Most animals fight for themselves. But some live in groups where certain members have one job: to fight for everyone else. Scientists call these members soldiers. Many soldiers never have babies of their own, and some are built so differently from their family that they look like a separate species.
Why would any animal give up having babies to fight? Almost always, it's because soldiers are protecting close family. Helping their mother or sisters survive passes on the same genes they carry. That's why soldiers keep showing up wherever animals live in tight family groups with a home worth defending.
Here are 7 animals that raise their own armies, starting with the most famous and ending with the strangest.
1. Ants
Ants are the best-known example. In many species, some workers grow into "majors," extra-large ants with giant heads and jaws. Big-headed ants (Pheidole) are named after their majors, which crush seeds and fight off invaders. Some carpenter ants have "door-head" soldiers with flat, plug-shaped heads. They sit in the nest entrance and block it with their face, like a living door.
2. Termites
Termites have soldiers too, and some come with built-in weapons. Many termite soldiers have huge jaws for biting. Others, called nasute termites, have heads shaped like a nozzle. They spray a sticky, toxic glue at attackers like ants, gumming them up so they can't move. These soldiers can't even feed themselves. Workers have to feed them.
3. Aphids
Soldiers aren't just a thing for ants and termites. In 1977, a Japanese scientist named Shigeyuki Aoki discovered aphids with soldiers. These tiny sap-sucking bugs often live in galls, hollow growths they trick a plant into building around them. Their soldiers stab predators, clean out waste, and in one species, squirt their own body fluid onto holes in the gall until it hardens like a scab, often dying in the process. Read more in Soldier Aphids.
4. Stingless bees
For a long time, scientists thought bees didn't have soldiers. Then, in 2012, they found them in the Jataí bee, a stingless bee from Brazil. Its guards are about 30% heavier than other workers, with bigger legs for fighting. They protect the nest from robber bees, which raid other hives to steal honey. Since then, soldiers have been found in several more stingless bee species. Read more in Stingless Bee Wars.
5. Snapping shrimp
Armies aren't only found on land. In the coral reefs of the Caribbean, the snapping shrimp Synalpheus regalis lives inside sponges in colonies of up to about 350 shrimp, with just one breeding female, the "queen." It was the first social animal of this kind found in the ocean. Snapping shrimp have a giant claw that snaps shut so fast it makes a loud pop. In this species, the big males do most of the snapping at intruders, so the queen and young shrimp can feed safely. When a stranger shows up, colony members can even snap together as a warning.
6. Thrips
Thrips are tiny insects, often smaller than a grain of rice. In Australia, some kinds live inside galls on acacia trees, just like gall aphids. In a few species, the first young raised in each gall become soldiers. They have short wings and huge front legs, which they use to fight off other thrips that try to invade and steal the gall.
7. Parasitic flatworms
The strangest army lives inside a snail. Trematodes are tiny flatworms that live as parasites. Inside a snail, one worm clones itself again and again, building a colony of thousands of identical worms. In 2010, Ryan Hechinger and his colleagues found that these colonies have two kinds of worms: big ones that make more worms, and small ones with large mouths that attack worms from other colonies that try to move into the same snail. They were the first flatworms found with soldiers. Because every worm in a colony is a clone, a soldier is fighting to protect copies of itself.
What all these armies have in common
Ants, termites, aphids, bees, shrimp, thrips and flatworms aren't closely related. But they all ended up with soldiers, because they all share the same situation: a family living packed together in one home that's worth stealing. When that happens, evolution keeps landing on the same answer. Some members stop having babies, and start standing guard instead.
Further reading
- Aoki, S. (1977). Colophina clematis, an aphid species with "soldiers." Kontyû.
- Grüter, C. et al. (2012). A morphologically specialized soldier caste improves colony defense in a neotropical eusocial bee. PNAS.
- Duffy, J. E., Morrison, C. L. & Macdonald, K. S. (2002). Colony defense and behavioral differentiation in the eusocial shrimp Synalpheus regalis. Behavioral Ecology and Sociobiology.
- Crespi, B. J., Carmean, D. A. & Mound, L. A. (1997). Phylogenetics of social behavior in Australian gall-forming thrips. Molecular Phylogenetics and Evolution.
- Hechinger, R. F., Wood, A. C. & Kuris, A. M. (2011). Social organization in a flatworm: trematode parasites form soldier and reproductive castes. Proceedings of the Royal Society B.
If you'd like to support this blog and are interested in ant supplies, check out canada-ant-colony.com for all your ant needs.