Communal-Breeding Alpine Tree Frogs Turn Out to Be Genetically Monogamous

A disease-ravaged population of alpine tree frogs in southeastern Australia has delivered a surprise to biologists: despite breeding in dense communal aggregations where males and females mingle for weeks, genetic parentage analysis shows that nearly every clutch of eggs was sired by a single male and mothered by a single female. The finding, reported for the alpine tree frog (Litoria verreauxii alpina), reveals a genetic mating system dominated by monogamy and remarkably low reproductive skew, in sharp contrast to what the species’ crowded breeding behavior would predict.

The alpine tree frog breeds in shallow alpine and sub-alpine wetlands, generally above 1200 meters elevation, during the early spring snowmelt. Breeding typically lasts six to twelve weeks, and frogs gather in aggregations at ponds, bogs, streamside pools, and drainage channels. At the focal breeding pond studied, a permanent manmade waterbody of just 7.1 square meters at roughly 1600 meters above sea level, males far outnumber females at the site and often remain there for extended periods, while females attend for shorter visits. Such conditions—high encounter rates and male-biased attendance—would ordinarily be expected to produce multiple mating and unequal reproductive success among males.

Complicating the picture is the fungal pathogen Batrachochytrium dendrobatidis, or Bd, which has reshaped the demography of remnant populations of this species. In L. v. alpina, Bd causes high adult mortality, shortened adult lifespans, and greatly reduced generational overlap. Because females mature later than males, these demographic changes could alter mate availability and reproductive opportunities. Although the study did not examine individual infection status, the disease-driven context makes the population an instructive case for asking whether reproductive opportunities are concentrated among a few individuals or spread more evenly across breeders.

To reconstruct the mating system, researchers sampled a population in the Victorian Alps weekly for six weeks during the 2018 breeding season. They captured 477 frogs over the period, representing 197 unique individuals—179 males and just 18 females. Each frog was photographed on the dorsal surface and groin for individual identification based on natural markings, a standard technique for this species because other tagging methods fail frequently. On first capture, a toe tip was collected for DNA. The team also surveyed and photographed 55 egg masses, ranging from roughly 10 to 550 eggs each, and sampled up to four eggs from the outside of each mass to limit disturbance while maximizing the chance of detecting multiple paternity.

Tissue and egg samples were genotyped using DArTseq, a reduced-representation sequencing approach, at Diversity Arrays Technology in Canberra. From an initial dataset of 85,374 single nucleotide polymorphisms, stringent filtering for call rate, reproducibility, and linkage disequilibrium produced 2953 SNP loci. Parentage was then assigned with the program COLONY 2, which uses a full-pedigree likelihood approach and was run under the assumption that both sexes were polygamous, with three replicate runs to confirm consistency. Assignments were retained only at a posterior probability of 0.95 or higher.

Genotyping succeeded for all 197 adults and for 128 of 165 eggs; the remaining eggs were lost to bacterial contamination by Herbaspirillum, a bacterium known to infect amphibian eggs, which mostly affected early-season masses. Among the 45 egg masses with at least one successfully sequenced egg, 97.7 percent had a definitive sire identified, while only 13.3 percent had a definitive dam, a gap the authors attribute to the small number of captured females available as candidate mothers rather than to genotyping failure. Paternity was assigned to 13.4 percent of captured males (24 of 179) and maternity to 16.7 percent of captured females (3 of 18).

The reconstructed pedigree identified 28 unique mating pairs across the genotyped egg masses, and the pattern was strikingly monogamous. Of the 24 males assigned paternity, 13 sired one egg mass, 6 sired two, 3 sired three, 1 sired four, and 1 sired six. Only one captured male was assigned offspring with more than one female across the entire season, and only one inferred, uncaptured female was associated with two different males. Crucially, within every egg mass where more than one egg was genotyped, all sampled eggs were assigned to the same sire, providing no evidence of multiple paternity. The authors caution that with only up to four eggs genotyped per mass and incomplete maternity assignment, low levels of undetected multiple paternity remain possible; they interpret genetic monogamy as the dominant reconstructed parentage pattern rather than proof that all pairings were exclusive.

Reproductive output shifted in character over the season even as its total stayed constant. The number of egg masses produced per mating pair rose significantly across the six weeks, with expected counts increasing by roughly 138 percent from week 1 to week 5, from a median of one mass to a median of three. Meanwhile, mean egg-mass size fell by about 75 percent, from 246 eggs in week 1 to 62 eggs in week 5. Total egg output per pair, however, did not change significantly with week, averaging 239 eggs across about 2.1 masses per pair. Because all masses attributed to a given pair appeared in the same weekly survey and were of similar developmental age, the authors suggest the shift reflects spatial rather than temporal bet-hedging: pairs spread their eggs across more, smaller deposits later in the season, plausibly buffering against localized desiccation, disturbance from road maintenance and off-road vehicle use, or pathogen risk as water levels decline.

Male reproductive success defied the usual predictors. Neither breeding-season timing, body size, nor recapture frequency significantly predicted whether a male sired offspring; the proportion of successful sires was nearly identical early (14.1 percent) and late (14.0 percent) in the season. This is surprising because male size is often a strong predictor of mating success in frogs, and prolonged site attendance is frequently linked to mating outcomes in other species. The authors suggest that female mate choice in this population may instead rely on other behavioral or physiological cues, noting that Bd infection can alter male coloration in this species, potentially affecting mate choice—though the mechanisms remain unresolved.

The genetic structure has conservation implications. Effective population size, estimated from the reconstructed sibship structure, was 43 individuals under a random-mating assumption (95 percent confidence interval 30–65) and 35 under non-random mating (95 percent CI 23–55)—below the threshold of 50 commonly associated with elevated risk of short-term inbreeding depression and reduced adaptive potential. Yet the low reproductive skew may offer some buffer: because realized breeding contribution was distributed across many males rather than concentrated among a few, the loss of genetic diversity may be slowed, particularly in a system where overlapping generations have likely ceased due to Bd-related adult mortality. The study also underscores a broader lesson: observable breeding behavior alone can mislead, and genetic parentage analysis is essential for revealing the true mating structure of wildlife populations, especially those under demographic pressure from disease. Maintaining multiple viable egg-laying sites across the breeding season, the authors suggest, could help preserve reproductive opportunities in disease-impacted alpine frog populations.

Subject of Research: Genetic mating system, reproductive skew, and seasonal egg allocation in the disease-impacted alpine tree frog

Article Title: Genetic Monogamy and Low Reproductive Skew in a Disease‐Impacted, Communal‐Breeding Frog

Article References: Wendt, A. S., Waddle, A. W., Hough, M. A., Berger, L., Skerratt, L. F., & Brannelly, L. A. (2026). Genetic Monogamy and Low Reproductive Skew in a Disease‐Impacted, Communal‐Breeding Frog. Ecology and Evolution, 16(10), Article e74510. https://doi.org/10.1002/ece3.74510

Image Credits: AI Generated

DOI: 10.1002/ece3.74510

Keywords: alpine tree frog, Litoria verreauxii alpina, genetic monogamy, parentage analysis, reproductive skew, amphibian, Batrachochytrium dendrobatidis, effective population size, bet-hedging, communal breeding, SNP genotyping, Australia