The other day I posted a picture of a weird planthopper that mimicked an ant, but only the male planthoppers. Females weren’t mimics at all. I also pointed out a similar case, but in the reverse direction: in the African butterfly Papilio dardanus (as in many butterflies), it is the female that’s mimetic and the male is not.
In the case of butterflies, I asked readers to guess why the mimicry was limited to females. The classic explanation, but not the only one, is that males are constrained from mimicking a distasteful model species because of sexual selection. That is, the females are genetically hard-wired to prefer a certain male appearance, while males will pretty much court anything. This difference in behavior allows the females to change their appearance in local populations, evolving to resemble other species of butterflies that birds learn to avoid because they’re distasteful (this is called Batesian mimicry). The males, however, are subject to countervailing selection; whle they could gain some advantage by also becoming Batesian mimics too, this is presumably offset by the reproduction they’d lose by becoming less attractive to their own females. Ergo female-limited mimicry.
Now this is only a provisional explanation. It sounds good, but as far as I know (and I may be wrong) it hasn’t been tested. And, at any rate, the planthopper I showed had male-limited mimicry: males mimicked ants and females did not. What could explain this reverse situation?
Again, we don’t know, but one explanation is that the sexes inhabit different niches, and males live in a habitat where selection is stronger to be mimetic. It’s not always true that males and females live in exactly the same places, or eat the same kind of food. Perhaps the male planthoppers live more often with ants, or in habitats that contain visible ants, which make them subject to stronger selection to resemble ants, which birds also avoid because they’re distasteful. Or perhaps females spend a lot of time laying eggs up in trees, where there are few ants.
These are all speculations, and ones that can be tested, but I’ve found at least one case of male-limited mimicry in a beetle where there seems to be an explanation based on niche differences.
Henry Hespenheide, in a paper published in 1975 (reference and link below), found a species of wood-boring beetle (a “bupestrid”) in which the males resembled another distasteful and brightly-colored red-and-black species, but the females did not. Here’s the two species: Chrysobothris humilis is the edible bupestrid species that is dimorphic, Saxinis deserticola mojavensis is the distasteful non-dimorphic species in which males and females are a striking red and black. Note that in the mimic species, only the males are red and black.
Hespenheide showed a niche difference between males and females of C. humilis males sit on the end of twigs waiting for mates. And this is where the models are found as well, and hence the males more likely to be subject to predation (and to be seen along with the model species) than are females, who stay more on the ground and presumably don’t encounter much predation. Thus selection would thus be stronger on males to look like the models. Females, on the other hand, might be better off begin cryptic, since they have different predators, or maybe very few of them. (Hespenheide suggests another explanation, involving dilution of the protection of mimicry by mimetic females, that sounds a bit dubious to me.) At any rate, there is an obvious niche difference between the sexes that could explain sex-limited mimicry.
Again, this is a tentative explanation and could be tested by various transplant experiments, moving beetles around in their environments, or by using “model” beetles made out of soft clay and painted different colors (a technique used to test mimicry in mice).
Hespenheide, H. 1975. Reverse sex-limited mimicry in a beetle. Evolution 29:780-784.