Field of Science

Sexual Dimorphism - Golden Orb Spiders

One of my favorite tropical organisms are the orb weaver spiders, and Queensland has some spectacular ones. Here's an image of one that had woven a big web in the corner of the car port of our beach house. When pointing the beast out, the Phactor refered to it as a "her", and was chastised by family and friends as being unable to sex spiders, but in this case the big ones are always female because these spiders have a tremendous sexual dimorphism: huge females and dwarf males, spiders so small that they are generally not noticed if you don't know what you're looking for. The female spiders are locally refered to as "bird-eating spiders" and you do occasionally see a feather stuck in the web, but while the web can hang on to the stray loose feather, even these substantial webs seldom can hold a bird. Old Shelob here has a body 4-5 cm long and a leg span of 20-21 cm! So just maybe she could "eat" a small entangled bird, but not sure how reliable some of the reports of bird catching are. While running into one of their webs and tangling with Shelob is a bit creepy, she's actually harmless to people, at least big people.
This particular spider's orb was also inhabited by a male, shown here in an inset (upper right) at approximately the same scale (Did you notice?). Now you may ask why is there such a huge difference in body size? In answer, remember sperm are not very big, so even a small male can make quite a few. So how do you mate with such a big fierce predator? Answer: carefully. Presumably a dwarf male is so small he isn't going to be mistaken for a meal, or even a snack, very often. In many invertebrates, females are bigger for egg production, so you start with somewhat smaller males who after all only make the much cheaper gametes, sperm. Such systems evolve when males of various sizes have different reproductive success. If larger males more frequently get eaten by the female before mating, and smaller males more frequently succeed in mating, then the average size of males decreases because those genes are passed on to offspring more frequently. At some point selection will be such that there no longer is any advantage to being still smaller. The dwarf males make a living by scavenging tiny prey caught and ignored by the big female, or by sneaking meals from larger prey items.


Jenn said...

I have the impression that it's typical for the male spider to be considerably smaller than the female, although your example here is fairly extreme.

Male spiders have been documented in elaborate dances, wooing the female - or at least distracting her from dinner - in order to approach.

They are fascinating critters. As long as they aren't in my house, mind you.

A. said...