Chromosomal inversions have long fascinated evolutionary geneticists because of what they can do structurally. When a stretch of chromosome breaks and reattaches in the opposite orientation, heterozygotes, individuals carrying one inverted copy and one standard copy, produce fewer viable recombinant gametes. The result is that genes trapped inside an inversion tend to be inherited together as a block, shielded from the shuffling effects of recombination. This makes inversions powerful vehicles for adaptation: co-adapted gene combinations that suit a particular environment can be preserved intact rather than broken apart each generation. Yet the same property raises a puzzle. If an inversion carries a combination of genes that is strongly beneficial in one setting, theory suggests it should sweep to fixation, eliminating the standard arrangement. Why, then, do so many inversions persist as polymorphisms, with both arrangements circulating in the same population for long periods?

The Mank lab’s study tackles this question in a system ideally suited to the task. Trinidadian guppies live in rivers where waterfalls create natural barriers, and above and below these barriers, populations experience radically different regimes of predation. Downstream, guppies coexist with predatory fish that exert intense natural selection. Upstream, predators are absent or rare, and the dominant selective force becomes sexual selection, as females choose among males on the basis of their colour ornaments. These paired high-predation and low-predation populations differ across a wide suite of morphological, behavioural and life-history traits, and they have replicated this divergence independently in multiple rivers, a natural experiment repeated by the landscape itself.

Rather than hunting for inversions at known locations, the researchers applied an unbiased method designed to detect them anywhere in the genome. They sampled guppies from three Trinidadian rivers, each containing a high-predation and a low-predation population. The scan revealed 22 inversions scattered across the genome. Using linked-read sequencing technology, which provides long-range information about how genetic variants are physically connected along chromosomes, the team was able to verify the precise breakpoints of all but one of these inversions, a level of validation that lends considerable confidence to the catalogue.

The ages of the inversions proved startling. Population-genetic analysis indicated that they range from roughly 1 to 6 million years old, and that they predate the colonisation of each of the three rivers by the guppies now living there. In other words, these are not recent mutations that arose locally in response to local conditions. They are ancient structural variants that arrived in Trinidad’s waters already established, and have been carried along as guppies founded new river systems. The inversions are also widespread, distributed across the genome rather than clustered on a single chromosome, suggesting that no part of the guppy’s genetic architecture is immune to this kind of structural change.

When the researchers tested whether inversion frequencies differed between high- and low-predation populations, three inversions emerged as significantly associated with the local adaptation syndromes that distinguish the two environments. Guppies from high-predation localities tend to be plainer, leaner and faster; those from low-predation localities are more colourful and carry traits favoured by mate choice. The association between specific inversions and these contrasting life histories suggests that the gene blocks locked inside them contribute to the coordinated shifts that predation pressure produces. Notably, however, none of the three inversions was reciprocally fixed across all three replicate rivers. The same inversion might be common below the waterfalls in one river and far less differentiated in another, a pattern that hints at the contingent, population-specific nature of adaptation even when the underlying selective pressures are similar.

Alongside these three adaptive candidates, the team observed seven additional inversions maintained in all three rivers without any evidence of association with the predation gradient. Their persistence demanded an explanation. Simulations modelling the expected level of inversion polymorphism under a neutral model, in which variants drift by chance alone, revealed that the observed diversity far exceeds what neutrality can accommodate. Something must be actively maintaining both arrangements of these chromosomes, preventing either from drifting to loss or sweeping to fixation.

The most striking clue came from a comparison with earlier work on guppy colouration. The researchers found a significant overlap between the polymorphic inversions and genomic loci previously implicated in variation in male ornament patterns, the pigmented spots and splashes that females scrutinise when choosing mates. This overlap suggests a mechanism long proposed in theory but rarely demonstrated with genomic data in the wild: negative frequency-dependent selection arising from female preference for novelty. If females favour males with rare or unusual colour patterns, then whichever arrangement of an inversion is currently common becomes disadvantageous, and the rarer arrangement gains a mating advantage. The two chromosomal arrangements are thereby trapped in a perpetual seesaw, each rising when the other falls, and neither ever winning outright. Such balancing selection can preserve structural variation for millions of years, exactly the timescales the team inferred for the guppy inversions.

The study’s broader significance lies in how it reframes the interplay between the two great engines of evolution. Natural selection, driven here by predation, clearly acts on some inversions, pushing their frequencies up or down in different environments. But sexual selection, driven by female choice, appears to be the force that keeps other inversions polymorphic over geological time, replenishing the raw material of male ornament variation on which mate choice feeds. The two processes do not simply act in opposition or in parallel; they interact in shaping the frequency dynamics of the same structural variants. For evolutionary biologists, the guppy results provide a vivid demonstration that the maintenance of genetic diversity is not merely a story of heterozygote advantage or environmental heterogeneity, but can hinge on the ever-shifting tastes of choosy females.

