The study, published today in Science, shows environmental factors exert a key influence on how foraging human populations and non-human species behave, despite their very different backgrounds.
The team of international researchers analysed data from more than 300 locations around the world, observing the behaviours of foraging human populations alongside other mammal and bird species living in the same place. Their findings show that for almost all behaviours, 14 of the 15 investigated, humans were more likely to behave similarly to the majority of other non-human species living in the same place than those elsewhere.
"Previous research has explored how environmental conditions shape the behaviour of closely related species. This is the first time a broad comparative perspective has been used to systematically compare very different species - humans, mammals, and birds - across a wide range of behaviours. Our evidence shows how remarkably pervasive and consistent the effect of the local environment is on behaviour," said author Dr Toman Barsbai, from the University of Bristol and the Kiel Institute for the World Economy. "The similarities are not only present for behaviours directly relating to the environment, such as finding food, where we might expect a clear correlation, but also for reproductive and social behaviours, which might seem less dependent on the local environment."
For example, when obtaining food, there are environments where humans get a significant proportion of their calories from hunting. In these locations it was shown there are much larger proportions of carnivorous mammals and birds than elsewhere. Similar associations were also identified for reliance on fishing, how far to travel to gather food, whether or not to store food, and whether or not to migrate between seasons - with each behaviour found to be more common in humans, other mammals, and birds in some locations than in others.
For reproductive behaviour, there are large differences across populations when individuals first reproduce. In some human populations, men on average have their first child when they are 30 years of age or older, whereas in other populations men might be younger than 20. At locations where humans have children later, the local mammals and birds are similarly on average older when they first reproduce than the mammals and birds living in places where humans reproduce early. The study also showed other variables were correlated across species, including the proportion of individuals having multiple partners, how far individuals move to live with new partners, and how likely couples are to divorce.
Regarding social interactions, there are some places in the world where offspring care is more equally shared between parents than in other places, places where group sizes are larger, and places where social classes, meaning some individuals are more dominant, are more common in both humans and non-human species.
The study findings strongly indicated these behavioural similarities were associated with the local environment. Knowing the environmental conditions of a place allowed the researchers to predict what behaviours to expect there. However, it is not yet clear which environmental factors are of particular importance for specific behaviours or what the mechanisms are linking them.
"We were surprised these associations appeared across humans, mammals, and birds," said author Dr Dieter Lukas, from the Max Planck Institute for Evolutionary Anthropology in Germany. "Different species could be expected to sense and interact with their environments in very different ways. Even if they end up with the same behaviour, they might have gotten there through different paths. In particular, the flexibility that allows humans to adapt behaviour to environments around the world is probably facilitated by relying on learning from other people and building on this information over generations."
The study focused on human populations who obtain most of their food by foraging in the environment where they live. "It would be interesting to see how many of these environmental restrictions shape other societies where individuals get food through agricultural specialisation and trading", said author Dr Andreas Pondorfer, from the University of Bonn and the Technical University of Munich. "Agricultural intensification is often thought to buffer humans from the environment. Nevertheless, individuals in these populations might not be as buffered as we think and behaviours might still reflect adaptations that occurred before the adoption of agriculture."
INFORMATION:
Paper
'Local convergence of behavior across species', by Toman Barsbai, Dieter Lukas and Andreas Pondorfer in Science
Notes to editors
The following images are available in high resolution on request: https://share.eva.mpg.de/index.php/s/XRznjcYtq3eMqSW
When the embargo lifts, the paper will be available here: https://doi.org/10.1126/science.abb7481
Please see below for detailed FAQs relating to the research.
All authors are available for interview and can be reached as follows:
Toman Barsbai (School of Economics, University of Bristol, Bristol, UK; Research Center International Development, Kiel Institute for the World Economy, Kiel, Germany): toman.barsbai@bristol.ac.uk, +44 74 344 457 56, https://sites.google.com/view/tomanbarsbai/
Dieter Lukas (Department of Human Behavior, Ecology and Culture, Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany): dieter_lukas@eva.mpg.de, +49 341 658 370 83, http://dieterlukas.mystrikingly.com/
Andreas Pondorfer (Department of Economics, University of Bonn, Bonn, Germany; TUMCS for Biotechnology and Sustainability, Technical University of Munich, Munich, Germany): andi.pondorfer@gmail.com, +49 163 788 41 75, https://andreas-pondorfer.mystrikingly.com/
FAQs
What is the question we are trying to address?
We humans exhibit an enormous diversity of behaviour, which has allowed us to colonize essentially all terrestrial environments around the world. But nonhuman animals also occur in a large variety of behaviours and environments. Which role do local environments play in explaining this huge global variation in behaviour? Does where one lives shape how one behaves and do the same environmental forces apply to both humans and nonhuman animals? In our study, we explore whether living in a specific local environment constrains the behaviour of humans and other mammal and bird species who share the same environment in a similar way. If so, we should expect humans and nonhuman species who live in the same location to behave similarly. Differences in local environments would then explain part of the observed global variation in behaviour across species.
What did we expect to find?
It was not clear to us what to expect as the theoretical prediction is ambiguous. On the one hand, species who live together in one place might specialize into different niches to reduce resource competition. In this case, we would expect behavioural diversity as different species would engage in different behaviours. On the other hand, local environmental conditions might only permit a certain range of behaviours. In this case, we would expect behavioural similarity as species with similar behaviours would tend to assemble in locations where these behaviours are most adaptive. It is even less clear what to expect for humans. There is evidence that cultural processes are responsible for the large variation in behaviour observed across human societies and that we humans build our own ecological niche. At the same time, however, ecological constraints that shape behaviour in other species might also apply to our own species. A priori, we thus did not have clear expectations about how these different forces play out in the real world.
How did we conduct our analysis?
