Just why does castration make spiders into better fighters?

For many male spiders, having sex means surrendering one or both of their genitals. In some species, up to 75% of males will experience what's known as sexual cannibalism. But this castration carries an advantage: the males become fearsome warriors.

We've talked about the phenomenon of castrated spider warriors before, but this is an important topic, dammit, and we'll just keep going back to this well over and over again until we find out everything there is to know. Since it's long since been established that spiders fight better without their genitals, the question that remains to be answered is why this should be.

Could the reasons be the spider equivalent of psychological, as the spider realizes it has nothing left to lose and will fight that much harder to protect the female that it presumably (and hopefully) managed to reproduce with? Certainly, that's part of the puzzle here, since the male spiders break off their genitals in the first place to block other males from copulating with the female. But still, why does all that make them better fighters than intact males?

In fact, the reason might be more physiological than psychology. As the great folks at NCBI ROFL have shared, researchers at the National University of Singapore have found evidence to support what's known as the "gloves-off hypothesis" after their work with the spider species Nephilengys malabarensis. Just know that when the researchers say "gloves", they are most definitely not talking about actual gloves:

The previously proposed ‘gloves-off' hypothesis, attributing eunuchs' enhanced locomotor endurance to the reduction in total body weight caused by genital mutilation, is plausible but has remained untested. Here, we tested the gloves-off hypothesis in N. malabarensis by comparing the time until exhaustion (i.e. endurance) of intact males with half- and full-eunuchs created experimentally. We found that by reducing body weight up to 4 per cent in half-eunuchs and 9 per cent in full-eunuchs through emasculation, endurance increases significantly in half-eunuchs (32%) and particularly strongly in full-eunuchs (80%). Our results corroborate the gloves-off hypothesis and further point towards the adaptive significance of male emasculation.

So the adaptive significance is increased endurance, eh? It's probably a good thing that this information came to light only after the Olympics started, you know?

Original paper at NCBI. Image of the related orb spider by zayzayem on Flickr.