Slow and steady wins the race, in nature at least, because speed often involves spending too much energy in a short amount of time. But some vital activities – such as capturing prey, defence and reproduction – do require sudden, explosive actions through ballistic movement.
What is ballistics?
You’ve probably heard the word 'ballistics' on TV shows when police match bullets to a gun, but ballistics is actually the study of how any object – including a projectile launched by an animal, plant or fungus – is rapidly propelled by mechanical force.
What do animals fire?
Solid projectiles are typically body parts (very few species are able to launch foreign objects, such as spears thrown by humans). Though porcupines shooting quills is a myth, some animals can launch spines: Cnidaria, the diverse group of aquatic invertebrates that includes anemones, corals and jellyfish, have cells called cnidocytes that sting food or foes by firing needles laced with toxins.
Other animals shoot liquid, including bodily fluids that move along pipes and exit via nozzles: spitting cobras spray venom, for instance, while bombardier beetles eject hot, irritating chemicals (making it possibly one of the most disgusting animals in the world).
By contrast, archerfish (which we named as one of the weirdest fish) hunt by firing shots of water to dislodge arthropods or even lizards from overhanging vegetation several metres away. The fish squeezes its gills to force water through a channel formed from an oddly-shaped tongue and a groove in the roof of its mouth, which creates a stream that becomes a bullet-like blob as it hits the target.
How are projectiles launched?
Ballistic movements are generated by storing force as elastic (potential) energy then releasing it as kinetic energy – like firing a rubber band by stretching it back then letting go. In animals, this is powered by muscle contraction and involves elastic fibres such as collagen.
But in plants and fungi, ballistic forces are typically powered by hydrostatic (turgor) pressure against the elastic walls of their cells. This is driven by osmosis, the movement of water in or out of cells so that they swell or shrink, often from absorption or evaporation. Releasing that pressure can launch projectiles, such as when squirting cucumbers explode and spread seeds in jets of slimy mucilage.
What do plants shoot?
Seeds, spores or pollen that carry reproductive cells. These future offspring are shot away from parents to reduce competition for resources, and may be eaten or catch the wind to be dispersed even further. A plant structure is often held in shape by water in its cells and deforms as hydrostatic pressure is released, moving any attached parts. For example, the fruit of hairy bittercress consist of turgid strips that, when dry, suddenly coil to catapult seeds away. This is explosive seed dispersal.
Shooting is challenging at microscopic scales as gas molecules become obstacles, which turns thin air into thick soup. To temporarily overcome drag, things must be shot at high velocity. In flowers of the white mulberry tree, filaments are held by threads that break when dry, causing a filament to spring outwards and fling the pollen it contains – like bowling a cricket ball – up to 6.6cm away at more than 600kph.
And what do fungi launch?
Like plants, fungi rely on ballistics for reproduction. The spores of most sac fungi (ascomycetes) shoot from the tip of pods, driven by pressurised sap. The other major group, club fungi (basidiomycetes), use condensation: each spore is attached by a peg and both collect water – when droplets merge, the centre of mass shifts and causes a spore to jump into the air. This transfer of momentum allows a mushroom to release as many as 30,000 spores per second, which equates to a total of billions over the lifetime of the fruiting body.
One unique case is the artillery fungus. Each of its 2mm-wide fruits rests on a cup whose surface flips from concave to convex, like the upward pop of a metal cap when you open a glass jar. This ‘snap-through buckling’ mechanism lets fungi propel fruits an astonishing distance of 6m, towards nearby plants and away from the ‘zone of repugnance’ that surrounds dung, making it more likely that a herbivore will swallow the fruits. The fungi pass through the gut and continue a reproductive cycle – nature’s never-ending race.
More wildlife stories from around the world
- “We’re not trying to recreate a species from ancient DNA scraps”: One of Earth’s rarest animals could be brought back from the edge of extinction
- “They eat everything – anything that fits into their mouth”: how scientists solved a deadly issue in the deep valleys of Yosemite
- Tuna carcass tied to seafloor sparks unearthly feeding frenzy
- Scuba divers went into a tidal cave in Bermuda and found a segmented creature lurking in the darkness
Main image: spitting cobra. Credit: Getty
