Weekly Wildlife Profile: Goanna

Goanna is an Aboriginal word used extensively in Australia to refer a group of lizards commonly called Monitor lizards elsewhere in the world. The best known monitor is the famous Komodo Dragon (Varanus komodoensis). The Komodo is also the largest lizard in world, growing in excess of three metres in length and as the name implies, it is found on the Indonesian island of Komodo.

There are approximately 30 species of monitors found in globally, mainly restricted to Africa, Asia and Australia. Twenty six species are thought to be endemic to Australia (Cogger, 2000; Swan et al., 2004). All goannas belong to the Family Varanidae, with all Australian species belonging to the Varanus genus (Cogger, 2000).

The forked tongue of a lace monitor in action (Photo credit: Rhianna Blackthorn)

Morphology

“Monitors are the most impressive of lizards. Unlike smaller scincids [sic: skinks], iguanids [sic: iguanas] and agamids [sic: dragons] that flee if they believe they are noticed, monitors have a deliberate, unafraid demeanour” (Molnar, 2004).

Snakes and monitors are believed to have shared ancient common ancestry. This is evident by the long, forked tongue of the goanna that is similar to that of a snake, and can be seen flicking in and out during periods of activity, transferring scent particles to the Jacobson Organ. Like snakes, they do not have the ability to chew their food, and feeding is infrequent. The diet of monitors varies with age; hatchlings and juveniles feed primarily on invertebrates while adults focus on mammals, birds and their eggs. It is known that carrion makes up for a large percentage of the adult diet (King et al., 1999b).

The limbs are powerful and well-developed in all species. There are five long digits on each foot with sharp, non-retractable curved claws (Swan et al., 2004). Goannas are able to avoid the heat of the day, staying deep underground in purposefully dug burrows allowing them preserve water (Green, 1972). A strong, powerful tail is used to whip would be attackers, often causing them to lose balance.

(Photo credit: Rhianna Blackthorn)

Conservation Status

Despite falling numbers and a generalised recognition of habitat reduction, just one species listed as considered threatened. The Rosenberg’s Goanna (Varanus rosenbergi) is listed as vulnerable under the NSW Threatened Species Conservation Act (1995) (Office of Environment and Heritage, 2011). It is perhaps because of this status that the Rosenberg’s goanna is the best studied Australian monitor (King et al., 1999a). The Rosenberg goanna is one of 20 species recognised internationally as threatened and listed with International Union for Conservation of Nature Red List (Bennett et al., 2010; International Union for Conservation of Nature, 2011).

This lace monitor walks past park picnickers (Photo credit: Rhianna Blackthorn)

Reproduction

During a 16 year study of wild, free ranging Rosenberg’s goannas on Kangaroo Island (off the South Australian Coast), the courtship, breeding, and egg maintenance of 205 individuals was observed (Rismiller et al., 2010). During the late spring, males engage in posturing and physical contact. When physical contact was engaged, fights lasted for a period exceeding 40 minutes (Rismiller et al., 2010). Courtship starts during the Australian summer with copulation occurring between the summer solstice and mid February. Each couple copulate for periods lasting 7 to 17 days. During this period, the pair will cohabit, although 40% to 80% of pairs remain exclusively monogamous throughout the breeding period.

Oviposition, (egg laying) occurs nocturnally between mid February and early March. About a week prior to egg laying, the female searches the mounds of one species of termite within her area. She spends approximately two days evacuating the mound to form an egg chamber approximately 50 cm deep into the mounds centre. It takes her up to four hours to lay an average of 14 eggs in her chamber. The chamber is then sealed by the termites within five hours of the female completing her egg laying, providing her eggs security and temperature stability.

A sub adult lace monitor gets ready to hit people getting to close to it with its powerful tail. (Photo credit: Rhianna Blackthorn)

Females, and occasionally the primary male remained close to the mound, guarding it against intrusion by other monitors. The defence of the mound is vigorous and aggressive, with the study observing sever injuries such as dislocated and broken limbs, ribs and spinal injuries. Incubation lasts up to three weeks and hatchlings are independent from birth.

Another study which focused on the lace monitor (Varanus varius) observed a similar reproductive pattern as described above (Carter, 1992). Very little reproductive studies exist outside of these two species, and little is known about the life cycle of newly hatched monitor lizards.

(Photo credit: Wikipedia)

Reference

Bennett, D., & Sweet, S. (2010). IUCN Red List of Threatened Species: Varanus rosenbergi. 2011.2. Retrieved 30 March, 2012, from http://www.iucnredlist.org/apps/ redlist/details/178031/0

Carter, D. B. (1992). Reproductive ecology of the lace monitor Varanus varius in south-eastern Australia. Unpublished Ph.D. Thesis. Australian National University. Canberra, Australia.

Cogger, H. (2000). Reptiles and Amphibians of Australia (6th ed.). Sydney, NSW: Ralph Curtis Books.

Green, B. (1972). Water Losses of the Sand Goanna (Varanus Gouldii) In Its Natural Environments. Ecology, 53(3), 452-457.

International Union for Conservation of Nature (2011). IUCN Red List of Threatened Species 2011.2. Retrieved 30 March, 2012, from http://www.iucnredlist.org/apps/redlist/search

(Photo credit: Wikipedia)

King, D., & Green, B. (1999a). Goanna. The Biology of Varanid Lizard. Sydney, Australia: Univesity of New South Wales Press.

King, D., & Green, B. (1999b). Goannas: The Biology of Varanid Lizards. Sydney, Australia: University of New South Wales Press Ltd.

Molnar, R. (2004). The long and honorable history of monitors and their kin. In E. R. Pianka, D. R. King & R. A. King (Eds.), Varanoid lizards of the world. Indiana, USA: Indiana University Press.

Office of Environment and Heritage (2011). Rosenberg’s goanna – vulnerable species listing  Retrieved 31 March, 2012, from http://www.environment.nsw.gov.au/determinations/RosenbergsGoannaVulSpListing.htm

Rismiller, P., McKelvey, M., & Green, B. (2010). Breeding Phenology and Behavior of Rosenberg’s Goanna (Varanus rosenbergi) on Kangaroo Island, South Australia. Journal of Herpetology, 44(3), 399-408. doi: 10.1670/09-066.1

Swan, G., Shea, G., & Sadlier, R. (2004). A Field Guide to Reptiles of New South Wales (Second ed.). Sydney, New South Wales: Reed New Holland.

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