That's because chocolate contains caffeine and a chemical called theobromine.

这是因为巧克力含有咖啡因和一种叫做可可碱的化学物质。

Smaller dogs will have more severe reactions, and the amount of caffeine and theobromine in chocolate varies.

体型较小的狗会有更严重的反应，巧克力中咖啡因和可可碱的含量也各不相同。

They process theobromine a lot more slowly, so it can easily build up and cause those dangerous effects.

他们代谢可可碱的速度很慢，所以可可碱很容易在体内累积并造成危害。

Your average adult human would need to eat about 8 kilograms of dark chocolate to get a lethal dose of theobromine.

一般成年人需要吃掉大约8公斤的黑巧克力，才会摄入份量足以致死的可可碱。

Early studies suggest small doses of very dark chocolate, rich in antioxidants, theobromine and caffeine may make us happier, healthier and less stressed.

早期的研究曾指出一小块的黑巧克力里含有丰富的抗氧化物、可可硷和咖啡因可使人心情愉悦、放松心情以及更健康。

Both molecules are dangerous for dogs but chocolate contains a lot more theobromine than it does caffeine so it's the molecule to watch out for.

咖啡因以及可可碱对于狗来说都相当危险，但巧克力中含有的可可碱成分相比咖啡因还要来得多，所以要注意的是这两种成分。

Also like caffeine, theobromine makes our heart pump faster, our blood vessels dilate, and some of our muscles get more energy, which sounds great!

就像咖啡因，可可碱也会促使我们心跳加快，血管扩张，以及让部分肌肉得到更多能量，这听起来很棒。

Whatever the reason, the same amount of theobromine stays in pets' bodies much longer, giving it more time to pile up and cause harm.

无论原因如何，等量的可可碱会停留在宠物的身体里更久的时间，使得可可碱更容易累积并造成伤害。

Like caffeine, theobromine is a type of alkaloid, which is a huge class of molecules that generally contain rings with at least one nitrogen atom in them, and alkaloids often have physiological effects on humans and other animals.

和咖啡因一样，可可碱属于生物碱的一种，生物碱是相当大的一类分子，一般来说它的环中会包含至少一个氮原子，生物碱通常会使人类及其他动物产生生理作用。

Sweeter chocolates have less theobromine, so the lethal dose is higher; about 5 kilograms of milk chocolate for dogs and about a third of a kilogram for cats, but they'd be sick long before eating that much.

较甜的巧克力所含的可可碱较少，所以足以致死的份量相对会较高，吃下约 5 公斤的牛奶巧克力会导致狗狗死亡，猫则是要吃下约 300 多公克，不过他们在吃下那么多分量之前就会先感到恶心了。

It's hard to know exactly why we evolved to be better at digesting theobromine, but it might be because alkaloids mostly come from plants, and our ancestors ate a lot more plants than early cats or dogs did.

我们很难确切了解为什么我们的身体会进化成能较容易消化可可碱，有可能是因为生物碱大部分都来自植物，而我们的祖先吃的植物比以前的猫和狗所吃的植物多很多。