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Tortoises
Tortoise or Turtle?
The names applied to freshwater Chelonians or Testudines
differ between countries. Australia, which has no true land tortoises, finds it convenient
to use the term tortoise, leaving turtle for the marine species. There is one exception to
the rule in the form of the Pig-nose Turtle (Carettochelys Insculpta), a recent discovery in Australia which indeed warrants the use of the term
freshwater turtle.
Often when the subject arises, Australian tortoises are called turtles. In some respects this is perhaps a better term. In many other countries the use of the term tortoise is restricted to the terrestrial, or land-dwelling species and, as a result, our "pet" tortoises are often deprived of the water which is essential for their survival.
Age and
Development
The growth rate of Australian tortoises in the wild has not been studied in detail.
However, they do grow quickly during the first five to ten years or until sexual maturity
is reached. At this point the growth in length begins to slow and a deepening of the shell
takes place as the weight continues to increase. After this stage is reached it is
impossible to assess the age.
The method of determining age by counting the growth rings on the scutes of the shell appears to be a reasonable guide to the age of immature individuals of some species. However some hatchlings are born with one or two growth rings and this adds to the difficulty of accurate determination of their age in the wild. Other factors such as food shortage and periods of decreased activity also have a bearing on the regularity of the growth rings. In addition the shedding of the scutes at intervals varying from two to four years means that the earlier growth rings are difficult to detect and at the next casting are often completely obscured.
Many of the reports indicating the possible age of tortoises are suspect. We have in the past been misled by the marking of incorrect dates on the carapaces of certain individuals. However, one record is that of the land tortoise that Captain James Cook left on the island of Tonga in 1773. It died in 1966, indicating a probable age of about 200 years. Among other authenticated reports is one of a common Box Turtle (Terrepene Carolina) which lived for 123 years.
Due to a lack of interest in the past, there are no longevity records available for Australian tortoises. Fifty years is a feasible estimate of the age attained by Australian tortoises in general, but this is purely conjecture.
A study of growth rates of tortoises from different climatic regions throughout Australia would be likely to produce different results. In captivity hatchlings grow quickly when they are kept in ideal conditions and given sufficient food. It would appear that hereditary factors may also have some bearing on the growth rate. Two Broad-shell Tortoises (Chelodina Expansa) which measured 3.5cm on hatching reached a length of 10cm in twelve months; two others from the Murray River measuring 3.2cm measured only 6.5cm after the same length of time. All four tortoises were kept together under identical conditions during the twelve month period. After the first year of living indoors in a heated aquarium with water temperature kept at 22ºC the original pair were obliged to hibernate during the winter months. Food was supplied to the four tortoises in similar quantities. It took five years for the tortoises in the outside enclosure to gain and extra 5.5cm, while those indoors increased by the same amount in two years.
In an attempt to gauge the growth rate of tortoises in their natural environment, studies have been taken involving a marking program on an Emydura species in a tributary of the Macleay River (NSW). Over 300 individual tortoises were marked by cutting notches in different combinations into the marginal shields.
After twelve months some of the tortoises were collected and remeasured. Comparing the initial figures with the second measurements produced a confusing result: the growth rate of some specimens was negative while others, originally the same size, showed considerable increases. Examinations of the marked tortoises during the following years produced similar inexplicable results and no uniform rate of growth emerged.
The smallest Australian tortoise is the Swamp Tortoise (Pseudemydura Umbrina) which can be considered full grown at a length of 15cm and the largest is the Pig-nose Turtle which reaches a length of more than 60cm. The largest long-neck tortoise is the Broad-shell Tortoise which has been recorded with a carapace length of 48.5cm. The largest short-neck, the Snapping Tortoise (Elseya Dentata), occasionally attains a length of 40cm and weighs up to 7.5 kg.
Scent Glands
All Australian tortoises have four scent glands, one situated at each
section of the bridge (where the carapace joins the plastron), and one alongside each back
leg. The objectionable odour some tortoises emit is a defense mechanism that is generally
employed when the tortoise is collected from the wild or is roughly handled. The species
most notorious for this is the Common Snake-neck Tortoise (Chelodina Longicollis),
but like all tortoises which follow this behavior it ceases to eject the pungent fluid
once it becomes accustomed to being handled. It is hard to understand why some individuals
emit this undesirable liquid, while others of the same species do not. The Saw-Shell
Tortoise (Elseya Latisernum) and the New Guinea Snake-neck Tortoise (Chelodina
Novaeguineae) appear to be the only species in Australia to emit a strong smelling
fear odour.
