Queen–worker caste ratio depends on colony size in the pharaoh ant (Monomorium pharaonis)
- 340 Downloads
The success of an ant colony depends on the simultaneous presence of reproducing queens and non-reproducing workers in a ratio that will maximize colony growth and reproduction. Despite its presumably crucial role, queen–worker caste ratios (the ratio of adult queens to workers) and the factors affecting this variable remain scarcely studied. Maintaining polygynous pharaoh ant (Monomorium pharaonis) colonies in the laboratory has provided us with the opportunity to experimentally manipulate colony size, one of the key factors that can be expected to affect colony level queen–worker caste ratios and body size of eclosing workers, gynes and males. We found that smaller colonies produced more new queens relative to workers, and that these queens and workers both tended to be larger. However, colony size had no effect on the size of males or on the sex ratio of the individuals reared. Furthermore, for the first time in a social insect, we confirmed the general life history prediction by Smith and Fretwell (Am Nat 108:499–506, 1974) that offspring number varies more than offspring size. Our findings document a high level of plasticity in energy allocation toward female castes and suggest that polygynous species with budding colonies may adaptively adjust caste ratios to ensure rapid growth.
KeywordsCaste Colony size Ergonomics Resource allocation Polygyny
We thank Christa Funch Jensen, Isabel Højgaard Rasmussen, Markus Drag, Mathilde Lerche-Jørgensen, Nathia Hass Brandtberg and Signe Lolle for help sorting, feeding, and measuring ants. The study was supported by The Danish National Research Foundation (AMS, JJB, JSP) and an EU Marie Curie International Incoming Fellowship (TAL).
- Anderson K.E., Linksvayer T.A. and Smith C.R. 2008. The causes and consequences of genetic caste determination in ants (Hymenoptera: Formicidae). Myrmecol. News 11: 119-132Google Scholar
- Bourke A.F.G. and Franks N.R. 1995. Social Evolution in Ants. Princeton University Press, Princeton, New Jersey. 529 ppGoogle Scholar
- Crozier R.H. and Pamilo P. 1996. Evolution of Social Insect Colonies: Sex Allocation and Kin Selection. Oxford University Press, New York. 306 ppGoogle Scholar
- Edwards J.P. 1986. The biology, economic importance, and control of the Pharaoh’s Ant. In: Economic Impact and Control of Social Insects (S.B. Vinson, Ed). Praeger Publishers, New York, pp 257-271Google Scholar
- Hölldobler B. and Wilson E.O. 1990. The Ants. Springer, New York. 732 ppGoogle Scholar
- Nonacs P. 1993. The effects of polygyny and colony life history on optimal sex investment. In: Queen Number and Sociality in Insects (L. Keller, Ed). Oxford University Press, New York, pp 110-131Google Scholar
- Oster G.F. and Wilson E.O. 1978. Caste and Ecology in the Social Insects, vol 12. Princeton University Press, Princeton, New Jersey. 352 ppGoogle Scholar
- Peacock A.D. and Baxter A.T. 1949. Studies in Pharaoh’s ant, Monomorium pharaonis (L.). 1. The rearing of artificial colonies. Entomol. Mon. Mag. 85: 256-260Google Scholar
- Porter S.D. and Tschinkel W.R. 1985a. Fire ant polymorphism (Hymenoptera: Formicidae): factors affecting worker size. Ann. Entomol. Soc. Am. 78: 381-386Google Scholar
- R Development Core Team. 2009. R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing, Vienna, Austria. ISBN 3-900051-07-0, URL http://www.R-project.org.