Insectes Sociaux

, Volume 58, Issue 4, pp 539–549 | Cite as

Colony demographics of rare soldier-polymorphic worker caste systems in Pheidole ants (Hymenoptera, Formicidae)

  • M. H. HuangEmail author
  • D. E. Wheeler
Research Article


Nearly all species in the ant genus Pheidole have dimorphic workers, with distinct small minors and larger soldiers. The size range of both castes is typically narrow. Just seven described species are soldier-polymorphic, with a broad soldier size range. Here, we characterize worker caste allocation and demography in the soldier-polymorphic P. obtusospinosa, P. rhea, and P. tepicana, and the dimorphic P. spadonia for comparison. The head allometry of soldiers in soldier-polymorphic species is strongly positive and that of dimorphic species is negative. Among soldier-polymorphic species, the soldier castes differ from each other in the degree of positive allometry. In addition, they differ in the number of size modes: P. obtusospinosa and P. rhea have two and P. tepicana has one. During colony ontogeny, P. obtusospinosa first has one mode and develops the second mode much later, while P. rhea produces multiple modes throughout. We also characterize worker caste systems based on the biomass allocation. For all three soldier-polymorphic species, the majority of soldiers are small soldiers. Pheidole obtusospinosa and P. rhea allocate roughly equal biomass to the two soldier classes, while P. tepicana allocates little to supersoldiers based on both biomass and caste ratio. These findings illustrate the interplay among caste ratios, biomass allocation, size frequency distributions, and allometry in the evolution of different worker caste systems. We conclude that soldier-polymorphic species may have evolved convergently in response to broad-scale factors, but differences among them suggest selection pressures in small-scale environments have been different.


Allometry Biomass allocation Caste ratio Colony ontogeny Competitive displacement Worker polymorphism 



We thank Anna Dornhaus, Yves Carriere, Jennifer Jandt and Anna Himler for their feedback on our manuscript and statistical advice. We would also like to thank two anonymous reviewers for their helpful feedback on an earlier version of our manuscript. We also thank K. Franklin for helping us identify some of our dimorphic Pheidole specimens.

Supplementary material

40_2011_176_MOESM1_ESM.doc (109 kb)
Supplementary Fig. 1 (DOC 109 kb)
40_2011_176_MOESM2_ESM.doc (48 kb)
Supplementary Table 1 (DOC 47 kb)


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Copyright information

© International Union for the Study of Social Insects (IUSSI) 2011

Authors and Affiliations

  1. 1.Department of EntomologyUniversity of ArizonaTucsonUSA

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