Question: Why are female spiders generally bigger compared to their male companions?

Keith Siew answered on 27 Jun 2013:

That’s a good question!

I think it has a lot to do with the fact the female needs to produce and guard the eggs. This is much more energy expensive and it would be advantageous to be a larger size to do this! Another reason I can think of would be to do with the fact that males tend to end up larger in other species because they are competing directly with other males to fertilise the females. So often the bigger stronger male wins the battle or is chosen by the female because he can protect her and the offspring…

When it comes to spiders though… they are solitary creatures and don’t come into contact with each other unless mating! So the chances of males encountering each other is slim… so the evolutionary drive isn’t there to cause them to be bigger than the females… Also another cool but freaky fact is that spiders are cannibals… so more often than not… its a very risky business for the males and the female will eat them… so I guess you could argue that by being small and easy to kill and eat… that you’ve provided the mother of the now fertilised eggs with a meal… increasing the chances of their survival and thus passing on the genes… so evolutionarily speaking this would keep the “small male” genes going!