An extended range

The Marbled Gecko (Christinus marmoratus) is a small, thick-tailed lizard, to about 14 cm long,   It is quite common in woodlands and heathlands of northern and western Victoria where it shelters by day under rocks or logs and hunts invertebrates by night.  In the first descriptions of the Marbled Gecko, in the 19^th century Prodromus of the Zoology of Victoria, Frederick McCoy describes its distribution as northern Victoria, no mention is made of any records near Melbourne.  Nevertheless the species is common in suburban Melbourne, particularly in the south-east, where it will often hide in rockeries, paling fences and beneath window ledges.  It will even enter houses in search of food.

The suburban populations are thought to be the result of introductions of animals with loads of rocks or firewood which have been transported from other parts of the state.  Once in the suburbs the animals adapted to the range of crevices and protective cover offered by gardens and houses and found food from invertebrates, many of which are attracted to the night-time lights.    

The Marbled Gecko is the only Victorian reptile that appears to have extended its range significantly since European settlement.  In fact reptiles are not known for being invasive species under most circumstances.  In the IUCN list of the most invasive vertebrates across the world there are 37 mammals and 24 birds but only 9 reptiles, most of which are simply escapees or releases from captivity in wetlands of the USA.  There has never been a reptile equivalent of the House Mouse (Mus musculus) or the House Sparrow (Passer domesticus) or the Cane Toad (Bufo marinus).

Of all the reptile groups that have invaded urban habitats the geckos stand out as the most successful.  The Mediterranean House Gecko (Hemidactylus turcicus), a native of southern Europe, has found its way into urban areas of northern and eastern Africa, the Middle East, Pakistan, India, Central America and most southern states of the USA.  The large Tokay Gecko (Gecko gecko), an arboreal and carnivorous species of south-east Asia, is common in urban areas, including major cities, throughout the region and have been successfully introduced to similar environments in the Hawaiian Islands, the Caribbean and Florida. The House Gecko (Hemidactylis frenatus), native to south-east Asia and northern Africa is very common in buildings throughout its natural range and has become well-established as an introduction across a large number of Pacific islands.  In many of these cases the gecko was probably transported to the urban areas amongst building materials or forest produce and they survived, and multiplied, because the urban landscape suited their hunting and breeding patterns.  A single pregnant female (all geckos produce just two eggs) could begin a new population.

Another reason that some gecko species have become successful invaders is that the females don't need to mate to produce viable eggs - a process known as parthenogenesis.  The best known example of this is the Mourning Gecko (Leptodactylis lugubris) a species for which no males are known, meaning that the species is effectively a giant clone.  Consequently, the transport of a single animal is enough to begin a new colony which the Mourning Gecko has done in many parts of the world, including the Northern Territory and Queensland.  It is believed that the most common way that reptiles develop a parthenogenetic reproductive strategy is through hybridisation where the progeny have a greater compliment of genetic material than the parents.  In the case of the Mourning Gecko all the animals are triploid - that is each cell nucleus has the normal genetic material from one parent but twice the amount from the other.  One of the side benefits of being triploid is that the progeny are often bigger and stronger than the parents.

Parthenogenesis is not known to occur in the Marbled Gecko but it is in another Victorian species, Bynoe's Gecko (Heteronotia binoei) which is found in the far north-west of the state as well across most of the rest of mainland Australia.  Normally Bynoe's Gecko reproduces conventionally with male-female intercourse preceding the development of eggs but there are populations in central Australia which are entirely female and, like the Mourning Gecko, are all triploid.  It is thought that this came about as a result of hybridising between different races of the species.  Whatever the mechanism, however, the parthenogenetic animals are bigger and there is evidence that their range is expanding to displace the sexual forms.  Clearly local biologists will need to be vigilant when assessing new records of Bynoe's Gecko in Victoria and check to see if the parthogenetic forms ever invade this state. 

© Paul Gullan, Viridans Biological Databases