In terms of size alone, this accolade of the animal with the biggest testicles goes to the right whale, whose testes weigh a massive 1,000kg.
Proportionately, however, it’s the tuberous bush cricket, whose testes amount to an impressive 14 per cent of its body mass. Imagine if a human's were 14 per cent of its body mass...
Testes tend to be proportionately larger in species in which the females are very promiscuous (female tuberous bush crickets mate with up to 23 partners), and there are two possible explanations.
First, they may allow a male to produce more sperm per mating in order to outcompete that of rivals; second, they may allow him to transfer a relatively small amount of sperm to a greater number of females. Research at the University of Derby supports the latter.