Worker division of labor and endocrine physiology are associated in the harvester ant, Pogonomyrmex californicus

Authors: DOLEZAL, ADAM - Arizona State University; Brent, Colin; HOLLDOBLER, BERT - Arizona State University; AMDAM, GRO - Arizona State University

Submitted to: Journal of Experimental Biology Online
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/29/2011
Publication Date: 2/1/2012
Citation: Dolezal, A.G., Brent, C.S., Holldobler, B., Amdam, G. 2012. Worker division of labor and endocrine physiology are associated in the harvester ant, Pogonomyrmex californicus. Journal of Experimental Biology Online. 215:454-460.

Interpretive Summary: In Pogonomyrmex californicus harvester ants, an age-based division of labor occurs in the worker caste, in which young workers perform nest tasks and older workers forage for food. While the behavioral aspects of worker division of labor are well characterized in numerous social species, the physiological underpinnings are often understudied. The investigation of this physiology, however, can be a useful means for understanding the way that behaviors can evolve. Here, we test whether worker task preference is associated with the expression of hormones normally coupled to reproductive physiology. We found that juvenile hormone content was elevated in foraging workers of both groups, and ecdysteroids were elevated in those ants that worked inside the nest. We conclude that P. californicus workers exhibit robust associations between behavioral phenotype and hormone content similar to those previously observed in founding queens of this species. These results support the hypothesis that the regulatory mechanisms underlying the worker division of labor may have evolved from controls for reproductive physiology and behavior.

Technical Abstract: In Pogonomyrmex californicus harvester ants, an age-based division of labor occurs in the worker caste, in which young workers perform nest tasks and older workers forage for food. While the behavioral aspects of worker division of labor are well characterized in numerous social species, the physiological underpinnings are often understudied. The investigation of this physiology, however, can be a useful means for understanding the proximate mechanisms of social evolution. An example of this type of approach is the reproductive ground plan hypothesis (RGPH), which posits that the reproductive regulatory mechanisms of solitary ancestors were co-opted to regulate worker behavior. Here, we test whether the behavioral dichotomy seen in P. californicus workers is associated with the expression of systemic hormones implicated in reproductive physiology and behavioral transitions observed in founding queens of this species. Whole body content of juvenile hormone (JH) and ecdysteroids were determined in 1) unmanipulated colonies, with workers transitioning across behaviors naturally and 2) single-cohort colonies, where entire colonies of same-aged workers were manipulated into age-independent division of labor. We found that JH content was elevated in foraging workers of both groups. This result supports previous suppositions that JH plays a central role in regulating social insect foraging behavior. Ecdysteroid content, on the other hand, was elevated in those ants that worked inside the nest relative to foragers. This finding provides the first documented link between ecdysteroid hormones and worker division of labor in a highly eusocial insect. We conclude that P. californicus workers exhibit robust associations between behavioral phenotype and hormone content similar to those observed in founding queens. This result supports the hypothesis that the regulatory infrastructure of worker division of labor is derived from co-opted reproductive regulators.