它们滞留在住房条件差、安全用水和卫生设施稀缺、环境肮脏以及带病昆虫和动物大量繁衍的地方。
They stay put in areas where housing is substandard, safe water and sanitation are scarce, environments are filthy, and disease-carrying insects and animals are abundant.
就像生活在没有树木的热带草原，你发现水果不够了，动物稀缺。
It's like living on the savannah and discovering you don't have enough fruit and the animals are scarce.
当食物稀缺并需要动物消耗体力去捕食时，增加对非偏好品的渴望能帮助动物吃下他们所获得的食物，即使尝着味道真的不怎么样。
When food is scarce and animals exert energy to retrieve food, increasing the desirability of less tasty food would help animals eat what they had caught, regardless of its inherent tastiness.
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