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