Lonesome George, the last of the Pinta Island tortoises, died in 2012, leaving no offspring. His solitude was imposed upon him by humans who killed the rest of his species and brought goats to his island in the Galapagos, destroying their habitat. But Lonesome George probably didn’t much mind being on his own. Tortoises are generally solitary, coming together only to mate. The same is true of most reptiles and even many mammals, including bears, moose, tigers, sloths, platypuses, rhinos and pangolins.
As social creatures ourselves, it is only natural that we are fascinated by animal societies, from wolf packs to ant colonies. But to understand sociality, we must look at the flip side: why do some animals prefer to go it alone? Surprisingly, researchers have paid little attention to this question. “People are only interested in group-living species with complex societies,” says at the Hubert Curien Pluridisciplinary Institute in Strasbourg, France. “But if you don’t understand why, in many cases, solitary living is the better solution, you also miss a total understanding of group living.”
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Now, Schradin and a handful of pioneering biologists are addressing this oversight. Already, their research reveals that being solitary isn’t simply the ancestral lifestyle for mammals, but an evolved strategy, a specialised way of living, with its own pros and cons. What’s more, animals considered solitary aren’t necessarily antisocial: it turns out that many of them have structured social networks, even if they spend most of their time by themselves. These creatures give us a new perspective on sociality, helping us see why other animals, including humans, evolved to live together.
Biologists have long assumed that being solitary was a primitive state in the evolution of animals needing no special explanation. Mammals evolved from reptiles around , so, like their reptilian forebears the cynodonts, these small, shrew-like creatures were thought to live alone. And this lifestyle was believed to have largely persisted to this day. “I grew up with the idea that, because most mammals are small, most are solitary living,” says Schradin. These assumptions seemed to be confirmed by a 2013 study indicating that .
However, many of the supposed asocial species in the analysis hadn’t been studied in the wild because they are nocturnal or elusive. So, Schradin decided to take a closer look at the evidence. Working with at the University of the Witwatersrand in South Africa, he focused only on mammals for which there was reliable information in natural populations. The research, published last year, found that less than a quarter – only – were mainly solitary, defined as both adult males and females sleeping and foraging alone most of the time.
The relative rarity of solitary living suggests it may not have been the lifestyle of the common ancestor of all mammals after all. Indeed, some fossil evidence adds weight to this idea: early cynodonts lived in burrows around 250 million years ago and some of these have been found to of several adults or adults with offspring, hinting at group living. Recent research in primates also supports the idea. The common ancestor of all primates was a small, nocturnal animal that lived in trees. Lemurs are among their closest living descendants, emerging some 70 million years ago, and although they are generally considered to be solitary, Schradin and his colleagues found that they are, in fact, . Furthermore, when they analysed field data from 493 populations of 215 primate species, it emerged that going solo was the primary way of life in just 3 per cent of populations and 6 per cent of species. Some other species were solitary foragers, but they shared their home range or sleeping site in pairs or groups. Based on these lifestyles of modern primates, the researchers concluded that their earliest ancestors were most likely to have .
Why go solo?
If solitary living isn’t the ancestral state of primates or other mammals, then it must have evolved as an adaptation to specific environments. So, what kind of environment drives animals towards this lifestyle? In an attempt to answer that question, some biologists have turned to social animals, which they know far more about.
One idea that has been put forward to explain why animals group together is called the resource dispersion hypothesis. If food is plentiful in localised patches and sparse elsewhere, those patches can feed many animals, and they will form groups to defend their resources. To flip this around, if food is evenly distributed and not shareable, that might favour living alone. A 2024 study found support for this hypothesis among martens and related animals, a group of carnivores considered to be highly solitary. , then at Cornell University, New York, and his colleagues analysed camera trap data from seven species across the world, taking the likelihood of animals appearing on camera in groups as a measure of their sociality. There was wide variation among them, with the yellow-throated marten (Martes flavigula) of East and South-East Asia seemingly the most sociable of the bunch. Crucially, though, the sociality of each species was , with animals more likely to form groups if they relied on patchy resources such as fruit, insect nests and large prey.
Nevertheless, this principle doesn’t explain all the variety in social systems between related species, says at the University of Göttingen in Germany. He has been in Madagascar for 30 years. “They’re in the same forest. They’re literally sitting in the same trees. Two of them are group living, three of them are pair living and three of them are solitary,” he says. “Maybe they’re feeding on slightly different things, but there’s nothing obvious that would come to mind [to explain their different social systems].”
Another factor commonly cited as a reason for group living is the risk of predation: strength in numbers can protect you from being eaten. But this is only really true for large animals, says Schradin. “Even when there are 100 small mice, they can do nothing against a jackal or a bird of prey.” For smaller animals, being a loner can be the safer option because you are harder to spot. Group-living animals must also cover long distances to forage, whereas going solo means you can find food within a small area, which reduces your risk of predation while moving around. Again, this can’t be the whole story because going solo can be a successful strategy for predators too. An estimated 80 per cent or more of species in the order Carnivora, which includes cats, dogs and bears, are described as solitary, socialising only to mate and bring up young.
