The Scientific History of Bokermann’s Casque-Headed Frog

The common name “Bokermann’s casque-headed frog” is applied to two distinct species of amphibians endemic to Brazil, a fact that necessitates careful examination when exploring their scientific history. This report aims to delve into the discoveries, taxonomic journeys, and research endeavors associated with these two fascinating members of the Brazilian herpetofauna, highlighting the contributions of the scientist for whom they are named. Understanding the scientific trajectory of these species requires a detailed look at their nomenclature, physical characteristics, ecological niches, behaviors, genetic relationships, and current conservation status.

Untangling the Names: A Tale of Two Species

The seemingly straightforward request for the history of “Bokermann’s casque-headed frog” immediately encounters a point of divergence: this common name refers to two separate species within the family Hylidae. This duality underscores a fundamental aspect of biological nomenclature, where common names, while often familiar, can sometimes mask the underlying diversity recognized by scientific classification. For precise scientific discourse, the use of specific scientific names is essential.

Species 1: Trachycephalus atlas Bokermann, 1966

The first species bearing this common name is scientifically known as Trachycephalus atlas. Its discovery and formal description were undertaken by the esteemed Brazilian herpetologist Werner C. A. Bokermann in 1966.¹ The original description was published in the journal Neotropica, in volume 12, spanning pages 120 to 124.⁶ The initial location from which the species was described, the type locality, is Fazenda Santo Onofre, situated in the municipality of Maracás within the state of Bahia, Brazil.³ Currently, Trachycephalus atlas is classified within the family Hylidae, commonly known as tree frogs, and belongs to the genus Trachycephalus, a group often referred to as casque-headed tree frogs.¹ Notably, the genus Phrynohyas was once considered distinct but has since been recognized as a synonym of Trachycephalus following a comprehensive systematic review conducted by Faivovich et al. in 2005.⁴ This taxonomic change reflects the dynamic nature of scientific classification, where new phylogenetic data often leads to revisions in our understanding of evolutionary relationships. This species is also commonly referred to as Bokermann’s casque-headed treefrog.¹

Species 2: Nyctimantis bokermanni (Pombal, 1993)

The second species known as Bokermann’s casque-headed frog was originally described under the name Aparasphenodon bokermanni by José Paulo Pombal Jr. in 1993.²⁰ This information is explicitly stated in resources like AmphibiaWeb.²⁰ Initially, this species was also placed within the family Hylidae and the genus Aparasphenodon.²⁰ However, a significant taxonomic revision occurred in 2021, when Blotto et al. redefined the genus Nyctimantis to encompass most species previously classified under Aparasphenodon, including A. bokermanni. This reclassification resulted in the species’ current scientific name: Nyctimantis bokermanni.²⁶ This taxonomic shift illustrates the ongoing process of refining amphibian classification based on modern phylogenetic analyses, which often incorporate both molecular and morphological data to better reflect evolutionary relationships. Despite this change in scientific name, the species continues to be known by the common name Bokermann’s casque-headed frog.²⁰

Table 1: Taxonomic Timeline of Bokermann’s Casque-Headed Frogs

Common Name	Original Scientific Name	Original Author(s) and Year	Current Scientific Name	Key Taxonomic Events
Bokermann’s casque-headed frog	Trachycephalus atlas	Werner C. A. Bokermann, 1966	Trachycephalus atlas	Phrynohyas synonymized with Trachycephalus (Faivovich et al., 2005)
Bokermann’s casque-headed frog	Aparasphenodon bokermanni	José Paulo Pombal Jr., 1993	Nyctimantis bokermanni	Genus Nyctimantis redefined to include A. bokermanni (Blotto et al., 2021)

