Meeting Abstract
We propose a practical concept that distinguishes the particular kind of weaponry that is used specifically during same-sex combats, which we term “intrasexually selected weapons” (ISWs). We offer hypotheses to answer the question: Why only some species have evolved weapons to fight for the opposite sex? We examined traits that seem to have evolved as ISWs in the entire animal phylogeny, restricting the classification of ISW to traits that: are only present or enlarged in one of the sexes, do not appear or enlarge until adulthood, and are used as weapons during intrasexual fights. We found that ISWs are only found in bilateral animals; appearing independently in nematodes, arthropods, and vertebrates. We establish that most ISWs: come in pairs, are located in or near the head, are overdeveloped structures of those found in females, are endo- or exoskeletal modifications, are modified feeding structures and/or locomotor appendages, are frequently used to guard females, territories, or both, and are also used in signaling displays to deter rivals and/or attract females. We also found that most taxa lack ISWs, that females of only few species possess better-developed weapons than males, that the independent evolutions of ISWs are not evenly distributed across the animal phylogeny, and that most vertebrates possessing ISWs have non-hunting habits (e.g. herbivores), or are carnivores that prey on very small prey relative to their body size (e.g. insectivores). We provide an evolutionary flow chart for ISWs and discuss biomechanical rationales not only for the appearances but also for the fashioning of the weapons. We provide a case study, with hummingbirds, linking functional morphology and performance measurements both of combat abilities and the relevant biological function affected by the ISW.
