How Mammals That Lost the Scrotum Keep Their Sperm Fertile

For most mammals, the recipe for making sperm includes one seemingly indispensable ingredient: a scrotum. The testes of the vast majority of mammalian species descend from the abdomen into a pouch of skin that hangs outside the body, keeping them several degrees cooler than core body temperature. The textbook explanation has long been that spermatogenesis, the elaborate process by which sperm cells are produced, simply cannot tolerate the warmth of the mammalian interior. Yet biology is full of rule-breakers. Dolphins, elephants, seals, manatees and several other lineages retain their testes inside the abdomen or groin, a condition known as natural cryptorchidism, and they do so while remaining fully fertile. How these animals manage to build functional sperm in an environment that would sabotage fertility in most of their relatives has been one of the more persistent puzzles in reproductive physiology. A new comparative genomics study published in BMC Genomics offers the most detailed molecular answer yet.

The research, led by Yu Zheng, Yixuan Sun and colleagues at Nanjing Normal University together with the Southern Marine Science and Engineering Guangdong Laboratory in Guangzhou, took a three-pronged approach. The team combined comparative evolutionary analyses of fertility-related genes across mammals with direct measurements of sperm morphology and laboratory functional assays of key proteins. The logic was straightforward: if naturally cryptorchid mammals have solved the problem of heat-exposed sperm production, the solution should leave fingerprints in their genomes, in the physical architecture of their sperm, and in the biochemical behavior of the proteins that orchestrate meiosis and flagellar construction.

The evolutionary analysis focused on genes previously associated with infertility. By comparing these genes across species with abdominal or inguinal testes and species with descended, scrotal testes, the researchers identified lineage-specific shifts in selective pressure in the naturally cryptorchid groups. In evolutionary terms, shifts in selective pressure mean that certain amino acid positions in these proteins have been changing in ways that differ from the background pattern of neutral drift, suggesting that the altered proteins may have been favored in lineages where the testes operate at elevated temperatures. Some genes carried signals of positive selection, while others showed candidate recurrent amino acid substitutions supported by ancestral sequence reconstruction, a technique that infers the likely identity of amino acids at key positions in the common ancestors of modern species.

When the team examined what these candidate genes actually do, a coherent functional picture emerged. Genes showing positive-selection signals or carrying the reconstruction-supported recurrent substitutions were significantly enriched for functions related to two biological processes: sperm flagellar organization and meiotic regulation. The flagellum is the whip-like tail that propels a sperm cell, and its assembly is one of the most architecturally demanding tasks in cell biology, requiring the precise coordination of hundreds of proteins into a microtubule-based motor. Meiosis, meanwhile, is the specialized cell division that halves the chromosome number and generates genetically diverse sperm precursors, and it is notoriously sensitive to thermal stress. That the genes under altered selective pressure cluster in exactly these two pathways suggests a two-front evolutionary response: remodeling the physical machinery of the sperm while shoring up the fragile genetic machinery of sperm production.

The morphological side of the study added a striking phenotypic dimension. Across the mammals surveyed, cryptorchid species consistently showed shorter sperm than their non-cryptorchid relatives. Sperm length is not a trivial trait; the dimensions of the head, midpiece and tail reflect underlying differences in DNA packaging, mitochondrial content and flagellar design. In the bottlenose dolphin, one of the representative cryptorchid species examined in detail, the researchers observed sperm with particularly prominent mitochondrial midpieces. The midpiece is the segment of the sperm just behind the head where mitochondria are concentrated to power the tail’s beating. Enlarged or emphasized midpieces in dolphin sperm hint at a possible energetic dimension to the cryptorchid solution, although the authors are careful to present these observations as representative findings rather than a fully resolved mechanism.

