Commensalism is a pseudo-partnership in which one species (the commensal) extracts benefits from another (the host) without causing it harm or benefit. The commensal organism obtains food, shelter, locomotion, or support. Commensalism can either be a brief interaction or a lifelong symbiosis.
This contrasts with mutualism, whereby both species prosper; parasitism, in which one species prospers at the cost of another; and amensalism, in which one organism is harmed while the other remains no better or worse for the exchange. There are four types of commensalism, depending on the relationship between the organisms.
- Inquilinism is a relationship where one body depends on the other for permanent housing, such as a bird living in a tree hole.
- Metabiosis is a relationship where one organism forms a habitat for the other. These include maggots living in dead bodies or hermit crabs using the shells of dead gastropod for protection.
- Phoresy is commensalism relationship where an organism attaches to the other for transport. Examples are millipedes on birds and pseudoscorpions on mammals.
- Microbiota organisms that form communities within the host organism. These include bacteria on human skin.
The following are ten intriguing examples of commensalism in the wild.
The term commensalism was initially coined by a Belgian scientist, Pierre-Joseph van Beneden, in 1876, specifically to refer to scavengers that tailed apex predators to dine on the leftover prey. One such example of this is in Golden jackals. When an unlucky individual is banished from the pack, it is no longer capable of hunting on its own. Instead, it will take to following tigers, staying back in the shadows so as not to disturb the hunter-in-action, and then moving in once the feline has had its fill.
Remora (Sucker Fish)
Another curious example of commensal scavenging is amongst remora fish (eight species that belong to the Echeneidae family). These "suckerfish" painlessly attach themselves to large marine animals, such as sharks, mantas, and whales, wait for them to feed, and then detach to snag some scraps. Though this is an example of commensalism, sometimes remora feed on their host's external parasites, becoming an example of mutualism.
Unlike the remora, Cattle egrets hang out on the backs of large grazing animals, such as cattle and horses, to capitalize on food. When the beasts of burden stir up the ground, the small birds hop down and snatch up exposed insects. They also follow farm machines for the same reason and even poke around airport-adjacent fields, waiting for planes to stir up the grass during takeoff and landing. These forms of opportunistic feeding account for about 50% of a Cattle egret's diet and about one-third of the energy expenditure.
If you have been hiking through the thick brush in North America, chances are you have picked a few of this weed's prickly brats off your clothes. The Burdock plant utilizes its spiky, globular flower heads to disperse its seeds across the land. When furry animals pass by, they naturally collect these bundles, which are sharp enough to attach to but not enough to penetrate the thick animal hides. This perfectly represents a special form of commensalism known as phoresy, whereby the commensal species uses a host specifically for transportation.
Pseuroscorpions also exhibit phoresy in that these arachnids attach themselves to exposed surfaces of host animals like the fur of mammals or beneath the wings of beetles and bees. In this way, these scorpion-like creatures, but without stingers, receive protection from predators. Their small size hardly creates any nuisance to the sheltering animal.
Hermit crabs famously seek shelter in acquired shells. Oftentimes, these squishy crustaceans rely on expired gastropods (snails) to protect themselves. Therefore, this qualifies as an example of metabiosis (another category of commensalism), whereby one species forms a habitat from a dead organism.
A less-appealing instance of metabiosis commensalism occurs when maggots inhabit a carcass. They not only feed on the available nutrients but utilize the moist environment to lay their eggs. The next generation of maggots relies on the immediate food source before making their way out of the dead animal's remains.
This frisky North American tundra-dweller will sometimes follow caribou around, which helps to indirectly summon preferred food sources. The stocky herbivores dig into the snow to scrounge up lichen plants. After the activity settles down, rodents may come to the surface, at which point the fox will swoop in for the kill. Arctic Foxes are also known to scavenge for remains left behind by polar bears.
Most owls are known to nest in the natural cavities in trees, cliffs, or buildings, as well as the abandoned nests of hawks or crows. In the first case, since they are not themselves burrowing into the woody plant, they cause no harm to the stalwart specimen. This approach is known as inquilinism, which is a form of commensalism whereby one organism uses another for permanent housing. By this definition, many birds exhibit inquilinism when they build their nests amongst the branches of trees.
Tree frogs also rely on arboreal friends for shelter and protection. Unlike nesting birds, these amphibians set up shop underneath large leaves in order to camouflage from predators and hide from the heavy rains. Tree frogs are found on every continent not called Antarctica, so wherever you are, take a peek at the underside of leafy areas, and you might just spot a few of these jumpy creatures.
The plants, animals, and critters of the earth have formed many creative partnerships amongst themselves. Though these can manifest in many ways, the aforementioned low-impact, net-beneficial interactions represent the pleasant side of the natural world.