Gray's monitor (Varanus olivaceus)

By Daniel Bennett

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.

An extract from my Little Book of Monitor Lizards

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, o要ly 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 o要 Luzon Island during the 1930s and triggered a series of visits to the Philippines which culminated not o要ly in the rediscovery of the species but also in o要e 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).

Gray's monitor gets its common name from its previous scientific name of Varanus grayi. It is found o要ly o要 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 5000 square km) 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. o要 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. o要 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.

Gray's monitor spends much of its time in trees, but finds almost all of its food o要 or below the surface. It shelters in thickets of vegetation o要 branches, in rock crevices and in tree hollows. That this species likes to wedge itself into tight crevices is demonstrated by the large numbers of scratches seen o要 their backs and bellies. Unlike other well known monitor lizards, Gray's monitor relies largely o要 its cryptic colouration for defence and when threatened tends to remain immobile amongst tangles of vegetation.

The most extraordinary feature of the ecology of this lizard is its diet. Juveniles feed largely o要 snails and crabs but between the ages of 12-18 months the diet changes to include a large number of fruits. No other monitor lizard is known to feed o要 plant matter in the wild, but in Gray's monitor it forms a major part of the diet. Other important foods are snails, crabs, spiders, beetles, birds and their eggs. Fruits are o要ly eaten when they are perfectly ripe and are collected from the forest floor. Both sugary and oily fruits are eaten, but the latter are consumed in larger numbers. Oily fruits are generally avoided by vertebrates because they contain chemicals that render proteins indigestible, so the ability to feed o要 them probably reduces competition between the monitor lizard and the other fruit eating birds and mammals with which it shares its range. Gray's monitor is sympatric with the water monitor V.salvator, and it has been suggested that it is o要ly able to survive competition from this non-specialized, gregarious animal by adapting to a largely frugivorous diet. Although a wide range of fruits are available in tropical forests, Gray's monitor selects o要ly a few of them. They appear to know the location of the trees that bear edible fruit within their home range. However although some trees bear large amounts of fruit over several months the lizards are very picky and rarely spend long feeding below an particular tree but move about to collect a variety of fruit. Nor do they gorge themselves with food in the manner of many large monitor species. Food in the stomach accounts for o要ly about 2% of their body weight, although many other species are known to consume individual prey items weighing 20% or more of their own weight. The need for a wide variety of different fruits is due to complex nutritional requirements and the need to avoid ingesting too much of any of the toxins present in the fruit. Least fruit is eaten between December and February, most sugary fruits are eaten between May and July and oily fruits most abundant from August to November and from February to March. The seasonal differences in amount of food available is reflected in the amount of body fat accumulated by the lizards. Fat levels are lowest from June to August and highest in October (the level varies somewhat between the sexes). Body fat may account for 5-12% of body weight depending o要 time of year.

As a consequence of its unusual diet the alimentary tract of Gray's monitor is unlike that of any other species. They possess a caecum in which microbial degradation of plant matter may occur and the large intestine is much longer than in carnivorous monitor lizards, suggesting a need for more complete digestion of food items. The ability to feed o要 sedentary food items present in large numbers is also probably responsible for the very small ranges in which these lizards are active; from just over 2000m2 to 27,100m2. However if the area of the trees used by the lizards is also accounted for, the mean activity range (14,800m2) increases ten-fold.

Little is known of the breeding habits of Grey's monitor. Reproductive activity occurs mainly between June and September, when day length is longest, temperatures lowest and rainfall highest. It is interesting that in Gray's monitor egg production seems not to depend o要 the use of accumulated fat, for fat bodies are smallest whilst ovaries are growing at their fastest rate. The first behavioural change associated with reproduction is bipedal combat between males which occurs as testes reach their greatest weight. Copulation has been observed in captivity and appear to be very gentle compared with that of other species. There is some evidence that long term pair bonding may occur in this species (see Chapter 5).

In the wild a single clutch of up to 11 eggs is produced between July and October. The eggs weigh 40-50g and measure about 6.7X3.6cm. They account for up to 18.6% of female body weight. In captivity two clutches of eggs (total 14) may be laid within four months of each other and Auffenberg believed that this may occur in the wild as well. Nothing is known of their nesting habits, but tree hollows seem the most likely nest sites. Nor is there any good data o要 the length of incubation. The smallest specimens are found from May to July, suggesting that the eggs may not hatch for over 300 days.

The skin of Grey's monitor is rarely used for commercial trade, but the meat is popular and the fat particularly so, because it is less liable to breaking down at high temperatures than that of the much more easily obtained water monitor. Apart from pythons, mankind appears to be the o要ly important predator of the adult lizards. o要 account of its very restricted distribution Gray's monitor has been placed o要 Appendix 1 of the CITES legislation and export of animals for commercial purposes is outlawed.

In captivity this species poses a number of unusual problems. A large, high enclosure should be provided with plenty of hiding places both above and below the ground. An enclosure with 9 square metres of floor space is sufficient to house a pair, but animals are often intolerant of each other or may refused to feed unless housed singlely. They enjoy being sprayed with water and should be .provided with frequent showers. Unlike most monitor lizards Gray's monitor does very little digging and appears not to ingest much substrate with its food. The fruit eaten by Gray's monitor are almost all endemic to the Philippine Islands and none are suitable for use as human food. Thus the frugivorous portion of this lizard's diet may prove almost impossible to replicate in captivity. It has been established that captives will survive for long periods o要 a diet of animals alone (Behler, Mitchell, pers. comm.) but it is unlikely that successful breeding can be accomplished o要 such a diet. The o要ly report of breeding comes from the Dallas Zoo (Card 1994a,b 1995c) where pulverised fruit, vegetable oil and fructose are used to supplement a diet of rodents. These additives must be injected into the prey animals; the o要ly fruits taken voluntarily in captivity are grapes. Females are capable of producing more than o要e clutch of eggs per year, but to date o要ly a single, short-lived juvenile has hatched, after 219 days at 28oC.

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