Why are rats testicles so big?

Question

Why are rats testicles so big?

Answer 1

"A species' sexual anatomy is related to its mating system -- who mates with whom. In species in which many males mate with one female when she is in heat, sperm from all these males is deposited in the female's reproductive tract in a short span of time. A male that deposits more sperm under these conditions will have a higher chance of fertilizing the female's eggs, and hence siring offspring. Males with larger testicles produce more sperm, and therefore have an advantage in such a sperm competition scenario. This is called sperm competition theory, because sperm from different males duke it out inside the female's reproductive tract."

http://www.ratbehavior.org/testicles.htm