Introduction

Mexico is one of the world’s most biologically rich countries (^{García-Frapolli et al., 2009}; ^{Mittermeier & Goettsch-Mittermeier, 1997}), largely due to the high biodiversity found in the southern regions of the Yucatán Peninsula, home to the largest expanse of mature seasonal tropical forests remaining in Mesoamerica (^{Carr III & Stoll, 1999}; ^{Vester et al., 2007}). The Calakmul Biosphere Reserve (CBR) is located in southern Mexico and together with adjacent state reserves Balam-Kú and Balam-Kin encompasses more than 1.2 million hectares of protected forest under jurisdiction and management of federal and Campeche state authorities, respectively. CBR is a UNESCO World Heritage Site of Culture and Nature due to the multiple ancient Mayan archaeological sites, including the major city of Calakmul and the remnants of the ancient Mayan agroforestry that has given place to a forest of outstanding biodiversity (^{UNESCO, 2016}). This reserve is composed of tropical semi-deciduous forest. In most of the reserve, canopy height ranges between 15 to 40 meters and 20% of trees lose their leaves during the dry season. Canopy in the northern parts of the reserve ranges from 8 to 20 meters in height and 40% of trees are deciduous (^{Chowdhury, 2006}). As with other areas of the Selva Maya (^{Ross & Rangel, 2011}), forest adjacent to ruin sites in Calakmul has notably larger trees, a different tree species composition and higher fruit production than other forested areas (^{Slater, 2019}). Surface water in CBR occurs only in some areas where the terrain allows the accumulation of water, which create semi-temporary natural ponds, locally known as “aguadas” (^{Barão-Nóbrega, 2019}). These “aguadas” are sustained by retaining water provided by the annual precipitation gradient and constitute the only source of water to both the fauna and the human communities of the region (^{Reyna-Hurtado et al., 2010}). These waterbodies are of particular importance to herpetofauna species that require aquatic habitat such as frogs, freshwater turtles, and crocodiles (^{Cedeño-Vázquez et al., 2010}; ^{Colston et al., 2015})

Operation Wallacea is a UK-based non-governmental organization specialized in biodiversity assessments and monitoring of protected areas. Utilizing the expertise of university academics and students, this organization has been performing annual biodiversity surveys in CBR since 2012. In 2015, an indexed species list of amphibians and reptiles of CBR was compiled and published, reporting the presence of 20 amphibian and 69 reptile species from 24 families (^{Colston et al., 2015}). Since then, taxonomic changes affected the nomenclature of many species, with some even subject to taxonomic division (e.g., Pseudelaphe flavirufa Cope, 1867 and Holcosus undulatus Wiegmann, 1834; ^{González-Sánchez et al., 2017}; ^{Meza-Lázaro & Nieto-Montes de Oca, 2015}). Operation Wallacea has continued performing annual biodiversity surveys in CBR and found 9 new herpetofauna records not previously reported for the area, with some even representing new records for the state of Campeche and the Mexican Yucatán Peninsula. We herein update and summarize current knowledge on the species of amphibians and reptiles in the region of Calakmul (CBR and surrounding areas) and discuss their conservation implications for the region. We also discuss other herpetofauna species that we believe may also occur in the region but have not been detected yet.

Materials and methods

Calakmul Biosphere Reserve (18°36’20.99” N, 89°56’39.98” W; WGS84; Fig. 1) is an expanse of tropical forest that covers an area of 723,000 hectares, and is part of the Selva Maya that encompasses Mexico, Guatemala and Belize, spanning over 10.6 million hectares and making it the largest continuous section of tropical forest in Mesoamerica (^{Vester et al., 2007}). The southern Yucatán Peninsula is characterized by a warm, sub-humid climate with a mean annual temperature of 24.6 ºC. A precipitation ecocline goes from the northwest (ca. 900 mm) to the southeast (ca. 1,400 mm) of the reserve (^{Vester et al., 2007}), over the 120 km from the north of the reserve to the Guatemalan border (^{Lawrence & Foster, 2002}), significantly influencing forest structure and tree species composition (^{Chowdhury, 2006}; ^{Martínez & Galindo-Leal, 2002}).

Figure 1 Location of Calakmul Biosphere Reserve within the southern Yucatán Peninsula in Mexico. Operation Wallacea’s annual survey sites are numerated from 1 to 6. All other localities represent opportunistic sightings or survey locations mentioned in previous literature (^{Calderón-Mandujano et al., 2003}, ²⁰¹⁰; ^{Colston et al., 2015}). Locations 7, 16 and 17 are located within Balam-Ku State Reserve. 

