Commensalism

Some animals live near others.

One animal gets help. The other animal is okay. It is not hurt. It does not get help either. It is like sharing a table.

Small fish can ride on big fish. They get a free ride and food.

Pigeons live near people too. They find food near us. Humans are not hurt by them.

Dogs once lived near hunters. They ate scraps left behind. This helped them live near us. Now they are our friends.

Some tiny bugs live on flies. They only use the fly for a ride.

This is a way to travel.

Nature has many ways to live together.

Some living things live together in a special way. This is called commensalism. In this way, one animal gets help. The other animal is not hurt. It does not get help, either. The name comes from a word that means "sharing a table."

Many animals use this to find food or travel. Remoras are small fish that stick to big fish or sea turtles. They get a free ride and eat leftovers.

Some animals follow humans to find food. Pigeons live near people and eat what we leave behind. Dogs may have started this way too. Long ago, wolves lived near human hunters. They ate scraps left at camp sites. Over a long time, these wolves changed. They became the dogs we know today.

Tiny bugs can also use this way to move. This is called phoresy. Some mites ride on flies just to travel. They do not hurt the fly. They only use it for a ride.

Commensalism is a special way that living things interact. In this relationship, one species gains a benefit. The other species, called the host, is not helped and is not harmed. This is different from mutualism, where both sides help each other. It is also different from parasitism, where one side is harmed. Some living things get food or shelter from their hosts. Others simply use a host to move from place to place. This way of living can be a short meeting or a long-term bond.

How this works depends on the type of living thing. One way is called phoresy, which is using another animal for transport. For example, tiny mites might ride on a fly to travel.

Another way involves getting food from a host. Remoras are fish that attach to sharks or sea turtles. They get a free ride and eat scraps or waste left by the host. In the ocean, brittle stars sometimes grip onto octocorals. This lifts the stars up into the water currents so they can feed more easily.

Scientists have studied these connections for a long time. The Belgian zoologist Pierre-Joseph van Beneden introduced the term "commensalism" in 1876. The word itself comes from a Latin phrase meaning "sharing a table." This describes how some animals live near humans to eat leftovers.

Pigeons are a great example of this. They lived alongside humans for thousands of years after being domesticated from rock doves. This helped them spread to many different parts of the world.

Many famous animals followed a commensal path to become pets. The dog was the first animal to be domesticated. This happened in Eurasia before the end of the Pleistocene period.

Some wolves likely lived near human hunter-gatherer camps. They were the less aggressive wolves that ate scraps left at the site. One famous piece of evidence is the Bonn–Oberkassel dog. This find dates back to about 14,000 years ago. This shows that dogs were with humans long before people started farming.

Commensalism also happens in tiny worlds we cannot see. Bacteria and fungi live on the human body as part of our natural flora. For instance, a bacteria called Staphylococcus aureus lives on 20 to 30 percent of people. It stays on the skin or inside the nose and ears without causing harm.

Even tiny microbes help each other through a process called nitrification. One type of bacteria turns ammonium into nitrite. Then, another type called Nitrobacter uses that nitrite to grow. It is a chain of events where one's waste becomes another's food.

Commensalism is a specific type of biological interaction known as symbiosis. In this relationship, one species, called the commensal, receives a benefit. Meanwhile, the other species, known as the host, is neither helped nor harmed by the interaction. This differs from mutualism, where both species benefit, and parasitism, where one species benefits while the host is harmed. Commensalism can range from brief encounters to long-term, intimate associations.

The commensal organism can gain several different advantages from its host. Some species obtain nutrients, such as food scraps or waste. Others find shelter or physical support to stay in a safe place. Some even use a host for locomotion, which is the ability to move from one place to another. In many cases, the commensal is much smaller than its host. The host remains unmodified, but the commensal may show great structural adaptations to suit its lifestyle.

Scientists categorize these interactions into different types based on how they work. One type is phoresy, where an animal uses another exclusively for transport. This is common among arthropods, such as mites riding on flies or beetles.

Another type is inquilinism, where one organism lives within the home of another. In the deep sea, brittle stars exhibit commensalism by gripping onto octocorals. This lifts the stars off the sea floor and into currents, which helps them feed through suspension. While this helps the brittle star, the octocoral is generally unaffected.

The term "commensalism" was introduced in 1876 by the Belgian zoologist Pierre-Joseph van Beneden. The word comes from the Medieval Latin "commensalis," which means "sharing a table." This reflects how some animals live near human habitats to eat refuse. This process can lead to domestication, a coevolutionary process where animals adapt to a new niche. Pigeons are a classic example, as they transitioned from rock doves to domestic birds by living alongside humans.

Human history shows how commensalism can change entire species. The dog was the first animal to be domesticated, occurring in Eurasia before the end of the Pleistocene. Archaeological evidence, like the Bonn–Oberkassel dog from approximately 14,000 years ago, shows dogs existed before agriculture. It is hypothesized that less aggressive wolves followed human hunter-gatherer camps to eat carcasses. Over time, these wolves may have developed physical changes, such as neotenization, where they retain juvenile skull features into adulthood. This selection for reduced aggression helped them move from commensals to partners.

Commensalism also occurs in the microscopic world of bacteria and fungi. For example, the bacterium Staphylococcus aureus is a metabiotic commensal for many people. It lives on the skin, in the nose, or in the ear canals of roughly 20% to 30% of the human population. Most of the time, it lives peacefully as part of the natural flora. In the soil and water, microorganisms use a process called nitrification to survive. First, bacteria like Nitrosomonas oxidize ammonium into nitrite. Then, Nitrobacter uses that nitrite as a source of energy to grow. This shows how the waste of one microbe becomes the food for another.

Despite its name, some scientists argue that true commensalism is rare. They suggest that most interactions are actually mutualistic or parasitic in subtle ways. For instance, epiphytes, which are plants that grow on other plants, can sometimes act as "nutritional pirates." They might intercept nutrients or shade the host, potentially reducing the host's rate of photosynthesis. Similarly, phoretic mites might make it harder for an insect to fly. Because of these complexities, biologists continue to study whether any biological relationship is truly neutral for both sides.