Among the most remarkable animal stories are those involving friendships where we might not expect them.  A friendship between man and dog is not surprising — after all, a dog is man’s best friend.  Nor is it surprising to find a friendship between two dogs — or two men. We expect intraspecies friendships.

But then we don’t.  A friendship between two fish would surprise us, or between two invertebrates, two amphibians, or two reptiles.  And we are surprised when we come upon apparent friendships between two very different species, such as cat and bird or duck and owl.

In this piece, I want to explore my own questions about animal friendships.

What is friendship?

Friendship between two people can involve many conditions, such as “affection, sympathy, empathy, honesty, altruism, mutual understanding and compassion, enjoyment of each other’s company, trust, and the ability to be oneself, express one’s feelings, and make mistakes without fear of judgment from the friend.”1  Such qualities might equally accompany friendships between different species — it is easy to imagine that a dog shares all of these qualities from its best friend and vice versa. But not all such qualities are necessarily present in every single friendship.  Two fish might enjoy each other’s company, and feel trust for each other, without any consideration of fearing judgment from each other.

In this paper, here’s how I will define friendship:

  • a preferential association between two individuals, wherein each is more likely to choose the other to associate with than other parties.
  • This association implies that each obtains some benefit from the other, and that each is likely to refrain from harming the other.

Our language can interfere with clear thinking on this subject. Friendship formation is no different than “bonding”, though scientists may imagine the latter term is better.  “Social animals” are those that bond, but it turns out that all animals are social at some time of their life or some time of the year.  But it is not science or language that impairs our ability to understand friendships in animals. It is our unnatural state.

As professional buffoons, we watch a flock of crows and don’t spot the families, the friendships. We see a small herd of deer and don’t see the family relations. We see a squirrel going into a nest in the winter, and don’t realize his brother is already in the nest. We don’t watch long enough, or closely enough to learn such things. We don’t note important details. And we hardly use our ears and nose at all when observing. So when we look at two animals interacting, we don’t know what they are doing because we don’t know who they are. We can’t guess the sex or age or family connection of most animals we see.  So let’s assume that a look at animal friendships is fraught with peril. Maybe intraspecies friendships will help us to distinguish the players and get our attention.

Intraspecies friendships

The apparent universality of friendship should make us realize that friendships in mammals must date back to the dawn of mammals, and friendships in birds must date back to the dawn of birds. And suddenly we find ourselves very circular: “Social” animals bond.  Bonding is what defines an animal as social. Bonding is nothing more or less than friendship formation.

Friends often have qualities in common — demographics, interests, values, occupations, etc. But two individuals that are seemingly very different can also sometimes choose each other.

I do not believe that opposites attract.  Opposites may find each other interesting. When there are no other birds of the same feather to flock with, odd couples may become more likely, as familiarity with the other individual builds, and breeds comfort.

It is the probability of evident similarity in friends that adds to our surprise when a giraffe befriends a mouse, or a predator befriends a species that is typically its prey.

What species form interspecies friendships?

If friendship is a preferential association, than we’d expect friendships to be limited to those species which normally form associations of schools, herds, flocks, packs, and the like.  For example, horses, donkeys, goats, and sheep all form herds, and so the same desires that draw one of these animals to others of a herd are likely available to draw that animal to a herding animal of another species.

Omitting mating season, amphibians all seem to live solitary lives, and thus may not be likely candidates for friendships.  In fact, there are some studies that indicate that tadpoles of the American toad can recognize their kin, and prefer unfamiliar siblings over familiar non-siblings.2

Snakes normally appear to be solitary and independent, but they bunk down for the winter in communal dens.  One study showed that hibernating snakes were segregated by size, rather than sex or species.3  The koi in our pond spend the winter dozing at the bottom with those who match in size as well.  For hibernating snakes and koi, size seems to matter in the winter bedroom.

In fact, there has been quite a bit of research on friendship in fish.  One experiment on minnows and chub showed that both species preferred their own kind when all fish were unfamiliar or when all were familiar, but preferred familiar fish of a different species over unfamiliar fish of their own species. As the authors note, “Preferential association with familiar shoal mates confers a number of potentially important benefits to individuals, including improved anti-predator effects and the reduction of aggression in competitive interactions.”4  In another study, researchers found that “fish preferentially associated with and exhibited lower aggression intensity directed towards familiar groups.”5  Such behavior fits with my earlier definition of friendship.

