We have all come across or actively sought out videos of unlikely animals putting aside their differences to nuzzle one another or engage in similar cute behavior, like when a larger animal gives a smaller animal a piggyback ride. There is of course an entire television show devoted to this phenomenon, plus thousands of viral videos and just as many news stories; browsing just the last few months of news on Google I found stories in reputable national outlets about a pig befriending a cat, a different cat befriending a guinea pig, and a dog befriending a deer. Of course these are isolated examples, they say nothing universal about life in the Animal Kingdom. If we want to know which species actually get along the best, on a population level, we must turn to those humans who know them best. Which is precisely what we’ve done for this week’s Giz Asks. Below, our experts weigh in.
Assistant Professor, Ecology, Evolution and Marine Biology, UC Santa Barbara
All kinds of species cooperate with one another in what we call a “mutualism,” or an interaction between species that benefits both parties. Mutualisms involve an exchange of all kinds of different services—some species provide housing for another in exchange for protection (think ant-plant mutualisms), others provide food in exchange for protection (think corals and their symbionts). But one of the most intriguing examples of two animals putting aside their differences and just getting along comes from cleaner shrimp-client fish interactions. Cleaner shrimp are small, tropical crustaceans that basically act like dental hygienists on a coral reef: they live at a stable location called a cleaning station, and fish, which we call clients, visit the station. When they get there, they pose motionless nearby and allow the cleaner shrimp to hop on and pick off ectoparasites and dead skin. Cleaner shrimp even go inside the mouths and gill slits of some fish, and all over their bodies, providing cleaning services. There are perhaps a dozen species of cleaner shrimp around the globe, and each species in turn services dozens of species of client fish.
One of the most intriguing things about these interactions is that half or more of the fish that visit and receive cleaning services are potential predators, meaning they can and regularly do eat crustaceans. So, in cleaner shrimp-client fish interactions, species that would otherwise be competitive (i.e. be predator and prey) cooperate instead: clients benefit from parasite removal, and maybe even from the tactile stimulation provided by the cleaner (perhaps like getting a massage), and cleaner shrimp receive a meal. Some work has shown that cleaner shrimp and client fish use visual signals when interacting. For example, cleaners might use signals to identify themselves as beneficial partners rather than a food item, and clients might use signals to indicate that they are seeking cleaning rather than a meal. Some cleaner shrimp species modify these signaling behaviors in the presence of potentially predatory clients: cleaner shrimp evaluate the risks posed by interacting with a certain client, and if the risks are high, they signal like crazy, perhaps sending a signal of cleaner identity that’s especially important to have noticed if a client could eat you! Thus, it appears that cleaner shrimp can strategically modify their behavior to try and maximize the chances of cooperation with the clients who might be most tempted to turn the cleaning interaction into a predation attempt.
There are other cleaner species in marine environments, primarily some species of gobies and wrasses; however, a great deal of work has shown that they sometimes “cheat” their clients, meaning that they eat healthy scales, mucus, or tissue instead of cleaning off parasites. In response, client fish sometimes punish fish cleaners by chasing them, or refusing to visit them again for a while. By contrast, we don’t have any evidence that cleaner shrimp cheat—it may be that they’re just too small to really cause any damage to a fish, or maybe the risks of cheating a potential predator are higher for cleaner shrimp than fish, too high to justify cheating. We don’t really know what keeps these two parties cooperating over time, but we do know that this interaction is extremely common and frequent: cleaner shrimp likely get the majority of their food every day from clients, sometimes servicing more than a dozen clients in an hour, and clients can spend a sizable percentage of their time every day getting cleaned.
Associate Professor, Psychology and Ecology and Evolutionary Biology, University of Michigan
Two examples come to mind immediately.
The first is the mutualism that seems to exist between warthogs and banded mongoose in Uganda. This is a very rare mutualistic relationship between two mammal species. The warthog has ticks all over it, they amble over to a group of banded mongoose (which live in relatively large social groups) and then the mongoose crawl all over them to consume the ticks on the outside of the warthog. It is fascinating and mind blowing to me. It’s an instance of mutualism because both benefit—the warthog gets the ticks removed, the mongoose gets a fat tick to eat. Sadly, there aren’t too many scientific studies on this topic yet.
