Keep your mimics close, but your mutualists closer: Mutualism and competition among Ithomiinae

While contentedly paddling around the river PLoS this morning, I happened upon a paper discussing niche convergence in the rainforest butterfly Family Nymphalidae . Diving into the article, I was pleasantly surprised to discover that Marianne Elias (et al), having been discontent with examining Müllerian mimicry from the usual perspectives, decided to study not only wing patterns and ring species dynamics but also the convergence of microhabitat in several neotropical species of butterfly;

"We showed that mutualism drives convergence in flight height and forest habitat, and that these effects outweigh common ancestry (which should lead related species to be more similar) and competition (which promotes ecological divergence). Our findings imply that species that benefit from one another might evolve to form more tightly knit local communities, suggesting that adaptation is a more important process affecting community composition than is commonly suspected."

That any two species, rather they be model and mimic species or co-mimic, should converge in terms of a behavior (such as flight pattern) didn’t seem all too surprising, nor did mentioning that adaptations could outweigh common ancestry; this at a very fundamental level would seem reasonable considering that biodiversity could be defined in terms of moving from, and breaking, the bonds of ancestral characteristics. However, a finding that co-mimics can converge to occupy the same microhabitat - now that’s something worthy of a second look!

Perhaps it’s a personal bias favoring competitive evolutionary drivers (which is by no means impossible) or some other inexplicable and diabolical thought process, but for some unknown reason, I found myself reminded of Sun Tzu’s advice (or was it Don Corleone’s?) regarding long term partnerships, “Keep your friends close, but your enemies closer.” In other words, I find myself harboring doubt as to just how deep this mutualism runs, and I’m curious as to the potential for apparent friends to become realized enemies; figuratively speaking of course.

Although certainly not in a position to argue to the contrary (or for that matter to even suggest it, wait…nevermind.), the idea that two genetically distinct species would develop a mutually beneficial mimicry (+,+) while occupying the same niche, the same microhabitat and while sharing (tough not reading as “competing for”) the same ecological resources seemed… Well it seems extraordinary.

Assuming that;
- they live in the same place (microhabitat)
- they’re converging morphologically (wing patterns)
- they’re converging behaviorally (flight characteristics)
- their larvae feed on the same plants (Solanaceae, almost exclusively)
- almost all males seek the same pyrrolizidine alkaloids for protection (bad bug taste)
- almost all males use the same alkaloids for use in pheromone signaling (come hither smell)

AND assuming that the only (recognized) mutual benefit is that they have the same predators (insectivorous birds; thereby sharing the cost of local predation)… Well it’s tough for me to wrap my head around… It’s as though some lepidopteran lawyers sat down and drew up a contract –

IN EXCHANGE FOR A 50% DECREASE* IN LOCAL MORTALITY, I HEREBY RENDER THE FOLLOWING TO THE INCOMING POPULATION OF POTENTIAL MUTUALISTS:
½ OF MY FOOD SUPPLY
½ OF MY LARVAE HOST PLANTS
½ OF MY CHEMICAL/ALKALOID SUPPLIES

FURTHERMORE, I RECOGNIZE AND ACKNOWLEDGE THAT THE POTENTIAL MUTUALISTS ARE NOT LIABLE FOR DAMAGES INCURRED FOR ANY TIME, ENERGY OR RESOURCES MISAPPROPRIATED ON MY BEHALF IN PURSUING PHEROMONE TRAILS, FLIGHT BEHAVIORS, WING COLORS OR WING PATTERNS OF WHAT I MISTAKENLY PERCEIVE TO BE POTENTIAL MATES, OR POTENTIAL RIVALS.

*VALUE BASED ON A “BAD DAY” FOR INCOMING MIMIC. YOUR ACTUAL PERCENT OF DECREASED MORTALITY MAY BE LESS.

There may be a light sarcasm effervescing from the text here, but I am sincerely having some difficulty in understanding why these habitat converging mutualists aren’t bunting heads and/or getting their antennae tied in knots?

Is there perhaps a temporal factor that I missed? If the mutualists utilize the same resource base, but only did so through varied circadian rhythms or timings, perhaps conflict (competition) could be minimized to the point of obscurity? It shouldn’t take much scheduled variance to make it worthwhile for both parties; does one species or the other prefer the early bird (or butterfly) special to evening seating? (On second thought, meals would be served on the providing plant’s timeline – which I would guess to be ecologically/adaptively linked to a specific species in order to maximize pollination or other benefit.)

Or, maybe there is some other unmentioned (or unnoticed by me) misfiring of apparent parsimony somewhere? Just because two species undergo convergence doesn’t necessarily require that they do so in a precise and complete manner. “Eye spot” patterns on the wings of mimics, for example, may appear as unappetizing as those of the model species and serve equally as well to dissuade would be predators – even if the spots aren’t the exact shade of brown, or of precisely the same shape, size or dimensions.

I realize that this blog is turning into an essay, but before closing there’s just one more minor note to be made - At the start, the article takes a markedly clear, and easily read look into the compounding effects of convergence by moving from homologies of wing pattern, through comparisons of flight height and then onto microhabitat convergence, but then in closing,

"Finally, our findings further support the idea of mimicry as a driver of speciation through ecological adaptation [59]. The implications of mimicry evolution extend far beyond the evolution of warning signals, with links to microhabitat use, including flight height and forest structure [24–26], but also to mate choice [60], flight physiology [61], and larval host plant use [24,27]. "

This seems to fit with the research and presented evidence, at least up to the “…but also...”

The last minute throwdown of mate choice, flight physiology and other “implications of mimicry” as inferred from various referenced studies, although quite possibly both legitimate and accurate, put my already swirling head into vertigo. If this process is indeed as I understand it to be, then I can’t help but think that such drives could only be found in localities exhibiting extremes of biodiversity, an excess of resources and a vast assortment of overlapping ranges and intermediary species; for example biodiversity hotspots such as those found in the Amazon where this study was conducted. How prevalent is this flavor of ecological convergence? And at what tempo and frequency does it operate? Is such a driver an Evolutionarily Stable Strategy – or does it possess a well concealed competitive underpinning?

In several places within the article, the authors state that this mode of adaptation “…is a more important process affecting community composition than is commonly suspected.” In regards to “common expectations” I readily concur, though admittedly if I were a model lepidopteran species, I wouldn’t be taking the Don up on his advice, rather I’d be keeping my recent mutualists closer than my closest of mimics.

Update: Please read George Beccaloni’s comment for additional clarification

Marianne Elias, Zachariah Gompert, Chris Jiggins, Keith Willmott (2008). Mutualistic Interactions Drive Ecological Niche Convergence in a Diverse Butterfly Community PLoS Biology, 6 (12) DOI: 10.1371/journal.pbio.0060300