Male morph coexistence in the bulb mite Rhizoglyphus robini A minor’s guide to reproduction

In numerous species, members of the same sex adopt different alternative reproductive tactics (ARTs). Typically, two male ARTs, or ‘morphs’, can be distinguished that differ in terms of physiology, behaviour or morphology: these are called ‘majors’ and ‘minors’. Majors in most species use weapons or ornaments to gain access to females. Minors in most species lack weapons or ornaments to gain access to females, which raises the question: how can majors and minors coexist? Coexistence of majors and minors requires that minors have some fitness advantages over majors. To examine potential fitness benefits of minors over majors, I used the male-dimorphic bulb mite Rhizoglyphus robini, where males are ‘fighters’ (majors) or ‘scramblers’ (minors), and performed several experiments. I demonstrated two fitness benefits that scramblers have over fighters: females mated to scramblers sired more offspring under some conditions, and daughters sired by scramblers had a higher fitness than daughters sired by fighters. I also observed that fighters frequently killed and/or cannibalised other mites. Though killing or cannibalising kin could incur an inclusive fitness cost to fighters, I showed that fighters tended to avoid killing and cannibalising kin. Finally, I provided evidence of a third male morph in the bulb mite: the ‘mega-scrambler’. My findings help to explain the maintenance of a male dimorphism, and thereby contribute to answering a key question in evolutionary biology of why variation is maintained in single populations.