In arthropods, most cases of morphological dimorphism within males are the result of a conditional evolutionarily stable strategy (ESS) with status-dependent tactics. In conditionally male-dimorphic species, the status' distributions of male morphs often overlap, and the environmentally cued threshold model (ET) states that the degree of overlap depends on the genetic variation in the distribution of the switchpoints that determine which morph is expressed in each value of status. Here we describe male dimorphism and alternative mating behaviors in the harvestman Serracutisoma proximum. Majors express elongated second legs and use them in territorial fights; minors possess short second legs and do not fight, but rather sneak into majors' territories and copulate with egg-guarding females. The static allometry of second legs reveals that major phenotype expression depends on body size (status), and that the switchpoint underlying the dimorphism presents a large amount of genetic variation in the population, which probably results from weak selective pressure on this trait. With a mark-recapture study, we show that major phenotype expression does not result in survival costs, which is consistent with our hypothesis that there is weak selection on the switchpoint. Finally, we demonstrate that switchpoint is independent of status distribution. In conclusion, our data support the ET model prediction that the genetic correlation between status and switchpoint is low, allowing the status distribution to evolve or to fluctuate seasonally, without any effect on the position of the mean switchpoint.
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Acknowledgments We are grateful to the staff of Intervales State Park for logistical support and to R.L. Paiva, T.M. Del Corso, N. Leiner, C.F. Lerche, P.M. Nassar, and T.M. Nazareth for helping in the fieldwork. Dr. William G. Eberhard helped this study since the beginning, through enlightening conversations about male dimorphism. Previous versions of the manuscript received useful comments by Drs. Leigh W. Simmons, Joseph L. Tomkins, Marcelo O. Gonzaga, and two anonymous reviewers to whom we are very grateful. This work was supported by Fundac¸ão de Amparo à Pesquisa do Estado de São Paulo (FAPESP, grant numbers 02/00381-0, 03/05427-0, 03/05418-1, 08/53737-2, 09/50031-4), Coordenac¸ão de Aperfeic¸oamento de Pes-soal de Nível Superior (CAPES), and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).
- Conditional strategy
- Environmental cue
- Male polyphenism
- Phenotypic plasticity
- Status dependence