The Enduring Contributions of Werner C. A. Bokermann

Werner Carlos Augusto Bokermann (1929-1995) was a highly influential figure in Brazilian herpetology.²⁸ His extensive research and numerous publications significantly advanced the understanding of the country’s rich amphibian diversity. Specializing in both herpetology and ornithology, Bokermann dedicated his career to collecting, describing, and studying various species, leaving an indelible mark on the field.²⁸ His remarkable contributions include the description of nearly 70 new species.²⁸ Furthermore, his impact is evident in the fact that several taxa have been named in his honor, including Aparasphenodon bokermanni, now known as Nyctimantis bokermanni.²⁸ This act of naming a species after a researcher serves as a profound recognition of their significant contributions to the field. Bokermann’s influence extended beyond his own research; he also served as the curator of amphibians at the Museu de Zoologia da Universidade de São Paulo until 1993, a position that allowed him to shape the direction of amphibian research in Brazil for many years.²⁸ His pivotal role in the discovery and original description of Trachycephalus atlas in 1966 ¹ further solidifies his central role in the scientific history of one of the frogs bearing his name. The naming of the other species, Nyctimantis bokermanni, in his honor by Pombal in 1993 ²⁰ serves as a lasting tribute to his significant contributions to herpetology. Bokermann’s involvement extended to the study and description of numerous other amphibian species, including Bokermannohyla martinsi ³⁰, Hylodes ornatus ³¹, and Bokermannohyla alvarengai ³², showcasing the breadth of his expertise and his profound impact on the understanding of Brazilian amphibian diversity.

Unraveling the Biology: Morphology, Habitat, and Distribution

Trachycephalus atlas is characterized by its relatively large size, with males reaching up to 98 mm and females up to 107 mm in snout-vent length.³ A key morphological feature is the presence of cranial co-ossification, where the skin of the head is fused to the underlying skull bones.³ Dorsally, this species typically exhibits a pale brown coloration with a distinctive dark blotch that often resembles an inverted Y.³ The limbs are marked with dark transversal bands on the forearms, arms, and legs.³ This species is endemic to Brazil and inhabits a variety of environments, including subtropical or tropical dry forests, moist savanna, subtropical or tropical dry shrubland, and intermittent freshwater marshes.¹ Its distribution is primarily concentrated in northeastern Brazil, with documented occurrences in the states of Bahia, Pernambuco, and Ceará.³ Research has expanded the known geographic range of T. atlas over time, with records extending from the initial type locality in Bahia to other regions in the northeast.³

Detailed morphological information on Nyctimantis bokermanni is less readily available within the provided snippets. However, as a casque-headed frog, it shares the characteristic of cranial ossification with other members of its former genus Aparasphenodon and its current genus Nyctimantis.²⁷ Related species, such as Aparasphenodon arapapa, exhibit a flattened and wide loreal region and a rounded canthus rostralis ²⁷, which may offer some insight into the general morphology of this group. A distinguishing feature of the Nyctimantis genus, as noted after its redefinition, is a red to dark-brown iris without reticulations and reduced foot webbing.²⁷ Nyctimantis bokermanni is also endemic to Brazil, but its habitat preferences differ from T. atlas, as it is found in subtropical or tropical moist lowland forests.²¹ Its distribution appears to be more restricted, with initial records from the Juréia-Itatins Ecological Station, another location 10 km away, and Rio Verde.²⁰ Subsequent research has expanded its known range to include areas in the state of São Paulo, such as Itanhaém, Cananéia, and Ilha do Cardoso, as well as municipalities in Santa Catarina like Guaramirim and Joinville, and Santa Catarina Island.²⁶ A record from Paranaguá in Paraná further delineates its distribution along the Atlantic coast.²² This species is also associated with arboreal restinga within the Atlantic Forest Biome.²⁷ The differing habitat preferences of these two species, one favoring drier, more open environments and the other inhabiting moist coastal forests, suggest distinct ecological adaptations.

