|From the study - Figure 1. Attendant Workers of |
Pristomyrmex punctatus standing on or around
Narathura japonica caterpillars
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:
- “Experienced” ants – had free access to the caterpillars and their DNO secretions
- “Inexperienced” ants – no caterpillar access, just some sugar soaked cotton balls
- “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)