Male-like females of a damselfly are not preferred by males even if they are the majority morph

Animals searching for prey and males searching for mates share similar problems of detection if their targets are diverse in colour or physical appearance. There is good evidence for predators switching their preferences for prey in a frequency-dependent way; predators focus on the most common form, and the decreased predation on rarer forms allows multiple forms to survive. Frequency-dependent mate selection has also been proposed to explain the maintenance of several female colour morphs in damselflies. However, the fact that one of the female morphs is coloured like a male (androchrome) and behaves similarly to males suggests the phenomenon of male mimicry in this system as an alternative explanation for the polymorphism. We compared androchrome frequencies in populations and mating pairs in Ischnura elegans, over a range of androchrome frequencies (8–90%). In 22 of 23 samples androchromes mated less often than expected (significantly in 13 samples). We found no evidence for males switching their preferences in a frequency-dependent way. A test of male preference for female morphs in a population with 85% androchromes indicated that males behaved indiscriminately and did not prefer the commonest (male-like) morph. Our results support androchrome male mimicry rather than learned mate recognition by males (a purely frequency-dependent model) as the main mechanism behind the maintenance of this sex-limited colour polymorphism.