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Basic Information

Mutualism is a cross-species interaction in nature where two species collaborate in some way that benefits both. There are several different varieties of animal mutualism partnership.

Hunting Partnerships

Some animals hunt together, or lead other animals to prey in hopes of scoring some tasty left-overs.

  • Coyote & Badger hunt prairie dog and ground squirrel together, especially in areas with dense bushes. Coyotes hunt above ground, and badgers below ground, and they channel fleeing prey towards each other.
  • Greater Honeyguide intentionally lead Baboon, Honey Badger or even Human to Beehive. The small bird loves the taste of honey, but cannot take on an entire beehive by themselves, so they seek out other animals (or people), making specific noises and displays to get their attention, and then lead them to the beehive. After the larger creature breaks open the hive and scatters the bees, the honeyguide will return for any honey left behind in the wreckage.

Burrow Partnerships

Some animals mutually benefit by sharing a burrow, nest, den or lair.

  • Goby live in Snapping Shrimp burrows. The Shrimp digs the burrow for them both, the gobie stands sentry for the nearly-blind shrimp. They communicate by touch.
  • There's a Frog species lives in Tarantula burrows. While tarantulas generally eat frogs, there’s a specific species of frog they don’t eat, which will live in their burrow, and eat any insect that might try to steal the tarantula’s eggs. The tarantula knows their frog friend by a chemical on their skin.
  • Clownfish live among Anemone clusters. The clownfish is immune to the anemone’s stings, and lives among them. In return, it protects them from the Butterfly Fish, which is also resistant to the sting.
  • Leaf-cutter Ant raises and farms Fungi in their colony. The ant brings pieces of leaf to the fungus, which feeds on them to grow. The ant in turn feeds on the fungus, essentially farming them.

Mobile Habitat Partnerships

Some animals ride around on other animals. This is an especially good deal for a sessile animal, but is often very beneficial for the load-bearing member of the partnership as well.

  • Crab rides on Sea Turtle. There's room for 2 oceanic crabs on a sea turtle butt. The crabs feed on parasites that would harm the turtle, and the turtle serves as their mobile habitat, which also helps the crabs stay monogamous.
  • Rove Beetle ride on Mouse, Rat or Possum. The rove beetles eat the fleas and ticks that would otherwise be biting the rodent
  • Mite rides on Carrion Beetle. The mites eating the nematodes and the larva of other insect species that were laid in the same carrion, reducing the competition the beetle faces.
  • Anemone ride on Crab. Each protects the other from things that would prey on it, including full-on predators much larger than the parasites.
  • Pearlfish crawls inside Clam, Sea Cucumber, Sea Squirt or Starfish - The pearlfish spends most of its day inside the invertebrate animals, where it is safe from predators. The pearlfish doesn’t harm its host, but it doesn’t particularly help them either, so this may be better termed as Commensalism rather than Mutualism.

Grooming Partnerships

Some animals live their own individual lives, but engage in short exchanges of beneficial behavior when they encounter each other. Often this involves grooming behavior, where the smaller animal eats the parasites that the larger animal can't reach.

  • Warthog seeking Mongoose for grooming. They live separately, but when they stumble upon each other, sometimes the Warthog will lay down and invite the mongoose to groom their fur and eat all the delicious parasites they can find.
  • Baird’s Tapir seeking White-nosed Coati for grooming, very similar to the Warthog and Mongoose situation.
  • Sunfish seeking sea bird or cleaner fish for grooming. The huge fish has a lot of parasites on their skin, so they follow flocks of seabirds, and then float on their side near the surface of the water, so the seabirds can land near them and eat parasites off the sunfish. They also seek out schools of cleaner fish for the same reason. Which they seek out depends on what part of the ocean they live in.
  • Marine iguana of the Galapagos Islands are groomed by Sally Lightfoot Crab. The iguana bask on the beaches in groups, and just let the crabs crawl all over and among them.
  • Oxpecker Bird grooms, but also feed on, large mammals like Giraffe, Hippopotamus, Rhinoceros, and Zebra. This one’s complicated, because while the birds do get rid of ticks and pests on the animals they land on, they also drink the blood that leaks from the wounds the parasites opened in the larger animal's hide. This vampire behavior is tolerated as the lesser evil, and the easily-startled oxpecker also serves as something of an alarm system for when predators are creeping up on the big mammal.

