What Animal Has the Biggest Testicles?

The North Atlantic right whale holds the record for the largest testicles of any animal on Earth, with a combined testicular mass that can reach roughly 1,000 kilograms, or about one ton. Each testis alone can outweigh an adult human. But “biggest” depends on how you measure. In proportion to body size, some tiny rodents and insects blow the right whale away, devoting a startling share of their body to reproductive tissue. The reasons behind that variation tell a story about mating, competition, and evolutionary pressure that stretches across the animal kingdom.

The Absolute Size Record

Right whales are not the largest whales. Blue whales dwarf them, yet blue whale testes are considerably smaller. The difference comes down to how each species mates. Female right whales mate with multiple males in quick succession, creating intense competition among sperm from different males to reach the egg first. Males that produce more sperm have a reproductive edge, and over millions of years that pressure has driven testicular size upward. The result is a pair of organs so massive they account for roughly one percent of the whale’s total body weight, an extraordinary figure for an animal that size.

Among land animals, the largest testes belong to African elephants, each one weighing several kilograms. That sounds impressive until you realize it represents a tiny fraction of the elephant’s multi-ton body. Elephants have a relatively low-competition mating system compared with some of the true record-holders in the relative-size category, which is why their testes are modest when scaled to body mass.

When You Measure by Proportion, Small Animals Win

Absolute weight is only one way to think about testicular size. Biologists often compare testes mass to total body mass, a ratio known as relative testes size. By that measure, some of the most extreme animals are tiny. A landmark study of mammalian testes found that among the smallest mammals, certain rodents devote as much as eight percent of their entire body mass to testicular tissue. For perspective, the same allometric relationship predicts that a ten-gram mammal should have testes making up about 1.8 percent of body weight, so those rodents at eight percent are off the charts even for their size class.¹

That pattern holds across the animal kingdom more broadly. Smaller species tend to allocate a larger share of their body and energy budget to testicular tissue than larger species do. The relationship is not linear: as body mass increases, testes grow too, but at a slower rate. So a mouse-sized animal will almost always have proportionally bigger testes than a horse-sized one, all else being equal. The interesting cases are the species that break even that generous curve, and they almost always turn out to be species where females mate with multiple males.

Why Sperm Competition Drives Everything

The single most powerful predictor of testicular size across the animal kingdom is the intensity of sperm competition. When a female mates with only one male per reproductive cycle, there is no race among sperm from different males, and natural selection does not push testes to be unusually large. When females routinely mate with several males, males that produce more sperm and can mate more frequently tend to father more offspring. Over evolutionary time, this selects for larger testes.

A large meta-analysis quantified this relationship across dozens of species and multiple ways of measuring sperm competition. The overall correlation between relative testes size and the level of sperm competition was strong, around 0.60, confirming that the link holds broadly rather than being an artifact of a few extreme species.² Testes that are large relative to body size are found in multi-male breeding systems, whether you call them promiscuous or polygynous, where several different males mate with each female during the same fertile period.¹

This is not just about producing more sperm cells. Larger testes also support higher mating frequencies, which matters in species where males need to copulate many times to maintain their share of paternity. The combined demand for volume and speed of production is what makes testes such a reliable marker of mating system.

Primates Tell the Story Clearly

Humans, gorillas, chimpanzees, and rhesus macaques make a natural comparison because they span the full range of primate mating systems. Chimpanzees live in multi-male groups where females regularly mate with many partners. Their testes are enormous relative to body size. Gorillas, on the other hand, live in single-male harems where one dominant silverback does nearly all the mating. With minimal sperm competition, gorilla testes are remarkably small for such a large-bodied primate. Humans and gorillas fall on the low end of relative testes size, while chimpanzees and macaques sit at the high end.

