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Sexual size dimorphism refers to the physical size difference between male and females of the same species. Spiders are regarded as one of the few terrestrial groups that show sexual size dimorphism, and one of the few sexually dimorphic groups that have females bigger than males, which is not typical. Typically, these characteristics are found in marine groups.[13] There is no broad explanation why this occurs in nature, however, there has been a lot of research that has been done to try to explain this phenomena. There are many hypotheses that have been suggested, however, none are sufficient alone.

In orb-weavers, the size dimorphism is typically known as extreme sexual size dimorphisms to do the extent of the difference in size. There are many different species of orb-weavers, however, the size difference ranges from the female being 1.5-9 times bigger than males.[13] Sexual size of the female is typically thought to be selected for fertility. The bigger the female, the more eggs she can hold, and the more offspring that she can produce.[14] Selection for male size are not as well-known and many different theories have been suggested, however, evidence does show support that female size has increased over time and that male size as stayed constant.[13]

In orb weavers, there is a phenomena, sexual cannibalism, that is a common throughout spider species. One hypothesis behind sexual size dimorphism is that sexual cannibalism and sexual size dimorphism are linked by a cyclic pattern in which each influences the other. One study showed that the greater the size difference between a male and female, the greater the chance for a sexual cannibalistic event to occur.[15] By females sexually cannibalizing smaller males, this should slowly rid the population of small males. However, ridding smaller males from the population is not so simple, as there are multiple selection pressures that are acting on both the males smaller size and the females larger size.