Mutualism a.k.a Caterpillars Drugging Ants To Do Their Bidding

From the study - Figure 1. Attendant Workers of
Pristomyrmex punctatus standing on or around
Narathura japonica caterpillars

The manuscript is done! Submitted! Summer interns are finished. Boot up Normal Life Mode, please. Recommence blogging. So many good papers have come out during my hiatus. Where to start…where to start…

If you have read this blog for any amount of time then you will come across my fascination with ant manipulation, particularly zombification. This is why my cursor stopped over a new paper in Current Biology about caterpillars manipulating ants to do their bidding.

Let’s start with mutualism. This is a topic that I have visited in the past, and in ants for that matter. It’s a nice little relationship between species that involves an exchange of goods and/or services. In the natural world, this often means food and protection.

In this study, the researchers chose the Japanese oakblue butterfly (Narathura japonica), a lycaenid belonging to the Theclinae subfamily of butterflies. Many in this group are myrmecophilic, meaning they associate (often mutualistically) with ants in some way. The Japanese oakblue caterpillar has a specialized exocrine gland, the “dorsal nectary organ (DNO),” that is located on the seventh abdominal segment and is flanked by tentacle organs (TO). The DNO secretes sugar- and amino acid-rich honeydew while the TO secretes scents to “talk” to the ants. A “Come on down!” or “Danger, Will Robinson!” type thing. The ants tend to the caterpillars and keep them safe for a nice, sugary food reward. But is that all to the story? Obviously not or this post would end here.

To do this experiment, butterfly eggs and their associated ants (Pristomyrmex punctatus) were collected and reared separately. Then three test situations were set up with 50 ants per treatment:

1. “Experienced” ants – had free access to the caterpillars and their DNO secretions
2. “Inexperienced” ants – no caterpillar access, just some sugar soaked cotton balls
3. “Unrewarded” ants – had access only to caterpillars that had their DNO’s blocked (a little bit of clear nail polish goes a long way)

After 3 days in their test situation, 10 ants from each treatment were moved to Petri dishes that were set on pieces of white paper with a line on it to divide the dishes into 2 halves. After the ants acclimated to their new little plastic arenas, they were observed to see how many times they crossed the center line (“locomotory activity”). Also at the 3 day time point, ants and caterpillars were frozen in liquid nitrogen until their brains could be dissected out, specifically removing the optic lobes. Now, I’ve done some pretty small dissections, but those come nowhere close to ant brain removal! Wow, just wow. Once those itty bitty brains were out, they were processed for liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) for serotonin, dopamine, octopamine, and tyramine. Very simply, that means making an ant-brain-aerosol that is then separated and identified by component.

They found that experienced ants had significantly less locomotory activity than the other two groups. So what does an ant walking, or in this case not walking, across a line even mean? Well, the fact that the ants are staying put signals that they are “standing guard” for the caterpillars. Okay, let’s say that standing means guarding, how do we know that this is really caterpillar-related and not just standing there? Well, first of all, it was only the experienced ants that did this. Second, the researchers observed that the caterpillars often “everted their TOs,” meaning that they turned them outward. This is typically a response the caterpillar makes when it is attacked by a predator – “Raise shields!” Experienced ants responded differently than the other two when they saw this caterpillar behavior in that they responded aggressively. This aggression is a response to the caterpillars’ alarm, one that has the ants defending against the predator. The fact that only experienced ants had these responses suggests that something in the DNO secretions is eliciting these defense behaviors.

So what is it about these secretions? That’s where the LC-MS/MS comes in. Biogenic amines are known function as neurotransmitters, neuromodulators, and/or neurohormones. This means that they can modify behavior in insects. DNO secretions contain biogenic amines. This analysis showed that experienced ant brains had low dopamine levels. Now, that’s important because dopamine has been shown to be involved in both locomotory activity and aggression in well studied organisms like fruit flies. Starting to see some links here, yes? To confirm the linkage, ants from each treatment were given reserpine, a small-molecule inhibitor that depletes dopamine but not serotonin in the brain. This test resulted the same behaviors, but the LC-MS/MS showed increased dopamine and serotonin in the ant brains. So same but different.

There is another aspect to consider: Who loses if the mutualism goes away? The honeydew is not the sole source of nourishment for the ants. They can leave and be still be fine. The caterpillar has much more to lose than the ant (its life via predation). So the caterpillars must be doing something besides sugar-loading their ants.  This is where the caterpillar gets sneaky - finding a way to make their ants to both stick around and defend against predators. As the authors put it, they they insert “manipulative drugs [into the honeydew] that could function to enforce cooperative behavior…from attendant ants.” Put that way, I’m okay calling it “ant mind control.”


Hojo, M., Pierce, N., & Tsuji, K. (2015). Lycaenid Caterpillar Secretions Manipulate Attendant Ant Behavior Current Biology DOI: 10.1016/j.cub.2015.07.016

p.s. The supplementary materials have a nice little video of ants in lined Petri dishes.

(image is Figure 1 from the above paper)

Higgs to Heavy Metal

What would the Higgs discovery sound like as a heavy-metal song?

As it turns out, pretty good.


Take two figures of Higgs Boson data and turn it into music and you get this:


See more at:

http://cylindricalonion.web.cern.ch/blog/201504/what-would-higgs-discovery-sound-heavy-metal-song

Did Someone Say Something?

