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.
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.
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.
As a rat owner you may have noticed that male rats have disproportionately large testicles. This striking anatomical feature has evolutionary reasons behind it. Let’s explore why rats have such big balls.
Male rats have testicles located in an external pouch called the scrotum The testicles produce sperm and hormones like testosterone They are attached to coiled tubes called epididymides that store mature sperm,
Rats also have other reproductive structures like the seminal vesicles and prostate located internally in the abdomen. These produce fluids that nourish sperm.
When Rats Reach Sexual Maturity
Rats mature quickly. Male rats reach sexual maturity and start producing sperm around 6-10 weeks of age. Their testicles dramatically enlarge during puberty between 4-6 weeks old.
By adulthood, rat testicles weigh around 6 grams each – that’s 4.4% of their body weight! Compare this to human testicles that only weigh around 50 grams each or 0.08-0.12% of our body weight.
Clearly, rat testicles are disproportionately large. But why did they evolve this way?
It’s All About Sperm Competition
The size of male animals’ testicles is strongly linked to their mating system. Species where one male mates with multiple females tend to have smaller testicles. The male faces no sperm competition, so doesn’t need to produce as much sperm.
But rats have a promiscuous mating system. Female rats mate with multiple males when in heat. This means intense sperm competition between males.
Rats with the biggest balls produce the most sperm. By flooding the female with sperm, they have the best chance of fertilizing more eggs.
Over evolutionary time, this sperm competition drove male rats to evolve larger and larger testicles. Their body size didn’t increase, so the testicles became disproportionate.
More Sperm Gives a Better Chance
Studies confirm rats produce more sperm when competing. Males inseminate more sperm when another male is present with a female in heat.
Across the animal kingdom, species with promiscuous mating habits tend to have the largest testicle-to-body weight ratios. Sperm competition offers a clear evolutionary reason for this pattern.
Big Balls Don’t Attract More Females
Surprisingly, big balls don’t make male rats more attractive to females. Females don’t prefer males with larger testicles or higher testosterone.
It appears female rats choose mates based more on scent and pheromones. But larger testicles still give a male rat an advantage by allowing him to flood a female with more sperm.
An Evolutionary Arms Race
As each male rat evolves larger testicles, this raises the bar for other males. They are forced to keep pace in the evolutionary arms race of ever-increasing testicle size.
This runaway selection driven by sperm competition explains why rats ended up with such comically large balls relative to their body size!
So rest assured – if your male pet rat has two huge, symmetrical lumps on his backside, these massive balls are completely normal for his species. While amusing to us, they are the product of intense evolutionary pressures to produce more sperm than the competition.
So why are their testicles disproportionately large?
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.
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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why are rat balls so large
FAQ
Why does my rat have a giant lump?
Some of the most common lumps that rats can develop are swellings from injuries or infection (abscess), they can get fatty growths (lipoma) and also mammary tumors which mainly are on one side or there body’s between there front and back leg.
Why do rats need a large caecum?
The caecum in the rabbit, rat and guinea pig is greatly enlarged to provide a “fermentation vat” for micro-organisms to break down the cellulose plant cell walls.
Why do some animals have huge balls?
In these cases, the evolution of large testes can be attributed to high copulatory frequency and sperm production and the competition among sperm of different males for fertilization of the same female.
At what age do rats’ balls drop?
The anus will be located centered behind the scrotum. The testes descend at about 30–40 days old, so they can be harder to identify in younger rats.Mar 15, 2024