When Charles Darwin presented the theory of evolution in the late 19th century, other biologists began to idealize the natural world as a realm driven by competition. Every living creature is striving to survive by any means necessary, these scientists thought, and it will happily push out, feed on, or otherwise out-compete any other creature that stands in its way. In actuality—as anyone who’s fallen down a “cute animals become friends” YouTube rabbit hole knows—the natural world is far kinder and far more complicated than the basic tenets of evolution would have it seem.
From the biggest whales to the tiniest fungi, living creatures depend upon each other to survive. Sure, one species “depending on” another can often mean that the first species needs to eat the second. But there are also flowering plants depending upon honeybees to spread their seeds, ants depending upon trees to provide homes for their colonies, corals depending upon microscopic algae to give them energy through photosynthesis, and even sea worms depending upon bacteria to help break down the bones of dead animals.
Scientists call a dependent relationship like this mutualism: two or more species that mutually benefit each other. Sometimes these relationships are more like casual friendships, where one species can help each other out occasionally, and other times they’re more like intense marriages, where both species need each other to survive. Mutualisms are important to biologists who look to the past and those who look to the future because studying these behavioral interactions can both provide clues as to how species evolved and help conservation researchers understand how to best protect endangered populations.
For this story, Stacker scoured the scientific literature (and surveyed a few of our science communication friends) to compile a list of 20 incredible mutualisms showcasing how different life forms can work together. Read on to learn about friendships from the plains of the Serengeti to the bottom of the ocean.
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Banding together to avoid predators is a useful tactic, even for animals as big as zebras and wildebeests. On the Serengeti, these two herbivores can graze together without getting competitive, because zebras like to eat longer, tougher grasses while wildebeests prefer shorter, more tender specimens. Plus, zebras, wildebeests, and impalas, a type of antelope, can all recognize each other’s warning calls and help each other escape danger. Scientists are still investigating this relationship to determine whether it is truly a mutualism or simply a convenient habitat-sharing interaction, but either way, it’s clear that the Serengeti’s plant-eaters are able to live in harmony.
In central India, langur monkeys and chital deer help each other avoid tigers and other predators. The langurs, long-haired gray monkeys with great eyesight, keep a look-out in the trees while the chital deer, brown spotted deer with an impressive sense of smell, watch out for predators on the ground. The deer also eat fruit that the langurs drop from the canopy.
In oceanic island habitats like the Galápagos, biodiversity is often limited—in other words, there just aren’t that many different species present. Plants in such environments often invite lots of pollinators to help them reproduce while pollinators benefit from less picky eating. This trend leads some bird species on the Galápagos islands to act as double mutualists: These birds consume nectar from plants’ flowers, and then later consume the very fruit that resulted from their own pollination.
Jens Olesen and colleagues, who described these interactions in a 2018 Nature paper, suggest that the conservation focus for island biologists should be on whole environments, rather than on particular species, in order to preserve these complex networks.
Zebra swallowtails, beautiful black-and-white-striped butterflies native to the eastern U.S. and Canada, are the state butterfly of Tennessee. For reproduction, these butterflies rely on pawpaws, a group of understory trees with large, yellow-green fruit. Zebra swallowtail caterpillars exclusively live on pawpaws, because they ingest compounds in the trees’ leaves that are poisonous to many predators. In return, the butterflies help the trees reproduce through pollination.
While acacia ants protect acacia trees, lemon ants protect duroia trees, flowering plants in the understory of the Amazon rainforest. A particular species of lemon ant, called Myrmelachista schumanni, nests in a particular species of duroia tree, called Duroia hirsuta. The ants are so determined not to let competitors threaten their nests that they produce formic acid, a deadly compound, and poison any other trees that come into the area. This deadly partnership creates areas of the forest that are populated solely by duroia trees. Indigenous people living in the Amazon called these strands “devil’s gardens” and believed they were created by evil forest spirits.
[Pictured: The swollen leaves and thin stem of a myrmecophyte from the forests of the Andaman Islands. One of the leaves has been sectioned.]
Like the yucca moths, acacia ants evolved alongside a group of trees. These ants depend on acacia trees for shelter; in fact, queen ants burrow into the large thorns at the base of acacia leaves, lay their eggs inside, and take nectar from the nearby leaves. When an ant colony grows large enough, every thorn of an acacia can become inhabited. In return, the ants defend their trees against rival insects and other predators.
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You know a friendship is special when two species groups share the same name. Yuccas, a group of tropical trees and shrubs known for their tough, sword-shaped leaves, and yucca moths, a group of small, nondescript moths, actually developed through natural history together in a process biologists call coevolution. The yucca plants rely on the moths for pollination, while the larvae of some moth species feed exclusively on yucca seeds.
Another small bird, big eater relationship can be found on the plains of sub-Saharan Africa: Oxpeckers perch on the backs of big herbivores, namely rhinoceroses and zebras, and eat the ticks and flies they find. While this relationship has long been hailed as a classic mutualism example, recent research reveals that it’s actually more sinister, as Kat Eschner reported in the Smithsonian. Oxpeckers not only eat ticks from their hosts’ backs, they also drink blood from the animals’ sores—making it harder for those wounds to heal.
