The Rosenberg's monitor (Varanus rosenbergi) is an Australian species of varanid reptile found in southern regions of the continent. They are large and fast predators with rugged bodies and long tails, having a combined length up to 1.5 metres, that will consume any smaller animal that is pursued and captured or found while foraging. They occur in the Australian Capital Territory, New South Wales, South Australia, Victoria, where it may be rare or locally common, and more frequently observed in Western Australia, where it is sometimes abundant.
Taxonomy The species was first described by German herpetologist Robert Mertens in 1957, named and recognized as a subspecies of Varanus gouldii; a revision of Western Australian monitors published in 1980 elevated the taxon to species status. It is now considered to be a member of the V. gouldii species complex. The holotype was collected at the Stirling Range in Southwest Australia. The specific name, rosenbergi, is in honor of German naturalist Hermann von Rosenberg.
The infrageneric classification of the species allies it to other taxa of the subgenus Varanus (Varanus). A geographically remote population in the eastern states has distinct characteristics that may represent a cryptic species. A description published as Pantherosaurus kuringai in 1985 is regarded as a synonym for this species. The common names include the heath monitor or the southern heath monitor, along with those names that are a legacy of their description as the sand goanna Varanus gouldii.
Description
Like most other monitor lizards, these are fast-moving predators with long tails, stout limbs and clawed feet. They resemble the closely related and widespread sand goanna (Varanus gouldii), to which they were previously allied as a subspecies, but distinguished by darker coloration overall, especially the markings at the tail tip and underside and the distinct black banding at the back and neck. Dull and vaguely defined yellow stripes appear across most of the tail's length, sometimes interrupting the blackish colour of its end. The feet and limbs are also very dark, and spotted with a creamy or brownish colour. The dark tone at the head is marked with yellow dots above the mouth, and the yellowish cream background colour of the underside displays a blackish reticular pattern. The largest specimens are those found at Kangaroo Island, near the coast of South Australia, these are also darker than those on the mainland and possess a greater number of scale rows at the ventral side. Fat layers in Rosenberg's monitor constitute around 7.6% of body weight, allowing energy reserves to be stored throughout the body and tail. This monitor can reach a length of 1.5 metres. It is dark gray with yellow and white spotting and black bands on its body and all the way down the tail. The juvenile is tinged with orange. The comparative ratio of head and body lengths between the sexes, often evident in the monitor subgenus Odatria, is not easily discernible in this species and their subgenus Varanus.
Behaviour
They are highly active carnivores, able to pursue large prey, and opportunistic generalists whose diet includes birds, reptiles, mammals, eggs, and carrion. Analysis of the island population in South Australia revealed that the majority of their diet is reptiles and mammals, with a third of their diet being invertebrates, primarily species of acridid grasshoppers, areneid sea snails, blattoid cockroaches and beetles. Some amphibians, mostly Limnodynastes frogs, and a small proportion of birds were also found to have been captured and consumed, but invertebrates would be found during its regular foraging activities through soft soils and detritus. This goanna has been observed eating road kill, including possums and wallabies, and consumption of these may account for a significant proportion of the mammal component of their diet. The native Australian bush rat is likely pursued and caught, but the records of consumption of the western grey kangaroo and brush-tail possum is likely the discovery of their carcasses. Water is obtained from prey when seasonal rainfall is low, and supplemented by drinking free water when available. Living in a sometimes saline environment, they are able to expel excess salts—primarily sodium chloride and a smaller amount of potassium—through gland structures at the nostrils. In winter months the water required by the goanna may be supplemented from free water or by pulmocutaneous exchange of moisture. The darker coloration of Rosenberg's monitor, a form of melanism, allows a greater degree of thermoregulation of their body in the cooler southern climates. They periodically bask in the sun throughout the day, or when the opportunity arises, laying in the soil if it is warm or clambering on to branches to avoid a net heat loss; heath monitors are mostly found in their burrows during the coldest parts of the year. Females may continue to be active at night, maintaining a body temperature 20 °C greater than the ambient temperature. Rosenberg's monitor conserves its energy requirements during cooler seasons of the year, entering a state of torpor, but demonstrates the ability to voluntarily arouse itself during these periods. The species will climb a tree to evade capture.
Reproduction
As a large amount of energy is expended in the production of and defence of eggs, females often (though not always) have a 1-2 year break between egg clutches. Males on the other hand usually mate every year. Reproductive behaviour, including guarding of nest sites, occurs during the summer over a four-month period of a sexually active adult's year, beginning before the December solstice and ending after the March equinox.
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