Why is one sex mimetic rather than the other?

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.

19 Comments

  1. Posted November 12, 2012 at 9:59 am | Permalink

    I smell a dissertation (or three)…isn’t this the exact sort of thing graduate students are for?

    b&

  2. Posted November 12, 2012 at 10:04 am | Permalink

    Well, if both are miming, then it’s really not miming so much as just improvisation, Or if you’re into tapping, trading. =^_^=

    • chascpeterson
      Posted November 12, 2012 at 11:17 am | Permalink

      mimicing.
      Not miming.

  3. Flo M
    Posted November 12, 2012 at 10:34 am | Permalink

    Another possibility is that sexual dimorphism may affect the way the mimicry is set up. your post made me think of this example:

    Zuniga magna is a jumping spider from Brazil. As in all spiders, the males’ pedipalps are enlarged, because they are used during mating for transferring spermatophores. Both males and females in these species mimic ants (as is very frequent among jumping spiders), albeit different species. The male uses its pedipalps to mimic the model ant’s head, while the females, which mimics a different ant species, have a constricted cephalothorax to mimic the model’s head.

    If you like to read more about ant mimics (apologies for the blatant self-promotion):

    http://www.cell.com/current-biology/fulltext/S0960-9822%2811%2900422-2#Summary

    Z. magna is described here: Ant‐mimicry in some Brazilian salticid and clubionid spiders (Araneae: Salticidae, Glubionidae)*
    PS OLIVEIRA – Biological Journal of the Linnean Societ

  4. thh1859
    Posted November 12, 2012 at 10:59 am | Permalink

    Makes you wonder how insects that aren’t distasteful, don’t mimic ones that are — or leaves or bird shit, live in the same niche as the mimics but just lie there survive at all.

    • rikkigumbs
      Posted November 12, 2012 at 12:20 pm | Permalink

      There are many other ways to avoid predation: crypsis, agility, morphological defence, chemical defence, etc. – mimicry is not the only option available!

      • thh1859
        Posted November 12, 2012 at 3:27 pm | Permalink

        Of course. But I’m thinking of the insects that have no discernible defence against predators. It seems to me that studying why they survive would be as interesting as studying insects with one of the many obvious (to us) survival atributes.

        • rikkigumbs
          Posted November 12, 2012 at 6:05 pm | Permalink

          I see what you mean. However, I do think one would be hard-pushed to identify many (if any) species that, following the most superficial of investigations, appear to have no discernible antipredator mechanisms. I’ve ran through the first ten invertebrate species that popped into my head and the antipredator mechanisms of all are pretty obvious (of course, my mind is most probably biased to my side of the discussion, and not an objective source of information in this – or any – instance). Many defences that we consider ‘obvious’ now may only be obvious in hindsight; I sure didn’t appreciate the aposematic nature of ladybirds as a child! (I can no longer work out whether I agree or disagree with your point)

        • Diane G.
          Posted November 13, 2012 at 1:30 am | Permalink

          They can also escape thru time (say, being nocturnal), space (say, being subterranean), fecundity (more offspring), employing defenders (aphids producing honeydew so ants protect them), varying life cycles (13-year cicadas)…The list is practically endless. I doubt there’s such as thing as what you propose–a completely vulnerable species. Not if there’s anything to this evolution biz.

          • thh1859
            Posted November 13, 2012 at 3:25 pm | Permalink

            Diane, thanks for your reply. I’ve had a metaphorical* Party card for 55 years which I would have to hand in if my position were that there is such a thing as a completely vulnerable species.

            It’s easily understandable why the attributes you list deter predators. I’m sure there are many more you know. The mystery for me is why the less endowed species survive.

            Putting my point, I hope, more clearly: Imagine a group of closely related species living in the same environment, eating the same food, equally tasty to predators, equally fecund and so on. In other words, and this is the essence, they are all equally vulnerable to predation. Question: Why do some have features clearly evolved to deter predators while others look as if they’re lying on a plate? The answer is that, by definition, those on the plate do have features that are equally effective against predators. But, it seems to me that those ‘hidden’ features are not studied. If they are, the results have never filtered down to me.

            The following example could be multiplied over a range of environments and species: Caterpillar (A) on an oak tree mimics a twig. Caterpillar (B) on the same tree doesn’t mimic anything, is not particularly well camouflaged nor has warning colours or other obvious indicators of predation deterrence. Both survive predators equally well. (A) mimics a twig to fool birds that would otherwise eat it. But what does (B) do to put birds off? My point now: I think that(B)’s conundrum is more interesting than twiggy’s and it’s study would lead to greater knowledge.

            *’Metaphorical’ because I’m not a card-carrying professional but I’ve been thinking about evolution for that long.

  5. IdoP
    Posted November 12, 2012 at 11:34 am | Permalink

    I have another theory. If the sexes are dimorphic to start with, then perhaps one sex is more likely than the other to acquire an initial mutation that makes that sex look more like the ‘model’ species. In other words, one sex is from the start ‘closer’ to the model in ‘morphospace’, if you like.

    • thh1859
      Posted November 12, 2012 at 3:16 pm | Permalink

      The most plausible theory I’ve heard so far.

    • Posted November 12, 2012 at 6:26 pm | Permalink

      Something like the neutral theory?I don’t think so because it can not explain why only one sex is mimic.

  6. rikkigumbs
    Posted November 12, 2012 at 12:15 pm | Permalink

    A similar pattern has been reported in SE Asian pitvipers; males of several closely-related species of Trimeresurus share red ‘warning’ markings whereas the females are typically more cryptic. The parallel evolution of the colouration in the males of the species suggests mullerian mimicry and is thought to be a result of sexual dimorphism in behaviour; active males benefit from warning colouration whereas sedentary females benefit from crypsis.

    This study does not investigate sexual dimorphism in activity patterns, microhabitat preferences, and/or predation rates (through models etc.), so I take the conclusions with a pinch of salt!

    The paper, if you’re interested:
    Sanders et al. 2006. Proc. R. Soc. B 273:1135-1141.

  7. Diane G.
    Posted November 13, 2012 at 1:32 am | Permalink

    I thought ecorocksmysocks suggestion from the last thread made a lot of sense:

    ecorocksmysocks

    Posted November 11, 2012 at 9:57 pm |

    My guess for the sexual dimorphism is that there is actually some psuedo-social / parental care behavior in some treehoppers. Females are known to stay on the same branch they oviposit onto and live alongside their offspring. The males are more mobile, I believe. Perhaps avoiding predators requires better aposematism for these males.

    Fits right in with the different niche hypothesis.

    • Diane G.
      Posted November 13, 2012 at 1:33 am | Permalink

      Add apostrophe ‘s’ after first “ecorocksmysocks.”

      One just can’t type that name enough…

  8. Posted November 13, 2012 at 1:47 pm | Permalink

    I don’t want to be nitpicky, but it should be buprestid (i.e. of the family Buprestidae), and not “bupestrid”.

  9. Posted November 13, 2012 at 3:23 pm | Permalink

    “That is, the females are genetically hard-wired to prefer a certain male appearance, while males will pretty much court anything.”

    If it moves, fuck it. :-)

    As Dave Mattacks from Fairport Convention explained comparing groupies with Kiss when the two bands met on tour: Kiss had really good groupies, and for us it was basically anything under 40 without a beard.


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