Living solitary is neither easy nor primitive

Makuya, L. and Schradin, C. (2024). Costs and benefits of solitary living in mammals. Journal of Zoology, https://doi.org/10.1111/jzo.13145

In a world full of hectic, stress and conflict, many of us dream of a simpler life: Being alone in a mountain hut, in the desert, out in nature, far away from the crazy crowds. But would living solitary really solve our problems, would it be an easy life? Or would it lead to other problems, other difficulties we don’t even think about?

For decades, field biologists have been going to exotic places to study why animals live in groups. Jane Goodall pioneered these studies, observing chimpanzees in the jungle of Tanzania. Many researchers studied large troops of baboons in the savannah, and Tim Clutton-Brock the adorable meerkats in the Kalahari. One basic assumption was always: Group-living evolved from a primitive solitary ancestor because it has many benefits. The fact that all these studies also showed significant costs of living together, with mean and egoistic conspecifics that try to dominate and use others, did not change this point of view: Group-living is superior to solitary living! Or is it? New emerging research starts to tell a different story, at least for mammals.

In our recent review in the Journal of Zoology, we show that solitary living is relatively rare in mammals, occurring in only about 22% of the studied species. In other words, less than 1 out of 4 mammal species is solitary living. And study after study finds that the ancestor of different groups of mammals was not a primitive solitary species: In elephant-shrews, insectivores, ungulates, primates, and even in marsupials, the ancestor might instead have been pair-living!

But if solitary living is not primitive and basic, then what? Then it must be a special adaptation? Maybe solitary living is not primitive, but an adaptation to specific environments. And indeed, solitary living species can be very successful. Take the harsh Kalahari in the middle of southern Africa, reached by only a few clouds from the oceans, such that rainfall is rather rare. The Kalahari semi-desert is famous for its highly social meerkats, and also home to the eusocial Damaraland mole rat. It has been argued that this complex form of group-living evolved as a response to the harsh environmental conditions of the Kalahari. But: Only 2 out of 90 mammal species living in the Kalahari are cooperative breeders, while 30 species are solitary living. Clearly, solitary living was overall more successful to cope with the harsh Kalahari than living in complex groups.

The problem is: We know very little about solitary living mammals. What are the problems they are facing, and what are the benefits? Meerkats warn each other of predators, but how do solitary mammals deal with this threat? On the other hand: Nearly all meerkats, apart from the dominant breeding pair, bear the extreme evolutionary costs of not being allowed to breed, while solitary living mammals can breed as they want. Or can they? What are the environmental constraints for the success of solitary species?

The basic answer is: We don’t know. For decades group-living species have been studied in detail, and solitary living species have been ignored. Because they are primitive and not interesting. Interestingly, without understanding the costs and benefits of solitary living, we will never understand why group-living has evolved. And we might fail in protecting the many solitary species in the world we are changing.

However, more and more studies on solitary mammals are emerging and they show fascinating and unexpected results. 1. Solitary individuals are not distributed randomly, but in kinship clusters. Females often have close female kin as neighbours, reducing competition. 2. Solitary mammals are often surprisingly amicable towards each other. In a few species like the European hamster, aggression and intolerance of conspecifics is the mechanism leading to solitary living. In other species it might rather be the absence of being attracted to each other. 3. Solitary species are not asocial but have complex social interactions with their neighbours, which are often characterised by tolerance and even amicable behaviours. In sum, it is likely that there are different forms of social systems that lead to solitary living.

Why is this important?

Solitary living in mammals is not a primitive default stage that needs no scientific explanation. Instead, we have to study why some species are solitary living in the same way as we investigated for decades why some species are group-living. Without a comprehensive understanding of solitary living, our understanding of mammalian social evolution is significantly constrained and conservation efforts for solitary mammals will often be inadequate. We hope our review will motivate research groups worldwide to study the socio-ecology and broad diversity of solitary living mammals.

Lindelani Makuya and Carsten Schradin