Thursday 2 April 2009

I swear, by my life and my love of it, that I will never live for the sake of another ant...

Not Exactly Rocket Science reports on a fascinating bit of evolutionary biology. Some ant species make slaves of other ants by raiding their nests, killing the adults and stealing the larva. The larva are then made to serve as worker slaves in the slavers' anthill. But, sometimes the slaves rise up. The genus Temnothorax sometimes rebels against their P.americanus overlords.
Achenbach and Foitzik collected 88 colonies of the slave-making P.americanus ant that had abducted workers from three species of Temnothorax. They found that the workers clearly care for the larvae, and nearly all of them were raised until their pupated. But at that point, the slaves' behaviour changed dramatically, taking on a more homicidal bent.

Two-thirds of pupae died before they hatched. The mortality rate was even higher (83%) for pupae containing queens, but very low (3%) for those containing males. The duo saw that the captives were deliberately killing the healthy pupae. In about 30% of cases, as in the photo, the workers would gang up to literally pull the developing ants apart. Another 53% of the pupae were killed by neglect, by workers who moved them out of the nest chamber.

These murders were solely the acts of the slaves. No P.americanus worker ever lifted a mandible against its own pupae. Nor are the deaths a reflection of a generally poor standard of care on the part of Temnothorax. In their own colonies, the majority of pupae hatched, with just 3-10% dying before that happened.
If the enslaved Temnothorax are close relatives to nearby Temnothorax colonies that could be targeted by P.americanus, the uprising increases the chances of those colonies surviving. Downloading the full article, we find a few potential strategies for Temnothorax. Escaping and returning home wouldn't work because they're enslaved as larvae, have P.americanus's colony odor and won't be recognized as friendly by Temnothorax on their return. An alternative strategy of going on strike - stopping working - may have negative consequences in forcing P.americanus to conduct new slave raids; the strategy then takes a long time to work. Rebellion is the only useful strategy.

Unresolved questions for me:
  • It seems odd to me that other P.americanus wouldn't attack the slaves killing their pupae, but that could be explained by ant defenses being triggered by chemical smells identifying ants as outsiders, and the slaves don't trigger those defenses.
  • Why don't Temnothorax destroy all of the P.americanus larvae? They kill the females and workers but leave the males alone. I can buy that there are higher benefits from killing the females and workers, but what's the constraint against which they're optimizing? Are there not enough of them to kill all the larvae in the time available?

Interesting stuff.

Reference: Achenbach, A., & Foitzik, S. (2009). FIRST EVIDENCE FOR SLAVE REBELLION: ENSLAVED ANT WORKERS SYSTEMATICALLY KILL THE BROOD OF THEIR SOCIAL PARASITE Evolution, 63 (4), 1068-1075 DOI: 10.1111/j.1558-5646.2009.00591.x

HT: ScienceBlogs


  1. I haven't read the article, but the question that leaps out at me if why enslavement occurs (i.e. is selected for) at all. Wouldn't a P.americanus that spends all that energy enslaving ants only to have their offspring destroyed at the worst possible moment find itself being outcompeted by a variant P.americanus that follows a non-enslavement strategy?

  2. Matt,

    They probably will, over time. Or they'll (P.americanus) evolve better strategies to manage their slaves.

    The situation seems sub-optimal because we're in the middle of history, not at the end of it. That includes the evolutionary history of ants :)

    Having said that, the genetic traits that cause Temnothorax to "rise up" are just frankly bizarre. But fascinating.

  3. Interesting indeed.

    "If the enslaved Temnothorax are close relatives to nearby Temnothorax colonies that could be targeted by P.americanus, the uprising increases the chances of those colonies surviving."

    It could also be that being rebellious when enslaved is a similar strategy to being toxic, discouraging the slave-making ants from enslaving that particular species.

  4. The uprisings seem to be an evolved response to a longstanding strategy of enslavement. Vader is right.