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In the spring of 1987, I stooped over the desk in my shared student office in Cambridge, England, running my finger across a map of Papua New Guinea and squinting at the tiny typescript. I was trying to establish the location of a cluster of tribes in the rainforest known as the Baining. Only one of those tribes was known to anthropology; the others were a mystery. As a young Ph.D. researcher, my ambition was to make friends with people as culturally different from me as it would be possible to find anywhere on the face of the Earth and to ask if I could live with them for two years.

After a very long series of flights and an arduous trek through the rainforest, I finally came face-to-face with the Mali—one of those elusive Baining groups I had been dreaming about. To my surprise, the Mali were much more like me than I could possibly have imagined. It wasn’t just that they had the same basic material needs and wants, but they also shared with me—and my compatriots back in England—fundamentally similar moral concerns, supernatural inclinations, loyalties and rivalries, as well as a deep appreciation of music, dance, and other artforms. Nevertheless, the Mali also had a great deal of knowledge and wisdom that was new to me. For one, they knew how to build very large families.

By large families I don’t just mean that they had lots of offspring—which was indeed true of many households in the village. What I mean is that they had the ability to conceptualize vast portions of humanity as one giant family, descended from common ancestors and related both physically and spiritually. Now, more than 35 years later, in a world increasingly torn apart by destructive regional conflicts and forms of polarization, I cannot help wondering if the wisdom of the Mali could help us to rethink the way we manage cooperation in the 21st century.

The Mali lived on a tropical island roughly twice the size of Wales, studded with mountains and active volcanoes and blanketed in jungle. They built their houses in small clusters around a village clearing of well-trodden earth. They used timber from the surrounding forest to create uprights and rafters, strips of interwoven bamboo for the walls, and long strands of grass to thatch their roofs.

They fed themselves on tubers and fruits grown using methods of slash-and-burn horticulture, supplemented by the meat of domestic pigs and fowls as well as bats, snakes, marsupials, and wild boar hunted in the forest. Some people grew coffee or copra to sell for small sums of money to traders. The fate of such crops was a mystery to their producers. Clearly you couldn’t eat the stuff. But the coins you received in return were very useful. You could buy bush knives and axes, which were more efficient at cutting down trees or butchering animals than traditional stone tools. But money also had another use. It could bring you closer to your ancestors.

I soon realized that families were more than just groups of living relatives. They encompassed a multitude of invisible spirits of the dead, who moved among us, taking an active interest in our comings and goings. As one of the leaders in my village put it, “The eyes of the ancestors see all our thoughts and deeds, and the things we say. The ancestors can sometimes discern observances and violations of the law, even though we are not aware of what we are doing” (my translation).

The red coloration should be created by lacerating our tongues and spitting out the blood.

The ancestors were thought to see into our hearts and minds and evaluate our behavior, as well as to observe everything we said and did. Every day the people in my village left out offerings of food and water in special temples so that the ancestors would come to eat and drink. When the spirits of the deceased gathered in this way, they were said to form the “village government.”

The village government comprised hundreds, possibly thousands of ancestors. Many were loved ones and remembered fondly—mostly victims of common diseases like malaria, which continually claimed people’s lives. But many were also remembered only by name, being from generations long past. The members of the village government loved them, and their living descendants loved them in return.

But the village members were always letting them down through their sinful ways. Somebody would forget to perform the right ritual, teenagers would sneak off to have sex in the forest, a man would promise to help his cousin in the garden and then fail to turn up. The ancestors saw it all. And that’s when money became most useful. There were special receptacles in the temple, where people could place the coins they had accrued from selling crops, and in return receive forgiveness for their sins.

This practice was inspired by Catholic missionaries. On his occasional visits, a priest would conduct a Mass and hear confessions in the village, but nobody believed his rituals did any good. People humored him out of politeness, but they knew that the real way to their ancestors’ hearts was through the temple rituals.