There are also practical implications for how researchers read the genome. Because inversions suppress recombination across large genomic neighbourhoods, association studies that ignore them can misattribute the effects of genes inside inversions, and conservation geneticists assessing the adaptive potential of fragmented populations may underestimate the hidden structural diversity those populations carry. The guppy work, conducted with an unbiased detection method and validated breakpoints, offers a template for surveying inversions in other natural systems where ancient polymorphisms may be quietly shaping traits, from plumage in birds to life histories in insects. For now, the humble Trinidadian guppy has added another chapter to its scientific biography: a fish whose colours have long charmed biologists turns out to owe part of that charm to chromosomal architecture laid down millions of years before any guppy swam a Trinidadian river.

Subject of Research: Chromosomal inversion polymorphisms and sexually selected traits in natural guppy populations

Article Title: Ancient inversion polymorphisms associate with sexually selected traits across natural guppy populations

Article References: Lin, Y., van der Bijl, W., & Mank, J. E. (2026). Ancient inversion polymorphisms associate with sexually selected traits across natural guppy populations. PLOS Biology, 24(9), e3004024. https://doi.org/10.1371/journal.pbio.3004024

Image Credits: AI Generated

DOI: 10.1371/journal.pbio.3004024

Keywords: guppies, chromosomal inversions, sexual selection, natural selection, local adaptation, balancing selection, frequency-dependent selection, Trinidad, population genetics, male colouration, PLOS Biology, structural variants

Uintatheres emerged roughly 58 million years ago, fewer than 10 million years after the asteroid impact that ended the age of dinosaurs, and they persisted in the fossil record for around 20 million years. They were among the largest land mammals of their time, notable for their bulk, their prominent horns and their large canine teeth. Because living relatives of such animals are long extinct, paleontologists have traditionally inferred differences between the sexes from clusters of size and ornament in fossil samples. If a collection of skulls seemed to sort into a big-horned group and a small-horned group, the natural interpretation was males and females. For uintatheres, that interpretation hardened into orthodoxy: males were depicted as much larger animals bearing far more dramatic cranial features than females, a picture repeated in museum halls, popular books and scientific literature alike.

The new research began, fittingly, with a puzzle far smaller than a meter-long skull. Kevin Mulcahy, a doctoral student at the University of Kansas Biodiversity Institute and Natural History Museum, and his doctoral adviser K. Christopher Beard, senior curator at the institute, were trying to identify a single broken upper molar collected during fieldwork in Montana, a tooth they suspected might belong to the uintathere group. To solve that problem, the two traveled to the American Museum of Natural History in New York, whose storied vertebrate paleontology collections hold entire shelves of uintathere skulls, each nearly a meter long. These are famous specimens, described and redescribed in the literature for decades, and the original goal was simply to compare teeth and pin down the identity of the Montana fossil.

What Mulcahy found instead was a pattern that did not match the textbooks. As he examined the celebrated skulls to compare their teeth, he noticed an enormous amount of variation — but it did not appear to fall into two neat categories. Working quickly, he took a series of measurements of the skulls with a ruler and began exploring them statistically. The logic of the test is straightforward: if a sample of fossils truly reflects a strongly dimorphic species, measurements of traits such as skull size, horn development and canine dimensions should fit a bimodal distribution, with two peaks corresponding to average males and average females. If the species is not strongly dimorphic, the same measurements should instead follow a single, unimodal distribution. When he ran the analysis, the expected two-peak pattern was nowhere to be found.

“It wasn’t like there’s one group of specimens that look bigger than another, and then there’s another group that are smaller with smaller horns and canines,” Mulcahy said. That observation raised an uncomfortable question: were uintatheres really such good examples of sexual dimorphism after all? Was there actual evidence in the morphology of the skulls — in the distribution of the ranges of morphology — that two sexes were being sampled? To find out, Mulcahy broadened the project, gathering additional measurements from specimens held at other institutions and applying newer statistical procedures. His final dataset included about 30 skulls of Uintatherium anceps, the species at the heart of the dimorphism story.

The results were underwhelming for the traditional view. “Maybe there were some little signals of dimorphism here and there, but nothing really concrete,” Mulcahy said. Yet a skeptic could still object that the methods themselves were too weak to detect dimorphism if it existed. Fossil samples carry a fundamental handicap: no one actually knows which specimens are male and which are female. Researchers can guess, assigning the most extreme individuals to one sex and the least extreme to the other, and then test whether the groups differ — but as Mulcahy points out, that procedure is deeply circular. Of course the groups will differ if you built them by sorting extremes in the first place.

To break that circularity, Mulcahy turned the same statistical techniques on a living animal whose sex could be verified with certainty: the modern bison. Bison offered an ideal comparison. They are another very large herbivore, they occupy an ecology broadly similar to what has been proposed for uintatheres, and they carry substantial horns, making them a plausible modern analogue for testing how dimorphism signals appear in skull measurements. The contrast with the fossils was striking. In the bison sample, the dimorphism signal was clear and strong: females clustered with females and males clustered with males in an almost perfect separation. The methods, in other words, were fully capable of detecting sexual dimorphism when it genuinely exists — which made its near absence in the uintathere skulls all the more meaningful.