Me...

via https://goo.gl/CVQ3VU

Why Do You Research?

All true...

(via Enzo Life Sciences and Trust me, I'm a Biologist, respectively. The latter doesn't cite an original source so if you know it then please let me know.

The Science of Beer Googles

It's Friday and you know what that means: Happy Hour!

Here's little infographic for your viewing pleasure all about the science of beer googles.

by NowSourcing.

From Visually.

- See more at: http://visual.ly/science-beer-goggles#sthash.X1vlduXe.dpuf

The Birds of Academia

Errant Science Comics gets it spot on (pun intended) with this little comic.

(via Errant Science on tumblr)

Vampire Plants: Sucking Life into the Community

I would like to introduce you to the yellow rattle (Rhinanthus minor). This little plant has pretty yellow flowers that belie a dark secret: it is a vampire plant. Okay, technically, it is a hemiparasite. The yellow rattle has green, photosynthetic leaves to make its own food, but its roots latch onto those of nearby plants to steal their water and nutrients. See? Vampire. I guess you could say that it hasn’t gone full parasite. So what’s wrong with a little leaching of material from your neighbor? Must be like stealing cable, right? Well, not exactly. Hemiparastitic plants can have impacts that are highly disproportionate to their small size. By stealing water and nutrients, they reduce the host plants’ photosynthetic rate and biomass, making them less competitive in their habitat. When you are less competitive in a highly-competitive environment then you don’t last all that long. But this is looking at it on an individual, plant-by-plant, level. Let’s scale it up a bit.

We’ll start by following the logic trail. Hemiparastic plants are widespread in ecosystems such as meadows where they attach to dominant grasses (in this case, dominant meaning numbers). As individual grass plants are weakened or killed, overall grass biomass decreases. When the dominants are gone there is more room for other types of plants to move in, increasing species richness. More types of plants attract more primary consumers (like insects) to feed on them. More insects attract more secondary consumers, and on and on. All in all that one little plant has caused indirect community level changes. A new paper by Hartley et al. in Ecology looks at how this one little plant can affect multiple trophic levels (or levels in the food chain).

The researchers laid out 13 blocks, each containing four 1 by 1 meter plots, in a field site in Sussex, UK. Three out of those four plots were allowed to be naturally colonized by yellow rattle. These “infected plots” were then randomly assigned to one of three treatments: yellow rattle removed, present, or enhanced. Then, during maximum vegetation and invertebrate abundance, plots were censused. Counts, ground cover, vegetation height, and species richness were measured for plants. Invertebrates were sampled with a Vortis suction sampler (a.k.a. bug vacuum) and hand counts. They were identified and classified into their trophic levels: herbivore, predator and detritivore.

They found that the density of yellow rattle had pronounced effects on the plant community. Grass cover was lower, plant diversity higher (although not richness), and plant height lower in plots where the hemiparasite was present or enhanced. Going up a trophic level, sap-feeding insects (Hemiptera) increased by 130%, caterpillars (Lepidoptera) by 217% and weevils (Curculionidae) by 188%. Whoa! Going up another level to the predators, spiders and harvestmen (Araneae and Opiliones) increased by 142% and bees and wasps (Hymenoptera) by 180%. Continuing with the whoa! Going down to the detritivore groups, wood lice (Isopoda) went up by 116% and springtails (Arthropleona) by 58%. Of course, not all animals fit into a single trophic group, but as you probably guessed by the trend you’ve been seeing, those groups went up too. Slugs and snails (Gastropoda) by 103% and mites (Acari) by 57%. That’s quite a list, but not everything increased. There were some non-significant groups including flies (Diptera), rove beetles (Staphylinidae), and springtails (Neelipleona and Symphypleona). It is also important to note that these numbers are from enhanced compared to removed plots. The present plots had increases in many of the same categories, though not as dramatic.

In scientist-speak these results showed that “the manipulation of a single sub-dominant plant species causes substantial changes in the abundance and diversity of organisms across four trophic levels in individual plots in a complex grassland community.” Basically, they demonstrated that grasses went down and just about everything else went up just by the addition of one, small and less numerous hemiparasite. This one little plant species caused striking (their term), rapid changes in the community. The less obvious effects of yellow rattle were how the effect was actually occurring. Was it on plant community composition or plant quality or stand height? The authors cite a greenhouse study that showed insects to prefer parasitized to unparasitized grasses when given the choice. Add to that that it is known that parasitic plants impact nutrient cycling, especially enhancing the nitrogen content of vegetation overall. This suggests that it may be plant quality that is the driving mechanism. So the observed, large increase of sap-sucking invertebrates, particularly the Hemiptera, with altered plant quality makes sense as does the increase in detritivores with newly nutrient-rich litter. At this point, perhaps you have considered the impact of these plant-attacking invertebrates as well. Good on you. They are probably themselves doing some damage on and affecting change on the plant community, but this work doesn’t go into that. Time for another study I think!


Hartley, S., Green, J., Massey, F., Press, M., Stewart, A., & John, E. (2015). Hemiparasitic plant impacts animal and plant communities across four trophic levels Ecology DOI: 10.1890/14-1244.1



(image via Urban Butterfly Garden UK)