Identify the Main Issue in the Following Passage

Essay

Identify the main issue in the following passage. If two or more issues are present, indicate how they are related-that is, does the settlement of one depend upon the settlement of the other?
Research has shown that the females of mammal species are more resistant to environmental hazards than are their male counterparts. That makes sense, because young mammals take so long to develop, both physically and mentally, and the females have to be able to function continuously if the species is going to survive.