“My sentence is for open War.”
John Milton, Paradise Lost (Book II, Line 51)
Most termites, like ants and bees, live in strict caste-based societies of closely-related individuals. Tasks like reproduction, work, and colony defense are divided among relatives within a strict hierarchy. Queens breed with drones to populate the colony. Her daughters include workers, soldiers, and the next generation of queens. She has a handful of sons too, but they don’t come in to this story.
For many ant and termite species, a soldier’s primary weapon are her mandibles, the sharp, strong, pinching, and crushing appendages of her jaw. Combined with her enormous size and strong armor, a mature soldier is a rather menacing sight to behold (I’ve inserted a picture of a few termite soldiers below). But, among some species, soldiers carry a second weapon, a weapon of last resort. In these species, a soldier can activate a series of glands along her back, causing a sticky (and presumably toxic) substance to burst from her back. She dies, and hopefully the explosive load also harms her foes.
Queens make soldiers to defend the colony. While it may not sound very efficient for natural selection to equip those soldiers with the insect equivalent of suicide belts (it takes a lot of energy and time to make a soldier, after all), it makes sense in certain situations. If the situation is hopeless (that is, if the soldier is overwhelmed, either by sheer number of enemies or the soldier’s increasing age) and she’s nearing defeat, better to give up her life and hope to take an enemy with her, rather than simply be mortally wounded by her foes. Besides, in that case, soldiers are still being soldiers: defending the colony at all cost.
What doesn’t make sense is to equip workers with a similar suicide weapon. True, among some termite species, workers rush to defend the colony when the situation is grim. But, lose a worker in battle and you lose an incredibly valuable mover and shaker for the colony. A termite worker is the ultimate jack-of-all-trades; she does everything form obtain food and supplies to repairing, maintaining, and cleaning the colony’s home. She’ll also rear the next generation of workers, soldiers, queens, and drones (those silly males that I promised wouldn’t come into this story, and you won’t hear of them again). The loss of a soldier weakens the colony in a time of war. The loss of a worker weakens the colony. Period.
Despite the increased value of workers toward a colony’s everyday maintenance and survival, ecologists and entomologists have documented soldier-like suicide defenses among workers in at least several termite species. Superficially, the similarities to soldier-based suicide defenses are striking. A cornered worker surrounded by enemies. Suddenly, her back swells and bursts, releasing a drop of liquid. She dies, and the caustic substance she extruded appears to harm her attackers. However, not all fighting workers resort to this suicide tactic. Why do some workers choose this route and others do not? How does the colony partition which workers are equipped with this suicide belt? How does it work, and what does it do to the termite’s enemies?
An international team of scientists from the Czech Republic, Belgium, and Japan decided to address these questions in a termite species from the tropics. They chose a species with workers known to take part in this “bursting” activity, Neocapritermes taracua. Led by J. Šobotník and T. Bourguignon, they collected workers from a single Neocapritermes colony in French Guiana and examined the potential for workers to activate their suicide responses in combat.
Their findings are striking, and should give us new respect for the value of age in our society. Šobotník, Bourguignon, and colleagues deciphered the age of workers by looking at the wear and tear on their mandibles. Worker mandibles aren’t as large, impressive, and menacing as the mandibles of soldiers but, since termites can’t regenerate their mandibles or repair damage to them by molting, mandibular shape, size, and sharpness are good indications of an individual’s age. Based on examination of the mandibles and observation of workers of all ages in encounters with workers from a hostile termite species, they concluded that older workers were more likely to engage multiple enemies in combat, and were more likely to end that combat quickly by “bursting” their backs to release that drop of sticky fluid. The technical name for this behavior is autothysis (literally “self sacrifice”). You can see some examples from Neocapritermes here.
Šobotník, Bourguignon, and peers also noted a physical difference between older and younger workers beyond mandibular wear and tear. Older workers have a pair of blue spots on their backs, while younger workers do not (see picture below). Closer examination and dissection of the blue spots on old workers showed that they’re storage compartments housing a blue crystal-like compound. The crystal-like compound contained at least one unique protein, and neither the blue crystals nor the proteins were found in compartments on the backs of younger workers.
The back pouches (or backpacks, as some like to call them) holding those mysterious blue crystals also harbored connections to the worker’s salivary glands. Šobotník, Bourguignon, and colleagues hypothesized that these pouches, combined with the glands, may be the suicide weapons of the older workers. The blue crystals within them, when triggered to react with secretions from the salivary glands, may start a chemical reaction creating that sticky fluid, which oozes from the ruptured backs of these workers. Younger workers, which lack blue crystals, can also “burst” on occasion, releasing a drop of sticky fluid. They do so less often (since they’re less aggressive toward enemies), but it still happens. So, the blue crystals obviously aren’t needed for the act of “bursting.” What are they good for, then?
As it turns out, the blue crystals are essential for the suicide weapon’s primary purpose: taking out the enemy. Šobotník, Bourguignon, and their team took drops of the sticky fluid from “burst” old and young workers, and tested the effects of those drops on workers from a hostile termite species. Drops from old workers (with blue crystals in their backpacks) were extremely toxic, often causing death or paralysis among workers from the hostile termite species. Drops from young workers (lacking those blue crystals) weren’t very toxic at all. But, when the scientists removed the blue crystals from the older workers before bursting, their drops became as harmless as those of their younger counterparts. When they took blue crystals from older workers, stuffed them into the backs of young workers, and then let those young workers burst, the drops from these young workers became more toxic and deadly to workers from the hostile species. These transplant experiments pretty much seal the deal: those mysterious blue crystals in the backpacks of older workers make their suicide attacks against enemies more effective compared to their younger counterparts.
Thus, among Neocapritermes termites, it appears that workers steadily accumulate a new role in the colony as they age. Older workers, with worn mandibles and decreasing practical “worker” value to the colony, take on a new role as citizen soldiers. Becoming army reserve, they show much more aggression toward enemies compared to young workers. They’re also more likely to take part in suicidal “bursting” attacks than young workers. This change in behavior is logical, since old workers also accumulate blue crystals in their back pouches, increasing the toxicity of the fluids they extrude when taking part in autothysis. For an older worker, with mandibles worn and effectiveness in its traditional worker tasks waning, this new role as a citizen soldier might help preserve its usefulness to the colony: a last line of defense against mortal enemies. As I scientist, I try to avoid anthropomorphizing other creatures too much. Still, there’s another word that keeps popping into my head as I think about these experimental results: brave.
Or perhaps it’s more appropriate to consider the phrase Šobotník, Bourguignon, and colleagues use to open the recent publication of their findings in Science: “We send our young men to war; ants send their old ladies.”
Termites, too. And they send them well armed.
- Šobotník J, Bourguignon T, et al. 2012. “Explosive Backpacks in Old Termite Workers.” Science. Volume 337: 436. [Subscription required to access full text, though supplementary data are open access.]
- Video of Neocapritermes taracua worker’s suicide attack against three Embiratermes neotenicus workers: available via Science magazine or YouTube.
Image credits: R. Hanus and Šobotník J, Bourguignon T, et al. 2012.