One day, the ancestors—gratified by the acts of devotion bestowed upon them in the temple rituals—would return from the dead and our giant family would be reunited. When this happened, it was said that the ancestors would have white skin and straight hair, just like the British, Germans, and Australians who had colonized New Guinea, bringing with them unfathomable wealth and technology.

As a young white man with passionate anti-racist convictions, I was horrified to learn about this dogma. For me, the obvious remedy was to eliminate the association between white skin, power, and wealth by establishing a more equal society in which all skin colors would be regarded with equal respect and pride. But for my Mali friends, the solution was to become one with the colonial invaders by themselves becoming white.

It was said that the returning ancestors would be rich beyond compare. And they would share this wealth with their descendants who, in turn, would peel off their earthly skin, and the many wounds and scars acquired from hard living, to discover unblemished white skin beneath. Then all of us would enjoy a period of eternal peace, health, and prosperity together.

The community I lived with frequently gathered in the village meeting house, to discuss how best to strengthen our bonds with the ancestors and hasten the miracle of their return. Sometimes people would ask me questions about England and listen in astonishment to my tales about the rituals of my college in Cambridge—especially the strange rules relating to dining etiquette, such as the wearing of gowns, saying grace in an ancient language, and passing the port in one direction only.

People wanted to know if the food was laid out in the same way as in our village temples? Did all the paintings on the walls around the dining hall depict our ancestors? Why was King’s College Chapel so big? How many ancestors did the college have? Sometimes people would ask me probing questions about why I was sent to live with the Mali. How did I acquire the seemingly endless supply of money in my pockets?

I explained that my doctoral research was funded by a grant from the Economic and Social Research Council—public funds, raised from taxes. There was no other way to explain this in the local language than to use the word “government.” But as soon as I uttered it, I realized I had been misunderstood. The entire community fell silent. “So, you were sent to us by the village government?” asked one of the elders.

Later, one of my closest confidants told me of a rumour that my presence in the village was a sign. “A sign of what?” I wanted to know. He said they believed I was an ancestor, and this was a portent that more would follow. It seemed to me quite incredible that anyone could mistake me for an ancestor. I was possibly the most incompetent person in the village when it came to almost anything of importance: gardening, hunting, cooking, or even speaking properly. “And what do you think?” I asked. He paused, apparently mulling it over. “I think you are an ancestor,” he said at last.

Becoming an ancestor was one way of joining the Mali family—albeit rather unusual and not without its drawbacks. Another way was to be initiated into the tribe. I first learned about this from one of the elders, who had become used to my endless questions about the arts of warfare and raiding, how to catch pythons without getting killed, or which kinds of magic were needed to find a lost dog. When I started asking about initiations, his voice dropped to a whisper.

Most of what he told me involved the secret techniques for making dancing costumes and masks. But two aspects of his account became lodged in my memory more firmly than the rest. One was that the red coloration on the masks should be created by lacerating our tongues and spitting out the blood to use as paint. The other was that we should have penis extensions fastened to us by a belt secured at our back by sharpened bones driven through the skin at the base of our spines.

When my time came to be initiated, I got off lightly. I was put through a sanitized version of the rituals, in which red paint was used instead of my blood and my dancing costume was secured by a knot, without having any sharp objects thrust into my body. But even though I was spared the most painful elements, I will never forget the pride I felt when I danced before the entire community, along with the other initiated men dressed in all our finery. I was part of the Mali family—whether or not I was also one of its progenitors.

The entire community fell silent. “So, you were sent to us by the village government?”

Then one day there was a terrible storm. As high winds and torrential rain swept through the village, some of us huddled together in the meeting house, trying to keep the fire going. People began to tell stories about tropical storms, and someone mentioned the time that a cyclone hit Australia, to the south of us.

Stories about Australians whose houses had been destroyed had traveled far and wide. To my astonishment, I learned that the people in the community and hundreds of others like it throughout the rainforest had been so appalled to hear of the suffering of their “brothers and sisters” in Australia that they decided to donate much of the money they had collected in their temples to help victims of the storm.

