The controversy over the level at which selection and evolution operate on social behavior is one of the most contentious in modern biology.
Are properties of animal groups a simple extension of individual characteristics that evolve through individual selection? Or must the effects of interactions among individuals and the group be considered to understand behavioral evolution?
Social traits, even those that are measurable as individual behaviors, may involve individual and group components of both inheritance and selection. A multi-level approach to evaluating the importance of social behaviors has the potential to transform the attention of behavioral ecologists from individuals to interacting groups.
Our most recent NSF grant is to use a multilevel selection framework called "social selection" to explore the effect each level of organization within social networks may have on the fitness of individuals. Our initial work on social selection used only dyadic interactions to quantified how the physical phenotypes (e.g. body size) of social partners can affect the fitness of focal individuals. This work also demonstrated that including social selection gradients in an analysis could drastically alter our estimates of total selection on phenotypes (Formica et al 2011).
We are currently exploring how different levels of social organization in social networks could affect the evolution of social behaviors and weaponry.
A figure from our most recent NSF grant to investigate the evolution of social networks and multilevel selection. Levels of selection in B. cornutus. (A) Traditional selection analysis regresses individual fitness on individual traits; however, using multi-level selection theory we also include the attributes of sub-networks (B) and entire networks (C) into this analysis.
Interactions with conspecifics have profound effects on an individual’s fitness. Whether through competition for resources, communication with potential mates, helping behavior, or interaction with kin, the social environment can mediate a large portion of the variation in fitness among individuals. The behaviors that generate the social environment are fascinating from an evolutionary perspective because they can be both the targets and agents of selection--in other words, the social environment itself can evolve. The goal of our lab is to understand how the process of selection shapes and is shaped by social behaviors. We explore social evolution through three lenses, social networks, multi-level selection, and natural history.
We use the forked fungus beetle (Bolitotherus cornutus) to address these questions. B. cornutus experience density-dependent sexual and social selection on male weaponry (Formica et al. 2011). Forked fungus beetles inhabit logs infected with shelf fungi, tend to remain on one log for a breeding season, conduct most of their social behaviors on the surface of these fungus shelves, and can live for many years. These properties allow us to conduct long-term mark recapture studies on B. cornutus populations in the wild.
In our explorations of social networks in B. cornutus, we have demonstrated that social network position does explain variation in fitness above and beyond that of typical behavioral phenotypes (Formica et al. 2012). During our initial work on social networks we also offered a new method for delineating social partners that uses probabilistic home range analysis (Formica et al. 2010).
We have also identified that individual position in social network is indeed a repeatable phenotype of an individual and that the structure of networks resets itself after a social disturbance (Formica et al. 2017).
Our most recent work focuses on mapping the social networks of 18 wild populations of B. cornutus across multiple years. With these data, we have begun to analyze how selection for network properties varies across space and time.
The realization that the position an individual occupies in a social network can profoundly affect its resources, risks, and fitness, has led to an escalating number of studies of networks in a wide range of animal species. Despite assertions that social networks are adaptive, it is unclear whether and how network position has the capacity to evolve through natural selection.
Because networks traits are expressed in the context of groups of interacting social partners, our lab takes a multilevel approach, asking:
(1) Which components of social network position are determined by properties of the individual and which are determined by interactions with conspecifics?
(2) Does individual social network position have the genetic capacity to respond to selection?
(3) What level of network organization has the greatest effect on individual differences in fitness?
A field assistant observing social interactions of forked fungus beetles on a wild population.
Social networks from nine wild populations of B. cornutus.
Even with the advances in theory and methodology we have made in the past century, we still know so little about the basic biology of most species. The longer I work on a system the more I realize the vast gaps in our knowledge. Appreciating the importance of natural history, our lab conducts curiosity driven studies to simply know more about the biology of forked fungus beetles.
Natural history studies also provide a space in the literature where new scientists can make important contributions. Because most of our lab members are undergraduate collaborators (Swarthmore is an undergraduate only institution), I have encouraged students to follow their own interests and design novel natural history projects that are not always in line with the lab’s larger program. Aside from allowing students to take a project from design to publication, these projects are often the most fun to work on.
These student driven projects have revealed that:
Male B. cornutus express strong and consistent mate choice for larger, more fecund females in the lab and wild (Formica et al. 2016)
Grip strength is an important performance trait affect by the morphology of male B. cornutus (Benowitz et al. 2012)
Differences in host fungi of a population do not result in differences in the immune response of B. cornutus (Formica and Chan 2015)
Using human forensic techniques, we can non-invasively extract DNA from the defensive excretions of beetles (Donald et al. 2012)
Please visit our People and Projects page for descriptions of our current Natural History projects in the Formica Lab.
A newly emerged B. cornutus male with large horns. Photo credit: Stan Malcolm.
This small-horned male beetle was part of an experiment to investigate male-male interactions. The tether allowed us to construct artificial social groups.