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In the end, Darwin came up with an entirely new theory to explain the extraordinary lengths many animals will go to in order to woo a potential mate. He called it sexual selection. Simply put, sexual selection is the evolutionary process that favors adaptations that increase an animal's chances of mating. Darwin identified two kinds. In the first, males compete fiercely with each other for access to females. This kind favors the evolution of secondary sexual characters, such as large size and armaments like horns, that enhance a male's ability to fight. In the second, males compete to win over a female.

In most animals, the drive to breed and produce offspring is strong. However, most males live their whole lives without having the chance to breed. The events leading up to mating can be very dangerous and also very costly to an individual. Some males have evolved elaborate structures, or weapons, as a result. The structures do help males in both combative situations and with attracting females, but ironically, the structures themselves come with certain costs. This flipped case study provides students with the opportunity to not only see how animal structures and functions are linked, but also to see how certain animal structures are needed and costly. There are videos that students are expected to view before the case. The case was initially designed for a second semester college general biology class for majors. However, it can also be used in non-major biology classes. Students should have some background knowledge of natural selection, specifically sexual selection as well as energetic demands of certain structures.