If the statistics were sound and the sample was adequate, then the flagship example of sexual dimorphism in the mammalian fossil record may never have deserved the title. Mulcahy suggests that the mistaken assumption has deep historical roots, reaching back to the late 19th and early 20th centuries, when so many fossil species were first described. This was the era in which evolutionary theory, sexual selection and natural selection were entering scientific discourse, and the field was dominated by male paleontologists working within a distinctly Victorian-era mindset. Within that intellectual climate, the idea that males would be bigger and more fearsome than females was less a hypothesis to be tested than an assumption to be illustrated. “When you step back and actually test that hypothesis, it doesn’t really hold up,” Mulcahy said.

The implications extend well beyond uintatheres. If a species long regarded as the classic, near-definitive case of dimorphism yields only weak and inconclusive signals under modern analysis, then other fossil species whose dimorphism was asserted on similar grounds may deserve re-examination as well. The finding also speaks to a broader debate about the evolution of size differences between the sexes across the mammalian family tree. Strong, big-male-style dimorphism was probably not ancestral at the root of the mammalian tree, Mulcahy noted; among living mammals where the pattern does appear, it has almost certainly evolved independently several times. The assumption that males are typically larger than females, he argued, reflects a long-standing bias in which animals scientists chose to study and which questions they found interesting.

That bias has been quantified in recent work on living mammals. As Mulcahy points out, a paper published just a couple of years ago showed that among modern mammal species, most either show no sexual dimorphism at all or actually feature larger females. The popular image of the animal kingdom as a place where males invariably outweigh and out-arm females, he said, is really an artifact of which creatures biologists in the past chose to look at. For paleontology, the lesson of the uintatheres is a methodological one: claims of dimorphism in fossil species need to be tested against explicit statistical expectations, ideally benchmarked against living species of known sex, rather than assumed from Victorian intuitions. A broken molar from Montana set that reassessment in motion, and the iconic horned giants of the Eocene may never look quite the same again.

Subject of Research: Sexual dimorphism in the extinct Eocene mammal Uintatherium anceps

Article Title: Study: ‘Classic example’ of sexual dimorphism in fossil record actually shows nuance between males and females

Article References: Study: ‘Classic example’ of sexual dimorphism in fossil record actually shows nuance between males and females. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: uintatheres, sexual dimorphism, paleontology, Eocene mammals, Uintatherium anceps, fossil skulls, statistical analysis, bison comparison, sexual selection, University of Kansas, PLOS One, mammalian evolution

The central problem the Exeter team set out to solve is a familiar one for behavioral biologists. When scientists want to know what an animal finds attractive in a mate, they typically present it with a handful of real or artificial options and record which one it approaches. The trouble is that such experiments can only ever sample a tiny slice of the possible variation in traits like body size, coloration, and ornament shape. The preferences an animal reveals may therefore reflect the limited menu it was offered rather than its true ideal. Kathryn Bullough, of the Centre for Ecology and Conservation at Exeter’s Penryn Campus, explained the motivation behind the new approach: studying mate choice in animals is difficult because experiments can present only a limited range of potential mates, so the team devised a way for fish to refine their preferences across many rounds of choice.

The solution borrowed a technique from computer science: the genetic algorithm. In the Exeter setup, each fish was placed in a tank with a screen at either end, each displaying a moving virtual mate rendered as a digital phenotype, a computer model whose characteristics could be varied systematically. Traits under algorithmic control included tail length, body size, body width, body color, tail color, and pattern. The fish was then free to swim for three minutes, and its position was tracked across three zones: a neutral central area and one zone near each virtual option. Time spent near a stimulus served as the measure of preference, a standard proxy in mate-choice research, since animals that favor a potential mate tend to linger close by.

From there, the evolutionary machinery took over. If a fish spent more time near one virtual mate than the other, that digital individual was declared the winner of the trial. Two new virtual fish were then generated based on the winner’s characteristics combined with those of another randomly selected winning mate from a separate trial, producing a fresh generation of candidate mates. These offspring were tested in the next round, and the process repeated. Over nine generations, the pool of virtual mates was progressively shaped by the accumulated choices of the fish, homing in on whatever combination of traits the choosers actually preferred. The method mimics sexual selection itself: traits that make an individual more likely to be chosen get passed on and amplified down the generations, exactly as attractive traits spread through real populations over evolutionary time.

The results revealed a sharp asymmetry between the sexes. Male green swordtails turned out to have simple, consistent tastes: they reliably chose larger females. The strength of this preference was visible in the data in a dramatic way. By the final generation, the virtual females favored by males were on average more than 24 percent larger than those in the first generation, a substantial shift produced in only nine rounds of selection. The researchers suggest a straightforward biological explanation: larger females can produce more offspring, so a male that prefers big females potentially sires more young, making the preference advantageous in evolutionary terms.