Nobody seemed to realize that their rich neighbors in Australia were mostly well insured or that their emergency services were well equipped to help them get through the disaster. Or maybe it didn’t matter. The way they spoke about it, the Australians were like family to them. And when family is in need, you do what you can. That’s what family is for.

I returned to Cambridge in the fall of 1989, and I was soon back at my desk surrounded by books. But I found myself frequently reflecting on this extraordinary act of generosity and especially the idea that family could be extended to include people from other cultures one will never meet and even to the dead who may one day return. As an anthropologist, I had read many theories about kinship, but none of them seemed to capture exactly the ways of thinking I had encountered in the rainforest. It gradually dawned on me that kinship boiled down to the sharing of something essential to oneself with other people.

This special “something” could be some feature of your body. Where I grew up in England, we talked about shared blood among family members. In other places it might be bones or even something intangible like the Polynesian concept of mana—the spiritual power or lifeforce associated with heritable positions of high social status. But however we conceptualize it—and whatever we call it—this essence is something we inherit from our ancestors. It connects us to them and to each other.

This is how we create tribes based on shared descent—and how we create the idea of ethnic groups whose shared ancestry is passed down in the form of traits we can see on the body. My family in the rainforest connected themselves in this way to me and even to people in Australia they had never met. This helped me to understand better their belief that we all share the same skin underneath.

But there was another way of sharing essences with other people. This way involved sharing something in the mind rather than in the body. It involved sharing experiences and memories that were essential to your autobiographic self, your sense of who you were as a person. That’s how the initiation rituals worked. They created memories of emotionally intense, life-changing experiences that were shared with other members of the group and with their ancestors.

If the tribe members in Papua New Guinea were capable of using these same methods of group-bonding to help people in faraway countries, then maybe they could teach us how to tackle cooperative problems on an even larger scale. By realizing that we are indeed one global family—descended from common ancestors—perhaps all of us may be capable of the same acts of generosity.

Lead photo:  The Mali, an Indigenous group in Papua New Guinea, in a ceremonial dance. Photo courtesy of Harvey Whitehouse.

• Harvey Whitehouse

  Posted on December 9, 2024

  Harvey Whitehouse’s new book is Inheritance: The Evolutionary Origins of the Modern World. He is an anthropologist and the director of the Center for the Study of Social Cohesion at the University of Oxford.

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In this issue of Nautilus, we bring you a new understanding of kinship—and how it can unite us, ocean waves, and even distant stars.

In the rainforests of Papua New Guinea, the Mali tribe has long practiced a valuable skill: building huge families. These aren’t genealogies created through blood and marriage, mapped out into discrete trees. As the British anthropologist Harvey Whitehouse discovered in his years of living among them, “they had the ability to conceptualize vast portions of humanity as one giant family … related both physically and spiritually.”

When a natural disaster struck Australia, a well-resourced country more than a thousand miles away, the tribe, which had lean economic resources, felt inspired to pool what they had and donate it to disaster relief. To the Mali, the Australians weren’t former colonizers. They weren’t even neighbors. “The way they spoke about it, the Australians were like family to them,” Whitehouse writes, wondering if this radical disregard for geopolitical, economic, and historical boundaries could help illuminate a different way forward in a deeply fractured century.

But kinship extends far beyond other humans, even other animals. We seem driven to find relatedness throughout the natural world, as Nautilus associate editor Kristen French discovered on a recent reporting trip to Brazil. There, she unpacked her surfboard and communed with the first ocean wave to be granted personhood status. It now has its own right to existence, protected under law. The wave is opaque with sediment, polluted, hard to surf. But as one local anthropologist and surfer told French, it “brings to us this kind of request for listening … to understand the language of nature,” an effort to remind people they are also a part of the planet and its cycles.  

