Above: a subadult Varanus bennetti, a newly described species from Losiep Island, Federated States of Micronesia ( photo by James Reardon).
The monitor lizards of Palau, the Western Carolines and the Mariana Islands have all been referred to as Varanus indicus, following Mertens(1942). Earlier, however, Kishida (1929) —a Japanese arachnologist and entomologist [21]—had described Varanus tsukamotoi from Saipan in a rather obscure natural-history journal. Subsequently, the name Varanus tsukamotoi was synonymized with V. indicus by Mertens on the basis of its similarity to that species and Kishida’s (1929) failure to differentiate his new species from indicus.
Recently Weijola et al. (2019) provided a molecular phylogeny for the Varanus subgenus Euprepiosaurus that included the Varanus indicus Group and showed the populations from Palau and the Mariana Islands are genetically distinct both from each other and from all other known species in the group. They also showed that all these assorted populations assigned to or recently partitioned from V. indicus form a monophyletic clade that diverged from each other sometime during the late Pleistocene (about 1–1.2 million years ago), prior to human presence in these islands.
This supports the biogeographic hypothesis that monitor lizards are indeed native to both Micronesian island
groups rather than recent human introductions, although this does not refute the possibility that humans have artificially expanded their distributions by introducing them to additional islands (Buden et al al. 2016). This is of especial importance in the Federated States of Micronesia, where many islands have introduced populations, other islands have populations of uncertain status (i.e. native versus introduced), and for which Weijola et al. (2019) lacked samples.
In a new paper Weijola et al. (2020) add additional species and populations to the sampling of Weijola et al. (2019), present additional new molecular sequences, and add morphological data to assess the evolutionary distinctiveness of the Palauan and Marianan monitor lizards. These include newly added Moluccan taxa that are critical for understanding the evolution and biogeography of the Micronesian monitors but that were absent in Weijola et al. (2019). They (Weijola et al. 2020) reinstate the name Varanus tsukamotoi Kishida, 1929 which has been in synonymy for more than 75 years—for those populations inhabiting the Mariana Islands (except Sarigan), and they describe the Palauan, Western Caroline and Sarigan populations as a new species. This increases the number of known endemic reptile species in Micronesia, demonstrates the exceptional ability for overseas dispersal of this group of lizards and argues for careful evaluation of the origin of each island’s monitor populations prior to considering eradication or control programs.
Weijola et al. (2020) present molecular and morphological evidence that monitor lizards of the Varanus indicus Group reached both Palau and the Mariana Islands sometime in the late Pleistocene and subsequently differentiated into two separate species endemic to each geographical region. One species is confined to the Mariana Islands, and for these populations, they revalidate the name Varanus tsukamotoi Kishida, 1929. The other species has a disjunct distribution in Palau, the Western Carolines and Sarigan Island in the Northern Marianas and is described as Varanus bennetti. Both species are most closely allied to each other, Varanus lirungensis and V. rainerguentheri, suggesting that colonization of Micronesia took place from the Moluccas.
Literature Cited
Kishida K. 1929 A new monitor from the island of Saipan, South Sea Islands. Lansania 1, 13–16.
Mertens R. 1942 Die Familie der Warane (Varanidae). Dritter Teil: Taxonomie. Abh. Senckenb. Naturforsch. Ges. 466, 235–391.
Weijola V, Vahtera V, Lindqvist C, Kraus F. 2019 A molecular phylogeny for the Pacific monitor lizards (Varanus subgenus Euprepiosaurus) reveals a recent and rapid radiation with high levels of cryptic diversity. Zool. J. Linnean Soc. 186, 1053–1066. (doi:10.1093/zoolinnean/zlz002)
Weijola V, Vahtera V, Koch A, Schmitz A, Kraus F. Taxonomy of Micronesian monitors (Reptilia: Squamata: Varanus): endemic status of new species argues for caution in pursuing eradication plans. Royal Society Open Science. 2020 May 13;7(5):200092.