See Also:


1. Video: Sci-Show
3. Video: BBC Earth
5. Animal Planet - article on badgers and coyotes hunting together.]

Game and Story Use

  • The big lesson here is that it's not just sentient or civilized creatures who know how to cooperate. Even something with "animal-level intelligence" may get organized, and/or have friends outside its own species.
  • It's surprising how little this actually shows up in RPG stat blocks, given how common it is in nature, and how weirdly specific monsters can be in campaign settings. You can have a world where predators have evolved to look like treasure chests, or where another creature lives inside a dungeon door waiting for some adventurer to put their ear to the door to listen for sounds on the other side… but rarely do you see something where a small critter inhabits the nest of a fearsome dragon cleaning their hide or treasure horde in exchange for not being dinner. Realistically, these sorts of ecological niches should be filled by creatures every bit as specialized as the mimic, piercer, or rustmonster.
    • They're probably left out because it's not particularly exciting to encounter a non-combatant critter that mostly exists to polish the scales of the endboss. The trick then is to make them not only uniquely evolved, but also interesting in the context of an adventure.
      • In "that RPG" Kobolds are prime candidates for this - bottom tier humanoids with a claim at draconic ancestry and a definite dragon obsession. Any dragon that tolerates them will have a small army of lookouts, lair cleaners and scale pickers, not to mention worshippers and booby-trap engineers.
      • As with the Oxpecker bird or the Goby, the low-level creature might serve partly as an alarm or lookout. Maybe the dragon sleeps all day, but his coin-cleaners don't, and they have a really good perception score so sneaking up is a lot harder. Clever players may see this as an opportunity. If they leave some really dirty money near the entrance of the dragon's lair, his cleaning crew will come out to gather or clean it. Then they can be attacked, or lured away with a trail of coppers. Once the lookouts are disposed of, sneaking in to ambush the big bad becomes much easier.
      • A sessile animal with special powers might hitch a ride on something more threatening. For example, it might make sense for a medusa to lair with creatures that eat all the stone it makes, but it's also possible the medusa may be covered by a symbiotic layer of a stone-eating slime-monster. Unless the PCs are made of stone, that's not particularly threatening, but maybe this slime layer also functions as armor or insulation, or the slime has a "stonepass" phasing ability that now allows the medusa to walk through a cliff to escape, or walk through a castle to reach its human prey.
    • Of course, it's possible for both species to be dangerous threats on their own, and therefore feel "relevant" to a gaming scenario.
      • A monster that destroys metal, like a rustmonster or certain types of oozes, may share a lair with a highly-mobile ravenous man-eating creature that's likely to be hunted by metal-using humanoids. On its raids against the human populace, the predator is most interested in bringing back the bodies of armored soldiers, so its mutualism partner can open up their metal shell and expose the sweet meats inside for the benefit of both monsters.
      • If one creature has an damage immunity to a particular type of attack, it would make sense for them to be paired together by evolution with a creature that uses that type of damage as a deterrent to other threats. Gaming is full of critters that are immune to poison, or magic, or disease. Match them up with a critter that has an area-effect attack that corresponds to their immunity, and you've got a real the makings of a great fight scene as well as an ecologically-sound mutualism niche.
  • Partnerships have to start somewhere. Sometimes it's a grouping that occurs at or near birth, but in other species it may be behavior picked up over time. It's likely to start at some place where animals frequent simultaneously: in the wild this often means at a river or watering hole, where herbivores gather and keep a common look-out against predation.
    • Mutualism is also increasingly common in places near where humans dwell, in liminal spaces, or near junkyards and landfills, as all that tasty food waste we leave sitting around is just too tempting. Animals get used to dumpster diving side-by-side. This can lead to one literally eating off the other, and into a new mutualism arrangement that they later teach to their offspring.
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