Wild marmots’ social networks reveal controversial evolutionary theory in action

It’s generally understood that close friendships enhance well-being. However, the influence of group affiliations on well-being is less clear. For instance, does workplace culture impact employee productivity? This explores the concept of multilevel selection and its broader implications beyond individual relationships, touching upon evolutionary biology and social behavior.

While a positive correlation between group influence and individual well-being might seem intuitive, the notion that a group’s structure or composition affects its members has sparked considerable debate in biology.

This concept, known as multilevel selection, extends the principles of natural selection. Natural selection posits that organisms with advantageous traits for their environment are more likely to survive and reproduce. Consequently, these beneficial traits – whether behavioral, morphological, or physiological – become more prevalent within a population over successive generations.

Traditional evolutionary perspectives emphasize natural selection acting on individual organism characteristics. For example, mammals with extensive social networks tend to live longer and produce more offspring. In this context, the number of social connections is the trait being selected.

Multilevel selection theory suggests that selection occurs simultaneously at both individual and group levels. Consider this illustration: membership in a highly social and interconnected group can benefit individual members, implying that group characteristics are also subject to selection. In natural settings, individuals within well-connected groups might exhibit increased longevity and reproductive success because these groups are more effective at securing limited resources or identifying threats. Here, the collective attributes of the group are under selection.

Multilevel selection might even favor seemingly contradictory traits at individual and group levels. For instance, it could promote reserved individuals within highly sociable groups, or conversely, outgoing individuals within more withdrawn groups.

Dating back to Charles Darwin’s 1871 publication “The Descent of Man,” the concept of multilevel selection has remained contentious, ever since he proposed that group dynamics likely influence individuals.

Empirical support for multilevel selection operating concurrently on individual social relationships and social groups has primarily come from controlled laboratory experiments. While crucial to scientific inquiry, the absence of field evidence in wild animal populations has perpetuated the long-standing debate. Driven by our interest in behavioral evolution as field biologists, we investigated multilevel selection in yellow-bellied marmots in their natural habitat.

Our recently published research lends credence to this debated theory, indicating that group structure within marmot communities might be as significant, or even more so, for survival than individual friendly relationships.

Observing Marmot Social Dynamics

Deciphering multilevel selection in natural populations has taken a century and a half, largely due to the extensive data required for statistically robust sample sizes.

Scientists at the Rocky Mountain Biological Laboratory in Crested Butte, Colorado, have conducted long-term research on marmots in the vicinity since 1962. This study represents the second-longest continuous observation of individually identifiable wild mammals globally.

Annually, researchers ensure individual marmot identification through tagging. Marmots are captured, fitted with unique ear tags, and marked with paint for remote identification. Trained researchers, affectionately known as “marmoteers,” dedicate approximately 1,000 hours annually to observing these substantial, cat-sized rodents using binoculars and spotting scopes.

Since 2003, the research team has specifically focused on marmot social interactions and bonds. Our multilevel selection analysis encompassed 42,369 distinct affiliative social interactions – including behaviors like play and grooming – among 1,294 marmots across 180 social groups, with group sizes varying from two to 35 individuals. We also tracked marmot lifespan, reaching up to 16 years, and annual offspring production for each individual.

This comprehensive dataset enabled us to map marmot social networks. Our objective was to quantify each marmot’s social connections, identify network structures, and characterize overall group organization.

Understanding these intricate marmot connections allowed us to address two pivotal questions: First, how do social relationships impact individual survival and reproduction – which individual traits are under selection? Second, how do social groups affect individual survival and reproduction – which group traits are under selection?

Crucially, we investigated these questions concurrently, recognizing that marmots are simultaneously influenced by their individual social relationships and the social groups they inhabit. Our statistical approach, known as contextual analysis, allowed us to assess the relative importance of social relationships and social groups.

Emerging Evidence Reshapes the Debate

Distinguishing group-level selection from traditional individual-level selection can be challenging. It represents a more intricate perspective on influential relationships. Rather than solely focusing on individual actions, the influence extends to the group – a collective entity impacting individuals.

Our recent findings provide evidence for multilevel selection of social behavior in a natural setting. We observed that both individual social relationships and social groups influence marmot survival and reproduction, with social groups exerting an equal or even greater influence. We calculated the selection gradient, a measure of selection strength on a trait, as 0.76 for individual traits and 1.03 for group traits.

Intriguingly, the nature of the impact on survival and reproduction varied across levels. In some instances, selection favored marmots with fewer social relationships while simultaneously favoring those in more social and interconnected groups. This can be analogized to an introverted person at a lively social gathering.

Evolution and multilevel selection are intricate natural processes, making such nuanced and complex results anticipated.

Multilevel selection extends its relevance to human groups, encompassing diverse forms like friendship circles, local communities, businesses, economies, and even entire nations. Our marmot study indicates that group-level consequences for individual success are not unique to humans.