On the flipside, some kinships we create can work against us. A growing view of artificial intelligence as kin to the human brain is eroding our sense of what’s special about our own abilities to reason and make good judgments. As philosopher and AI ethicist Shannon Vallor explains to Nautilus contributor Philip Ball, large language models that employ artificial intelligence are superficial analogies to the brain. In fact, Vallor says, LLMs are more like “mindless toasters that do a lot of cognitive labor without thinking.”

In the stories ahead, we’ll explore these unexpected minefields of our quest to relate to AI, surf the protected wave in Brazil, and travel to Papua New Guinea to learn what the concept of family can truly mean. We’ll also learn about what inspires animals to adopt, discover siblings among stars, and more.  

It turns out that what connects us is bigger than we thought.

Lead image: melitas / Shutterstock

• Katherine Harmon Courage

  Posted on December 9, 2024

  Katherine Harmon Courage is the executive editor at Nautilus.

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Scattered across the universe are stars that shine subtle hints of a shared ancestry. They might be thousands of light-years apart, almost indistinguishable from other, nearer stars that surround them, but similarities—their ages, their compositions, and their orbits—reveal a common origin story. And astronomers can now use these stellar kin-groups to reconstruct the chaotic, often violent, history of galaxies.

Galaxies are the bustling cities of the universe, home to almost all its stars, and through their histories, we can learn about the grand forces that shape the organization of matter.

To trace these stellar genealogies, scientists look to the stars’ conception. Stars are rarely born alone. Instead, a single massive cloud of gas and dust—called a giant molecular cloud—filled with a particular proportion of elements will spontaneously collapse and fragment, popping out tens, hundreds, and sometimes thousands of stars at once. The famous Orion Nebula, a delight to backyard astronomers, is one such active star-forming region.

But shortly after their birth, these stars begin to leave the nest. While initially tightly knit, they do not possess enough gravity to bind to each other. Any little disturbance—a passing neighbor nebula, gravitational interactions with each other—sends the stars farther away from each other on a path chosen through sheer random chance, like spilled salt scattering on the counter. In only a few million years, the stars, once birthed from the same gas cloud, can be tens of thousands of light-years apart, each one set to follow its own orbital trajectory around the galaxy.

It’s in the details—a similar orbital origin point, a mutual age, a common abundance of heavy elements—that stars retain a blueprint of their birthplace. And scientists are now able to start charting these pasts.

Shortly after their birth, stars begin to leave the nest.

By matching these properties to that of the sun, for example, in 2014 astronomers were able to find our own first-known stellar sibling. Located in the constellation Hercules and sitting 110 light-years away, the star, named HD 162826, can be seen with a small telescope. The discovery came as a surprise. Even though the sun likely has thousands of siblings, they were thought to be so far away by now as to elude confirmation of a match. But HD 162826 just happened to be close enough that the team could confirm its abundance of elements and reconstruct its orbital motion, indicating that four and a half billion years ago, that star and our sun likely shared a point of origin.

Essentially all stars belong to one such family group or another. If the group is young enough, such as the well known Pleiades cluster, then the identification is easy. There simply hasn’t been enough time for the members to scatter away, and the stars still live in the same neighborhood. But most stars cannot be matched to their siblings; it takes extensive surveys covering millions, if not billions, of stars to tease out the signatures of a few kin groups. The vast majority of those identified groups are easily identified as members of the Milky Way.

But among the 300-odd billion stellar denizens of the Milky Way, there are some groups that simply … don’t belong. They have an odd abundance of elements, or strange orbital properties. They don’t share the genetic properties of the bulk of our galaxy’s stars.

Take, for example, the oddly named Gaia Sausage, discovered in 2018. The “Gaia” comes from the European Space Agency’s Gaia spacecraft, which has amassed, so far, a catalog of more than 2 billion stars with detailed information on their properties. The “sausage” part is less obvious. When astronomers plot the circular motion versus the radial motion of stars in our galaxy, a large elongated, sausage-like lump appears in the plot, made up of a collection of stars unlike the others in the Milky Way.

