Rats are great pets – they are often fun, charismatic and very intelligent. If you own a male rat, you may be worried about the presence of (often very!) large soft lumps around their back end as they reach sexual maturity and around adulthood. You may be surprised to find that these are normal anatomical landmarks of a male rat – his testicles and scrotum. 

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Rat Anatomy 101

The testis (singular for testicles) and the epididymis (a structure within the scrotum that is attached to the side of the testis; it’s a coiled segment of the spermatic ducts that stores spermatozoa while they mature) are located in a scrotum (the external pouch that in most mammals contains the testes), this is the area you can see to the back end of your male rat.

The seminal vesicles, whose function is to produce a fluid that is high in fructose, a sugar that provides nutrients for sperm cells, as well as other proteins, enzymes, and mucus, and the different lobes of the prostate are located in the abdomen.

Rats also have a fairly large fat pad present within the scrotum; which can make the testes look even bigger than they actually are.

Sexual maturity

Male rats reach sexual maturity at about 6 to 10 weeks of age; females reach maturity at 8 to 12 weeks. From this age onward, females and males should be housed separately.

From about 4-6 weeks old, the testicles will grow dramatically as the rat proceeds through puberty. And he will become fertile around about 50 days old. By adulthood, the average rat’s testicles come in at about 6g each, or 4.4% of their body weight; we can compare this to the average of 50g each, or 0.08 – 0.12% for humans.

So why are their testicles disproportionately large?

Does it make them more likely to find a female?

The characteristics that make attractiveness in male rats remain unidentified although they are being investigated. However, some previous studies have ruled out some possible male traits such as weigh more, have larger testes, or exhibit higher levels of urinary testosterone; female rats do not consistently prefer males that have these traits. They believe scent of the male, and chemicals he produces, are a bigger factor.

So why then?

It has been said that Rats (Rattus norvegicus) are a promiscuous species, they have lots of partners! The size of these organs comes from the fact that there is significant competition. Rats are frequently polygamerous (typically having more than one mate). 

Males with larger testicles produce more sperm. They therefore have an advantage in such a sperm competition scenario, this is called sperm competition theory. 

Essentially, a male rat wants to reproduce, he has to out-compete the other male rats who will also mate with the female. Therefore, he produces more sperm to “outnumber” his rivals. But to make more sperm, you need bigger testicles. Of course, all the other rats are evolving in the same direction – so the testicles get disproportionately bigger and bigger. In fact, there’s even evidence that male rats have some control over how many sperm they ejaculate when mating – and studies show that they inseminate a female with more when another male is visible than when on their own!

Of course, if you are ever concerned about lumps and bumps on your rats, it is always worth asking, but rest assured that if your adult male rat has two gigantic symmetrical lumps on their underside at their back end, then they’re probably his testicles!

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