Research suggests humans, Neanderthals interbred.

研究证实人类和尼安德特人有过杂交。

More interesting, when these ticklish rats were interbred for four generations, the offspring chirped twice as often as their great-grandparents.

更有意思的是，这些喜欢痒痒的老鼠，经过4代之后，发出吱吱声的比率竟可达到它们的曾祖父母的2倍。