The influx of cats prompted a period of dramatic decline among the dog clan. This has left North America with just nine species of wild dog today, comprising different variations of wolves and foxes, according to the fossil analysis published in the journal PNAS.

猫科动物的涌入导致犬科动物数量在一段时间内大幅下降。依据这项发表在美国国家科学院院刊(PNAS)上的化石分析，这造成了如今北美洲仅存九个犬类物种，包括狼和狐狸的不同变种。

Dr Silvestro says it is unclear exactly why, when times were tough, the cats were able to see off dogs so comprehensively. But he believed it could be something to do with the retractable claws that ancient cats have passed down to their domesticated descendants, but which dogs don’t have.

西尔韦斯特罗博士称，目前还不是十分清楚在当时的艰难岁月，猫科动物为什么能将犬科动物“驱逐得那么彻底。但是他认为这可能跟古代猫科动物伸缩自如的爪子有关，这是犬科动物所不具有的。古代猫科动物还将这种爪子遗传给了自己的后裔家猫。

Millions of years ago, some dog species, such as wolves, were effective killing machines, chasing down their prey at high speed and devouring it. All surviving dog species fall into this "runner" camp.

数百万年前，某些犬科物种(比如说狼)是有效的杀戮机器，能以高速追捕猎物并吞食掉。所有幸存的犬科动物都属于“奔跑系猎手阵营。

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