Female preferences proved far more complex and far more individual. Rather than converging on a single shared ideal, different females steered the algorithm in different directions. Some favored longer tails, others brighter colors, and the trajectories of their choices diverged across the generations. This individual variation is itself an interesting finding, because it suggests that female mate choice in this species is not a uniform filter but a mosaic of personal tastes, which in a wild population could help maintain the diversity of male traits rather than driving them all toward a single optimum.

One female’s choices stood out as particularly puzzling. Over successive generations, her selected mates looked increasingly like females rather than males. The reasons remain unclear, but the researchers offer a plausible hypothesis: this individual may have avoided threatening male characteristics, moving away from them and therefore, incidentally, toward more female-like traits. The case illustrates one of the strengths of the algorithmic approach, which can surface unexpected preference patterns that conventional two-option experiments would never reveal, while also highlighting that interpreting those patterns still requires careful behavioral reasoning.

The choice of species was no accident. Green swordtails, native to Central America and named for the males’ distinctive sword-like tail fin, are unusually well suited to this kind of work. Dr. Laura Kelley of the Exeter team noted that the fish respond to videos of other fish, which makes virtual stimuli effective, and that they provide no parental care for their young. That second point matters for interpretation: without the complication of assessing a partner’s future contribution to raising offspring, mate choice in this species is relatively straightforward, with fish primarily looking for good genes to pass on to their offspring. The sword-like tail, a classic textbook example of an ornament shaped by female preference, made the species an apt subject for a study of how such preferences operate in the first place.

The method also promises practical gains for the wider field. As Kelley pointed out, a comparable study using real fish would take years, because breeding and measuring generation after generation of live animals is slow and logistically demanding. The digital approach compresses that timeline dramatically, providing what the researchers describe as a powerful tool for understanding how wild animals choose their mates. Because the virtual mates are fully parameterized, the researchers know exactly which traits changed across generations and by how much, allowing preferences to be quantified trait by trait rather than inferred indirectly. The technique could in principle be adapted to other species that respond to visual stimuli, opening the door to comparative studies of how mate preferences evolve across different ecological and social settings.

Beyond its technical novelty, the study carries a pleasing conceptual symmetry. Sexual selection is usually imagined as a slow force acting over countless generations in rivers and streams, gradually sculpting the extravagant tails and vivid colors of animals like the green swordtail. Here, the same logic was run in miniature, with fish acting as the selective agents and algorithms as the hereditary machinery, and the imprint of choice emerged within weeks rather than millennia. The fish, in a very real sense, designed their perfect partners, and in doing so they showed scientists exactly what perfection looks like to them: for males, a simple rule favoring size; for females, a rich and varied set of tastes that no single experiment could have uncovered. The work, entitled Exploring male and female mate preferences using genetic algorithms and digital phenotypes, appeared in Royal Society Open Science on 29 September 2026.

Subject of Research: Mate preference evolution in green swordtail fish studied with genetic algorithms and virtual mates

Article Title: Fish choose perfect partners in dating game study

Article References: Fish choose perfect partners in dating game study. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: green swordtail fish, mate choice, sexual selection, genetic algorithms, digital phenotypes, University of Exeter, Royal Society Open Science, animal behavior, evolutionary biology, virtual mates, female preference, behavioral ecology

The species at the center of the work, Quasipaa spinosa, is a large ranid frog prized in traditional cuisine and increasingly threatened in the wild, which makes understanding its reproductive biology more than an academic exercise. During the breeding season, males clasp females in a tight embrace called amplexus, and the chest spines are thought to help the male anchor himself to the often slippery female in fast-flowing stream water. But while the spines were long noted by field biologists as a striking secondary sexual trait, the structural and molecular machinery that builds them each year had never been systematically characterized. The new study set out to close that gap by combining classical histology with modern high-throughput molecular tools.

The team began by examining the chest skin of males at different points in the annual reproductive cycle: before breeding, during breeding, and after breeding had concluded. Using AB-PAS histological staining, a technique that distinguishes different classes of carbohydrates and secretory products in tissue sections, they revealed that the chest skin of Q. spinosa is not ordinary frog skin. It contains specialized glandular structures and a dermal framework unusually rich in collagen, the fibrous protein that gives skin its tensile strength. This pre-existing scaffold, the authors argue, provides the structural foundation on which the seasonal spines are subsequently built, rather than the spines arising de novo from undifferentiated tissue.

To understand what changes molecularly as the spines grow, the researchers performed quantitative proteomic analysis, comparing the protein content of chest skin across the pre-breeding, breeding and post-breeding stages, and also contrasting chest skin with abdominal skin from the same animals. The comparison identified large numbers of differentially expressed proteins, and the patterns were far from random. Principal component analysis showed that samples from each reproductive stage clustered distinctly, indicating that the chest skin undergoes a coordinated molecular remodeling program as the breeding season approaches, peaks, and passes. Among the protein categories that shifted most dramatically were components of the extracellular matrix, the fibrous and gel-like material that surrounds cells and gives tissues their shape.