The second is a hunting mutualism between a fish (grouper) and moray eel. Many animals live in groups, and this can help them find prey or have higher hunting success, such as a larger pack of wolves hunting large animals like bison or moose. Each of the animals may have a specialized role in the hunt. Here, we have the moray eel that lives in a coral reef, which flushes out fish that the grouper then eats. The grouper might flush the fish into the reef, which the eel eats. Both seem to benefit and have different roles. What’s especially cool is the grouper even seems to use gestural communication where they point to where a fish might be located and the eel comes over to flush it out.
Professor of Biology and Psychology at Clarkson University, who studies cognition and social behavior in birds and other animals
In winter, where I live in New York State, a family group of black-capped chickadees will be joined by a pair of downy woodpeckers, red-breasted and white-breasted nuthatches, brown creepers, and golden-crowned kinglets. This mixed-species flock of small birds will travel through the forest, searching for insects and seeds. Mixed-species flocks have more eyes and ears, so are more likely to detect hunting hawks or other predators. The birds forage in different ways, which reduces competition and makes them better at detecting different predators. For example, the creepers move up a tree trunk, and tend to look upward, while nuthatches go down the tree trunk, and tend to look downward. Each species has a warning call that it gives upon sighting a predator. The predator warning calls used by the members of a mixed-species flock are mutually understood, so when one bird makes a chirp upon sighting a predator, all flock members head for safe cover and then scan for the predator.
Mixed species of small birds occur in forests all over the world. In the Amazonian rainforests, up to thirty species may move together in a single flock, and individuals of four to ten core flock species associate together year-round and throughout their lives. Besides the benefit of faster and more reliable predator detection by being a mixed-species flock member, birds benefit by improved foraging success. Insects or other small prey may fly or hop to successfully evade one bird, only to be pursued and captured by another—often a species that forages in a different manner.
That said, some mixed-species flock members—while generally getting along with the others—are not above occasionally deceiving a flock mate to snatch a tasty insect. Shrike-tanagers in Amazonia and drongos in Africa are both species that are very vigilant, good at detecting predators, and usually first to warn their flock mates. However, from time-to-time, when another bird has found a desirable prey item, the shrike-tanager or drongo will give an alarm call, causing the other bird to flee for cover, and the ‘bird who cried wolf’ will steal the deceived bird’s prey.
Small birds aren’t the only animals that travel together in mixed species groups for safety and improved foraging success. Monkeys in Africa and South America forage in mixed species troops. Some coral reef fish form mixed species schools. In India, chital deer and langur monkeys forage together.
It is clear that some animals become familiar and get along with group members that happen to be other species, but do they become friends? We don’t know, but it is possible. It would require that group members recognize members of other species as individuals, remember their interactions with them, and preferentially seek out and associate with individuals with whom they are most familiar. Given what we do know about the cognitive capacities of birds, in my view interspecies friendships among mixed-species flock members are plausible.
Associate Professor, Biology, Georgetown
One of my favorites is the relationship between ants and aphids.
Aphids are a herbivore that feeds on plants. They have a really long beak, and they stick it all the way down into where the plant makes sugar, and they suck that sugar up. Then, they poop out sugar water. Ants have chewing mouth parts—there’s no way they could get at that sugar water on their own, because if they tried they’d wind up ingesting all kinds of nasty secondary chemicals. Ants are predators, and they need carbs, and they get those carbs from aphids’ poop. In return, they protect the aphids from predators.
In the tropics, this same thing happens between ants and treehoppers. In fact, in the tropics the ants will actually move the treehoppers around to maintain better and better sugar sources—they’re like cowboys wrangling their herd. They’ll keep the babies in a little ant-nest.
On their own, it’s worth noting, aphids are worthless—a predator like a ladybird beetle can come along and just wipe them out in no time at all. Some of them take 15 minutes to withdraw their beak from the sugar source, by which time the beak’s the only part of them that’s left—the predator’s eaten the rest of them. But the ants, thankfully, protect them.
Professor of Psychology and Director of the Canine Science Collaboratory at Arizona State University and author of Dog is Love: Why and How Your Dog Loves You
When our son was small it seemed like every story we read him involved different species of animals who were friends with each other. There were baddies in the wild too, of course, but by and large, beasts in Bambi’s forest and Nemo’s ocean were buddies. Unfortunately, this is a far from realistic picture of nature.
