定義： A depression caused by erosion by water or ice. Low-lying land bordered by higher ground; especially elongate, relatively large gently sloping depressions of the Earth's surface, commonly situated between two mountains or between ranges of hills or mountains, and often containing a stream with an outlet.
定義： Description of the relationships among individuals within a population of organisms, including reproductive relationships (mating system), relationships within each sex, adult – young interactions, helpers and cooperation (group living).