Sounds and Secrets: Behavioral Research

Research on the behavior of Trachycephalus atlas has included studies on its vocalizations. The advertisement call of this species consists of 10 to 15 multipulsed notes, with a mean duration of 0.005 seconds and a mean interval between notes of 0.009 seconds.³⁵ The mean emission rate is approximately 71 notes per second, and the call duration ranges from 0.14 to 0.22 seconds, with a mean interval between calls of 0.35 seconds. The dominant frequency of the call is around 1.51 kHz.³⁵ Comparative studies have examined the acoustic characteristics of T. atlas‘s call in relation to other Trachycephalus species, contributing to our understanding of species-specific communication signals.³⁶ While research on geographic variation in the calls of other Trachycephalus species exists ³⁷, similar studies specifically focusing on T. atlas were not evident in the provided material. In terms of reproductive behavior, Trachycephalus frogs generally breed in tree cavities that accumulate rainwater, with males using these cavities as calling sites to attract females, where eggs are subsequently deposited.⁴

In contrast, the provided research material reveals a notable lack of information regarding the vocalizations of Nyctimantis bokermanni. It is explicitly stated that the advertisement calls for all taxa within the former genus Aparasphenodon remain unknown.²⁷ This represents a significant gap in our understanding of the communication strategies of this species. However, a key behavioral trait associated with Nyctimantis bokermanni and other casque-headed hylids is phragmosis.²⁷ This behavior involves the frog using its heavily ossified head to block the entrances of burrows or crevices it inhabits, providing a physical barrier against predators and potentially helping to reduce water loss in its environment.²⁷ This defensive mechanism is a significant adaptation for these arboreal frogs. The disparity in behavioral knowledge, with some aspects of T. atlas‘s behavior being studied while the vocalizations of N. bokermanni remain unknown, highlights areas for future research.

Exploring the Evolutionary Path: Genetic and Phylogenetic Studies

Genetic and phylogenetic studies have played a crucial role in understanding the evolutionary relationships of both species. The genus Trachycephalus is characterized genetically by 37 transformations in nuclear and mitochondrial protein and ribosomal genes.⁹ Phylogenetic analyses of the Lophyohylini tribe, to which Trachycephalus belongs, have provided a broader context for its evolutionary placement within Hylidae.³³ The synonymization of Phrynohyas with Trachycephalus was largely based on molecular data revealing their close evolutionary relationship.⁴ Interestingly, despite some morphological differences, the genetic divergence in DNA sequences among some Trachycephalus species, including perhaps those closely related to T. atlas, can be remarkably small, suggesting relatively recent evolutionary splits.³⁴

The reclassification of Aparasphenodon bokermanni into the genus Nyctimantis in 2021 was a direct result of molecular and morphological phylogenetic studies.²⁶ These analyses demonstrated that the former genus Aparasphenodon was not monophyletic, meaning it did not contain all the descendants of a single common ancestor. The redefined Nyctimantis now forms a well-supported clade that includes the majority of the former Aparasphenodon species, along with Argenteohyla and Corythomantis galeata.²⁷ This taxonomic revision reflects the power of phylogenetic analyses in uncovering true evolutionary relationships and refining our classification systems. These ongoing genetic and phylogenetic investigations continue to shape our understanding of the evolutionary history of these and other amphibian species.

Assessing the State of Survival: Conservation Status

The conservation status of the two species known as Bokermann’s casque-headed frog differs according to the IUCN Red List. Trachycephalus atlas is currently listed as Least Concern (LC).¹ Despite this relatively favorable status, habitat loss is recognized as a threat to this species ², underscoring the importance of continued habitat protection and monitoring.

In contrast, Nyctimantis bokermanni is currently classified as Data Deficient (DD) by the IUCN Red List.²⁰ This designation indicates that there is insufficient information available to assess its risk of extinction. The species’ restricted distribution, as documented in the research material ²⁰, likely contributes to this status, as limited geographic ranges can often correlate with higher vulnerability. The Data Deficient status highlights a critical need for further research to gather more information on the population size, distribution, trends, and specific threats facing Nyctimantis bokermanni to accurately assess its conservation needs. The contrasting conservation statuses underscore the importance of evaluating each species individually, even when they share a common name.