No animal is born knowing what species it belongs to. All young beasts must learn who are their kind during a crucial early phase of life known technically as the critical period for social imprinting. Whatever a cub, pup, calf, chick, kid, or anything sees, smells, and hears in its first days of life it will thereafter seek out for its mate (in both the Australian and biological senses). This happens so early that the young are necessarily huddled close with their mothers and siblings and are very unlikely to interact with members of any other species.
And that’s a good thing.
Think about it. If an animal from a prey species were to try and make friends with an individual from a predator species, she would pretty quickly become that predator’s lunch. And if a member of a predator species tried to make friends with members of his prey species, he would soon starve to death.
The only exceptions to this general rule of biology are our friends the domesticated species. One of the key impacts of domestication is an extension of this critical period for social imprinting from days to months so that dogs, cats, and the others, easily make friends with the other beings around them—especially human beings. That’s lovely for us, and good for them, but we should not let our experience of beloved cats and dogs mislead us about the true state of affairs out in the wild. Keep your dogs and cats close: let nature be nature outside.
Teaching Associate Professor, Animal Science, University of Minnesota
My first thought was about mutualistic relationships between species—i.e., when two species associate in close proximity to each other, and each receives benefits from the interaction. A common example of this would be smaller animals “riding” on larger animals to pick parasites and other little critters off the large animals’ bodies (thus performing a hygiene service for them), while the smaller animal gets quick and easy free meals. See: cleaner wrasse (a type of fish) and their “client” reef fish or sharks.
In the domestic animal world, we have a lot of examples of housing different species who get along fairly well together—for example, using animals like llamas, dogs, and donkeys to guard flocks of sheep, or giving companionship goats to horses who may otherwise have to be housed individually for whatever reason. And of course there are the diverse species we keep as pets and who can get along really well together (though a big caveat to that is that many people may misinterpret a lot of their pets’ behaviors as “cute” or otherwise indicative of liking or tolerance when the reality may be far from it). However, generally I’d also say that many or most of those species would still prefer to form relationships with their conspecifics (members of their same species) over other species, so we should still aim for allowing animals to form bonds with their own kind. But individual exemptions always apply.
In terms of examples of “interspecies friendships,” I can think of a lot of one-off examples of individual animals appearing to bond and play together. One of the most famous examples of unexpected bonds between radically different species might be the hippo Owen and tortoise Mzee, whose relationship spurred children’s books. However, outside of individual examples or mutualism, I’m less certain as to whether there are entire species whose members tend to form long-term, affiliative “friendships” with members of other species, where both species benefit. That’s not to say they wouldn’t exist, but I just happen to not be aware of them.
Associate Professor, Wildlife Ecology, University of Delaware
Members of the order Carnivora (think lions and tigers and bears, plus, oh my, so many others) are typically rather fierce animals. Sure, some of them work in single species social groups to take down bigger prey, but when two different Carnivora species interact, it usually ends in bloodshed. The idea of lions getting along with hyenas, or a brown bear hanging out with a few wolf buddies, is laughable. Which is what makes the friendship between coyotes and badgers so astounding and so special.
Astounding but not new. Native Americans have long told stories about badgers and coyotes hunting together. Settlers scratched descriptions of the odd relationship into their journals. All well before modern scientists broke the companionship down into cold dead percentages, rates, and scientific terms that are hard to follow. “Complementary morphological adaptations,” “interspecific tolerance,” “mutualism,”all point to the idea that when a coyote partners with a badger it is simply because they are both more likely to be successful in hunting their prey. This fits nicely with our understanding of natural selection, and conveniently gives the notoriously grouchy badger and the sneaky coyote a scientific reason to hang out together. It also saves us from using the term “friends” for something so far removed from our primate selves.
But recently a video went viral and the footage suggests that this may be a bit more than a professional relationship. In the clip, a coyote jumps into the universal “play bow” of canids, wags its tail, and invites a badger to follow it down a drainage culvert. The badger, at complete ease, saunters after the coyote, and possibly, just possibly, has a little extra wiggle of happiness to its gait. You be the judge. Cold hard assassins off to make a kill? Or just a couple of pals, playing around, as they head to the local diner? I know which I choose to believe.
The sheer audacity of these two mammalian predators forming such a close bond is why I choose them as the two species that get along the best.
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