If a human year really were equivalent to seven dog years, then people would reach reproductive age by seven, and some would live past 150.

For more than 50 years, scientists and dog lovers have been trying to debunk the dog-years myth. Yet, it persists in books, news articles and the popular imagination. No matter how you measure it, this numerical notion has impressive longevity.

'You can't really kill the seven-year rule,' says Kelly M. Cassidy, curator of a biology museum at Washington State University, who in her spare time maintains an online compilation of dog-longevity studies.

The rule's one-size-fits-all simplicity makes it a compelling way for people to track their pets' development, or to monitor their own lives through their pets. That simplicity, however, is also the rule's undoing -- the seven-year glitch.

Scientists would prefer more-nuanced conversions. Typical lifespans among the hundreds of canine breeds can range from 8 to 16. And dogs grow quickly in the first couple of years, with bigger breeds reaching the equivalent of U.S. voting age in toddlerhood, by age two. 'Eight years in one breed is not equivalent to eight years in another,' says David J. Waters, associate director of Purdue University's Center on Aging and the Life Course.

It remains mysterious to scientists why big dogs die younger. Across different species, bigger animals tend to live longer: Compare men with mice. Within species, an inverse relationship sometimes takes hold: Smaller rats live longer than big ones. Prof. Waters prefers a physiological explanation for small dogs' longer lifespans.

Tracing the dog-year mythology to its source is difficult. An inscription at Westminster Abbey -- no relation to the dog show -- from the 13th century puts the ratio at 9 to 1, noting that dogs live nine years and men 81, with other life forms living longer by ratios of three, up to the predicted planet lifespan of 19,683 years. A similar ratio was calculated by 18th-century French naturalist Georges Buffon, who reported that dogs can live to 10 or 12, and man to 90 or 100.

Somewhere along the way, it seems likely to several veterinarians, typical lifespans were pegged at about 70 for humans and about 10 for dogs. Thus, the seven-year rule was born. 'My guess is it was a marketing ploy,' says William Fortney, a veterinarian at Kansas State University, 'a way to educate the public on how fast a dog ages compared to a human, predominantly from a health standpoint. It was a way to encourage owners to bring in their pets at least once a year.'

The rule has endured in many corners of world culture. In May, unverified reports of a 29-year-old mixed-breed dog in Chesterfield, England, headlined its supposed 203-year-old age. The notion has even been adopted by the Internet culture to explain its faster-than-life pace. The book '21 Dog Years' is about the author's three years at Amazon.

Vets who tested the rule found several problems. Some 55 years ago, researcher A. Lebeau studied life-stage markers common to dogs and humans, such as puberty, adulthood and maximum lifespan, and found that aging in dogs can proceed 20 times as fast as human aging before age 1, gradually slowing to a ratio of about five. Since then, scientists have used veterinary-hospital records and breed-club surveys to refine the relationship further, by breed and by weight.

The improved formulas have appeared in general-interest books such as 'Dogs for Dummies' and recent editions of 'Old Farmer's Almanac.'

But the new orthodoxy is itself based on uncertain numbers. There is no equivalent to the National Center for Health Statistics for dogs. Instead, there are three main sources for data on their longevity: pet-insurance companies, breed-club surveys and veterinary hospitals.

The first two may be biased toward longer-living dogs, because owners who belong to clubs and buy insurance may spend more to prolong their pets' lives. Dr. Cassidy adds that surveys require dog owners to recall their pets' lifespan, a number they tend to exaggerate. She has documented the gap by comparing ages from death notices on a poodle email list she belongs to with birth records in a poodle database. (Like other investigators in the field, Dr. Cassidy is a dog owner -- three in all, ages 2 to 10 actual years.)

Meanwhile, hospitals may be biased toward shorter lifespans, because they tend to admit the toughest cases, not healthy dogs. 'It's not good data, but it gets you in the ballpark,' says Prof. Fortney.

The true numbers are moving targets, adds Jeff Sampson, canine-genetics consultant to the Kennel Club in the U.K. As veterinary medicine improves and more dogs are immunized, fewer die young of distemper and parvovirus today than 30 years ago, Mr. Sampson says.

To dog lovers, fixed mathematical ratios of lifespan matter far less than the comfort that their companion lived a long and happy life.