That focus on the extracellular matrix, or ECM, is the conceptual heart of the paper. Pathway analysis of the differentially expressed proteins pointed to significant enrichment of ECM-related processes, including focal adhesion signaling and ECM-receptor interactions, the molecular junctions through which cells grip and remodel their surrounding scaffold. In practical terms, the data suggest that spine formation is not simply a matter of cells producing more keratin, but of the entire dermal environment being reorganized: collagen deposited and cross-linked, cell-matrix attachments adjusted, and tissue architecture reshaped to support the emerging keratinized structures. The spines, in this view, are the visible output of a deep remodeling of the skin’s connective-tissue framework.

The proteomic findings were reinforced at the level of gene expression. Using real-time quantitative PCR, the team validated the behavior of genes connected to the TGF-beta/Smad signaling pathway, a canonical regulatory cascade known from developmental biology to control cell growth, differentiation and the production of matrix components. The expression of these genes, along with others involved in matrix remodeling, changed dynamically across the reproductive cycle, rising and falling in step with the growth and subsequent regression of the spines. This concordance between protein-level and transcript-level evidence strengthens the conclusion that TGF-beta/Smad-mediated matrix remodeling is a central mechanism in the seasonal construction of the chest spines, and it offers a concrete molecular handle for future work on how amphibian secondary sexual characters are hormonally and environmentally regulated.

But the most consequential part of the study is its experimental test of function. Correlation between a showy trait and mating success is easy to claim and hard to prove, because attractive males may simply be older, larger or healthier in ways that independently boost their reproductive output. The researchers addressed this directly with reproductive competition experiments in which multiple males competed for access to females, combined with paternity testing based on microsatellite markers, short repetitive DNA sequences that act as genetic fingerprints and allow each offspring to be assigned to its biological father. The results were unambiguous: males whose chest spines were better developed sired more offspring than their rivals, and when the researchers artificially blunted the spines, those males’ reproductive success dropped.

This experimental design matters because it isolates the spines themselves as a causal factor in male reproductive success rather than a mere correlate. The paternity data, generated with SSR markers listed in the study’s supplementary tables, tie the physical condition of the spines directly to the number of offspring each male produced under competitive conditions. Together with the morphological measurements of spine phenotype recorded for each male, the experiments demonstrate that the seasonal spines function as a genuine sexually selected weapon or grip aid, most plausibly by improving a male’s ability to maintain amplexus with a female in turbulent stream environments where rivals and currents alike conspire to dislodge him.

Beyond its evolutionary interest, the work carries practical implications for conservation and aquaculture. Q. spinosa is heavily harvested across its range, and captive breeding programs are increasingly important both for commercial production and for population supplementation. The authors point out that chest spine development could serve as a simple, non-invasive visual indicator of male reproductive performance, allowing hatchery managers to select broodstock males with well-developed spines and thereby improve the genetic output of breeding groups. In wild populations under conservation management, the same trait could be used to gauge the reproductive fitness of males, linking an evolutionarily selected secondary sexual character to the day-to-day business of managing amphibian populations under pressure.

The study also opens broader questions about tissue plasticity in adult vertebrates. The idea that a region of skin can be repeatedly rebuilt each year, dismantled and reconstructed through extracellular matrix remodeling governed by a well-known developmental signaling pathway, resonates with research on antler regeneration, seasonal plumage and nuptial pads in other amphibians. By supplying a full pipeline from histology through proteomics and gene expression to functional paternity testing, the Zhejiang Normal University team has provided one of the most complete accounts to date of how a seasonal secondary sexual trait is built and why it matters. For a species whose survival increasingly depends on managed breeding, knowing that the spines on a male’s chest are both a molecular barometer of his readiness and a reliable predictor of his success as a father turns a curious anatomical feature into a practical conservation tool.

Subject of Research: Seasonal chest spine formation and its role in male reproductive success in the spiny frog Quasipaa spinosa

Article Title: Formation and reproductive significance of chest spines in Quasipaa spinosa

Article References: Peng, X., Piao, Z., Yi, Z., Min, C., Ben, L., & Rongquan, Z. (2026). Formation and reproductive significance of chest spines in Quasipaa spinosa. Frontiers in Zoology. https://doi.org/10.1186/s12983-026-00634-1

Image Credits: AI Generated

DOI: 10.1186/s12983-026-00634-1

Keywords: Quasipaa spinosa, chest spines, extracellular matrix remodeling, TGF-beta/Smad signaling, reproductive success, sexual selection, amphibian conservation, paternity testing, proteomics, secondary sexual traits, broodstock selection, Frontiers in Zoology

Alternative reproductive tactics, known in the literature as ARTs, are among the most striking manifestations of sexual selection. Instead of every male competing on the same terms, populations evolve two or more discrete behavioural or morphological strategies within one sex. In countless species, from salmon to dung beetles to frogs, large ‘bourgeois’ males guard territories, court females and care for offspring, while smaller ‘sneaker’ or ‘parasitic’ males forgo courtship entirely, darting into rival nests to release sperm at the decisive moment. The payoff for each tactic is frequency dependent:bourgeois males do well while they are rare, sneakers thrive when they can slip past the guards. Although ARTs have been documented across most major animal groups, their evolution in freshwater fishes remains poorly understood, and within the Gobiidae, the goby family, the evidence has been remarkably thin.