The benefits of friendship

I once had a Russian tortoise who often had free range in the house. Once I placed her at one end of the living room, and I lay down at the other end, on my side with my bottom arm extended.  Norma came running straight toward me — or rather for my arm pit.  She plowed into my arm pit, and fell asleep.  While I was flattered to be so adored, I realized then that she had spotted a dark warm spot, perfect for sleeping. On previous occasions, when Norma and I had committee meetings on the floor, I had introduced her to my arm pit, and so her speed crossing the living room was more the result of a good memory (and the fact that there were no places to burrow in the carpeting, no other warm spots.)  But I had given Norma what she needed at that moment — darkness and warmth — and she had given me what I needed — a sense of being appreciated.

Providing mutual warmth is likely a common benefit of animal friendships. The web is loaded with images of dogs, bunnies, cats, and mice snuggling and warming each other.

Dog and cat snuggle.

Dog and cat snuggle, warming each other.

But in the image below, our hen is brooding, and the capillaries in her chest are overheated, a design that facilitates heat transfer to her eggs. She seeks to cool her chest, and while the coolest side of her eggs usually does the trick, this puppy is the perfect solution. She cools, and the puppy warms.

Hen broods puppy.

Hen cools, puppy warms. Both are happy.

Warming and cooling can be important mutual benefits of friendship.  So can grooming.  Nearly all animals have places that are hard to reach, and thus places where parasites have evolved to frequent.  Social birds may get lice, for instance, that are clever enough to choose such places on the head and neck. Mutual grooming helps form and maintain social bonds, and often requires that some degree of trust and affection is in place before it begins.  Mutual grooming may sometimes deliver small snacks to the groomer in the form of salt or tasty parasites, but likely goes much deeper than that: as a youngster, birds and mammals were groomed by their parents and they quickly learned to feel good about it.  Mutual grooming provides mutual benefits.

Hen broods puppy.

Mutual grooming in horses.

Hen broods puppy.

Lion grooms a fawn. Both are happy.

Play provides one of the most arresting opportunities for mutual delight in a friendship.  All birds and mammals seem to play when young, and dogs — who may play throughout life because of neoteny6, are particularly likely to turn up in stories of interspecific play.  Writing this piece was inspired, in fact, by my recent reading about a wild wolf who loved to play with dogs.7

Hen broods puppy.

Polar bear and sled dog at play. Both are happy.

Protection and nurturing is something that moms do with their youngsters.  Exercising a “maternal instinct” must give mothers a good feeling, and most of us feel good when we care for a dependent.  As a dependent, we feel good when we are cared for. I enjoy feeding the birds (squirrels, raccoons, foxes, opossums, fish, etc.)  And surely they enjoy being fed.

Warming, cooling, grooming, play, protection and nurturing all happen in friendship. But the benefits of friendship go beyond the immediate moment. As Seyfarth and Cheney write, “Friendships are adaptive. Male allies have superior competitive ability and improved reproductive success; females with the strongest, most enduring friendships experience less stress, higher infant survival, and live longer.”8

How common are interspecies friendships?

In one of my salt water aquariums, there are just 3 fish. Two of them, a Yellow Tang and an Engineer Goby, have been friends for over a year. “Friends” may be a stretch here. The Tang clearly chooses to spend a great deal of time in the caves of the tank with the Goby, and retreats to them when she becomes nervous. The Goby, for his part, doesn’t ever follow the Tang about, but does show restraint: he could easily eat her if he chose to, as he did a previous fourth fish in this tank.  In a much larger tank, another Yellow Tang and Engineer Goby swim, but with much more space, and many other fish to consort with, neither ever chooses to be close to the other.

Circumstance seems to have a disproportionate affect on the likelihood of an interspecies friendship.  Let mom raise baby ducks, and they become baby ducks.  Raise the little ducks with a big dog and no mom, and the baby ducks will happily choose the dog for company.9 Raise a baby hippo with its mom and family, under natural circumstances, and the baby will not likely form a friendship with any tortoises or other species in the area. But separate the baby from its family, and confine it with a tortoise about its size, and friendship might be inevitable. This was the case with Owen and Mzee, a famous friendship story available online and in books.10

Dog and her ducklings.

Dog and her ducklings.

Owen and Mzee.

Owen and Mzee.

Chelsea Marshall documents 21 interspecies friendships11 with photos. They include a yellow lab and duckling, cat and squirrel, boxer (dog) and kid (goat), Springer Spaniel and lambs, Border Collie and Vietnamese Pot-Bellied Piglets, an owl and gosling, a German Shorthaired Pointer and an owlet, a Great Dane and a fawn, a dog and a badger, a foxhound and fox kits, a cow and lambs, a dog and a baby llama, a boxer and a piglet, a chimpanzee and puppies, etc. In each story, the two animals were birds or mammals, were captive, the first usually a female with good mothering tendencies, and the second a young orphan.  Such circumstances may make interspecific friendship almost inevitable.

