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1 November 2015 Movement, sex ratio, and population density in a dwarf male spider species, Misumenoides formosipes (Araneae: Thomisidae)
G. N. Dodson, A. G. Anderson, L. M. Stellwag
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Crab spiders in the subfamily Thomisinae rank among the most extreme animals in terms of sexual size dimorphism (SSD). Hypotheses regarding the apparent selection for dwarfing of males relative to females generally reference advantages of small male size for mobility. Specific proposals claim that selection should be strongest in species with limited male-male combat, which would otherwise favor larger males. We aimed to determine if the predicted traits of low densities, female biased sex ratios, more movement by males, and limited male-male conflict characterized a population of Misumenoides formosipes Walckenaer 1837. New and previous assessments of these characteristics in this extremely dimorphic spider revealed a mix of support and discordance with the predicted set of traits. Repeated plot censuses over 2 years, together with daily monitoring of females and collections of males, documented relatively low densities with males outnumbering females by as much as 2.37:1. The movements of marked males were measured upon rediscovery during daily searches using two methods: tracking individuals from their point of discovery and trials in which males were moved to predetermined positions relative to females. Female movements were measured by marking their hunting positions followed by daily searches of these locations. Female average tenure across their locations was twice that of males (5.05 versus 2.45 days) and the initial moves made by marked males in trials were six times further than initial moves by monitored females (1.76 versus 0.29 m). Male-male conflicts over positions near females are frequent and intense in M. formosipes. By contrast, male fights are rare in the female biased populations of Misumena vatia, a species with similarly extreme SSD. Thus, while extreme SSD may be associated with enhanced mobility of small males during searches for females, it is not precluded by extensive male agonistic encounters.

The American Arachnological Society
G. N. Dodson, A. G. Anderson, and L. M. Stellwag "Movement, sex ratio, and population density in a dwarf male spider species, Misumenoides formosipes (Araneae: Thomisidae)," The Journal of Arachnology 43(3), 388-393, (1 November 2015).
Received: 2 February 2015; Published: 1 November 2015
crab spider
gravity hypothesis
sexual size dimorphism
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