The discovery matters because parthenogenesis, while not unheard of in ticks, is rare and has profound consequences for how a parasite spreads, evolves, and responds to control measures. Asexual populations can expand rapidly from a single founder female, since every individual is capable of reproducing, but they carry limited genetic diversity. Bisexual populations, by contrast, generate genetic variation through recombination, which can fuel adaptation to new hosts, climates, and pesticides. Knowing which reproductive strategy dominates in a given region is therefore not an academic curiosity; it directly shapes predictions about disease transmission and tick management across the Neotropics, where A. rotundatum parasitizes a wide range of cold-blooded vertebrates, from amphibians to reptiles.

The fieldwork focused on green iguanas, Iguana iguana, captured in the Amazon biome of Maranhão. Ticks collected from these reptiles were identified as A. rotundatum using a combination of morphological examination and molecular characterization, a dual approach that guards against the misidentification that can easily arise in a species complex. In total, the researchers collected 517 ticks across all life stages: 82 larvae, 95 nymphs, and 340 adults. Among the adults, 130 were males and 210 were females, yielding a sex ratio of roughly one male for every 0.6 females—a statistically balanced ratio that immediately signaled something unusual for a species in which males are considered rare occurrences in nature.

The team did not stop at counting adults. Twenty-five engorged and semi-engorged nymphs were taken from wild iguanas and brought into the laboratory, where seventeen of them successfully molted into adults: ten males and seven females, a ratio of approximately one male to 0.7 females. This second, independent line of evidence confirmed that the male-female balance observed among field-collected adults was not a sampling artifact. Nymphs feeding on wild hosts, when allowed to complete their development under controlled conditions, produced both sexes in comparable numbers, indicating that the bisexual trait is intrinsic to this population rather than a chance aggregation of individuals on a particular host.

To test whether the pattern would persist across generations, the researchers established a laboratory colony using cane toads, Rhinella marina, as hosts. Three toads were artificially infested with larvae, nymphs, and adults at standardized doses—approximately 500 larvae per host, 25 nymphs per host, and four adults per host—and the colony was maintained for two consecutive generations. Toads were housed individually in glass aquaria in a room held at 27 degrees Celsius and roughly 90 percent relative humidity, while the free-living tick stages were kept in desiccators inside an incubator under the same conditions. These parameters mimic the warm, saturated microclimates that Neotropical ticks experience in leaf litter and burrows, and they allowed the team to track each developmental transition with precision.

The laboratory results mirrored the field data strikingly. Approximately 27 percent of engorged larvae successfully molted into nymphs, and 61 percent of those nymphs completed development to the adult stage, producing 23 males and 18 females—a ratio of one male to 0.8 females. The mean egg incubation period was 33.7 days, with a standard deviation of 1.5 days, and the mean hatching percentage reached 90 percent, with a standard deviation of 7 percent. Taken together, the field counts, the molting of wild nymphs, and two generations of controlled rearing all pointed to the same conclusion: this population of A. rotundatum generates males and females in similar proportions, fulfilling the biological definition of a bisexual population.

Why does this matter beyond the taxonomy of a single tick species? A. rotundatum is a parasite of ectothermic vertebrates and is widely distributed across Central and South America, where it has also been recorded feeding on hosts well outside its usual repertoire, including domestic and wild endotherms in some circumstances. Ticks in the genus Amblyomma are significant vectors of pathogens affecting wildlife, livestock, and humans, and reproductive mode influences everything from population growth rates to the maintenance of vector competence. A parthenogenetic lineage can colonize new areas from a single individual, whereas a bisexual lineage requires both sexes to establish itself but can evolve more rapidly once established. Documenting a bisexual population in the Amazon biome adds a crucial data point to maps of tick reproductive diversity and may help explain regional differences in the species’ ecology and epidemiological role.

The study also carries implications for how scientists interpret historical records. Because A. rotundatum has been considered parthenogenetic, males encountered in the field have often been treated as anomalies or as evidence of occasional sexual reproduction superimposed on an asexual baseline. The new findings suggest that at least some populations may be consistently and functionally bisexual, raising the possibility that the species encompasses a mosaic of reproductive strategies across its vast Neotropical range. Similar mosaics have been documented in other parthenogenetic invertebrates, where asexual lineages coexist with, and occasionally arise from, sexual ancestors. Untangling that structure in A. rotundatum will require sampling across multiple biomes, and the Maranhão population now serves as a reference point for such comparisons.

From a practical standpoint, the confirmation of sexual reproduction opens experimental avenues that were previously impractical. Controlled crosses between males and females allow researchers to study inheritance of traits such as host preference, acaricide resistance, and pathogen transmission, work that is far more difficult in strictly asexual lines. The successful maintenance of two consecutive generations on a laboratory host, with high hatching rates and well-characterized developmental timings, provides a reproducible rearing protocol that other laboratories can adopt. Standardized life-history parameters—such as the roughly 34-day egg incubation period and the molting success rates reported here—also give ecologists baseline figures for modeling population dynamics under changing climatic conditions in the Amazon and beyond.

The research, led by Ruth M. de Moraes e Silva of the Federal University of Maranhão together with colleagues at institutions including the Federal Rural University of Rio de Janeiro and the University of São Paulo, was conducted under permits from Brazil’s biodiversity authorities and approved by the university’s animal experimentation ethics committee. Funded through Brazilian programs supporting Amazonian biodiversity research and animal parasitology, the study exemplifies how careful field collection combined with disciplined laboratory rearing can revise long-standing assumptions about familiar organisms. For a tick that has spent decades in the scientific literature as a reproductive loner, the discovery that males and females thrive together in the eastern Amazon is a reminder that even well-studied parasites can still surprise the people who study them.

Subject of Research: Discovery of a bisexual population of the parthenogenetic tick Amblyomma rotundatum in the Brazilian Amazon

Article Title: Bisexual population of the parthenogenetic tick Amblyomma rotundatum: bioecological data from the field and laboratory

Article References: de Moraes e Silva, R. M., de Almeida, E. B., DallAgnol, L. T., Nogueira, R. M. S., Santos, J. M. S., de França, A. C. M., de Coelho, L. F. O., Pereira, J. R. F., Sousa, D. M., Barbosa, C. N. F., Faccini, J. L. H., McIntosh, D., Martins, T. F., Silito, I. S., Martins, M. P., Labruna, M. B., Costa-Junior, L. M., & Luz, H. R. (2026). Bisexual population of the parthenogenetic tick Amblyomma rotundatum: bioecological data from the field and laboratory. Parasites & Vectors. https://doi.org/10.1186/s13071-026-07682-w

Image Credits: AI Generated

DOI: 10.1186/s13071-026-07682-w

Keywords: Amblyomma rotundatum, ticks, parthenogenesis, bisexual reproduction, Amazon biome, Parasites & Vectors, Iguana iguana, Rhinella marina, tick biology, Brazil, parasitology, sex ratio