The male orb-web nephilid spider often castrates himself during sex, reducing his body weight by up to 9%. This could increase his endurance when defending his mate from competitors.
Daiqin Li at the National University of Singapore and his colleagues partly or fully castrated male spiders (Nephilengys malabarensis; fully castrated pictured right) and then assessed their stamina by repeatedly poking them with a paintbrush to goad them into moving.
Spiders that retained their genitals, or palps, quickly became exhausted, whereas the much lighter eunuchs showed 80% greater locomotor endurance. Spiders with one remaining palp kept going for 32% longer than intact males (left).
Biol. Lett. http://dx.doi.org/10.1098/rsbl.2012.0285 (2012)