But these stars in the Gaia Sausage haven’t remained clustered together. They are currently scattered all around the Milky Way, their thin threads of commonality all that betrays their shared history. They have a common proportion of heavy elements, or “metallicity.” And their orbits are extremely elliptical, bringing the stars brushing up against the core of the galaxy then flinging them back out more than 60,000 light-years away. In their commonality, they tell the story of a once-mighty galaxy eventually consumed and obliterated.

Cosmologists give this process an innocent-sounding name: hierarchical galaxy formation. But what that means is that our contemporary Milky Way, like all massive and proud galaxies in the universe, has lived a violent life, and only acquired its present-day bulk through devouring its neighbors.

Billions of years ago, there were no galaxies. In this period, some 100 million years after the Big Bang, there weren’t even any stars. There was just a relatively smooth continuum of hydrogen, helium, and dark matter spread across the universe. But within that continuum, there were small density differences, places of slightly higher or lower density. Over time—hundreds of millions of years—those regions of higher density accreted more and more material, forming the gravitational seeds that would eventually grow up to become galaxies and form stars of like-histories. As smaller galaxies venture too close to large ones, their constituent stars get consumed and scattered within them.

We can only understand the details of galactic mergers through computer simulations that recreate these titanic events, tracing them through the shared lineages of these ancient, subsumed stars. Through these simulations, cosmologists see just how cataclysmic the process is.

In the case of a minor merger, when a large galaxy such as the Milky Way consumes a small one, the process might look something like this: The smaller galaxy feels a different gravitational pull at different locations, causing it to stretch out. It elongates, turning from a dense spherical clump to a long, thin stream. This stream then plunges headlong into the consuming galaxy, where its individual stars are then strewn about into random orbits.

In the details, stars retain a blueprint of their birthplace. 

After a few billion years, you would never know that anything had ever happened.

The only clues we have today are the common origin of those far-flung stars. But the Gaia Sausage is not the only dispersed ancestral group. There are other such strands of these hidden histories. The Sagittarius Stream, discovered in 2002 is a “ghost” remnant of the Sagittarius Dwarf Elliptical Galaxy. It consists of a thin strand of stars and gas ripped from its parent galaxy looping around the core of the Milky Way; we seem to have caught this merger event towards the end, but not yet at its final moments.

And there are more such streams: the Arcturus, the Helmi, the Palomar 5. There is even the Monoceros Ring, a community of stars that make three complete circles around the Milky Way. Each stream represents a distinct kinship, a shared heritage from a once-intact galaxy otherwise lost to history.

Some former galaxies were defeated and swallowed by the Milky Way so long ago that we can barely discern them. Hypothesized in 2020, the Kraken galaxy is thought to have contributed roughly 10 percent of the Milky Way’s present-day collection of globular clusters—tight dense clumps of stars that retain their own shape. In this case, the remnant population of the Kraken resisted further obliteration.

These smaller galaxies consumed by the Milky Way also alter their host; they change the composition, structure, and chemical makeup of their newfound home. Another name for the Gaia Sausage is Gaia Enceladus, named after the mythological giant buried under Mount Etna and responsible for its earthquakes. For the Milky Way, the collision with Gaia Enceladus disrupted its prominent thin disk of stars, puffing it out and causing the formation of an ancillary, thicker disk.

Some ancient star cousins might even be in our neighborhood of the galaxy. Recently, a team of astronomers discovered 20 stars within a few thousand light-years of the sun that all might share a common ancestry. Given their particular makeup of heavy elements and the scattered nature of their orbits, the astronomers think that they are the leftovers of a truly ancient galaxy, named Loki, which merged with ours when the Milky Way was first coalescing in the darkness of the early universe, some 11-12 billion years ago.

