Borneo Earless Monitor - The Holy Grail of the Reptile World

The earless monitor lizard (Lanthanotus borneensis) is a semi-aquatic, brown reptile native to the Southeast Asian island of Borneo. It is the sole surviving species in the family Lanthanotidae and is closely related to the true monitor lizards. This unique lizard was first documented in 1878 by Franz Steindachner. The genus name Lanthanotus, meaning "hidden ear," and the species name borneensis, indicating its origin from Borneo, were assigned to it. Steindachner promptly recognized its distinctiveness and classified it into its own family, Lanthanotidae. However, in 1899, George Albert Boulenger...

Earless Monitor Lizard 

( Lanthanotus borneensis )

Borneo earless monitor by Matthijs Kuijpers via Flickr

    
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
Superfamily: Varanoidea
Family: Lanthanotidae
Steindachner, 1877
Genus: Lanthanotidae
Steindachner, 1878
Species: L. borneensis

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The earless monitor lizard (Lanthanotus borneensis), aka Borneo earless monitor is a semi-aquatic, brown reptile native to the Southeast Asian island of Borneo. It is the sole surviving species in the family Lanthanotidae and is closely related to the true monitor lizards.

This unique lizard was first documented in 1878 by Franz Steindachner. The genus name Lanthanotus, meaning "hidden ear," and the species name borneensis, indicating its origin from Borneo, were assigned to it. Steindachner promptly recognized its distinctiveness and classified it into its own family, Lanthanotidae. However, in 1899, George Albert Boulenger reassigned it to the family Helodermatidae, alongside the beaded lizards and Gila monster, based on morphological similarities. Subsequent studies in the 1950s revealed that while there is a relationship with Helodermatidae, it is relatively distant, likely due to convergent evolution. Consequently, they are considered separate families within the broader Anguimorpha group, although their exact relationships have been debated.

Earlier studies grouped the earless monitor lizard with Helodermatidae and Varanidae (true monitor lizards) in Varanoidea, but recent genetic evidence indicates its closest relative is Varanidae. While most authorities maintain their separate family status due to deep divergence, some suggest classifying the earless monitor lizard as a subfamily, Lanthanotinae, of Varanidae. Together, they form a clade, with their sister group being Shinisauridae; at a higher taxonomic level, the sister groups of these three include Helodermatidae and other families within Anguimorpha. The divergence of their most recent common ancestor occurred in the mid-Cretaceous period.

The extinct Cherminotus, identified from Late Cretaceous fossils in Mongolia, has been proposed as a member of Lanthanotidae, although this assertion is contested.

Distribution & Habitat

Range map of Lanthanotus borneensis from iNaturalist.org, CC BY-NC 4.0.

The earless monitor lizard is native to the Southeast Asian island of Borneo, where it is known from Sarawak in East Malaysia, as well as West and North Kalimantan in Indonesia. Until late 2012, its documented range in North Kalimantan was a part of East Kalimantan. It was also confirmed to inhabit Brunei for the first time in 2022, in the country's Temburong District. Prior to this discovery, it was believed that they might occur there and has been recorded approximately 100 km (60 mi) from the border. There are no records from Sabah, Central Kalimantan, or South Kalimantan.

It is found in lowlands at elevations below 300 m (1,000 ft) near streams and marshes. These habitats are typically in rainforests, but it is also encountered in streams flowing through degraded environments such as agricultural land, mature fruit tree gardens, and palm oil plantations, and reportedly may be present in rice paddies. The streams it occupies are often rocky. Its habitat is tropical with air and water temperatures that generally range from about 22–29 °C (72–84 °F), and captives reportedly prefer temperatures between 24–28 °C (75–82 °F). At a site with a high density of earless monitor lizards, the water was clear and had a neutral pH. It shares the same microhabitat as Tropidophorus water skinks, and in some areas, its range overlaps with T. brookei.
 
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Diet
They typically prey on earthworms, crustaceans, and fish. In captivity, they will consume fish (both whole and in pieces), earthworms, squid, shrimp, tadpoles, yolk from green sea turtle eggs, pieces of pig and chicken liver, baby mice, and mussels, but will not accept bird eggs or frog legs. In captivity, adults typically feed once or twice per week, but may occasionally go through longer periods without feeding. Unusually for a lizard, they are capable of swallowing prey while submerged underwater. They seem to accomplish this by expelling water from their nostrils, akin to turtles.

