Over the years I’ve written here about several kinds of mimicry. The most common subjects have been Batesian mimicry, in which the evolutionary scenario involves three species: an easily identifiable and noxious or toxic model, a predator that learns (or has evolved) to avoid the model (signal receiver), and an edible mimic that evolves to resemble the model. You can easily see how an edible species would leave more offspring if it accumulated mutations that made it resemble the model, for it would be avoided by the predator more often. Here’s one example: a harmless and edible fly that mimics a bee, almost certainly to avoid bird predation:
The second form of mimicry I’ve often discussed is Müllerian mimicry, in which a group of easily identifiable species, all of them toxic or unpalatable, evolve to “converge,” or resemble each other. Such mutual resemblance gives members of the similar-looking species an advantage, for it’s easier for the predator to learn and avoid one pattern rather than several.
Here’s one example, a group of butterflies in the genus Acraea, Such mimicry needn’t just involve one group of organisms: similar patterns have been described in mimicry “rings” that involve butterflies, beetles, “true bugs” (Hemiptera), and wasps. All of them can achieve some protection from predation by adopting similar colors and patterns.
For both sorts of mimicry to evolve, the signal receiver must encounter both model and mimic, so they all have to live in the same area. (There is one scenario in which model and mimic can live in different areas, but I’ll leave you to figure out how that works.)
A classic case of what was thought to be Batesian mimicry involves moths or butterflies that look like bees or wasps. The resemblance between model and putative mimic is sometimes astonishingly precise. Here’s a moth that for all the world looks like a wasp. Note all the features of the moth that have evolved to resemble the wasp: the wings are clear, are folded longitudinally like those of a wasp, the body is narrow, striped and colored like that of a wasp, and has a very thin constriction between thorax and abdomen (“petiole”) that regular moths don’t have. Believe me, if you saw this, you would mistake it for a wasp and avoid it, just like predators do.
But this may not be a classic case of Batesian mimicry, or so claims a new paper in Ecology and Evolution by Michael Boppré et al. (reference below, free download). There may not be a “signal receiver” that has learned to avoid the moth because it resembles a stinging wasp. Rather, as Boppé et al. suggest, the predator is said to be the “model” itself: a predatory wasp, and the scenario involves innate avoidance rather than learning.
Here’s how it works. The wasps in question, yellowjackets (wasps in the genera Vespula and Dolichovespula) are abundant social insects that make their living as predators (and scavengers) of other insects. Here’s a yellowjacket attacking a praying mantis:
We know that on their hunting expeditions yellowjackets won’t attack members of their own species; that, after all, would be a maladaptive behavior, since members of your own species may well be members of your own nest. Further, if you attack another yellowjacket, you yourself could get stung to death.
Besides this observation, the authors noted that the mimicry between palatable moths like the one above and the yellowjackets is astonishingly precise: far too precise, they say, to have evolved to fool birds. (They argue that birds will avoid a prey simply by longer-distance recognition of general patterns like color and striping.) Here’s are two more examples of the precision (read the caption); the first photo shows two species of wasps and a moth. Can you tell which is which? (Hint: the antennae are a giveaway).
And here’s another example of precise mimicry of body shape, color, and pattern. Again, inspection of the antennae shows that the wasp is on the left and the moth on the right:
The authors’ hypothesis is both clever and simple–so clever and simple that, in fact, I’m surprised that nobody has suggested it before. It is this:
The moths aren’t mimicking the wasps to fool birds; they’re mimicking the wasps to fool the wasps themselves. That’s because the wasps are predators, and will avoid attacking any insect that looks very similar to their nestmates, because you don’t get a food reward by attacking another wasp.
In other words, the predatory wasps have an innate aversion toward attacking animals whose appearance they’ve evolved to recognize (presumably because they’re eusocial and live in groups where they help each other); and the “model” takes advantage of this, evolving a precise resemblance to the wasp. The authors like this hypothesis because they think that the wasps scrutinize potential prey much more closely than do birds, a trait that “forces” the models to evolve a very precise resemblance—a resemblance that, they say, couldn’t be explained by the scrutiny of birds alone.