That gap mattered to the researchers behind the new study because gobies are no minor sideshow. The family comprises roughly 2,000 species across 212 genera, making it one of the most species-abundant vertebrate families on Earth. Most gobies are small, typically under 15 centimetres, and are distinguished by fused pelvic fins that form an adhesive disc. Many exhibit substrate-associated spawning, with males guarding nests of developing eggs, a breeding system that in other goby lineages has repeatedly spawned sneaker tactics. South African freshwater gobies, which occur naturally along the eastern seaboard from northern Mozambique to the Cape region, are believed to share this nest-guarding habit, but their mating systems had never been examined in detail. The River Goby, an important component of freshwater fish assemblages in Southern African rivers, was an ideal candidate for a closer look.

The study focused on a population living in ten man-made ponds in the semi-arid Lower Sundays River Valley of the Eastern Cape Province. Between winter 2013 and autumn 2015, the team sampled fish seasonally using a 30-metre seine net, measuring total length to the nearest tenth of a millimetre and wet body weight to the nearest tenth of a gram. In the laboratory, specimens were dissected, gonads were assessed against a standard five-stage maturity classification, and reproductive investment was quantified using the gonadosomatic index, or GSI, the ratio of gonad mass to eviscerated body mass expressed as a percentage. The work was ethically approved by the NRF-South African Institute for Aquatic Biodiversity Animal Ethics Committee, with specimens collected under permits from the Eastern Cape provincial authorities.

One of the study’s most practical contributions is a simple, non-lethal method for sexing these fish. Examination of the urogenital papilla, a small external structure, proved completely reliable: females possess broader, blunt papillae, while males have shorter, pointed ones. Chi-square analysis confirmed that every one of the 392 individuals identified as male externally was verified as male by gonadal dissection, and all 399 individuals with the female-type papilla were confirmed as females, a 100 percent accuracy rate. Because live-bearing-free sexing usually requires sacrificing and dissecting fish, a validated external marker opens the door to behavioural and longitudinal studies that would otherwise be impossible on a species whose reproductive tactics are still being pieced together.

The sex ratio of the sampled population itself carried a signal: 392 males, 399 females and 37 juveniles produced a female-dominated ratio of 1 male to 1.21 females, a significant departure from parity. But the headline finding lay in the males. Based on pelvic fin coloration, the team identified three morphologically distinct morphs. Dark-morph males, displaying deep pigmentation, were significantly longer and heavier than light-morph males, while intermediate males fell between the extremes. Among the 98 reproductively ripe males captured, light morphs were the most common, followed by intermediates and then dark males, a distribution that foreshadowed the functional differences the analysis would later expose.

The decisive evidence came from the pattern of gonadal investment across body sizes. A loess regression of GSI against total length revealed sharp discontinuities between the morphs. Light-morph males showed elevated gonadosomatic indices at small body sizes, in the 70 to 85 millimetre range, while dark-morph males achieved peak GSI only at larger sizes, between 95 and 105 millimetres. In evolutionary terms, this is the classic signature of a trade-off. Small males that cannot win territory or attract mates appear to channel a disproportionate share of their energy into testes development, priming themselves for sperm competition inside another male’s nest, while large, dark, presumably territorial males invest in size and dominance displays rather than sheer gonadal output. Light morphs were, in fact, heavier than medium morphs, further hinting that the intermediate form is not simply a scaling of the same strategy.

Seasonal abundance data added a temporal dimension. Dark morphs peaked in spring, with 355 individuals recorded, and summer, with 202 individuals, a window that corresponds precisely to the known reproductive season of the species in the Sundays River system, which runs from early spring in September to mid-summer in December. Light morphs, by contrast, were most abundant in winter, with 142 individuals recorded outside the breeding season. The authors interpret this as evidence that the morphological and physiological traits underpinning sneaker tactics are established before the breeding season begins, allowing pale males to be ready to exploit spawning opportunities the moment they arise. Such preparation is consistent with the idea that reproductive tactics are not improvised at the nest but develop over months in anticipation of competition.

