Behavioral evolution in the major worker subcaste of twig-nesting Pheidole (Hymenoptera: Formicidae): does morphological specialization influence task plasticity?
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Polymorphism frequently correlates with specialized labor in social insects, but extreme morphologies may compromise behavioral flexibility and thus limit caste evolution. The ant genus Pheidole has dimorphic worker subcastes in which major workers appear limited due to their morphology to performing defensive or trophic functions, thus providing an ideal model to investigate specialization and plasticity. We examined worker morphology, brood-care flexibility, and subcaste ratio in 17 species of tropical twig-nesting Pheidole by quantifying nursing by major workers in natural colonies and in subcolonies lacking minors, in which we also measured brood survival and growth. Across species, majors performed significantly less brood care than minors in intact colonies, but increased rates of brood care 20-fold in subcolonies lacking minors. Brood nursed by majors had lower survival than brood tended by minors, although rates of brood growth did not vary between subcastes. Significant interspecific variation in rates of brood care by major workers did not lead to significant differences in brood growth or survival. Additionally, we did not find a significant association between the degree of major worker morphometric specialization and rates of nursing, growth, or survival of brood among species. Therefore, major workers showed reduced efficacy of brood care, but the degree of morphological specialization among species did not directly compromise task plasticity. The compact nests and all-or-nothing consequences of predation or disturbance on colony fitness may have influenced the evolution of major worker brood-care competency in twig-nesting Pheidole.
KeywordsCaste evolution Division of labor Polyethism Response threshold Trade-off
We thank the directors and staff of Tiputini Biodiversity Station for their assistance in obtaining permits and support in the field. We thank Clifton Meek, Megan Johnson, Brian Henry, Noah Reid, Scott Appleby, Elise Koncsek, Frank Azorsa Salazar, Wendy Mertl, and Winston McDonald for field and laboratory assistance. We are very grateful to Stefan Cover for confirming Pheidole identifications, to Dr. Gary Alpert for instruction on digital imaging, to Dr. Marcio Pie for suggestions on morphometric analyses, and to Dr. Marc Seid for assistance developing ethogram techniques. We thank Dr. Michael Kaspari, Dr. Michael Sorenson, and two anonymous reviewers for constructive comments on the manuscript. This work was funded by a National Science Foundation Graduate Research Fellowship awarded to ALM. JT was funded by NSF Grants IOB 0725013 and 0724591 awarded to J. Traniello and W. Gronenberg. Voucher samples were collected and transported under permit 017-IC-FA-PNY-MA issued to ALM by the Ecuadorian Ministry of the Environment. This work complied with all current laws of Ecuador and the USA. The authors declare that they have no conflict of interest.
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