This difference runs deeper than anatomy. Research comparing gene activity in the testes of these four species found that testicular gene-expression patterns cluster by mating system rather than by how closely the species are related. Gorilla testes resemble human testes in their gene activity, and macaque testes resemble chimpanzee testes, even though gorillas are more closely related to chimpanzees than to humans on the evolutionary tree. The convergence in gene expression between species that share a mating strategy, despite being on different branches of the family tree, is strong evidence that reproductive competition has shaped testicular biology at a molecular level.³

Birds and Dramatic Seasonal Swings

In many bird species, testes do not stay the same size year-round. They can swell to hundreds of times their resting volume during the breeding season and shrink back down afterward. A songbird’s testes in winter can be barely visible specks; by spring they may account for a significant fraction of its body weight. This seasonal ballooning is driven by changes in daylight length that trigger hormonal cascades.

Among birds, waterfowl stand out for particularly intense reproductive competition. In species where forced copulations are common, males have evolved not only larger testes but longer and more elaborate intromittent organs. A study of 16 waterfowl species found that testes size, phallus length, and the complexity of surface structures on the phallus all increased with the frequency of forced extra-pair copulations.⁴ In response, females of those same species have evolved more complex vaginal anatomy, creating a coevolutionary arms race between the sexes.⁵

Bird testes also show an interesting quirk: the left and right testis often differ in size. Research has demonstrated that this asymmetry is not random. When one testis fails to develop or is damaged, the other can compensate by growing larger, maintaining total sperm output. This compensation can be complete, essentially making the single remaining testis do the work of two.⁶

Insects and Other Invertebrates

The conversation shifts again when you leave vertebrates entirely. Among fruit flies in the family Drosophilidae, testicular architecture varies enormously, both within and between species. Some species invest in producing a huge number of small sperm, while others produce fewer but much longer sperm cells. The number of primary sperm-producing cells in the testis is strongly tied to sperm length: species with longer sperm have fewer of these cells, which in turn affects overall sperm output. The testis essentially trades quantity for quality, or vice versa, depending on the species’ reproductive ecology.⁷

Some fruit fly species produce sperm cells that are longer than the fly’s entire body. In those species, the testes are coiled structures that take up a substantial portion of the abdominal cavity. The investment is extraordinary for an animal that weighs a fraction of a gram. These examples illustrate that “biggest” in the testicular world is not always about mass. Sometimes it is about internal architecture, the length of the cells being produced, or the proportion of the body cavity filled by reproductive tissue.

Keeping Testes Cool Without a Scrotum

Most mammals keep their testes outside the body cavity in a scrotum, where temperatures run a few degrees below core body temperature. Sperm production is sensitive to heat, and even modest temperature increases can damage developing sperm. But not all mammals have the luxury of a scrotum. Dolphins and whales keep their testes deep inside the abdomen, which poses an obvious thermal problem for animals with core temperatures similar to ours.

Dolphins solve this with a specialized network of blood vessels called a countercurrent heat exchanger. Arteries carrying warm blood toward the testes are bundled alongside veins returning cooled blood from the dorsal fin and tail flukes, surfaces that shed heat to the surrounding water. The cool venous blood chills the arterial blood before it reaches the testes, keeping reproductive tissue below core temperature even though it sits deep in the belly.⁸ Anatomical studies have confirmed that this two-layered arterio-venous arrangement is a genuine heat exchanger, not just a coincidental proximity of vessels.⁹ During exercise, when core temperature rises, the system works harder to maintain testicular cooling.¹⁰

Why most land mammals externalized their testes in the first place is still debated. One hypothesis links it to the evolution of warm-bloodedness: as mammals developed higher core temperatures, the heat-sensitive process of sperm production needed cooler surroundings. A more specific version of this idea suggests that externalization also tempers testosterone production. The enzyme pathway that stimulates testosterone in the testes is less active at the cooler scrotal temperature of about 32°C than at core body temperature of 37°C, which may prevent excessive masculinization of male offspring.¹¹

When Heat Goes Wrong

The sensitivity of testes to temperature is not just an evolutionary curiosity. Experimental work has shown that sustained high ambient temperatures cause measurable damage to testicular tissue. In mice exposed to prolonged heat, testes and epididymis lost weight, the area of sperm-producing tubules shrank, and the thickness of the cell layers that generate sperm decreased. Sperm themselves suffered: density dropped, viability fell, membrane integrity declined, and malformation rates climbed.¹² These findings help explain why animals have evolved such elaborate cooling strategies, from scrotal descent in land mammals to vascular heat exchangers in cetaceans.