The parallels with other gobies are striking and suggest convergent forces at work. In the invasive Round Goby, Neogobius melanostomus, researchers have documented ontogenetically distinct tactics in which small males adopt sneaker behaviour with higher relative gonadal investment while larger males defend territories and provide parental care. In Caspian gobies, three male types, large territorial courters, intermediates and small cuckolding parasites, have been described in multiple species. The recurring appearance of this tripartite system across geographically separate gobiid lineages points to strong and consistent selective pressures, most likely intense male-male competition for limited spawning sites in a family where nest-guarding males effectively monopolize reproduction. Whether the same pressures shaped the Sundays River population remains an open question, but the morphological fingerprints are remarkably consistent.

Important caveats remain, and the authors are careful to flag them. Direct behavioural observations of spawning have not yet been conducted for G. callidus, so the assignment of tactics to morphs rests on morphological and physiological evidence rather than on watching dark males guard nests or light males dart into them. It is also unknown whether the morphs represent fixed genetic strategies or condition-dependent plastic responses, with individual males potentially switching tactics as they grow, as has been suggested for condition-dependent tactics in other gobies. Phenotypic plasticity allows genotypic expression to vary with environmental conditions, and life-history theory holds that reproductive traits evolve through trade-offs in which the evolution of one trait constrains or influences another. Genetic analyses, sperm quality assessments across morphs and direct observations of spawning competition are the logical next steps, along with determining whether the intermediate morph is a transitional stage en route from sneaker to parental or a distinct tactic in its own right. Whatever the answers, the study delivers the first evidence of alternative reproductive tactics in this endemic Southern African goby, and it underscores a broader lesson for freshwater ecology: beneath the surface of even the humblest pond fish, evolution may be running several different games at once, each with its own winners, losers and rules of engagement.

Subject of Research: Alternative reproductive tactics in the River Goby Glossogobius callidus

Article Title: Alternative reproductive tactics and strategies of Glossogobius callidus (Teleostei: Gobiidae)

Article References: Cembi, O., Mutshekwa, T., Munyai, L. F., Ndaleni, P., & Mofu, L. (2026). Alternative reproductive tactics and strategies of Glossogobius callidus (Teleostei: Gobiidae). Discover Ecology, 2(1), Article 3. https://doi.org/10.1007/s44396-025-00021-w

Image Credits: AI Generated

DOI: 10.1007/s44396-025-00021-w

Keywords: alternative reproductive tactics, River Goby, Glossogobius callidus, Gobiidae, sneaker males, parental care, sexual selection, gonadosomatic index, South Africa, freshwater fish, pelvic fin coloration, reproductive strategies

The male choice hypothesis, developed by Apostolou and colleagues in a series of papers beginning in 2017, proposes that female same-sex attraction persists in populations because men find some degree of same-sex attraction in women appealing, and because such women may be more willing to engage in same-sex behavior that satisfies male partners. In this framework, male preference functions as a selective force: if men preferentially choose women with certain patterns of sexual flexibility, and if this preference translates into higher mating success or relationship stability for those women, then genes and developmental pathways associated with female same-sex attraction could be maintained by positive selection rather than eliminated by its reproductive costs. The hypothesis sits within a broader program of research by Apostolou on sexual selection under parental choice, which argues that for much of human evolutionary history parents, rather than prospective mates themselves, exercised substantial control over mating decisions.

Wilczkowski’s 2026 target article, titled Male choice hypothesis: Shortcomings, inconsistencies, and proposed alternatives, argued that the evidential foundations of this account are weaker than its proponents suggest. According to the structure of the debate, the critique pressed on multiple fronts: that the survey findings showing male approval of female same-sex attraction are modest in magnitude and vulnerable to methodological artifacts; that male preference in a modern, largely WEIRD sample does not establish an ancestral selective pressure; and that the hypothesis sits awkwardly alongside the substantial body of evidence indicating that exclusive or predominant female homosexuality is associated with reduced reproductive output. When a trait demonstrably lowers the number of offspring produced, the argument goes, a preference-based benefit must be strong and consistent enough to offset that cost, something Wilczkowski contended the existing data do not demonstrate.

Apostolou’s response takes up that challenge by reassembling the evidence base and contesting the interpretation of several key datasets. A central empirical pillar of the male choice program is work using nationally representative and convenience samples showing that a substantial proportion of men report that a partner’s same-sex attraction would not deter them, and that many men find the prospect of a female partner engaging in same-sex behavior appealing. Follow-up cross-cultural work by Apostolou and Wang extended these findings beyond Western samples, an important move because evolutionary hypotheses ultimately rest on claims about ancestral and species-wide selection pressures rather than the idiosyncrasies of contemporary college students. From this perspective, the presence of the preference across disparate cultural settings is precisely the kind of pattern one would expect if male choice had played a genuine selective role.

