Research

The research in my lab focuses on the consequences of sociality on the evolution of individual, population, and species characteristics. The evolution of social groups is a fundamental transition that has occurred in a diverse array of organisms, from bacteria to primates. I use the tools of ecology, ethology, and genetics to understand how social life affects the evolution and dynamics of ant populations, primarily in the southwestern and southern US. Ants are obligately social animals, organized into highly structured colonies containing one or more reproductive queens and a large number of female but reproductively sterile workers. The only component of the ant life cycle in which solitary life is possible is when young queens leave their parent nests to found their own new colonies, yet even at this stage queens of many species choose to join forces with other queens to form group nests rather than live solitarily. By living together, ants interact with their environment as a group, which can dramatically change their ability to tolerate environmental extremes, their interactions with members of their own and other species, and their patterns of dispersal and reproductive behaviors.

Our investigations focus on two main study systems. The first is the desert harvester ant Messor pergandei, in which the behavior of queens switches abruptly from solitary colony founding to group colony founding across its range in southern California. Current projects include: 1) quantifying gene flow across the behavioral transition zone, 2) measuring the types and strength of natural selection acting on queen behavior, both between regions and within the transition zone, and 3) the effects of social behavior on queen body size, morphology, and physiology.

Our second project is social insect hybrid zones, where the interaction between social structure and interspecific hybridization has led to some surprising and unique forms of interspecific interactions. We study two hybrid zones, one in the harvester ant genus Pogonomyrmex and the other between two North American fire ant species in the genus Solenopsis. In both these cases, hybrid offspring almost always become sterile workers, while non-hybrids are invariably reproductive offspring. Ongoing projects include the evolutionary origin and geographic distribution of genetic caste determination, how differences in caste fate of genetically different offspring are mediated within the colony, the role of selection in maintaining genetic caste systems, and the current and historical genetic relationships between hybridizing populations.

Representative Publications

• Helms, K. R. and S. Helms Cahan, in press. Divergence in mating flight patterns of the seed-harvester ant Pogonomyrmex rugosus Emery, (1895) in the western Mojave Desert. Myrmecological News.
• Helms Cahan, S., Daly, A.M., Schwander, T., Woods, H.A. in press. Genetic caste determination does not reduce colony growth rates in Pogonomyrmex harvester ants. Functional Ecology.
• Helms Cahan, S., Julian, G. E. 2010. Shift in frequency-dependent selection across the life-cycle in obligately interbreeding harvester ant lineages. Evolutionary Ecology 24:359-374.
• Schwander, T., Helms Cahan, S., S. Suni, Keller, L. 2008. Mechanisms of reproductive isolation between an ant species of hybrid origin and its parents. Evolution 62:1635-1643.
• Schwander, T., Humbert, J.-Y., Brent, C. S., Helms Cahan, S., Chapuis, L., Renai, E., Keller, L. 2008. Maternal effect on female caste determination in a social insect. Current Biology 18:265-269.
• Schwander, T., Keller, L., Helms Cahan, S. 2007. Two alternate mechanisms contribute to the persistence of interdependent lineages in Pogonomyrmex harvester ants. Molecular Ecology 16:3533-3543.
• Schwander, T., Helms Cahan, S., Keller, L. 2007. Characterization and distribution of Pogonomyrmex harvester ant lineages with genetic caste determination. Molecular Ecology 16:367-387.
• Helms Cahan, S. Julian, G.E., Schwander, T., Keller, L. 2006. Reproductive isolation between the harvester ant Pogonomyrmex rugosus and two lineages with genetic caste determination. Ecology 87:2160-2170.
• Julian, G.E., Helms Cahan, S. 2006. Behavioral differences between Pogonomyrmex rugosus and two dependent lineages (H1/H2). Ecology 87:2207-2214.
• Schwander, T., Helms Cahan, S., Keller, L. 2005. Genetic caste determination in Pogonomyrmex harvester ants imposes costs during colony founding. Journal of Evolutionary Biology 19:402-409.
• Helms Cahan, S., Rissing, S.W. 2005. Variation in queen size across a behavioral transition zone in the ant Messor pergandei. Insectes Sociaux 52:84-88
• Helms Cahan, S., Julian, G. E, Rissing, S. W., Schwander, T, Parker, J. D., Keller, L. 2004. Loss of phenotypic plasticity explains genotype-caste association in harvester ants. Current Biology 14:2277-2282.
• Helms Cahan, S., Keller, L. 2003. Complex hybrid origin of genetic caste determination in harvester ants. Nature 424:306-309.
• Helms Cahan, S., Vinson, S. B. 2003. Reproductive division of labor between hybrid and non-hybrid offspring in a fire ant hybrid zone. Evolution 57:1562-1570.
• Helms Cahan, S., Parker, J. D., Rissing, S. W., Johnson, R. A., Polony, T. S., Weiser, M.D., Smith, D.R. 2002. Extreme genetic differences between queens and workers in hybridizing Pogonomyrmex harvester ants. Proceedings of the Royal Society of London, Series B 269:1871-1877.
• Helms Cahan, S., Blumstein, D.T., Sundstrom, L., Liebig, J. and Griffin, A. 2002. Social trajectories and the evolution of social behavior. Oikos 96:206-216.
• Helms Cahan, S., 2001. Ecological variation across a behavioral transition zone in the ant Messor pergandei. Oecologia 129:629-635.
• Helms Cahan, S. 2001. Co-operation and conflict in ant foundress associations: insights from geographical variation. Animal Behaviour 61:819-825.

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