Male and female orbspider
The small male approaching the much larger female of the golden orb spider, Nephila clavipes. Photo: T. Hesselberg.
A new study explains the observed size difference between females and males (sexual dimorphism) in spiders by referring to gravity. Gravity gives rise to an optimal size above where climbing speed is reduced. Since male spiders are often required to climb to access females located in their webs in the vegetation, large spiders should thus have larger sexual dimorphism. The scientists were able to show that this is true for a wide range of different spiders.

Whereas males in many vertebrates are larger than females, they are usually smaller in invertebrates. This is especially the case in many spiders, where the female can be many times larger than the male. Several hypotheses have been proposed to explain this. The sexual cannibalism hypothesis states that it is an advantage for males to be so small that they can easier escape from the female or be too small to be considered prey. However, it is also an advantage for males to become mature faster and needing fewer moults since this increase the possibility they survive into adulthood. Finally the gravity hypothesis (GH) states that it is advantageous for males to be small since smaller spiders have a higher climbing speed. Males, especially, in spiders that build webs in the vegetation needs to climb a lot.

 

A team of biologists led by Jordi Moya-Laraño from CSIC in Spain investigated the claims of the comparing the climbing speed of spiders from a range of families and of different ages and sizes. They discovered that the GH is only partly true. There exists an optimal body size of around 7.4 mm for climbing. Both smaller and larger spiders show lower climbing speed. Larger spiders face the consequences of a higher gravitational force and smaller spiders suffer from a too small step frequency in relation to step length. The scientists could then show that web-building females smaller than the optimal size had males of a similar size, whereas females larger than the optimal size had smaller males. The larger the spider, the greater the sexual dimorphism.

Thus the gravity hypothesis can at least partly describe the observed significant differences between males and females in large web-building spiders.

Source:
J. Moya-Laraño, D. Vinkovic, C. M. Allard & W. Foellmer. (2009). Optimal climbing speed explains the evolution of extreme sexual size dimorphism in spiders. Journal of Evolutionary Biology 22: 954-963.