Double the mutualists, double the fun?

ResearchBlogging.orgFor all living things, information is critical to survival. Where’s the best food source? Is there a predator nearby? Will this be a good place to build a nest? It probably shouldn’t be surprising, then, that lots of animals do what humans do when faced with a host of hard-to-answer questions—they take their cues from their neighbors.

Red-backed shrikes place their nesting sites near where other shrike species have set up territories. Many bird species recognize each other’s predator alarm calls, and respond appropriately. And a new natural history discovery published in the latest issue of The American Naturalist shows that treehoppers let one species of butterfly know where to find ants that will tend its larvae [$a].


The ant-tended butterfly (Parrhasius polibetes, above) looks for ant-tended treehoppers (Guayaquila xiphias, below) to know where to lay her eggs. Photos from Kaminski et al. (2010), figure 2.

The treehoppers help out the butterfly inadvertently, because both of them are dependent on a common resource: ants. Like many true bugs, treehoppers make their living sucking the sap of a host plant. This gives them a surplus of simple sugars and water, which they excrete as “honeydew” to attract ants for protection. As it happens, the larvae of the butterfly Parrhasius polibetes do the same thing—so the new study’s authors hypothesized that P. polibetes females might prefer to lay their eggs on plants where treehoppers were already present, since those would likely already have ants ready to protect butterfly larvae.

To test this, the authors set up experimental pairs of host-plant branches, one occupied by ant-tended treehoppers, and one not. They excluded ants from accessing the unoccupied branch with Tanglefoot, a water-resistant glue used in insect traps. After 48 hours, they checked the experimental plants for newly-laid butterfly eggs, and found that P. polibetes was both more likely to lay eggs, and laid more eggs at a time, on branches occupied by treehoppers.

To assess the fitness benefit of laying eggs on treehopper-occupied plants, the authors compared the survival of newly hatched P. polibetes larvae artificially introduced onto branches occupied by treehoppers to the survival of larvae introduced to branches unoccupied by treehoppers (and with ants excluded, again, using Tanglefoot). The larvae placed with treehoppers had substantially better odds of survival—about six times better.

These two experiments confound the effect of treehoppers with the effect of ants, however—so the authors performed one additional experiment. In this one, they set up paired branches with and without treehoppers, but allowed ants to reach both the occupied and unoccupied branches—and the general result from the earlier experiment held. Larvae placed near treehoppers were three times more likely to survive for the duration of the experiment even when larvae placed on a branch without treehoppers were able to attract ants on their own.

So it looks like P. polibetes is able to freeload on the treehoppers’ ant-attracting efforts, and benefits from that freeloading. What effect does that freeloading have on the treehoppers, or the ants, or the host plant? It’s hard to say based on the data presented in the current paper, but I’d guess that the treehoppers don’t lose much—in fact, they might gain from having another ant-attracting insect nearby, just as the butterfly larvae do. Similarly, it’s probably helpful for the ants to have more honeydew-producing species in the same location. It’s almost like that commercial for … what was the product?

(I’ll leave it to you, dear reader, to decide which insects correspond to which gendered pair in that video.)

I’d think, though, that this pile-on isn’t so good for the host plant, if plants already hosting treehoppers are more likely to have to deal with butterfly larvae, too. Untangling all the different ways these four species—ants, treehoppers, butterflies, host plants—exert direct and indirect natural selection on each other should keep the authors busy for a long time to come.

References

Hromada, M., Antczak, M., Valone, T., & Tryjanowski, P. (2008). Settling decisions and heterospecific social information use in shrikes. PLoS ONE, 3 (12) DOI: 10.1371/journal.pone.0003930

Kaminski, L., Freitas, A., & Oliveira, P. (2010). Interaction between mutualisms: Ant‐tended butterflies exploit enemy‐free space provided by ant‐treehopper associations. The American Naturalist DOI: 10.1086/655427

Magrath, R., Pitcher, B., & Gardner, J. (2007). A mutual understanding? Interspecific responses by birds to each other’s aerial alarm calls. Behavioral Ecology, 18 (5), 944-51 DOI: 10.1093/beheco/arm063

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Why do butterflies have four wings?

ResearchBlogging.orgIn this week’s PNAS is a tidy result that demonstrates what you ca get away with when you study invertebrates: butterflies and moths can still fly if their hindwings are amputated, but they can’t take evasive action [$-a]. That summary tells you just about all you need to about the reported experimental result; but the rest of the article has some interesting speculation.


Photo by me.

The authors, Jantzen and Eisner, start from the premise that the large, showy wings of butterflies should (very generally) make them major targets of predation. They note, however, that butterflies are also marked by highly erratic flight – an extreme maneuverability that makes it difficult for a predator to guess where a butterfly is going to be in the future based on its current trajectory. Maybe showy wings actually act as a kind of inverse protective coloration:

A bird, we suggest, could learn or inherently know that brightly colored airborne prey, discernible from afar, is not worth the chase. Too elusive to catch and, because of their [wing] scales, too slippery to hold … Birds might simply write butterflies off, and … relegate them all to the category of the undesirable, treating them as they treat noxious insects that they disregard.

Jantzen and Eisner’s experiment, in which they amputated butterflies’ hindwings, confirms that butterflies can still fly without the second pair of wings, but fly less erratically. Does the result confirm the authors’ major hypothesis, though? I’m not so sure. There are plenty of other reasons to have big, showy wings – mate attraction, or as placement for eye spots to fool predators – and plenty of butterflies and moths are comparatively small and non-showy. Jantzen and Eisner’s hypothesis smells more than a bit like adaptive story-telling, though it provides some food for thought.

Reference

B. Jantzen, T. Eisner (2008). Hindwings are unnecessary for flight but essential for execution of normal evasive flight in Lepidoptera PNAS, 105 (43), 16636-40 DOI: 10.1073/pnas.0807223105

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