Conclusion: A Continuing Journey of Discovery

The scientific history of the amphibians known as Bokermann’s casque-headed frog reveals a fascinating journey of discovery, taxonomic refinement, and ongoing research. From the initial description of Trachycephalus atlas by Werner C. A. Bokermann in 1966 to the more recent taxonomic reclassification of Aparasphenodon bokermanni as Nyctimantis bokermanni in 2021, our understanding of these species has evolved significantly. The enduring contributions of Werner Bokermann to Brazilian herpetology are evident not only in his description of one of these species but also in the honor bestowed upon him by naming the other. While research has provided valuable insights into the morphology, habitat, distribution, behavior, and genetics of both species, gaps in our knowledge remain, particularly concerning the vocalizations and conservation status of Nyctimantis bokermanni. The differing conservation assessments, with Trachycephalus atlas listed as Least Concern and Nyctimantis bokermanni as Data Deficient, highlight the need for continued research and conservation efforts to ensure the long-term survival of these ecologically significant amphibians within the rich biodiversity of Brazil. The ongoing scientific inquiry promises to further illuminate the unique characteristics and ecological roles of these remarkable frogs.

Bibliography

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2. Faivovich, J., C. F. B. Haddad, P. C. A. Garcia, D. R. Frost, J. A. Campbell, and W. C. Wheeler. 2005. Systematic review of the frog family Hylidae, with special reference to Hylinae: phylogenetic analysis and taxonomic revision. Bulletin of the American Museum of Natural History 294:1–240.  
3. Bokermann, W. C. A. 1966. Una nueva especie de Trachycephalus de Bahia, Brasil. Neotropica 12(39): 120-124.
4. Frost, D. R. 2011. Amphibian species of the world: an online reference version 5.5 (12 May, 2011). Electronic Database accessible at <http//research.amnh.org/herpetology/amphibian/index.php>. American Museum of Natural History, New York, USA.  
5. Roberto, I. J., S. C. Ribeiro, L. M. B. Bezerra, and P. B. de Macedo Carneiro. 2011. Amphibia, Anura, Hylidae, Trachycephalus atlas Bokermann, 1966: distribution extension and geographic distribution map. Check List 7(3): 326–327.  
6. Pombal, J. P., Jr. 1993. New species of Aparasphenodon (Anura, Hylidae) from southeastern Brazil. Copeia 1993(4): 1080–1086.
7. Blotto, B. L., M. Lyra, C. Cardoso, J. Rodrigues, V. S. Dias, D. M. Marciano, F. D. Vechio, V. G. Orrico, R. Brandão, E. B. de Assis, V. S. Lantyer-Silva, M. G. Rutherford, T. Gagliardi-Urrutia, M. Solé, D. Baldo, I. Nunes, R. Cajade, A. Torres, T. Grant, K.-H. Jungfer, H. R. Silva, C. F. B. Haddad, and J. Faivovich. 2021. The South American and West Indian casque-headed treefrogs (Hylidae: Hylinae: Lophyohylini): phylogeny, morphology, biogeography, and a new taxonomic arrangement. Cladistics 37(1): 1–90.
8. Santos, A. S. R. dos. 1995. In memoriam: Werner C. A. Bokermann (1929-1995). Boletim Centro de Estudos Ornitológicos 9: 3-4.  
9. Freitas, M. A., and T. F. S. Silva. 2007. A herpetofauna das caatingas e áreas de altitudes do nordeste brasileiro. Pelotas: USEB. 384 p.  
10. Bokermann, W. C. A. 1956. Sobre um novo gênero de hilídeos do Brasil (Amphibia, Anura). Boletim do Museu Nacional, Rio de Janeiro (N.S.), Zoologia 128: 1-20.  
11. Bokermann, W. C. A. 1967. Hylodes ornatus, nova espécie do Estado do Rio de Janeiro (Amphibia, Salientia, Leptodactylidae). Revista Brasileira de Biologia 27(2): 183-185.
12. Bokermann, W. C. A. 1965. Três novos batráquios da região central de Mato Grosso, Brasil (Amphibia, Salientia). Revista Brasileira de Biologia 25(3): 257–264.  
13. Dos Santos-Silva, C. R., S. F. Ferrari, and F. A. Juncá. 2012. Acoustic characteristics of the advertisement call of Trachycephalus atlas Bokermann, 1966 (Anura: Hylidae). Zootaxa 3424: 66–68.  

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