The difference between this and “classic” Batesian mimicry is that a.) the signal receiver and the model are the same species: the wasp; and b.) there is no learning involved: wasp recognition of its self-pattern is an evolved trait, probably evolved the same way most animals develop a recognition of members of their own species.
This hypothesis is clever, and when I read the paper I had the same reaction that Thomas Henry Huxley had after reading Darwin’s On the Origin of Species: “How extremely stupid not to have thought of that!” After all, we’ve known about mimicry since shortly after The Origin was published, and yet it took people over 150 years to come up with this simple idea.
Yes, it’s a clever idea, and may well be right, especially if the chance of predation by a wasp is much higher than by a bird. But is the hypothesis true? What’s the evidence for it?
Sadly, there isn’t much yet beyond the authors’ speculation that mimicry this precise could not be mediated by visual bird predation, but requires the acute vision of a wasp. Further, the authors describe yellowjacket wasps preying on moths in the wild but not on other yellowjackets or on moths mimicking yellowjackets. (This leads to the idea that the similar black-and-yellow striped pattern of different wasp species could be a case of “quasi-Müllerian” mimicry, but one in which they’ve evolved to resemble each other because it’s injurious to attack each other. In this case again, there is no predator learning involved, nor any signal receiver.)
Now the authors’ hypothesis doesn’t rule out that this is also a case of true Batesian mimicry: that selection occurred both by wasps avoiding attacks on prey that look like themselves, and also by birds having learned to avoid attacking anything that looks like a wasp. Both factors could be involved, and I suspect are. But how do you test whether yellowjacket predation was a driving force of selection?
The authors note that it’s hard to test that:
However, it seems likely that (2) general, visually oriented predators such as birds are additional selecting agents shaping similarity of other insects to wasps. Thus, in “wasp mimicry” two sorts of selecting agents (with different life-styles) are plausibly acting. Then, the relative abundance of predatory wasps (individuals and species) that recognize look-alikes as non-food versus various predators that learn through experience could explain the accuracy and non-accuracy of potentially profitable mimics. We would observe combinations of innate protective masquerade and learned Batesian and Müllerian mimicry, and recognize different sorts of selecting agents, namely those which respond innately and those which learn by experience. Thus, accurate mimics would be protected against both wasps and birds, whereas inaccurate mimics would be protected mainly against educated birds (which to a certain extent generalize a learned pattern) but not so well against wasps. In theory, proof could only come from studies in habitats where wasps prey on insects but learning predators do not occur—however, such places cannot be found.
But you could think of other tests. For example, put mimetic moths and non-mimetic moths in a cage with yellowjackets. If the mimetic moths are attacked less often, that would support the authors’ hypothesis. Or you could efface certain parts of the mimics’ pattern with paint and see if they get attacked more often. I’m sure that, with some thought, other tests are possible. Here’s one I just thought of: if a palatable moth resembles something that doesn’t attack it, like a bumblebee, then it’s likely that this is a case of true Batesian mimicry rather than the new form of mimicry (not given a new name) described by Boppré et al. And in such cases you’d expect the mimicry to be less precise than that of yellowjacket mimics, because (according to the authors), birds don’t need to look as closely at potential prey as do yellowjackets.
A final point: the authors note that sometimes the moths themselves may be toxic: some species eat plants and, like monarch butterflies, sequester the unpalatable alkaloid compounds in their bodies. If this were the case (and that would be relatively easy to test), then the mimicry becomes more complicated, and harder to understand. Why wouldn’t the wasp predators evolve to recognize whatever pattern a toxic moth had in the first place—that is, why did such precise mimicry evolve? Perhaps in this case it would prevent the initial strike of the moth that kills the mimic, even if the moth then recognizes that the mimic isn’t fit to eat. Or, the authors could be wrong about how birds recognize potential prey: birds may be more sharp-sighted than we think.
At any rate, this is a novel hypothesis and well worth considering in the case of mimics that look like their predators. I was once fooled by one of these moths that had gotten into my house in Maryland, and I had to get my fly net to catch it as I was afraid of getting stung. Only after I caught it did I realize the ruse. If a moth can fool someone who works with insects, it’s likely that it can fool a predatory wasp or bee.
h/t: Matthew Cobb