The woman biologist stayed in Africa studying wild animals for 13 years before she returned.
在这里studying一般看作是现在分词作伴随状语,这里可以改成：
The woman biologist stayed in Africa and studied wild animals for 13 years before she returned.
如果改成：The woman biologist stayed in Africa to study wild animals for 13 years before she returned.意思与原句 有出入.不定式短语是目的状语
=to study wild animals,the woman biologist stayed in Africa for 13 years before she returned.