Physical Characteristics

Earless borneo monitor By Chien C. Lee via Wild Borneo Photography
Earless monitor lizards possess a cylindrical body, elongated neck, short limbs, elongated sharp claws, diminutive eyes, partially transparent lower eyelids, and six longitudinal rows of prominently keeled scales. Despite their name, they are capable of hearing, although they lack a tympanum, an ear opening, and other externally visible signs of ears. The upperparts exhibit an orangish-brown hue, while the underside displays a mottled pattern of dark brown and whitish, pale yellowish, ochre, or rusty tones. The tail is prehensile, and if lost, it is not regenerated. The skin is shed infrequently, possibly less than once a year, with reports of shedding occurring either in one piece (similar to snakes) or in smaller fragments (typical of lizards). Overall, the sexes resemble each other, but males feature a notably broader head and tail base compared to females. These differences become evident from around the age of three. At times, they may vibrate their throat (reminiscent of frogs), and their forked tongue is occasionally flicked (similar to snakes). They are capable of emitting a gentle, squeaky vocalization. 

Adult earless monitor lizards typically exhibit a snout-to-vent length (SVL) of about 20 cm (8 in), and a total length of approximately 40 cm (16 in). In a study of eighteen wild individuals, comprising six males and twelve females, their SVL ranged from 15.6 to 22 cm (6.1–8.7 in), while their tail length ranged from 17.4 to 22.1 cm (6.9–8.7 in) (excluding one individual missing much of its tail), and their weight ranged from 48 to 120 g (1.7–4.2 oz) (excluding one sickly and emaciated individual). Among these, the largest total length (SVL+tail) was observed in a male measuring 44.1 cm (17.4 in), which also stands as the longest recorded in the wild. A specimen collected in the 1960s measured a total length of 51 cm (20 in), and shortly before its demise, an individual housed at the Bronx Zoo from 1968 to 1976 had a total length of 47 cm (18.5 in) and weighed 209 g (7.4 oz), although it was severely obese. Upon hatching, the young typically measure about 7–14 cm (3–6 in) in total length. By the age of one year, their total length reaches approximately 23–25 cm (9–10 in).


Behaviour
Earless monitor lizards are primarily nocturnal creatures, though occasional daytime sightings in open areas have been documented. They typically spend their days near water in burrows, which can extend up to 30 cm (1 ft) in length, or under logs, rocks, or vegetation. Although generally sedentary and not particularly agile, they can exhibit rapid bursts of movement when startled and swiftly capture prey items placed before them. In a study involving 19 individuals located during the night, approximately half were found in the water, while the rest were situated near water on land. In captivity, they may remain nearly motionless underwater for extended periods, periodically surfacing to breathe with their noses. While submerged, their semi-transparent lower eyelids are typically closed, shielding their eyes. There is speculation that their prehensile tail is used to anchor themselves to stones, roots, and other objects underwater, preventing them from being swept away during floods.

Although generally calm and inactive when handled, males are typically more assertive than females when caught. In one instance, a scientist received a deep bite on his finger, yet did not exhibit any symptoms indicating the presence of venom in the bite (unlike the related and venomous beaded lizards, Gila monster, and some monitor lizards). This corroborated decades-old dissection studies that found no venom glands or grooves in the teeth. Others kept in captivity were observed to bite frequently, resulting in relatively deep wounds (in comparison to those from similar-sized lizards) that can bleed extensively, with blood clotting reportedly occurring at a slower rate than in typical wounds. Recent studies have identified both venom glands and toxic compounds in the bite of this species. The primary components are kallikreins (with CRISPs to a lesser extent), primarily affecting fibrinogen cleavage, which is crucial for blood clotting. However, this effect is relatively mild in the venom of the earless monitor lizard compared to that of many other venomous reptiles, including some of the true monitor lizards.

Breeding & Life-Cycle

Similar to their closest relatives, they are oviparous, although limited information exists regarding their reproduction. Based on captive observations, a pair will mate repeatedly over a few months, with each session lasting for hours. In one instance, a single mating endured for 44 hours. They mate in aquatic environments. In the wild, mating activity has been observed in February, and a female captured in April was likely gravid. The 2–12 (average 8) oval eggs measure about 3 cm (1.2 in) in length and feature a leathery white shell. They are laid on terrestrial surfaces. In captivity, the eggs hatch after approximately three months at a temperature of 27 °C (81 °F). Adult males are likely territorially aggressive, as a survey of a locality found twice as many females as males, and most of the males (but no females) had various injuries, such as loss of toes or tail, and scarring on the head or neck. In captivity, young individuals up to 6 months old have been kept in groups, while adults have been kept singly, as pairs, or a single male with several females; housing more than one adult male leads to issues. The lifespan remains unknown, but—despite the very limited knowledge of reptile keeping at the time—an individual that entered captivity as a young adult in the 1960s lived for more than 7.5 years after its capture (growing from 38.2 to 47 cm or 15.0 to 18.5 in in total length), and others have reportedly surpassed a decade in captivity.

