So, why are rat testicles so large?

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. When a female rat comes into heat, she may be mated by multiple males in a short span of time. The male with the biggest testicles will produce the most sperm, which means he'll have a higher chance of fertilizing some of her eggs.

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