monitor lizards have the highest standard metabolic rates of all extant reptiles. Like snakes, monitor lizards have highly forked tongues that act as part of the "smell" sense, where the tips of the tongue carry molecules from the environment to sensory organs in the skull. The forked apparatus allows for these lizards to sense boundaries in the molecules they collect, almost smelling in "stereo". Monitor lizards have a high aerobic scope that is afforded, in part, by their heart anatomy. Whereas most reptiles are considered to have three-chambered hearts, the hearts of monitor lizards – as with those of boas and pythons – have a well developed ventricular septum that completely separates the pulmonary and systemic sides of the circulatory system during systole. This allows monitor lizards to create mammalian-equivalent pressure differentials between the pulmonary and systemic circuits, which in turn ensure that oxygenated blood is quickly distributed to the body without also flooding the lungs with high-pressure blood. Anatomical and molecular studies indicate that all varanids (and possibly all lizards) are partially venomous. The venom of monitor lizards is diverse and complex, as a result of the diverse ecological niches monitor lizards occupy. Monitor lizards are oviparous, laying from seven to 38 eggs, which they often cover with soil or protect in a hollow tree stump. Some monitor lizards, including the Komodo dragon, are capable of parthenogenesis. Evolution Varanus is the only living member of the family Varanidae. Varanidae last shared a common ancestor with their closest living relatives, earless "monitors", during the Late Cretaceous. The oldest known members of Varanidae are known from the Late Cretaceous of Mongolia. During the Eocene, the varanid Saniwa occurred in North America. The closest known relative of Varanus is Archaeovaranus from the Eocene of China, suggesting that the genus Varanus is of Asian origin. The oldest fossils of Varanus date to the early Miocene. Many of the species within the various subgenera also form species complexes with each other: V. indicus species complex (V. indicus, V. cerambonensis, V. caerulivirens, V. colei, V. obor, V. lirugensis, V. rainerguentheri, V. zugorum) V. doreanus species complex (V. doreanus, V. finschi, V. semotus, V. yuwonoi) V. gouldii species complex (V. gouldii, V. rosenbergi, V. panoptes) V. bengalensis species complex (V. bengalensis, V. nebulosus) V. acanthurus species complex (V. acanthurus, V. baritji, V. primordius, V. storri) V. exanthematicus species complex (V. exanthematicus, V. albigularis, V. yemenensis) V.