Conservation Status


The earless monitor lizard has been classified as endangered by the IUCN due to its limited range (covering less than 500 km2 or 190 sq mi). The species is typically considered very rare, but it is easily overlooked, and until 1999, confirmed sightings were only recorded in Sarawak. Confirmation from Kalimantan came later. In some areas, locals are unaware of its presence or consider it rare, while in others, it may be more prevalent. At one site in West Kalimantan, 17 out of 21 locals surveyed were aware of its presence, and most of them considered it common. At three other sites in the region, the majority surveyed were aware of its presence, but less than half considered it common. In West Kalimantan, a survey of a 400 m (1,300 ft) long section of a stream, along with two adjacent streams, revealed 19 earless monitor lizards, indicating an unusually high density for a lizard of this size. Despite this high density in a stream used by locals for washing, fishing, and as a source of drinking water, they reported seeing the species very rarely, and some had never seen it. Nevertheless, at present, the earless monitor lizard is only confirmed to inhabit a relatively small number of sites.

Approximately 100 museum specimens are known, and most major natural history museums have one or more in their collection. These specimens were primarily collected in the 1960s–1980s or earlier, often during floods when earless monitor lizards were swept along the current and ended up in fishing traps. From the 1960s to the 1990s, small numbers entered the pet trade. In 2012, it was featured in a Japanese reptile keepers magazine, leading to a larger number of individuals entering captivity in subsequent years. From May 2014 to October 2015, at least 95 earless monitor lizards appeared in the trade in various regions across Asia, Europe, and the United States. Collecting the species from the wild is illegal; it has been protected in Malaysia since 1971, in Brunei since 1978, and in Indonesia since 1980. Penalties for illegal trade range from fines to imprisonment. 

As a precaution, some scientists have refrained from providing exact locations of discoveries, citing fears of alerting wildlife traders. In 2015, a smuggler was caught in an Indonesian airport with 8 individuals, and in 2016, another was caught with 17 individuals. This trade is driven by the species' high price; when initially entering the market in Japan, a pair sold for ¥3 million (more than US$25,000). Although the price has since fallen significantly, it remains valuable. Unlike all other monitor species, the earless monitor lizard was not listed on CITES, which would restrict international trade. In 2016, it was proposed to be placed on CITES Appendix I, and in 2017, it received protection when placed on Appendix II (with a zero export quota for wild individuals for commercial purposes).

The first confirmed captive breeding occurred at a zoo in Japan in 2014. A few years later, several European zoos initiated breeding programs; in 2017, it was bred at Schönbrunn Zoo in Austria, and in 2018, at Prague Zoo in the Czech Republic and Moscow Zoo in Russia. There have been other reports of breeding by private keepers, and captive-bred individuals have been offered for sale, although some of these cases may not involve genuine captive breeding.

Habitat loss poses another serious threat, as Borneo's forests are rapidly being replaced by oil palm plantations. However, the earless monitor lizard can thrive in high densities in areas surrounded by degraded habitats, including oil palm plantations, and rocky streams, possibly its preferred habitat, are relatively unaffected by human activities.


References


  • Das, I.; Auliya, M. (2021). "Lanthanotus borneensis". IUCN Red List of Threatened Species. 2021: e.T102342572A102342580. doi:10.2305/IUCN.UK.2021-2.RLTS.T102342572A102342580.en. Retrived on Feb 11, 2024, from
    https://doi.org/10.17161/randa.v30i1.20089.
    •  
  • Bauer, Aaron M. (1998). Cogger, H.G.; Zweifel, R.G. (eds.). Encyclopedia of Reptiles and Amphibians. San Diego: Academic Press. pp. 156–157. ISBN 0-12-178560-2. Retrived on Feb 11, 2024.
  • Wiens, John J.; Hutter, Carl R.; Mulcahy, Daniel G.; Noonan, Brice P.; Townsend, Ted M.; Sites, Jack W.; Reeder, Tod W. (2012). "Resolving the phylogeny of lizards and snakes (Squamata) with extensive sampling of genes and species". Biology Letters. 8 (6): 1043–1046. doi:10.1098/rsbl.2012.0703. PMC 3497141. PMID 22993238.
    Retrived on Feb 11, 2023, from
    • https://royalsocietypublishing.org/doi/10.1098/rsbl.2012.0703.
  • Pyron, R. Alexander; Burbrink, Frank T.; Wiens, John J. (2013). "A phylogeny and revised classification of Squamata, including 4161 species of lizards and snakes". BMC Evolutionary Biology. 13 (1): 93. doi:10.1186/1471-2148-13-93. PMC 3682911. PMID 23627680.
    Retrived on Feb 11, 2023, from
    • https://bmcecolevol.biomedcentral.com/articles/10.1186/1471-2148-13-93.
  • Douglas, Michael E.; Douglas, Marlis R.; Schuett, Gordon W.; Beck, Daniel D.; Sullivan, Brian K. (April 2010). "Conservation phylogenetics of helodermatid lizards using multiple molecular markers and a supertree approach". Molecular Phylogenetics and Evolution. 55 (1): 153–167. doi:10.1016/j.ympev.2009.12.009. PMID 20006722.
    Retrived on Feb 11, 2023, from
    • https://www.sciencedirect.com/science/article/abs/pii/S1055790309005156?via%3Dihub.
   
          
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