题干分析：讲座主旨。

原文定位：

So let's go back to our discussion of how animals defend against predators. Here's an interesting phenomenon that caught the attention of some researchers. Female mule deer aggressively defend their young against predators. Not exactly shocking news, but there's more. They defend the young of other deer too, even ones of a different species. And not only that, they will even leave their own young alone to do so.

选项分析：教授提到骡鹿不仅仅会保护自己的孩子，还会保护其他骡鹿，甚至其他种族的孩子，甚至做到丢下自己的孩子的地步，随后文章就在讨论这种利他行为的几种可能解释，可知应选C选项。文中讲到的互惠利他行为仅限于骡鹿，A选项错误；B选项未提及；文章主要讲的是骡鹿和互惠理他行为，D选项属于次要细节。