But those who live by the sword, die by the sword—even galaxies. The Milky Way has merged, acquired, destroyed, and cannibalized countless neighbors, engulfing their surviving stars and matter within its vast bulk. But those conflicts were all unequal, against foes far smaller than our own galaxy.

Sitting 2.5 million light-years away is a true peer to the Milky Way, the Andromeda Galaxy, resplendent with more than 1 trillion stars, and roughly equal to our own galaxy in total mass. And it’s headed right for us. In about 5 billion years, our two galaxies will begin to merge together, a process that will take more than half a billion years to complete.

The new mega-galaxy will be unrecognizable. No spiral arms, no flat disk. Just an elliptical lump of stars mixed together in randomized orbits. Perhaps some future astronomer, living in an age well after our own sun has died, will scan their heavens and find a population of stars that share a common age and metallicity; a family born in the Milky Way but flung distantly through that reorganized cosmos, the faintest of patterns tying them back to a vanished homeland.

Lead image: McCarthy’s PhotoWorks / Shutterstock

• Paul M. Sutter

  Posted on December 9, 2024

  Paul M. Sutter is a research professor in astrophysics at the Institute for Advanced Computational Science at Stony Brook University and a guest researcher at the Flatiron Institute in New York City. He is the author of Your Place in the Universe: Understanding our Big, Messy Existence.

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The Gir Forest, a sere landscape of teak, acacia, and jujube trees in western India, is home to the world’s last remaining wild Asiatic lions. Park rangers track the 650 cats’ every move to protect them, and scientists have been following the endangered population’s triumphs and tragedies since the mid-1990s in one of the longest-running carnivore studies in Asia.

In December 2018, rangers spotted a Gir lioness with an odd litter of cubs: One of them was a young leopard. “I got the information, went there, and just couldn’t believe my eyes,” says Stotra Chakrabarti, a behavioral ecologist at Macalester College in Minnesota, who was then a graduate student doing field work in the area.

Lions and leopards compete for the same resources, so the usual reaction of an adult of either species encountering a youngster of the other would be to kill it on the spot. But for the next 45 days, the rangers and researchers watched as the leopard cub nursed from the lioness, noshed on kills she made, and romped with his adoptive lion siblings.

This offbeat family idyll was short-lived: In February 2019, the leopard cub turned up dead near a watering hole. His small body looked perfect from the outside, unscathed, but an autopsy revealed a femoral hernia—a birth defect that may have caused his biological mother to abandon him and eventually led to his death.

Such relationships seem like an evolutionary paradox at first glance.

Years later, Chakrabarti remains “completely stumped” by the unusual adoption. “I will probably never see this again,” he marvels.

The word adoption “is very anthropomorphically charged,” Chakrabarti acknowledges. Adoption is known widely in human societies but is more likely to be thought of as a behavior that sets us apart from other animal species rather than a practice that unites us. Yet relationships in which an adult animal takes on a parental role for a young one that is not her or his own offspring, while rare, are widely documented across the animal kingdom. Sea otters adopt, as do elephant seals, gulls, dolphins, elephants, cheetahs, penguins, storks, African wild dogs, and a whole troop of primate species.

Such relationships seem like an evolutionary paradox at first glance. Feeding and protecting a juvenile is costly. Evolution is supposed to be about the promulgation of one’s genes; why invest time and energy in the survival of another’s, potentially at the expense of your own? Yet animal adoption may also illuminate the underpinnings of a sensibility common to humans and other animals.

Most animal adoptions occur within species, often within extended family groups. Orphaned chimpanzees may be cared for by older siblings, and among red howler monkeys in Venezuela grandmothers sometimes step in to care for a daughter’s infant. These observations are easily explained by the evolutionary principle of kin selection, which holds that seemingly altruistic behavior is evolutionarily advantageous when it benefits an individual’s relatives, who after all share many of the individual’s genes.

