Boar testicles are large because pigs evolved in a mating system where females mate with multiple males, creating intense competition between sperm from different boars. Each testicle in a mature boar weighs roughly 170 to 180 grams, and the pair together accounts for about half a percent of total body weight. That ratio is high among mammals, and it comes down to one core evolutionary pressure: the more sperm a male can produce, the better his odds of fathering offspring.
Sperm Competition Drives Testicle Size
Wild boars and feral pigs live in groups where a single sow in heat will often mate with more than one male. When that happens, the ejaculates from rival boars are literally competing inside the female reproductive tract to reach the egg first. This is called sperm competition, and across the animal kingdom, it is one of the most consistent predictors of testicle size relative to body weight. Species where females mate with many males tend to evolve larger testes; species with strict monogamy or a single dominant male who guards access tend to have smaller ones.
Bigger testes produce more sperm per unit of time. But size alone isn’t the whole story. Research published in Biology Letters found that sperm competition also selects for a faster rate of spermatogenesis, the process of manufacturing new sperm cells. So boars don’t just have bigger factories; those factories run at a higher speed. The combination means a boar can flood the reproductive tract with an enormous number of sperm cells in a single mating.
The Numbers Behind Boar Reproduction
A single boar ejaculate averages around 220 milliliters, roughly the volume of a coffee mug. That’s vastly more than most mammals of comparable size. The sperm concentration in that ejaculate averages about 365 million sperm per milliliter, which means a single mating delivers tens of billions of sperm cells. Of those, about 96% are actively motile, swimming forward with purpose.
This output is only possible because of how much testicular tissue is devoted to sperm production. In a young piglet at 40 days old, sperm-producing tubules make up just 36% of the testicle’s volume, with hormone-producing cells occupying about 40%. By the time a boar reaches roughly 190 days of age (a little over six months), those tubules have expanded to 72% of the total volume. The hormone-producing cells shrink to about 10% of testicular volume by 250 days. In other words, the mature boar testicle is essentially a sperm-making machine, with the vast majority of its tissue dedicated to producing as many sperm cells as possible.
How Fast Boar Testes Grow
Boar testicles grow remarkably fast during puberty. The diameter of the sperm-producing tubules roughly quadruples between 40 and 250 days of age, going from 65 micrometers to 236 micrometers. Hormone production peaks earlier than full reproductive maturity. The cells responsible for testosterone output hit their highest production capacity around 130 to 160 days of age, which is when the hormonal surge of puberty is at its strongest. After that, testosterone production per cell levels off, but the sheer increase in testicular mass keeps overall hormone output high.
This rapid growth is part of why boars can become sexually active before they’re fully grown. A young boar at six or seven months already has functioning, substantial testes, even though the animal may continue adding body mass for another year or more.
Larger Testes Mean More Pheromones
Testicle size doesn’t just affect sperm output. It also directly influences the production of androstenone, a potent pheromone that boars produce in their testes and store in fat tissue. Androstenone levels correlate positively with both testosterone levels and reproductive organ weight. In the pork industry, this matters because androstenone is one of the main compounds behind “boar taint,” an unpleasant smell and flavor in meat from uncastrated males.
Research on immunocastrated pigs (boars given a vaccine that suppresses reproductive hormones) found that testicular weight was a reliable indicator of which animals still had high androstenone levels. Animals whose testes weighed more than roughly 450 grams were flagged as “at androstenone risk” on the slaughter line. This is a practical example of how testicle size serves as a proxy for overall reproductive and hormonal activity.
Why Farmers Care About Testicle Size
In pig breeding, larger testes aren’t just a curiosity. They’re a selection tool. Research has found that testicle size in boars is a useful predictor of semen production, which matters enormously in modern pork operations where a single boar’s semen may be collected and used to artificially inseminate hundreds of sows.
More surprisingly, testicle size in boars shows a positive genetic correlation with reproductive traits in their female offspring. Studies have found correlations as high as 0.39 between a boar’s testicular size and the total number of piglets born to his daughters. When breeders selected for larger litter sizes over generations, they inadvertently also selected for larger testes in the males. The two traits are genetically linked, so a boar with bigger testes is statistically more likely to produce daughters that have larger litters. This makes scrotal size one of the more practical measurements a breeder can take on a young boar to predict the reproductive value of his entire genetic line.
How Pigs Compare to Other Animals
Relative testicle size varies wildly across mammals, and pigs sit on the larger end of the spectrum. The pattern holds across species: chimpanzees, which live in multi-male groups where females mate promiscuously, have testes that are proportionally much larger than those of gorillas, where a single silverback monopolizes mating. Boars follow the chimp pattern, investing heavily in sperm production because their ancestral mating system never guaranteed exclusive access to a female.
Domestication hasn’t reduced this trait. If anything, selective breeding for fertility has reinforced it. The evolutionary logic that built large testes in wild boars has been amplified by human breeders choosing the most reproductively productive males for generation after generation. The result is an animal whose reproductive anatomy is optimized, both by natural selection and by artificial selection, to produce sperm in extraordinary quantities.