Monitors, Mammals, and Wallace’s Line

Although varanid lizards have achieved parity with mammals in many parts of the world, they exploit very different metabolic strategies and sensory mechanisms, and are properly regarded as convergent on the lifestyle of small predatory mammals. In parts of island southeast Asia and in Australia small monitors appear to fill ecological niches occupied by cats, mustelids and viverrids in other parts of the world. Species diversity of large monitor species (>.3 m TL) does not change across Wallace’s Line, which is the distributional boundary for nearly all small, carnivorous placental mammals, but the diversity of small monitor species (<.2 m TL) increases abruptly from this line east. Small monitor species can coexist with dasyurid marsupial carnivores, but apparently not with placental carnivores (this is being tested now with the establishment of feral cats in Australia). Because juveniles of large monitor species can survive in communities with placental carnivores to the west of Wallace’s Line, we speculate that predation rather than resource competition has influenced the observed pattern. Large monitors produce large clutches and their juveniles soon grow out of the vulnerable size range, whereas small monitor species (despite being more fecund, when body size is factored out) produce fewer eggs, are always subject to mammalian predation, and may not be able to coexist over evolutionary time with placental carnivores.