of the most preferred combinations,” McGrane says. “It really seems to hit that umami sweet spot.”
That jibes with Toda’s personal experience. When she was a veterinary student, she got cats with no appetite to eat by sprinkling their food with dried flakes of bonito—a common umami ingredient in Japan and a close relative of tuna. “It worked very well!” she says.
Indeed, one application of the work could be developing foods that are more palatable to cats, McGrane says. He also thinks a spoonful of umami (figuratively speaking) could help feline medications go down easier—welcome news for anyone who’s almost lost a finger trying to pill a cat.
Why cats have a hankering for tuna in the first place remains a mystery. They evolved in the deserts of the Middle East about 10,000 years ago, where fish of any kind was unlikely to be on the menu.
It may have been a taste cats developed over time. As far back as 1500 B.C.E., cats are depicted eating fish in the art of Ancient Egypt. And by the Middle Ages, felines in some Middle Eastern ports were consuming large quantities of fish—including tuna—likely because they were feasting on the scraps left by fishers. In both cases, cats that evolved a taste for fish—and perhaps tuna in particular—may have had an advantage over their comrades, says Fiona Marshall, a zooarchaeologist at Washington University in St. Louis.
“We’re at a starting point—it’s not a finished story,” McGrane admits. “But all of this work is building up to our basic understanding of what it means to be a cat.”
注释：
receptor: n
表示"受体；接受器"，means "a cellular structure that is postulated to exist in order to mediate between a chemical agent that acts on nervous tissue and the physiological response"，如：Placebo effect tied to brain receptor activity. 安慰剂的效果与大脑内受体活动度有关。
umami: n
表示"鲜味"，如：MSG is a flavor enhancer that excites the fifth taste sense umami (the others being salt, sweet, sour, bitter). 味精作为一种佐料可以增加第五种味觉-鲜味（其它是咸，甜，酸，苦）。
obligate: v
表示" 使负义务；强制"，means "force somebody to do something"，如：I would not obligate society to treat me a certain way,and I never thought like that. 我不去强求社会应该对我怎样,我从来没想过。
carnivore: n
表示" 食肉动物；食虫植物"，means "a terrestrial or aquatic flesh-eating mammal; any animal that feeds on flesh"，如：Lions and tigers are carnivores. 狮子和老虎是食肉动物。
palatable: adj
表示" 美味的；使人愉快的"，means "acceptable to the taste or mind"，如：Mother provides palatable meals for our family every day. 母亲每天为家人烹制美味的三餐。
euthanize: v
表示" 使安乐死"，如：To euthanize unwanted pets has made his wife sick to her stomach. (对不要的宠物进行安乐死，令他老婆很恶心
palate: n
表示"味觉；口味;上颚"，means ""，如：I have a good palate for fine wine. 我善于品尝好酒。There is something wrong with my palate. 我的上颚有些不适。
glutamic: adj
表示"谷氨酸的"，如：This paper studies the effect of interfacial agent in the yield of glutamic acid extracted by using isoelectric point method. 本文介绍了表面活性剂对等电点提取谷氨酸的收得率之影响。
nucleotides: n
表示"核苷酸"，means " the basic structural unit of nucleic acids (DNA or RNA"，如：Reduced nicotinamide nucleotides can function as reductants to nitrogenase. 还原的烟酰胺核苷酸对固态酶可以起还原剂的作用。
jibe: v
表示" 嘲笑；嘲弄"，means "be compatible, similar or consistent; coincide in their characteristics;"，如：She jibed his folly. 她嘲笑他的愚行。
veterinary: n
表示"兽医"，means "a doctor who practices veterinary medicine"，如：A veterinary surgeon is a doctor of a sort. 兽医是一类医生。
hankering: n
表示" 渴望"，means "a yearning for something or to do something"，如：Thus we experience the most severe hankering for You.这样，我们感受到最剧烈地渴望你。
zooarchaeologist： n
表示" 动物考古学家"
中文简要说明：
猫咪的祖先最早源自于1万年前的中东沙漠，多年来科学家一直相当好奇，为何来自沙漠的猫咪会特爱金枪鱼或任何海鲜类，如今科学家发现，猫咪的味觉感受器会与金枪鱼中2种浓度特别高的化学物质结合，这些化学物质强化了猫咪对鲜味的感受，导致猫咪更爱鱼腥味。
综合期刊《科学》（Science）旗下新闻网、科技新闻网站「生活科学」（Live Science）报导，8月发表在期刊《化学感官》（Chemical Senses）的一篇研究显示，猫与人类一样，会使用味觉感受器来感测鲜味，鲜味是甜、酸、咸、苦以外5种基本味道中的一种，赋予食物咸味或肉味。
先前的研究显示，猫咪吃不出甜味，因为牠们的甜味味觉感受器基因失去作用，专家假设猫咪尝不出碳水化合物及甜味，是因为这些巨量营养素（macronutrients）对牠们的生存来说并非必须。
带领这项研究的英国沃尔瑟姆宠物营养中心（Waltham Petcare Science Institute）感官科学专家麦格雷恩（Scott McGrane）说，自从发现猫咪没有甜味的味觉感受器后，团队确实开始思考猫咪对什么有反应。
麦格雷恩与团队透过解剖一只6岁公猫，来观察猫咪舌头上的基因表现，人类舌头上有TAs1r1、Tas1r3等2个基因，这2个基因共同作用后让人类感受到鲜味，科学家原本就知道猫咪舌头上有Tas1r3，但不清楚是否有TAs1r1，这份研究显示，猫咪拥有感知鲜味所需的2种基因。
团队进一步观察TAs1r1、Tas1r3等2种基因编码的蛋白质，发现与人类基因有很大不同，当中2种影响人类尝出鲜味最重要的胺基酸结合点位发生突变。
于是麦格雷恩团队进一步改造细胞，在细胞上创造出猫咪舌头上能感受鲜味的味觉感受器，并进一步让这些细胞暴露在一系列胺基酸、核苷酸（nucleotide）中，发现这些细胞确实对鲜味有反应，但作用方式与人类细胞不同，在人类细胞中，胺基酸先与细胞结合，核苷酸再放大反应，不过在猫咪的细胞中，核苷酸先启动味觉感受器后，胺基酸再进一步强化作用效果。
麦格雷恩团队后续又进一步让25只猫咪分别暴露在装有胺基酸、核苷酸的水碗及单纯装水的水碗中，发现猫咪们对前者有更强烈的兴趣，显示鲜味确实是驱动猫咪食欲的主要味道。
麦格雷恩进一步说明，在所有鲜味中，猫咪们对添加了组胺酸（Histidine）及次黄嘌呤核苷酸（Inosine monophosphate）等2种化合物的水碗特别感兴趣，这2种物质尤其在鲔鱼中有较高含量，麦格雷恩说：「这是最受欢迎的组合之一。」
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