浅谈为什么狗不能活得像人类一样长?
Dogs are mammals like humans, and they have a heart, lungs and all the things you need to survive.
狗是像人类一样的哺乳动物，它们拥有心脏、肺和所有生存所需的东西。Our pet dogs give us so much happiness, but then so much sadness when they die. So why don't they survive as long as us?
我们的宠物狗给了我们如此多的快乐，但是当他们死去时，我们是那么的悲伤。那么为什么他们不能活得像我们一样久呢？Scientists worked out a long time ago that the lifespan of animals is mostly related to their size. Bigger animals usually live longer than smaller animals.
很久以前，科学家就研究出动物的寿命与它们的体型大小有关。较大的动物通常比较小的动物寿命更长。
The main reason people used to think big animals live longer is because of something called metabolic rates.
人们过去认为大动物活得更长的主要原因是因为代谢率。A metabolic rate is like how much petrol a car uses -- cars that use up their petrol more slowly can drive for longer, a bit like animals with lower metabolic rates.
代谢率就像一辆汽车使用多少汽油，更慢地使用汽油的汽车可以开得更久，有点像代谢率较低的动物。Smaller animals usually have higher metabolic rates, which lead to shorter lifespans, like a car that uses up its petrol very quickly.
较小的动物通常具有较高的新陈代谢率，这导致寿命缩短，就像汽车很快耗尽汽油一样。
The problem is that this doesn't work for all animals. Some parrots have high metabolic rates but can live for more than 80 years! A metabolic rate is related to heart rate, and some parrots have a heart rate of 600 beats per minute. (Your heart beats around 70 to 100 times per minute.)
问题是这并不是对所有的动物都起作用。一些鹦鹉的代谢率很高，但可以活80年以上！代谢率与心率有关，一些鹦鹉的心率为每分钟600次。（你的心脏每分钟跳动70到100次左右。）And dogs are one of the animals that do not follow this rule. A 70kg Great Dane is lucky to reach seven years, but a 4kg Chihuahua can live for 10 years or more.
而狗是不遵循这个规则的动物之一。一只70公斤的大丹犬，如果幸运，寿命能达到7年，但是4公斤的奇瓦瓦狗可以活10年或更长时间。We still don't really know why this happens. But if you want a dog that lives longer, you should choose a small breed.
我们仍然不知道为什么会发生这种情况。但是如果你想要一只活得更长的狗，你应该选择一个小品种。People now think it might depend on the "evolutionary pressures" that each animal faces. An evolutionary pressure includes things like other animals that want to kill you for food.
人们现在认为这可能取决于每只动物所面临的“进化压力”。进化压力包括其他想要猎杀你的动物。Because they are so big, large animals like elephants and whales are less likely to be attacked in the wild than small animals like guppies or mice.
因为非常大，所以像大象和鲸鱼这样的大型动物，不会像野生动物或老鼠这样的小动物在野外那么容易受到攻击。