Herpetofauna surveys in CBR were carried out each year between June and August from 2015 to 2019 through transects, timed searches around waterbodies, and opportunistic funnel trapping in 6 different sampling localities (Dos Naciones, KM20, KM27, KM40, Mancolona and Hormiguero; Fig. 1), as part of Operation Wallacea’s annual biodiversity monitoring project in the region. For further detailed information on surveyed locations and methods, please see ^{Colston et al. (2015)}. All data were collected by teams of students led by university academics and local indigenous experts. Due to permit restrictions within CBR, no specimens were collected but digital photographs for all new records were taken and represent digital vouchers (^{Colston et al., 2015}), curated by Operation Wallacea (José António L. Barão-Nóbrega and Pedro E. Nahuat-Cervera) and University of Texas (Eric N. Smith and Gregory Pandelis). Fieldwork was performed in compliance with the protocols described by ^{Beaupre et al. (2004)}. Research permit was yearly granted to Operation Wallacea’s long-term monitoring project, in collaboration with Pronatura Península de Yucatán, A.C., by Secretaría de Medio Ambiente y Recursos Naturales (Semarnat; SGPA/DGVS/005403/18).

We compiled an updated species list for the region of Calakmul by combining our herpetological survey results between 2015-2019 in CBR with ad libitum herpetofauna sightings from JALBN’s field-work activities in the region, opportunistic amphibian and reptile records from the iNaturalist community (research grade records only; ^{iNaturalist, 2019}) and herpetofauna species information from literature review on previous studies conducted within and around the reserve (^{Calderón-Mandujano et al., 2003}, ²⁰⁰⁸, ²⁰¹⁰; ^{Colston et al., 2015}; ^{Neri-Castro et al., 2017}). We decided to base our taxonomic classification according to ^{González-Sánchez et al. (2017)}, ^{Lee (2000)}, ^{Meza-Lázaro and Nieto-Montes de Oca (2015)}, ^{Wilson, Johnson et al. (2013)}, and ^{Wilson, Mata-Silva et al. (2013)}, and we updated the scientific names of species until October 2021, based on ^{Frost (2021)} for amphibians and ^{Uetz and Hošek (2021)} for reptiles.

Results

Operation Wallacea’s herpetofauna survey records, pooled together with data from pre-existing literature and opportunistic sightings, indicate the occurrence of 109 species of amphibians and reptiles within the region of Calakmul (Table 1). Of this total, 23 are amphibian species belonging to 9 families, while the remaining 86 species are reptiles distributed amongst 25 families. For amphibians, the richest family in terms of species is Hylidae (35% of the total amphibian species richness detected), while for reptiles the richest families are Colubridae (23%), Dipsadidae (20%), and Dactyloidae (8%). Approximately 38% (n = 41) of the herpetofauna species reported to occur in Calakmul (Table 1) are listed under a threat category by the environmental Mexican legislation (NOM-059-SEMARNAT-2010). Of these 41 species, 71% (n = 29) are indicated as in need of Special protection, 24% (n =10) as Threatened, and 5% (n = 2; Cachryx defensor Cope, 1866 and Claudius angustatus Cope, 1865) as in Danger of extinction.

During this study, we encountered a total of 11 herpetofauna species (9 in CBR and 2 in the surrounding areas) that had never been previously reported for the region. The frog Engystomops pustulosus (Cope, 1864), the lizards Ctenosaura similis (Gray, 1831) and Holcosus gaigeae (Smith & Laufe, 1946), and the snakes Coniophanes bipunctatus (Günther, 1858), Pseudelaphe phaescens (Dowling, 1952), and Stenorrhina freminvillei (Duméril, Bibron & Duméril, 1854) represent new records for the region of Calakmul (Figs. 2A, C, D, 3A-C). The snakes Tantilla cuniculator (Smith, 1939), Tantilla schistosa (Bocourt, 1883), and Scaphiodontophis annulatus (Duméril, Bibron & Duméril, 1854) represent new records for Campeche (Fig. 4A-C). The frog Eleutherodactylus leprus (Cope, 1879) and the snake Sibon dimidiatus (Günther, 1872) represent new records for the Mexican Yucatán Peninsula (Figs. 2B, 4D). All new records mentioned above correspond to adult individuals. Number of observed individuals goes as following: 1 individual of E. pustulosus, C. similis, C. bipunctatus, S. dimidiatus, S. freminvillei, and T. cuniculator; 2 individuals of S. annulatus, T. schistosa, and P. phaescens; more than 5 individuals of E. leprus and H. gaigeae.

Figure 2 New records encountered in Calakmul: A) Engystomops pustulosus (UTADC 9453), B) Eleutherodactylus leprus (UTADC 9452), C) Ctenosaura similis (UTADC 9451), and D) Holcosus gaigeae (UTADC 9454). Photos by PENC. 

Figure 3 New records encountered in Calakmul: A) Coniophanes bipunctatus (UTADC 9450), B) Pseudelaphe phaescens (UTADC 9455), C) Tantilla cuniculator (UTADC 9459). Photos A) and B) by PENC, and C) by A Ross. 

Figure 4 New records encountered in Calakmul: A) Stenorrhina freminvillei (UTADC 9458), B) Tantilla schistosa (UTADC 9460), C) Scaphiodontophis annulatus (UTADC 9456), D) Sibon dimidiatus (UTADC 9457). Photos A) by PENC, B) by AT, C) and (D) by JO. 