How common are interspecific friendships in nature?  Probably very rare.  Intraspecific friendships are likely to be more mutually gratifying and to better meet the needs of the would-be friends.

The curious relation between soldiers, parrots, horses, and dogs.

While interspecies friendships may be most likely with mother-infant combinations, the benefits of friendship are so great that adults of some species can be counted on to connect.  Where traditional treatments fail, therapy horses12, therapy dogs13, and therapy parrots14 may come to the rescue.  While a traumatized soldier might bond more quickly with an animal he or she learned had been traumatized (the birds of a feather principle), a dog, horse, or parrot who had never been traumatized might do just as well at providing friendship.

Perhaps it is not friendship, but humans seem drawn to aquariums, and there are a variety of health benefits that have been documented by watching fish in them.15 Often the fish in these aquariums are drawn to those watching them (yes, of course, they think they will be fed), but perhaps they experience comparable health benefits.

Not all that glitters

Sometimes people get confused. Not every cute photo on the Internet is about interspecies friendship. Consider these, and see if you agree:

  • Frog hugging goldfish, dog humping duck: not love, sex.
  • some pictures, particularly where all contestants are lined up facing the camera: likely posed, though there must be tolerance, and perhaps friendship, among the subjects.
  • remora attached to turtle: likely closer to parasitism than mutual attraction.
  • cat paws crayfish in the middle of the road: what’s a crayfish doing in the middle of the road?  and what does the picture taken a few minutes later look like?
  • cockatoo pulls spaghetti from pot, hands to dog:  OK, OK.  Friendship.

For Further Reading

  • Buckley, Carol. “Tarra & Bella: The Elephant and Dog Who Became Best Friends”.
  • Dagg, Anne I. “Animal Friendships”. 2011
  • Holland, Jennifer S. “Unlikely Friendships: 50 Remarkable Stores from the Animal Kingdom” 2011. “Unlikely Loves: 43 Heartwarming True Stories from the Animal Kingdom”. 2013. “Unlikely Friendships for Kids: The Dod & the Piglet: And Four Other Stories of Animal Friendships” 2012.
  • Seyfarth, Robert M. and Dorothy L. Chene “The Evolutionary Origins of Friendship” Annual Review of Psychology.  Vol. 63: 153-177 January 2012.
  • Thimmesh, Catherine. “Friends: True Stories of Extraordinary Animal Friendships” 2011.
  • Zimmer, Carl. “The Surprising Science of Animal Friendships”. Time Magazine Feb 20, 2012. Discussed here.

Show 15 footnotes

  1. from Wikipedia.
  2. Waldman, B. “Preference for unfamiliar siblings over familiar non-siblings in American toad (Bufo americanus) tadpoles.” Animal Behavior. 34:48-53.
  3.  William S. Brown, William S. Parker and John A. Elder “Thermal and Spatial Relationships of Two Species of Colubrid Snakes during Hibernation” Herpetologica Vol. 30, No. 1 (Mar., 1974), pp. 32-38
  4.  Ward AJ, Axford S, Krause J. “Cross-species familiarity in shoaling fishes.” Proc Biol Sci. 2003 Jun 7;270(1520):1157-61.
  5. Edenbrow, M. and D.P. Croft “Kin and familiarity influence association preferences and aggression in the mangrove killifish Kryptolebias marmoratus” Journal of Fish Biology. Volume 80, Issue 3 March 2012 Pages 503–518
  6. see Man’s Best Friend
  7. Jans, Nick. “A Wolf Called Romeo”.  Mariner Books. 2015. Read about it first here.
  8.  Seyfarth, Robert M. and Dorothy L. Chene “The Evolutionary Origins of Friendship” Annual Review of Psychology.  Vol. 63: 153-177 January 2012.
  9. “Imprinting” likely launches such a friendship, as Konrad Lorenz discovered.  But friendship seems to quickly develop.
  10. Craig Hatkoff, Isabella Hatkoff (Authors), Peter Greste (Illustrator) “Owen and Mzee: The True Story of a Remarkable Friendship” Scholastic Press. 2016. Also YouTube videos such as this.
  11. Marshall, Chelsea “The 21 Most Touching Interspecies Friendships You Never Thought Possible” BuzzFeed.
  12. see these images.
  13. see these images.
  14. see these images.
  15. See “Aquarium Therapy” in Wikipedia.