The reply also engages with the physiological and psychometric literature on female sexual response. Studies of sexual arousal, including work by Chivers and colleagues and more recent research on genital and subjective arousal patterns, have repeatedly found that women’s arousal is less category-specific than men’s, responding to a broader range of sexual stimuli. For proponents of male choice, this nonspecificity is consistent with a capacity for sexual fluidity that male preference could have shaped. Apostolou’s broader account of female same-sex attraction as relatively fluid, weakly canalized, and responsive to social context draws on behavioral-genetic findings from twin studies in Sweden and elsewhere, which indicate modest heritability for same-sex sexual behavior alongside substantial environmental influence, and on longitudinal cohort research showing that women’s sexual identity and attraction shift over time more often than men’s. The massive genome-wide association study published in Science in 2019 reinforced the picture of a highly polygenic, non-deterministic architecture for same-sex sexual behavior, leaving room for accounts in which developmental plasticity interacts with social selection.

Against this, the critique and the reply must both confront the reproductive cost data directly. Apostolou’s own 2022 analysis of two nationally representative U.S. samples documented a direct reproductive cost associated with same-sex attraction, meaning that women reporting such attraction had fewer children on average. This is a crucial technical point in the dispute: the male choice hypothesis does not deny the cost, it argues that the cost was historically offset by fitness benefits flowing from male preference, such as greater access to mates, improved partner retention, or cooperative alliances with other women that enhanced survival and resource acquisition. Evaluating this balancing act requires reconstructing ancestral mating markets, and it is here that the parental choice framework does much of the work. If parents historically controlled whom daughters married, as evidence from historical societies and hunter-gatherer marriage practices suggests, then women’s own preferences were under weaker sexual selection, and traits that made a woman more acceptable to the men who influenced mating decisions could plausibly spread.

Methodological disagreements run through the exchange as well. The response addresses concerns about survey design, including the treatment of midpoint responses on Likert scales, an issue with a technical literature of its own, and about the gap between stated preferences and actual mating behavior. Critics of preference-based selection arguments note that what men say they find appealing in hypothetical scenarios may bear little relation to the mate choices they make, and that self-reported attraction measures diverge to a degree from genital measures of arousal, as meta-analytic work by Chivers and colleagues has shown. Defenders counter that preference measures, when replicated across cultures and supplemented by behavioral proxies such as pornography consumption patterns and reported willingness to satisfy partners, collectively point in the same direction, and that the absence of any single perfect measure is not grounds for discarding convergent evidence. The dispute thus becomes a case study in how evolutionary behavioral scientists adjudicate between competing bodies of imperfect evidence.

The stakes of the debate extend beyond a single hypothesis. Any account of female same-sex attraction must square three robust empirical regularities: the trait is partially heritable but not strongly genetically determined; it is more fluid and less category-specific in women than in men; and predominant homosexuality carries measurable reproductive costs. The male choice hypothesis is one of several positive-selection proposals in the literature, alongside Kanazawa’s arguments about female sexual fluidity arising from generalized arousal mechanisms and Diamond’s work situating same-gender sexuality within a broader framework of fluidity and diverse forms of sexual diversity. Alternative accounts, including weak negative selection, where mildly deleterious variants persist because selection against them is too weak to eliminate them quickly, and byproduct explanations, in which same-sex attraction emerges as a side effect of selection acting on other traits, remain live competitors, and Wilczkowski’s article is explicitly framed as proposing alternatives to male choice rather than merely attacking it.

What the exchange ultimately illustrates is the current state of evolutionary hypotheses about human sexuality: rich in explanatory ambition, but constrained by the difficulty of testing claims about ancestral selection with modern data. Apostolou’s letter argues that the accumulated evidence, from male preference surveys across cultures, from the psychophysiology of female arousal, from behavioral genetics, and from anthropological records of parental control over mating, does not come close to refuting the male choice hypothesis, and that rejecting it requires a higher standard of disproof than Wilczkowski applies. Whether the hypothesis survives this scrutiny will depend on future work that can quantify how large a preference-based fitness benefit would need to be, measure male preference with methods that map onto actual ancestral mate choice, and test the predicted patterns in populations and datasets not yet examined. For now, the journal’s correspondence pages have become the arena where one of the more provocative ideas in evolutionary psychology is being stress-tested in public, and readers of sexual science are witnessing how a contested hypothesis earns, or loses, its place in the literature one rebuttal at a time.

Subject of Research: Evolutionary explanations of female same-sex attraction and the male choice hypothesis debate

Article Title: Does the Evidence Refute the Male Choice Hypothesis? A Response to Wilczkowski (2026)

Article References: Apostolou, M. (2026). Does the Evidence Refute the Male Choice Hypothesis? A Response to Wilczkowski (2026). Archives of Sexual Behavior. https://doi.org/10.1007/s10508-026-03635-7

Image Credits: AI Generated

DOI: 10.1007/s10508-026-03635-7

Keywords: male choice hypothesis, female same-sex attraction, evolutionary psychology, sexual selection, sexual fluidity, Apostolou, Wilczkowski, Archives of Sexual Behavior, parental choice, reproductive cost, mate preference, behavioral genetics