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Varanus olivaceus Print E-mail

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Gray's monitor, butaan

Varanus olivaceus  Hallowell 1857

Varanus olivaceus
The extraordinary ability of monitor lizards to escape notice by ecologists and other field workers, despite their large size, is exemplified best by the case of Gray's monitor lizard. For 130 years, between 1845 and 1975, only a preserved juvenile and the skull of an adult were known. Despite several thorough investigations of the Philippine Islands in which they were believed to live no further specimens were found, and the species was generally considered to have become extinct. Finally another specimen was discovered in the collection of the American Museum of Natural History (Auffenberg 1976). It had been collected on Luzon Island during the 1930s and triggered a series of visits to the Philippines which culminated not only in the rediscovery of the species but also in one of the most comprehensive studies of the ecology of a lizard species ever performed. Almost all that is known about this extraordinary monitor lizard can be found in Auffenberg (1988).

ImageGray's monitor gets its common name from its previous scientific name of Varanus grayi. It is found only on southern Luzon and Catanduanes Island in the Philippines, where it inhabits the forested slopes of low mountains. Possibly its range was much larger in the past, but the development of agriculture has removed its habitat from most valleys. The hillsides frequented by the lizard are covered with thick rainforest and have many cliffs and rock outcrops. Although its total range is small (about 5000km2) Gray's monitor appears to be common in many areas. However its very secretive habits and cryptic colouration enable it to evade detection.

As in many other monitor species, male Grey's monitors grow larger than females. Maximum size reported by Auffenberg in 73cm SVL (176cm TL) and over 9kg. On average adult males measure 65cm SVL, 6.7kg whilst adult females measure 51cm SVL and weigh 2.6kg. Hatchlings weigh about 25g and measure about 35cm TL. Tail length in adults varies from 118-162% of SVL. A very large and apparently old specimen was delivered to the San Diego Zoo during the 1980s. Its total length was estimated at 200cm. The lizard was unconscious when received and never recovered. On death it was sent to the Florida State Museum (Auffenberg, pers. comm.).

The forests inhabited by Gray's monitor allow it to be active throughout the year. Temperatures are relatively constant and never high enough to prevent activity. Climatic data can be found in Chapter 4 of Auffenberg (1988). Body temperatures range from 27.8-38.2oC and a critical temperature of 41.6-42.4oC has been established. Males are more active than females and move throughout the day, whereas females' movements tend to be restricted to the mornings. Also males grow faster than females. Average growth rates are 3.5cm per month for juveniles and 0.3cm per month for adults. They reach sexual maturity by the third year of life, at 40cm SVL and 1kg for females and 45cm SVL and 1.5kg for males.



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William Oliver

William Oliver. Champion of biodiversity and its students. So many of us benefited from his advice and expertise. What a character. RIP.


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The Butaan Project
The Butaan Project - Research
butaan3.jpgThe only obligate fruit-eaters among reptiles are three species of monitor lizard that live in the Philippines. Frugivorous vertebrates tend to be able to fly (almost all are bats and birds) and so these lizards have a unique ecological role as highly specialized and relatively immobile fruit eaters. Before this project started, the only studies of this unique giant and endangered lizard had involved killing the animals. We have developed a set of techniques that allow us to learn about these animals in a completely non-destructive way.


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