But sometimes the connections underlying adoptive relationships are more nebulous. When a young mountain gorilla loses their mother, the rest of the troop comes together to support them. This includes older siblings, same-age peers, and especially the troop’s dominant male, who will often share his nest to help the youngster stay warm at night.

“Care for the orphan does particularly fall on the dominant male,” says Robin Morrison, an evolutionary biologist at the University of Zurich in Switzerland who also works with the Dian Fossey Gorilla Fund’s Karisoke Research Center in Rwanda. Even if the male is not related to the adopted juvenile, he and his genes may benefit: Female mountain gorillas prefer to mate with males who are good with babies.

Animals sometimes adopt unrelated young even from outside their community. Researchers at the Luo Scientific Reserve in Wamba, Democratic Republic of the Congo witnessed two cases in which female bonobos adopted infants from neighboring social groups. Similar behavior has also been seen in Angola black-and-white colobus monkeys and Taihangshan macaques.

Those adoptions are hard to square with classic evolutionary theory. Harder still are cross-species adoptions. In addition to the lioness who adopted the leopard cub, scientifically documented examples include a troop of capuchin monkeys taking in a baby marmoset; a bottlenose dolphin caring for a melon-headed whale calf alongside her own baby; and an Icelandic killer whale seen traveling alongside a long-finned pilot whale calf. And amateur observations of cross-species adoptions—cats adopting puppies, dogs adopting fawns, and so on—abound in the form of cute animal videos on the Internet, suggesting that the propensity for these relationships may be much more widespread than had previously been acknowledged.

Among Lake Erie’s ring-billed gulls, who breed in large, chaotic colonies, about 8 percent of chicks leave their natal nest each year, sneak into a nearby one, and are accepted by the neighboring parents. This so-called reproductive error, a kind of glitch whereby animals become confused or fail to discriminate between other offspring and their own, is frequently invoked to explain animal adoption. In the big evolutionary scheme of things, it seems, it’s better for gull parents to occasionally raise a chick that’s not their own than to be hypervigilant about interlopers, which could sometimes result in pushing their own young out of the nest.

A similar hullaballoo characterizes the elephant seal birthing colony at Año Nuevo, California, where one-quarter to one-half of pups each year become separated from their mothers due to storms, high tides, or the disruptive movements of galumphing males. About one-quarter of these “orphans” are subsequently adopted or frequently cared for by other females. Most often, the adoptive mothers have lost or become separated from their own pup, and many of them are young and inexperienced. (The Gir lioness, too, was relatively new to motherhood, raising only her second litter of cubs, when she adopted the leopard.)

Lack of experience may contribute to the failure to discriminate between one’s own and other young, but researchers have suggested another benefit to adoption for these mothers: It’s good practice, yielding improved parenting skills that help the animal’s own young later on.

Surging maternal hormones may contribute to a new mother’s willingness to take responsibility for an unrelated mouth to feed, either in addition to her own young or as a substitute if she has recently lost a baby. Most of the time, an animal’s intense maternal instincts will be directed at her own offspring, says James Serpell, professor emeritus at the University of Pennsylvania School of Veterinary Medicine. But every so often, she’ll simply “glom on to whatever comes in handy.” This may sometimes lead to Raising Arizona-style capers documented in the scientific literature in which animals adopt young who don’t actually need adopting—a Tibetan macaque mother, for example, who had recently given birth, kidnapped another infant and ended up raising both babies as her own.

Maybe our tendency is to look at evolutionary forces too narrowly.

In the southwestern United States, biologists working to restore endangered Mexican wolves have put these widespread proclivities to use in a fostering program that places captive-born wolf pups into wild dens with litters of the same age.

“We rub them all together in this little puppy pile” so that captive-born pups acquire their newfound siblings’ scent, explains Allison Greenleaf, a senior U.S. Fish and Wildlife Service biologist on the Mexican wolf recovery program. Then the new, expanded litter is put back in the wild den. Mother Nature—or mother wolf, with assistance from the rest of the pack, who cooperate to raise a single breeding pair’s litter—does the rest.