We build our analysis around an ethnographic database that provides detailed information on the behaviour of 339 human hunter-gatherer populations living in diverse environments around the world (Africa n=20, Asia n=28, Australia n=56, North America n=215, and South America n=20). The database was compiled by the archaeologist Lewis Binford and relies on anthropological observations from the 19th and 20th centuries. Our focus is on small-scale, foraging human populations because they are generally tied to a specific location and acquire food from the available local resources. For each of the human populations, we first identified all mammal and bird species that lived in the same location (i.e., within a 25km radius around the centroid of each human population). We then identified 15 behavioural variables encoded in the human database for which closely comparative data exist for the nonhuman species. We assigned the typically observed behaviour to each species and computed average mammal and bird behaviours at the different locations. We were thus able to analyse the association between human, mammal, and bird behaviours across locations.
Which behaviours did we look at?
We looked at a wide range of behaviours that reflect three broad behavioural domains: (i) foraging behaviour (i.e., reliance on meat diet, reliance on fish diet, extent of food storage, day range, migratory distance, distribution range), (ii) reproductive behaviour (i.e., age at first reproduction, extent of polygyny, existence of patrilocality, existence of exogamy, divorce), and (iii) social behavior (i.e., extent of paternal care, population density, group size, existence of social classes).
What do we find?
We find that *foraging* human populations and nonhuman mammal and bird species who live in the same location behave similarly. This local convergence of behaviour applies to foraging, reproductive, and social behaviour. In total, we found a systematic relationship for 14 out of the 15 behaviours we analysed.
What are the implications of our findings?
Local environmental conditions appear to filter for which behaviours occur at a specific location. Only those behaviour that are beneficial, or at least do not have a substantial cost, can be expressed. Most importantly, our findings show that these ecological filters operate similarly across species from very different taxonomic groups including our own species. We were surprised that local environmental conditions appear to select for similar variants of foraging, reproductive, and social behaviours in humans, mammals, and birds. Despite our behavioural flexibility that has allowed us to live in very diverse environments we humans thus rely on similar behavioural strategies as nonhuman animals in their respective local environments. Our findings offer new insights into why there is variation in behaviour across environments and cultures.
How do we know that the observed relationships reflect ecological conditions?
First, the observed relationships in behaviour across species weaken considerably when we account for variables that capture ecological conditions (i.e., biomes, latitude, altitude, and proximity to coast) in our statistical analysis. This is consistent with the idea that ecological conditions constrain behaviour. Second, associations between the same ecological variables and the different behaviours are very similar across humans, mammals, and birds. This is consistent with the idea that specific ecological conditions consistently shape behaviour across species. Third, in line with this argument, human behaviour from one location is similar to nonhuman behaviour found at other locations with similar ecological conditions.
Do our findings mean that humans are just like other animals?
From an evolutionary perspective, humans are part of the animal lineage. Humans could thus be tied to the local environment in similar ways as other species. However, humans exhibit enormous variation in behaviour and are the only mammal that occurs in essentially all terrestrial environments. Variation in behaviour also exists in other species, but to a much lower extent than for humans. The flexibility that allows humans to adapt their behaviour to local conditions and the extreme reliance on learning from others and building on this information over generations set humans apart.
Do our findings imply that human culture does not matter for how humans behave?
No. Our results only suggest that human culture *also* emerged out of adaptation to local ecological condition, in addition to independent non-ecological processes.
Are these foraging human populations simple because they behave like animals?
No, quite the opposite. While the behaviour might superficially look the same, human foraging societies generally developed sophisticated techniques to deal with the challenges they encounter in their environments. For example, where humans and other mammals and birds rely more on fishing, human populations have acquired detailed knowledge and sophisticated technology on how to obtain fish. Foraging human populations persisted in their local environments precisely because they had identified the most appropriate set of behaviours to live in environments, where other societies, which we might erroneously consider more advanced, could not persist. In addition, many nonhuman animals are as well much more sophisticated than we usually acknowledge.
What do our findings tell us about industrial human societies?
Not much. On the one hand, agriculture, market integration, and technology might weaken the response of human behaviour to local environmental conditions. On the other hand, there might be path dependencies (e.g., via cultural transmission of behaviour across generations), which constrain the subsequent evolution of behaviours. In this case, the behavioural diversity of industrial human societies might still reflect past adaptations to the local environment. Our evidence on foraging human societies does not allow us to answer this question.
Do our findings reflect natural selection based on genes?
Our study cannot reveal what caused the differences in behaviour, either across human populations or across other species. In general, the expression of any behaviour is influenced by an interplay of a large number of genes, changes through development and flexibility, and experiences of diverse environmental factors. The exact genetic regions and environmental factors involved, and the interplay among them, are unknown for the behaviours we analysed. It is unlikely that the differences in behaviour we observe across human populations can be traced back to single regions within the genome. First, there are no clear genetic differences that separate all individuals in one population from all individuals in another population. Second, for all the behaviours that we analysed, individuals can change their behaviour as soon as they move to a new society. This happens regularly among humans, e.g., to form marriages. Third, the rules that govern behaviour often change quite rapidly, e.g., when societies adopt new religious traditions.
Do our findings imply that there is a single dominant behaviour in a specific environment?
No. There might be multiple niches at a given locality and different ways to cope with ecological challenges. Indeed, there are multiple ways to behave in all environments we looked at. Instead, our findings suggest that local ecological constraints act like a filter making specific behaviours more likely to occur in those environments where they are adaptative or at least do not have a substantial cost (but without determining the outcome). For instance, food storage is more beneficial in environments with strong seasonality that makes it difficult to source food in winter. Food storage is hence relatively frequent there. But this does not imply that food storage is a must or is not found in other environments with a more stable food supply throughout the year. It is only less frequent in those environments.
