ISLAND MAINLAND BODY SIZE DIFFERENCES IN AUSTRALIAN VARANID LIZARDS

Island varanids seem to be an exception to the rule that territorial v ertebrate taxa often become gigantic relative to mainland relatives wh en on islands, whereas non-territorial species become dwarfed (Case 19 78). However, no systematic island/mainland studies have examined the empirical size trends in this group of carnivorous lizards. We perform such an analysis for the Australian region and critically evaluate va rious selective agents that might be responsible for size changes in s everal island populations. Insular gigantism occurs at least four time s among the island populations examined. The magnitude of size change is positively correlated to prey abundance on the islands (as indirect ly measured through a condition index of the lizards, essentially a me asure of how fat they are) and the size of prey: islands with large pr ey have large varanids and vice versa. Since the island population wit h the largest size change, the Reevesby Varanus rosenbergi, was introd uced less than 100 years ago, these size changes can be quite rapid. T his might indicate that selective coefficients are strong, however, we can not exclude the possibility that these size differences have no g enetic component and simply reflect environmental differences in growt h rate and shifts in age structure between island and mainland locatio ns.