Original Paper

Evolutionary Ecology

, Volume 25, Issue 2, pp 331-349

Conditional male dimorphism and alternative reproductive tactics in a Neotropical arachnid (Opiliones)

  • Bruno A. BuzattoAffiliated withCentre for Evolutionary Biology, School of Animal Biology (G.09), The University of Western Australia Email author 
  • , Gustavo S. RequenaAffiliated withPrograma de Pós-graduação em Ecologia, Instituto de Biociências, Universidade de São Paulo
  • , Rafael S. LourençoAffiliated withDepartamento de Estatística, Instituto de Matemática e Estatística, Universidade Estadual de Campinas
  • , Roberto Munguía-SteyerAffiliated withDepartamento de Ecologia, Instituto de Biociências, Universidade de São Paulo
  • , Glauco MachadoAffiliated withDepartamento de Ecologia, Instituto de Biociências, Universidade de São Paulo

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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.


Conditional strategy Environmental cue Male polyphenism Phenotypic plasticity Status dependence Threshold