I keep seeing people posting photos of their rats with nuts larger than their legs. I also went to the pet store recently to pick up something for my cats, and saw a bunch of rats in a cage with fat hangers. What is that about? Is that common in rodents?
In: Biology
It is because rats are very gregarious and they mate throughout the year. They live in very large groups and those groups have many males present which are able to mate with females whenever the females want. And although they do fight for dominance, the dominant males are not able to have exclusive control over the females.
So it became evolutionarily beneficial to be able to produce a ridiculous amount of sperm at a time. That way you out-compete the other males inside the impregnated females. And they did that by developing very large testicles.
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