Commercial harvesting of giant lizards: The biology of water monitors Varanus salvator in southern Sumatra

Abstract

The Asian water monitor Varanus salvator is the second-largest lizard species in the world (to > 1 m SVL, 2·5 m total, 20 kg), and is heavily exploited (> 1 million skins per annum). In the course of three trips between August 1993 and April 1995, we gathered information on 166 water monitors captured in southern Sumatra for the commercial skin trade. Relatively equal numbers of males and females were captured, but the males were almost all adults whereas half of the females were juveniles. Sex ratios and body sizes did not vary significantly among the three trips. Males grow larger than females, but the largest animals are not used in the leather trade. Males mature at around 40 cm SVL (= 1 m total, 1 kg), and females at around 50 cm. Maturation thus occurs at a small proportion of maximum size, as is typical for large species of reptiles. Adult males are more heavy-bodied than females, and have longer tails, but fat stores did not differ between the sexes. Prey items included crustaceans, rats and other varanids, but most lizards were kept for so long prior to slaughter that the stomach was empty of food. All adult-size males had active gonads, but testes were larger in April than in October. All adult females in the August and April samples were reproductively active, but less activity was evident in October. The egg-laying season extends from April to October (at least), and most female water monitors in southern Sumatra produce multiple clutches each year. Larger females begin to breed earlier in the year than do smaller animals. Clutch sizes ranged from five to 22, and were positively correlated with maternal body size. We measured stretched and dried skins from processed lizards to establish a predictive equation linking lizard SVL to skin width. The persistence of water monitors in southern Sumatra, despite intense harvesting, reflects the large area of suitable habitat with low human densities, combined with the monitors' ecological flexibility (in habitat and diets), their high reproductive rate (early maturation and frequent reproduction), and (perhaps) the concentration of commercial harvesting on adult males. At current levels, the commercial trade may extirpate varanids from local areas but will not drive the species to extinction.