These are my choices???

Tidarren sisyphoides.  Cute little spider. The females are much larger, and perhaps because of this, the males have very large testicles.

Female spiders are notoriously mean after mating, and like other spiders these females will attack and kill their amorous counterparts, just post-copulation.  (How rude!!) 

The males are weighted down by the aforementioned testicles.  They have a hard time running away from the attacking females.  So …

So ….

http://www.cbc.ca/quirks/archives/03-04/images/tidarren.jpg

They pull off one of their own testicles, to allow themselves to run faster! It is an evolutionary advantage – remove a testicle, run faster, live to copulate another day.