Discussion

The 109 species of amphibians and reptiles found in CBR and surrounding areas represent, respectively, approximately 84% and 75% of total species richness registered for Campeche and the Mexican Yucatán Peninsula (^{González-Sánchez et al., 2017}), and approximately 60% of the observed herpetofauna richness in the entire Selva Maya (188 species; ^{Lee, 2000}). Of the total number of species found in Calakmul, nearly 21% are endemic to the Yucatán Peninsula (e.g., Bolitoglossa yucatana Peters, 1882, Cachryx Defensor, Tantilla cuniculator, Terrapene yucatana Boulenger, 1895). Field observations and data from Operation Wallacea’s monitoring project between 2014 and 2018 in CBR indicate a decline in overall herpetofauna abundance and diversity, paired with a significant reduction in water availability across the region due to the ongoing prolonged drought the reserve is experiencing (^{Slater, 2019}). As site conservation might be one of the most effective means of reducing biodiversity loss (^{Eken et al., 2004}), our data highlight the importance of increasing our knowledge on the assemblage and distribution of herpetofauna species within the Yucatán Peninsula, when taking measures to preserve the fauna of this region.

Our results from 2015 to 2019 in CBR point out that only about 65% of the species we found were recorded in 3 or more areas. This result is not very different from the percentage previously reported for the reserve (50%; ^{Colston et al., 2015}) and further suggests significant differences in species richness and diversity within Calakmul. Topography and latitude-correlated rainfall differences across this region could be accountable for differences in local microhabitat differences (^{Lyons & Willig, 2002}; ^{Stevens, 1989}), which might result in species variation among areas (^{Garda et al., 2013}; ^{Vitt et al., 2007}).

The significant percentage (38%) of herpetofauna species occurring in Calakmul listed in the risk categories proposed by the Mexican environmental legislation (NOM-059-SEMARNAT-2010) highlights the importance of considering amphibians and reptiles when planning future development projects that would involve habitat loss or alteration in the region (e.g., Mayan train; ^{Pskowski, 2019}), as degradation and habitat loss has been indicated as one of the major threats to amphibian and reptile species conservation in Mexico (^{Flores-Villela & García-Vázquez, 2014}; ^{Parra-Olea et al., 2014}). Worthy of mention is that unlike CBR, which has federal jurisdiction and management, other areas of the Calakmul region (e.g., Balam-Kú and Balam-Kin state reserves) are not under the same legal protection framework, which restricts the conservation efforts of environmental authorities operating in these areas.

Although the frog E. leprus and the snake S. dimidiatus represent new records for the Mexican part of the Yucatán Peninsula, it was speculated that the distribution of these species could encompass the region of Calakmul, as they have both been reported in Uaxactún (northern Guatemala), which is relatively close (70 km; ^{Lee, 2000}) to the southern portion of CBR (^{González-Sánchez et al., 2017}). This situation might also be the case for other amphibians and reptiles, hence we believe that species typically reported in neighbouring regions (e.g., Tabasco) or countries (i.e., Belize and Guatemala), such as the lizards Celestus rozellae (Smith, 1942), Norops capito (Peters, 1863), Norops uniformis (Cope, 1885), Sceloporus serrifer (Cope, 1866), Sphaerodactylus millepunctatus (Hallowell, 1861), the snakes Adelphicos quadrivirgatum (Jan, 1862), Conophis lineatus (Duméril, Bibron & Duméril, 1854), Masticophis mentovarius (Duméril, Bibron & Duméril, 1854), Oxyrhopus petolarius (Linnaeus, 1758), Tretanorhinus nigroluteus (Cope, 1861), Bothriechis schlegelii (Berthold, 1846), Porthidium nasutum (Bocourt, 1868), and the turtle Dermatemys mawii (Gray, 1847) might also occur within the region of Calakmul. Some of these species require very specific habitat characteristics to thrive (^{Lee, 2000}), which would likely limit their distribution to very confined areas/habitats within Calakmul. For example, species like D. mawii and T. nigroluteus inhabit permanent and semi-permanent waterbodies with a relatively slow water flow (^{Chaves et al., 2016}; ^{Lee, 2000}; ^{Vogt et al., 2006}). Information gathered during author JALBN’s PhD activities in the region indicates the presence of areas across Calakmul, away from our regular survey sites, that seem to offer such conditions (^{Barão-Nóbrega, 2019}).

The list presented in this work increases to 109 the number of herpetofauna species known to occur in the region of Calakmul, and to 135 the number of amphibians and reptiles in Campeche, which therefore becomes the Mexican state of the Yucatán Peninsula with the highest herpetofauna diversity, followed by Quintana Roo with 133 species. Furthermore, our study increases to 148 the total number of amphibian and reptile species recorded for the Yucatán Peninsula (^{González-Sánchez et al., 2017}; ^{Ortiz-Medina et al., 2019}). Future studies should be performed during different seasons and in other areas within Calakmul, focusing sampling efforts on fossorial amphibians and reptiles (e.g., genera Tantilla and Tantillita), which are difficult to detect and, therefore, deeply understudied.