While wrasse fish have some preferences for which larger animals they’d like to clean, they are generally equal-opportunity feeders. Egyptian plover birds, on the other hand, have a mutualistic relationship with one specific predator: the Nile crocodile. When a Nile crocodile gets food stuck in its teeth, it will sit in the sun with its mouth open, and a Plover bird will swing by to pick the extra food out. The crocodile is thus protected from rotting food and infections.
Exploring a coral reef is a hazardous task; not only do fish have to avoid big predators, they also risk having their scales clogged by smaller, parasitic creatures and ocean gunk. When the situation gets particularly dire, reef fish visit “cleaning stations” run by blue-streaked cleaner wrasses and other similar species. At these “cleaning stations,” basically the aquatic equivalent of a dentist’s office, wrasses eat the parasites, displaced scales, and other gunk from their so-called clients. The larger fish get spa treatment while the wrasse gets a meal.
Protection from predators is a common theme in nature’s collaborations. Clownfish, recognizable as the titular character from “Finding Nemo,” have evolved a layer of protective mucus that allows them to live in anemones, flowering ocean plants with poisonous tentacles. The clownfish enjoy a safe haven from the open ocean while defending the anemones from other fish that would feed on the plants.
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How did carrier crabs (Dorippe frascone) get their name? The moniker comes from a mutualism: These crabs have five pairs of legs, including an especially strong back pair which allows them to carry other sea creatures…such as deadly sea urchins. An urchin’s spine protects a carrier crab from predators, while the urchin enjoys a ride to new feeding grounds. You can see this collaboration in action on National Geographic’s YouTube channel.
[Pictured: Carrier crab, Dorippe frascone, carrying an upside-down jellyfish, Flores Indonesia.]
Speaking of burrowing: Osedax annelid worms, also called boneworms and zombie worms, eat by digging into the bones of dead animals on the seafloor. These worms have neither mouths nor stomachs, so they rely on a group of bacteria called Oceanospirillales to degrade the bones down into materials the worms can process. The bacteria, meanwhile, get “access to fresh bone material,” as Jessica Carilli put it in an article for Nature’s Scitable blog.
[Pictured: Osedax antarcticus.]
Animal partnership can happen beneath the waves as well as on land. In the Atlantic, Pacific, and Indian oceans, goby fish and pistol shrimp help each other survive. The shrimp builds shelter for both creatures, digging a burrow in the sand on the seafloor, while the goby keeps a lookout for predators with its superior eyesight. Partnerships between individual shrimps and gobies form when the animals are young and usually continue into both species’ adulthood.
Three-toed sloths spend most of their lives eating and napping high in the trees—except for a few hours each week when they descend to the ground to defecate. A slow climb down a tree trunk and return back to a leafy perch costs a sloth a lot of energy and exposes it to predators, making defecation the #1 cause of sloth mortality. So, why do sloths risk everything to climb down, rather than just letting go from high in the air?
The answer is tied to a complex partnership between sloths, algae living on the sloths’ hairy backs, and several species of pyralid moth, as Jonathan Pauli and colleagues explained in a 2013 Royal Society paper. The sloths climb down to the forest floor in order to facilitate the moth life cycle, and the moths bring nutrients from the soil to the sloths’ backs, in turn facilitating the growth of algae, which the sloths and moths both like to eat.
Panic grass, a flowering grass group, is found across central and eastern America. But one special variety (scientific name Dichanthelium lanuginosum var. thermale) is able to grow in the geothermal soils at Yellowstone National Park, where temperatures get as high as 150 degrees Fahrenheit (65 degrees Celsius). The fungus species Curvularia protuberata makes this extreme lifestyle possible by transferring a heat-tolerance virus to the plant, while the grass gives the fungus nutrients and a place to live.
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You might remember from high school science classes that cows have four stomachs. Cows, like sheep, goats, and buffalo, are part of a group called ruminants, large mammals with unique digestive systems that are able to break down grass and other coarse plant materials. And what do these unique digestive systems rely upon? Microorganisms, of course. Bacteria in ruminants’ stomachs break down big sugar molecules for the animals while taking some of the nutrients in return.
Corals, those vibrant animals that make up coral reefs, are another animal group that depends on a partnership with microscopic algae. The algae, called zooxanthellae, are able to conduct photosynthesis (the plant-cell process that makes sugar from sunlight), and they provide the corals with food in return for a safe place to live. Zooxanthellae also give coral reefs their distinctive bright colors; in fact, when corals become stressed, the animals push the algae off and turn white in a phenomenon known as coral bleaching.
Lichen: they’re little plants that grow on trees and rocks, right? Wrong. Lichen are actually formed by collaboration; colonies of microscopic algae live inside of fungi, with each organism providing the other with nutrients. And the interaction is even more complex than that, as Ed Yong described in a 2019 Atlantic article: some lichen colonies are harboring multiple fungi species, and scientists are still working to understand the role of each one.
The last mutualism in this story actually helped build plant and animal life as we know it. Biologists hypothesize that both the mitochondrion, a tiny apparatus in our cells that makes energy (also known as the “powerhouse of the cell”), and the chloroplast, a similar apparatus specific to plant cells that turns the sun’s light into food, were both once independent cells.
These independent cells were engulfed by host cells, and inside their hosts, tiny invaders took advantage of the new, safer living environment while providing these hosts with energy. That partnership, which biologists call the Endosymbiotic Theory, evolved into the complex eukaryotic cells that make up our bodies today.
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