Perhaps the most compelling result came from the functional assays. The team focused on HORMAD1, a meiosis-related protein that had shown positive-selection signals in cryptorchid lineages. HORMAD1 is known to play a role in the surveillance and repair of DNA double-strand breaks during meiosis, a checkpoint function that is essential for producing viable gametes. When the researchers compared the behavior of HORMAD1 proteins in the laboratory, the version associated with cryptorchid lineages exhibited enhanced thermal stability. In practical terms, the protein maintained its structural integrity better under heat than would be expected, suggesting a possible role in preserving meiotic function at the elevated temperatures that abdominal testes experience. This is the kind of direct biochemical evidence that comparative genomics studies rarely achieve, and it transforms the argument from correlation to plausible mechanism.

The significance of the HORMAD1 finding extends beyond one protein. Meiotic arrest is one of the best-documented consequences of testicular overheating in scrotal mammals, which is why heat stress is a recognized risk factor for temporary infertility in livestock and humans alike. If cryptorchid lineages have evolved meiotic proteins that tolerate heat, they have effectively decoupled sperm production from the thermal constraint that drove the evolution of the scrotum in the first place. The study’s authors frame this as part of a broader pattern: molecular changes in meiotic regulation and sperm architecture, together with the enhanced thermal stability of proteins like HORMAD1, may jointly contribute to fertility maintenance in the absence of scrotal cooling.

It is worth noting the methodological caution embedded in the work. The enrichment analysis produced a nuanced result: when the sixty-five candidate genes were themselves used as the statistical background, no significant functional enrichment was detected, and the significant enrichment for flagellar and meiotic functions appeared only when the candidates were tested against a mouse testis-expressed gene background. This kind of sensitivity to background choice is a familiar challenge in comparative genomics, and the authors report it transparently. It does not undermine the central findings, but it underscores that the evolutionary signal, while consistent, is drawn from a finite set of candidate genes and should be interpreted as a framework rather than a finished mechanism.

The study also benefited from an unusual and ethically light-touch sampling strategy. The bottlenose dolphin semen sample was obtained from an eight-year-old male at Nanjing Underwater World through voluntary ejaculation following routine husbandry training, while the bovine comparison material consisted of archived cryopreserved semen from a single male. No invasive sampling or experimental manipulation of live animals was conducted for the study. This matters both ethically and scientifically: it demonstrates that meaningful reproductive genomics can be done with material collected during routine animal care, an approach that could be extended to other marine mammals in aquaria and rehabilitation settings where repeated sampling opportunities are rare and precious.

Taken together, the findings sketch a comparative evolutionary framework for understanding reproductive persistence under chronic thermal constraint. Naturally cryptorchid mammals, the study concludes, exhibit molecular and morphological changes associated with sperm architecture and meiotic regulation, and these changes, alongside the enhanced thermal stability of HORMAD1, may explain how fertility survives without the cooling benefit of a scrotum. The work also reframes the scrotum itself. Rather than viewing external testes as the universal prerequisite for mammalian fertility, the evidence suggests that the scrotum is one solution among several, an adaptation that most lineages adopted but that a handful of lineages, from dolphins diving through cold ocean water to elephants in tropical heat, have circumvented through compensatory molecular evolution. For researchers studying heat-related infertility, livestock breeding in warming climates, or the reproductive biology of endangered marine mammals, the genomes of these natural experiment-holders may hold practical lessons that are only beginning to be read.

Subject of Research: Molecular evolution of fertility-related genes and sperm thermal adaptation in naturally cryptorchid mammals

Article Title: Evolution of fertility-related genes provides insights into morphological remodeling and enhanced thermal stability of sperm in naturally cryptorchid mammals

Article References: Evolution of fertility-related genes provides insights into morphological remodeling and enhanced thermal stability of sperm in naturally cryptorchid mammals. (n.d.). https://doi.org/10.1186/s12864-026-13369-4

Image Credits: AI Generated

DOI: 10.1186/s12864-026-13369-4

Keywords: cryptorchid mammals, molecular evolution, spermatogenesis, sperm morphology, thermal adaptation, HORMAD1, meiosis, flagellum, bottlenose dolphin, comparative genomics, fertility, testicular descent