Since there is a strong correlation between these variables and a country's wealth, they controlled for GNP per head by measuring whether it alone accounted for the difference in mating strategy.

由于这些指标与一个国家的财富密切相关，他们以人均国民生产总值为控制变量来看它是否能够单独解释择偶策略的差别。

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But important questions remain, he says. For example, male humpback whales sometimes sing as they move in and out of competitive groups, but researchers don't know what role the songs play in mating.

但重要的问题还是，他说，比如，雄性驼背鲸有时候在进入或离开一个竞争性群体时唱歌，但研究者不清楚唱歌在其中起什么作用。

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But important questions remain, he says. For example, male humpback whales sometimes sing as they move in and out of competitive groups, but researchers don't know what role the songs play in mating.

但重要的问题还是，他说，比如，雄性驼背鲸有时候在进入或离开一个竞争性群体时唱歌，但研究者不清楚唱歌在其中起什么作用。

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