Sexual dimorphism is a concept where-in the males and the females of the same species show vast differences in terms of their size, color or behavior. One such unmistakable example of size based dimorphism is amongst a group of spiders known as the orb-weaver spiders. There are many species of these orb-weavers and all of them seem to exhibit this morphological difference in quite a dramatic way. The males seem to be less than one-tenth the size of a female, looking more like her meal.
The first time I saw this dramatic disparity was when I came across the male of an orb-weaver spider, commonly known as the giant wood spider in the rainforests of Agumbe. The tiny male was moving around the web looking more like the female’s little one ! He seemed to be quite wary of her as well, so as not be mistaken as her prey.
Here’s another one that I saw more recently, a huge female signature spider and the tiny male alongside.
Curious to know as to why this huge difference in size, read up some material on the web. There are many theories around it. Some theories suggest that the males are small, so as to avoid being preyed upon by the bigger females, that the smaller males might not be detected or might be ignored by the females as potential prey. Another theory suggests that the females are much bigger to be able to produce more offspring. However, all these don’t seem to be necessarily true, because, only some species of spiders show this extreme size difference between sexes, while many others do not.
A closer look at a male signature spider which sits quaintly on the disconnected section of the web. It’s just so tiny in front of the huge legs of the female.
A more recent study on this hypothesizes that the size difference could be due to a spider behavior known as ‘bridging’.
‘Bridging’, is a means of transportation for spiders living in the trees and other vegetation of forests. In bridging, a spider throws out a strand of its silk into the air and waits for it to be carried to another plant near by. The spider then tightens the strand and uses it to crawl to its new place. It keeps moving from place to place in search of food or a potential mate.
The study thus found that bridging would be much easier for the tiniest of male spiders than for their slightly larger counterparts, as they would have to move around bracing the effect of gravity. Hence the males that get to move around more, tend to get better chances for mating and hence more opportunities to spread their genes.
This, the researchers feel could have driven the evolution of male spiders toward smaller sizes. A really interesting theory, building a bridge for more opportunities.