A lifestyle with surprising benefits
While food resources and predation risk might explain why some animals have evolved to be solitary, another possibility is that it brings some surprising benefits. Foraging in a smaller range, for example, means that an individual doesn’t need to expend so much energy in search of food. Schradin and Makuya propose that this is one of the three main advantages of solitary living. The second is that living alone reduces your chance of picking up parasites or infections. Third, being a loner also means you avoid the stress that comes with social interactions – including competition over mating. Loners have a better chance of reproducing, especially compared with gregarious species, such as meerkats, where a few dominant individuals tend to monopolise mating.
Many of these solitary animals have a kind of social network that we have no idea about
As well as different possible reasons for going it alone, it turns out there are . For a start, there is huge variation in how aggressive solitary animals are to other individuals. Hamster mothers, for example, even attack their own offspring when they reach maturity and chase them away, whereas bush Karoo rats (Myotomys unisulcatus) are far more accommodating. They live in a semi-desert region of South Africa and are solitary most of the time, but they form small family groups after the breeding season. The rats build stick lodges and offspring inherit these from their mothers, so rats that live near each other tend to be related and they share feeding grounds. “They are also very tolerant of each other,” says Makuya, who . “This is in contrast to what is believed about solitary species, that they are solitary because they are aggressive.”
In fact, many apparent loners turn out to have hidden social lives. One of the most surprising examples comes from a . Camera traps and GPS data showed they frequently shared their kills with unrelated individuals – and the beneficiaries often reciprocated. Brown bears can be surprisingly social too. When a team led by at the University of South-Eastern Norway analysed in Sweden, most of the interactions were found to be meetings between males and females during the mating season. However, the researchers also saw males and females and family groups socialising throughout the year. “When they’re not hibernating, they seem to be associating with multiple individuals,” says Heeres. “But we don’t know what the reason is for these associations that happen outside the mating season.” Perhaps males are keeping an eye on their rivals or checking out females that will soon be ready to mate. Another possibility is that they are congregating around food sources. In North America, this happens at salmon streams and garbage dumps, he notes, although such features weren’t present in his study area.
“I think because solitary animals are so elusive most of the time, we have underestimated their sociality in general,” says Heeres. They may interact in ways that are less obvious, through scents and sounds, for example. “I think that’s one of the biggest gaps in what we should learn from them. I think we would find some remarkable examples – that these solitary animals are having a whole kind of [social] network that we have no idea about.” Kappeler makes a similar point. “I don’t think we have a full understanding of the variation that exists out there,” he says. “Certainly, only a tiny minority of species is truly solitary.” Given what we now know, he suggests that labels like solitary and group-living may be too simplistic.
A better understanding of how other animals live won’t just reveal unexpected variety and clarify the evolutionary origins of their social lives, it could also shed light on the neurological mechanisms that control social and antisocial behaviour. “One of the motivations for studying solitary behaviour is that even in social creatures like us, not everyone is social at the same level. We have a spectrum,” says at the Weizmann Institute of Science in Israel. Some of us suffer when we are cut off from other people, as we were during covid-19 lockdowns, for instance, whereas others are much happier on their own. Traditionally, mice and rats have been used as a lab model to study the mechanisms that control human social behaviour because they share much of our genetics, neurology and physiology. However, both are social animals. To get the full picture, we need to look at a range of species with different degrees of sociality, says Kimchi. “I think if we understand this in animals, we can understand better about social-related neuropsychological conditions [such as autism],” she says. “It can give us more hints where to look.”
Hypersocial and antisocial mole rats
Mole rats might be a good place to start because seemingly similar species can have very different social lives. Middle East blind mole rats (Spalax ehrenbergi), for example, are hypersolitary. They live alone in underground tunnels and are extremely hostile to others. “If they accidentally interact, one of them will be injured or even dead,” says Kimchi. This is in stark contrast to naked mole rats (Heterocephalus glaber). They have a similar subterranean lifestyle, but live in colonies with dozens or hundreds of individuals. Like those of some ants and bees, these colonies have a queen and a small number of breeding males, while the rest are workers. Why the two species are so different is still a mystery, but Kimchi suggests we should look at them another way. “You can think about [the colony] as a superorganism. And this superorganism has a solitary lifestyle,” she says. Although the colony residents tolerate one another, they attack and kill invaders from other colonies with as much aggression as blind mole rats display towards other individuals. This raises interesting questions about the extent and power of sociality that might also be relevant to our relationships.
So much about solitary animals remains to be discovered. And the potential payoffs of this research are intriguing. To help us get closer to understanding why some animals club together while others go it alone, Makuya and Schradin have recently set up a virtual where researchers can share their findings and ideas. “There are more and more people studying solitary species and coming up with the correct questions, but they’re not connected yet,” says Schradin. He believes that with more knowledge about these creatures, we will be better able to protect them. Like Kimchi, he also thinks studying them can help us understand ourselves.
“One big lesson is that you don’t have to be asocial to be solitary,” says Schradin. “You can live alone but still have meaningful social interactions with your neighbours.” For humans, social isolation is strongly linked to ill health, but we can also benefit from spending time alone, and solitary animals could help us better understand these benefits. “Maybe we should see that, for some people, or for yourself at some stages of your life, it’s also OK if you’re looking to be left alone and to avoid social conflict.”
