Rare-male Effect in Nature Raises Intriguing What to Do and How

questions. The most obvious is, why do female guppies prefer males with uncommon colour patterns? Here we have more hypotheses than data sets. One theoretical analysis suggests that greater survival of rare male types, which has been found in guppies, might contribute to the evolution of such a preference, even if it is costly. Interestingly, the same analysis points out that the negative feedback inherent in sexual selection for rarity (such selection causes rare types to become common) can ultimately prevent preferences for rare males from becoming ubiquitous. It remains to be determined whether preference for rarity varies among guppy females in experimental populations such as those used in this study. Hughes et al. point out that the rare-male effect might also emerge if females avoid mating with males that share the colour patterns of their previous mates, to increase the genetic diversity of their offspring. Alternatively, preferences for rarity could have evolved because they lead to lower rates of inbreeding if populations are small and intermittently isolated. It has even been suggested that mating preferences for rare types provide no benefit for females and may be a manifestation of a preference for novelty that has evolved in other contexts. The clever manipulations of natural populations used by Hughes et al. may, as the authors suggest, be a promising avenue for exploring the maintenance of variation by frequencydependent selection in other systems. It might well be the case that, when put to the test, preferences for rarity might themselves not prove to be so rare. ■