Wednesday, May 13, 2020

[Herpetology • 2020] Varanus bennetti & V. tsukamotoi • Taxonomy of Micronesian Monitors (Reptilia: Squamata: Varanus): Endemic Status of New Species argues for Caution in Pursuing Eradication Plans

Varanus bennetti  
Weijola, Vahtera, Koch, Schmitz & Kraus, 2020

‘Bennett's Long-tailed Monitor’  DOI: 10.1098/rsos.200092

In the light of recent phylogenetic studies, we re-assess the taxonomy and biogeography of the Varanus populations distributed in the Micronesian islands of Palau, the Western Carolines and the Marianas. Whether these populations are of natural origin or human introductions has long been contentious, but no study has fully resolved that question. Here, we 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, we revalidate the name V. tsukamotoi Kishida, 1929. The other species has a disjunct distribution in Palau, the Western Carolines and Sarigan Island in the Northern Marianas and is herein described as Varanus bennetti sp. nov. Both species are most closely allied to each other, V. lirungensis and V. rainerguentheri, suggesting that colonization of Micronesia took place from the Moluccas. We discuss the biogeographic distributions of both species in the light of the likely colonization mechanism and previous arguments for human introduction, and we argue that bounties for Palauan populations are ill-advised and plans for eradication of some other populations must first demonstrate that they are, in fact, introduced and not native.

Keywords: eradication, alien species, Caroline Islands, Mariana Islands, trans-marine dispersal

Figure 9. Varanus bennetti sp. nov., Rock Islands, Palau
(photo by Thibaud Aronson).

Figure 10. Varanus bennetti sp. nov., Ngarchelong, Palau
 (photo by Thibaud Aronson).

Figure 11. Subadult Varanus bennetti sp. nov., Losiep Island, Federated States of Micronesia
(photo by James Reardon).

Figure 12. Adult Varanus bennetti sp. nov., Losiep Island, Federated States of Micronesia
 (photo by James Reardon).

Varanus bennetti sp. nov. 

Diagnosis: Varanus bennetti can be distinguished from all other members of Euprepiosaurus by its unique combination of (i) dorsum black and evenly speckled with yellow scales, sometimes arranged in small groups of yellow scales, (ii) tongue dark blue/grey, (iii) venter cream coloured with pale grey cross-bands, (iv) tail exceptionally long (F/SVL mean = 1.76, range = 1.60–1.89), high XY scale counts (148–160), (v) a clear yellow temporal stripe present in about half of the studied specimens, and (vi), in life, peach colouring on the throat.

 Etymology: The specific epithet is a genitive singular patronym in commemoration of the late Dr Daniel Bennett, 1966–2020, and his life-long commitment to the study and conservation of monitor lizards in Africa and Southeast Asia. As a vernacular name we suggest ‘Bennett's Long-tailed Monitor’.

  Distribution: We have examined specimens of V. bennetti from Koror, Ngeaur and Ngcheangel islands in the Palau archipelago, from Yap and Losiep islands in the Federated States of Micronesia (FSM), and from Sarigan Island in the Commonwealth of the Northern Mariana Islands (CNMI) (figure 1). Crombie & Pregill [1999] also list this species (as Varanus cf. indicus) from an additional two islands in the Palau group: Ngeriungs and Babeldaob.

Ecology: Crombie & Pregill [1999] remarked that the monitors on Palau are decidedly terrestrial and prefer, when possible, to take refuge in terrestrial refuges rather than in trees. On Ngeaur, they are reportedly most common in the rugged limestone interior of the island [1999]. Both features are atypical for species in the V. indicus Group, which usually seek refuge in trees and attain their highest densities in coastal habitats [Weijola, 2010]. On Sarigan Island, the most common food items found in dissected lizards (n = 16) were rats (Rattus exulans), insects and lizards [Vogt, 2008]. In addition, that author found a high proportion of males among the specimens examined by him: only four of 16 specimens were females.

Figure 1. Map of the Pacific region showing the distribution of
Varanus tsukamotoi (white dots), Varanus bennetti sp. nov. (red dots),
  V. lirungensis (yellow dot) and V. rainerguentheri (green dot).

 Figure 7. Mature Varanus tsukamotoi on Guam
(photo by Peter Xiong). 

 Varanus tsukamotoi Kishida, 1929 

 Diagnosis: Varanus tsukamotoi can be distinguished from all other members of the V. indicus group by its unique combination of: (i) dorsum black and covered with evenly distributed yellow scales, (ii) tongue dark blue/grey, (iii) yellow temporal stripe usually absent, (iv) low scale counts around the head (P: 31–40), tail base (Q: 54–74) and midbody (S: 101–126), and (v) usually prominent dark pigmentation in the gular region.

 Etymology: Kishida named this species in honour of Dr Iwasaburo Tsukamoto, who supported his expedition to the South Sea Islands, and proposed ‘Saipan monitor’ and ‘Tsukamoto Ohtokage’ as the English and Japanese vernacular names. We suggest the common name ‘Mariana monitor’ as it more accurately describes the distribution of this species.

 Ecology: Dryden [1965] examined the stomach contents of 84 animals dissected on Guam. The prey items found (in order of frequency) were giant African snails (Achatina sp.), miscellaneous arthropods (insects, insect larvae and millipedes), rats (Rattus mindanensis and R. exulans), shrews (Suncus murinus), hermit crabs, earthworms, slugs, bird eggs, skink (1) (Emoia cyanurum), gecko (1) (Hemidactylus frenatus), blind snake (1) (Indotyphlops braminus), and a skink egg. Specimens removed from Cocos Island foraged at the island's dump and frequently included chicken bones in their stomachs (FK, personal observation). Wikramanayake & Dryden [1988] studied the reproductive biology of Varanus on Guam and considered males to be sexually mature at 320 mm and females at 275 mm SVL. Mature males averaged almost three times the mass of mature females. Reproduction appeared to be seasonal, with mating taking place during the early dry season (December–April) and eggs presumably hatching during the wet season (April–December).

Figure 6. Hatchling Varanus tsukamotoi on Cocos Island
(photo by Björn Lardner). 

Valter Weijola, Varpu Vahtera, André Koch, Andreas Schmitz and Fred Kraus. 2020. Taxonomy of Micronesian monitors (Reptilia: Squamata: Varanus): Endemic Status of New Species argues for Caution in Pursuing Eradication Plans. Royal Society Open Science. 7(5) DOI: 10.1098/rsos.200092