The fostering program, the researchers hope, will enrich the genetic diversity of the wild Mexican wolf population, which is descended from just seven captive-raised individuals. It’s also thought that wild-raised pups will be more wary of humans and thus less likely to become nuisance animals than captive-raised wolves released into the wild as adults, Greenleaf says.

Since 2016, 126 captive-born Mexican wolves have been fostered into 48 dens, and at least 20 have survived to reach breeding age at 2 years old—a survival rate comparable to that of wild-born pups raised by their biological parents. Fostered pups have produced 30 litters of their own, and some have even become foster parents themselves.

Adoption may be more complex, but also simpler, than this search for evolutionary explanations makes it out to be. People reflexively look to immediate biological advantages as a rationale for animal behavior, says Judith Benz-Schwarzburg, an ethicist at the University of Veterinary Medicine in Vienna, Austria. But adoption shows that other animal species, especially those who care for their young, are “driven by lots of reasons”—just like humans are.

Chakrabarti, the biologist who studied the leopard cub-adopting lioness, would agree. Animals are not “automatons,” he says. “Each of them are rational, sentient beings,” with individual agency, preferences, and personality. We may never know why some feel moved to adopt.

Or maybe our tendency is to look at evolutionary forces too narrowly, on too fine a timescale. Humans and other animals with extended parental care have “had hundreds of thousands of years of evolutionary pressure to care for infants and look after them,” Morrison says. “Having strong caring instincts has been really valuable across our evolutionary history.”

Those evolutionary forces will be especially strong in the case of species, like our own, that exhibit cooperative parental care. Perhaps that helps explain why we’re the absolute champions at cross-species adoption. Although dogs and cats might have started as working additions to households, many have become more akin to “fur babies,” as the contemporary term goes; in many hunter-gatherer societies it’s common for people to bring young animals into the village and raise them as pets. This behavior may be “a natural consequence of a human propensity to care for other individuals,” as Serpell puts it.

In fact, one theory of dog domestication holds that the process began with Pleistocene hunter-gatherers bringing young wolf pups, perhaps orphans, back to camp to be suckled by human women and raised as part of the community.

An early domestication of dogs might have even facilitated other cross-species adoptive relationships. “I strongly suspect that the capacity of a nursing mother dog to accept puppies that aren’t her own may have helped in the domestication of the cat,” Serpell says. “Adult dogs are not very accepting of cats, and it would have been difficult to domesticate cats in a community that already had dogs. But if you had a nursing mother dog, you could certainly foster a kitten onto her and she would adopt it.”

Several thousand years later, these phenomena remain embedded in our daily lives. Consider the three cats who live in my house: two full sisters and a half-sister who is a few months older, we’ve been told, with a foundling backstory involving a plastic tote in the woods. As in orphan chimpanzee families, the older girl has become a mother figure for the younger two, grooming them, putting them in their place with a swat when necessary, patrolling the ramparts of the windowsills for threats to her territory.

Sometimes I watch her, stretched out on her side in the pose of a nursing mother cat, the other two lying crosswise on top of her even though all three are now fully grown. In her posture, at once patient and resigned, I can see the echoes of a lioness in a clearing in the Gir Forest. I can see a half-wild wolf suckling a half-wild kitten, and I can see a fully wild wolf unfazed by the sudden growth in the size of her litter.

Then again, maybe my own devotion to the older sister, my desire to see the caretaker well cared for, also has something in common with that lioness. Maybe animal adoptive parents are not just a curiosity but a model for human behavior: we affectionate silverbacks, we Gir lionesses, weaving relationships both genetic and not, united by a drive to care.

Lead image: vvvita / Shutterstock

• Sarah DeWeerdt

  Posted on December 9, 2024

  Sarah DeWeerdt is a freelance writer in Seattle covering biology, medicine, and the environment.

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