Thursday, 3 August 2006 - 5:40 PM

Subcaste Morphology and Behavioral Plasticity in a Community of Tropical Pheidole

Amy L. Mertl and James F.A. Traniello. Biology, Boston University, 5 Cummington St., Boston, MA 02215

Ecological specialization is expected to involve a trade-off between behavioral flexibility and morphological change. However, examining such trade offs in the field is difficult. The major subcaste of the highly diverse ant genus Pheidole provides an interesting model system to test this hypothesis. Pheidole species are dimorphic, with the sterile workers divided into two distinct subcastes - minors and majors. Minors generally perform foraging and brood care, whereas majors are specialized for defense, food storage or seed milling. Majors are morphologically distinct from minors, and their repertoires vary among species. We studied the relationship between the morphological specialization of majors and their behavioral flexibility (the ability to perform subcaste-atypical tasks) in an assemblage of Amazonian Pheidole. Intact queen-right colonies of 17 species of twig-nesting Pheidole were collected at Tiputini, Ecuador (n =1-12 colonies per species).  Colonies were established in artificial nests consisting of one central and two side chambers separated by fine mesh. Subcolonies with equal numbers of ants and brood inhabited the side chambers, one of which had only majors and the other only minors. Rates of brood care by majors and brood survival were measured over a two-week period. The degree of morphological divergence between the majors and minors was analyzed for each species using synchroscopy images and automontage software. Subcolonies composed of major workers had significantly lower rates of brood survival, although there was no significant difference in rates of brood care. This pattern varied greatly among species. There was a trend for major workers that were more morphologically distinct than minors to have lower brood survival, suggesting that morphological specialization is associated with a loss of task flexibility. Significant differences in colony size and caste ratios were found between species and subgroups. Results are discussed in light of the ecology and behavior of the study species.

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