This week, we will begin to think about distinguishing and differentiating the selective forces that have led to evolution of extreme structures. In a recent review paper, Doug Emlen and co-authors wrote this: “Sexually selected ornaments and weapons are among the most elaborate and diverse traits in the animal kingdom, and their origin and maintenance remains an active research area among evolutionary biologists and behavioral ecologists. Sexual selection arises from competition (typically among males) for access to mates, and can take the form of male–male competition, female mate choice, or both. Although both mechanisms are credited with the evolution of exaggerated sexual traits, most sexual selection studies focus on female mate choice. The unfortunate consequence of this bias is that the terms ‘sexual selection’ and ‘mate choice’ are now often used interchangeably, and the term ‘ornament’ often refers generally to all sexually selected traits.”

We will begin our own exploration of these distinct forms and uses of structures by introducing the claws of male fiddler crabs to add to our earlier consideration of horns and antlers.