The vulnerability to heat also matters in a changing climate. Species that already operate near their thermal limits for sperm production could face fertility problems as average temperatures rise, a concern that wildlife biologists are only beginning to investigate seriously.

The Energy Cost of Big Testes

Growing and maintaining large testes is expensive. They are metabolically active organs that consume energy continuously to fuel sperm production. For small animals, especially those that devote a large fraction of body mass to testicular tissue, the energy drain can be significant enough to force trade-offs with other survival needs.

Edible dormice provide a vivid example. Sexually active males with larger testes face higher thermoregulatory costs, essentially burning more energy to stay warm while also fueling reproduction. To cope, males with bigger testes tend to form larger sleeping groups, huddling with other dormice to share body heat. This behavioral adjustment reduces individual energy expenditure by almost 40 percent, enough to offset the metabolic burden of maintaining large reproductive organs.¹³ The finding is a reminder that testicular size is not free. It carries physiological costs that ripple into behavior, social structure, and energy balance.

Bigger Testes Do Not Always Mean More Sperm Per Gram

You might assume that a large testis is simply a scaled-up version of a small one, but the internal composition varies dramatically between species. A comparison of human and rat testes found that rats produce nearly seven times more sperm per gram of testicular tissue than humans do. Rat testes contain a higher proportion of the cell types directly involved in sperm production, including the seminiferous tubules and germinal cells that churn out spermatids. Human testes, by contrast, are disproportionately made up of non-sperm-producing components like connective tissue, interstitial cells, and support cells.¹⁴

This means that comparing raw testis weight between species does not tell you everything about sperm output. A smaller but more efficiently packed testis can outperform a larger but less densely productive one. The internal architecture, how much of the organ is devoted to actual sperm assembly lines versus support structures, matters as much as total volume. For species under intense sperm competition, selection appears to favor not just bigger testes but testes that are more densely packed with productive tissue.

What Testes Can Reveal About Extinct Animals

Testes do not fossilize. They are soft tissue, gone within days of an animal’s death. Yet biologists can make educated guesses about the mating systems of extinct species by using the tight relationship between testicular size and mating behavior in living animals. If you know the body size of a fossil species and can infer its social structure from other clues, like sexual size differences or the ratio of males to females at fossil sites, you can estimate where it probably sat on the testicular-size spectrum.

These inferences are inherently uncertain, but they are grounded in a real and well-documented pattern. The correlation between mating system and relative testes size is one of the most robust relationships in comparative biology. It has held up across mammals, birds, insects, fish, and amphibians. When researchers find a fossil primate with low sexual dimorphism, for instance, one reasonable inference is that it lived in a multi-male group with high sperm competition, and therefore likely had relatively large testes for its body size. The logic is indirect but far from baseless, because the link between behavior and anatomy in living species is so consistent.

Antlers, Ornaments, and Honest Advertising

In some species, you can get a rough sense of a male’s reproductive investment without ever looking at his testes. Secondary sexual traits like antler size in deer appear to function as honest signals of fertility. Males with larger, more elaborate antlers tend to have larger testes and faster-swimming sperm.¹⁵ The connection makes intuitive sense: both antler growth and sperm production are energetically costly and sensitive to a male’s overall health and nutritional status. A male in poor condition cannot afford to grow impressive antlers and simultaneously maintain high sperm quality, so the ornament becomes an external flag of internal reproductive capability.

This link between visible ornaments and hidden reproductive organs adds another layer to the story of testicular evolution. Natural selection is not just shaping testes in isolation. It is shaping entire suites of traits, from the size of a whale’s testes to the branching of a deer’s antlers, all in service of the same underlying competition for paternity.