Evol Ecol Res 8: 1393-1408 (2006)     Full PDF if your library subscribes.

An optimality approach to male and female body sizes in an extremely size-dimorphic cichlid fish

Dolores Schütz,1,2 Geoffrey A. Parker,3 Michael Taborsky2 and Tetsu Sato4

1Konrad Lorenz Institut für Vergleichende Verhaltensforschung (KLIVV), Austrain Academy of Sciences, Vienna, Austria,  2Department of Behavioural Ecology, Institute of Zoology, University of Bern, Bern, Switzerland,  3Population and Evolutionary Biology Research Group, School of Biological Sciences, University of Liverpool, Liverpool, UK and  4Lake Victoria Project, Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, Yokohama, Japan

Author to whom all correspondence should be addressed.
e-mail: michael.taborsky@esh.unibe.ch


Question: Can intrasexual selection on male size and natural selection on female size in the shell-brooding cichlid Lamprologus callipterus explain the greatest male/female sexual size dimorphism (SSD) reported to date among animals?

Mathematical methods: (1) Mortality model to predict male and female body sizes. (2) Gain rate maximization and (3) combined mortality/gain rate maximization model to predict female size. We compared observed body sizes in the field with predictions from these models.

Key assumptions: (1) Body size at sexual maturity is determined by a trade-off between survival to sexual maturity and (a) number of mates in males and (b) fecundity in females. (2) Female body size is constrained by increasing search costs, because (3) shells large enough for breeding become limited as females become larger.

Results: (1) Male body size predicted by the mortality model matches size as found in the field closely, but the model strongly overestimated female size. (2) The female body size predicted by the gain rate maximization model fitted well within the range of sizes observed in the field. (3) Including the effect of mortality into this model hardly changes the prediction.

Conclusions: (1) Intersexual selection can explain male body size, but fecundity selection alone cannot explain female size. (2) The breeding substrate strongly constrains female size. (3) The extreme male-biased SSD is due to strong selection on the body sizes of both sexes in opposite directions.

Keywords: ecological constraints, ESS model, natural selection, sexual size dimorphism, shell brooding.

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