如果狗活一年相当于人活七年，那么人七岁就该进入生育年龄，而一些人甚至有可能会活过150岁。

50多年来，科学家和爱狗人士一直试图解开狗的年龄之谜。不过，七年公式已经在书刊、新闻报导和公众印象中广为流传。无论你怎么看，这个公式都有着令人印象深刻的生命力。

华盛顿州立大学(Washington State University)生物博物馆馆长凯利·卡西迪(Kelly M. Cassidy)表示，你没法彻底否定这个七年公式。她在空余时间一直在网上从事狗寿命研究。

这条公式简单易记，因此当人们测算自己宠物的成长，或通过宠物监控自己的年纪时，首先就会想到这个公式。不过，这个公式的简单化也正是其软肋所在。

科学家可能倾向于更为精细的年龄换算公式。对数百个品种的狗统计发现，它们的寿命一般在8到16岁不等。狗在生命前两年长的非常快，体型较大的品种两岁就能发育成熟，达到相当于美国选民法定年龄的水平。普渡大学(Purdue University)年龄和寿命研究中心副主任大卫·沃特斯(David J. Waters)说，同样是八岁的狗，不同品种之间也会有所区别。

科学家现在还不清楚为什么体型大的狗死的更早。横向比较各个物种发现，体型较大的动物通常寿命更长：比如说人比老鼠寿命长。但具体到一个物种内部，则会出现相反的规律：个子小的老鼠比大老鼠活的更久。沃特斯倾向于从生理学方面解释小老鼠长寿的原因。

要对有关狗年龄的七年公式探根溯源并不容易。伦敦西敏寺的一篇13世纪的铭文提到人狗年龄比率是9比1，狗活9年相当于人活81岁；而其他寿命较长的生命形式可以是三倍比率，最高达到19,683岁。18世纪法国自然学家乔治·布丰(Georges Buffon)也进行了类似的年龄比例推算，他报告称狗可以活10到12岁，而人可以活到90到100岁。

此后有数位兽医学家也沿着这一方向进行了研究，他们认为人类寿命一般在70岁左右，而狗大约在10岁上下。七年公式由此而生。堪萨斯州立大学(Kansas State University)兽医学家威廉·福特尼(William Fortney)表示，他猜这条定律只是一种营销策略，主要是想从健康的角度让人们知道，狗的寿命和人相比多么短暂，从而鼓励宠物主人至少每年带狗看一次兽医。

这条公式也在世界各地面临着挑战。今年5月，媒体刊登了一则未经证实的报导，称英国切斯特菲尔德有条29岁的混血狗，算起来相当于人活到了203岁。甚至在网上，也有人采用了这个公式来形容生活节奏之快。《21个狗年》一书讲的就是作者在亚马逊(Amazon)渡过的三年时光。

质疑这条定律的兽医们发现了几个问题。大约55年前，研究者勒布(A. Lebeau)研究了人和狗之间共同的生命阶段，例如青春期、成年期和最高寿命；结果发现1岁之前狗的发育速度相当于人的20倍，这个比例随后逐渐放缓至5倍左右。科学家们此后研究了兽医院的资料和繁殖犬舍的调查数据，通过不同品种和体重进一步确定年龄关系。

《Dogs for Dummies》等通俗读物以及近期出版的《Old Farmer's Almanac》已经刊登了进一步研究得出的狗类年龄公式。

但这个新公式本身也是基于不确定数据得出的。它没有采用美国健康统计中心(National Center for Health Statistics)的狗类数据，而是依靠三个主要的数据来源：宠物保险公司、繁殖犬舍调查和宠物医院。

前两类机构提供的狗寿命可能更长，因为犬舍俱乐部的成员和买了宠物保险的主人可能会花更多钱来延长爱犬寿命。卡西迪谈到，调查要求狗主人回想自己宠物的寿命，他们可能会夸大这一数字。她记录了自己参加的狮子狗俱乐部开出的死亡报告上的狗寿命，将其与一个狮子狗数据库上的出生纪录进行对比核实。同该领域的其他调查者一样，卡西迪自己也养了三条狗，年龄从2岁到10岁不等。

另一方面，医院提供的狗寿命可能会比较短，因为医院遇到的多是健康状况糟糕的狗。福特尼说，这不是精确的数据，但能让你略知一二。

英国养犬俱乐部(Kennel Club)的犬类遗传顾问杰夫·桑普森(Jeff Sampson)指出，狗的实际寿命数据也在不断变化。他说，随着宠物医疗条件的改进，越来越多的狗得到免疫，现在因为犬瘟和细小病毒而死的壮年狗数量比30年前少多了。

不过对爱狗人士来说，只要自己的狗伴侣活得开心长寿，这比什么寿命公式都重要。
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