It’s sad but true, male Chimpanzees have bigger testicles than humans, and they’re even bigger than a Gorillas.

Most people think that a gorilla would be hung like an ape, and they’re surprised to find out that they’re wrong.

Despite being the largest primate, weighing in at 350 to 450 pounds, the gorilla has testicles that weigh only a bit more than 1 ounce total for the pair. Although humans and orangutans are lighter than gorillas, they are better endowed, weighing in at 1.5 ounces. What’s even more surprising is that the 100-pound chimpanzee’s testicles tip the scales at a whopping 4 ounces.

Biologists and insecure male humans have long puzzled over the mystery of why that might be. Finally, after a lot of research that involved invading the privacy of an awful lot of primates, they finally, once and for all, figured it out.

Not too surprisingly, it turns out that the size of a primate’s testicles are a really good indicator of sperm count. What researchers didn’t count on was that sperm count is a good indicator of how secure a male is in the love of a good female primate.

Females, although infinitely divine, are by nature notoriously fickle things. Females in species after species, primates and otherwise, hedge their reproductive bets by mating with whatever males they can find. This is good for them and the propagation of said species, because it ensures that their eggs get fertilized. However, what’s good for them is profoundly unsettling to the males, who can never be sure that they are the real fathers of the children of their mates.

So what does this have to do with the size of ape testicles? Generally, the more promiscuous the women are within a species, the larger the testicles of the males who love them. Based on the same principle that buying more tickets ups your chance of winning the lottery, the males with the biggest testicles, and sperm counts, tend to have a better chance of reproducing with promiscuous females.

Given that, you can fairly accurately infer each species’ sexual behavior by measuring the males’ testicles. For example, male gorillas are pretty secure. They live in closed societies in which they constantly watch the females and make a point of running off any male interlopers.

So gorillas can easily get by with small testicles and sperm counts to match, about 65 million of the little guys per ejaculation.

Chimpanzees, at the other extreme, mate promiscuously. A female chimp in heat will take on suitors left and right, and the males are equally ready to jump when the opportunity presents itself.

The males with the highest sperm counts have competed best in the reproductive lottery that has continued over millions of years.

As a result, those well-endowed chimps and other promiscuous male primates have testicles the size of cannonballs, and routinely issue forth billions of sperm with each ejaculation.

How do humans shape up in comparison, then? Someplace in between, considering our levels of both female fidelity and male fertility. An average human male sends off between 200 and 500 million sperm per ejaculation.

Interestingly, even among individual humans, the sperm levels are also somewhat dependent on how much security the male feels regarding the fidelity of his partner. Males in couples that spend most of their time together have lower sperm counts; yet the same males, when separated from their partners for more time, develop higher sperm counts, even when the interval between ejaculations remains constant.

Also, that same fact holds true in adulterous relationships, where time together is usually rare and irregular; as a result, a disproportionate number of children born to adulterous human females are sired by their lovers instead of their husbands.