Saturday, 17 March 2012

Turtles and Tortoises 1: The Pancake Tortoise

This series of posts will be about one of the most famous groups of reptiles, the chelonians, variously referred to as turtles, tortoises or terrapins. Although they are subdivided into various taxonomic groups, these do not actually correlate with the various English names. In American English, they are almost all referred to as turtles. In British English, the names differ by habitat – tortoise is used to refer to terrestrial animals, turtle to marine forms, and terrapin to freshwater animals. Why we have so many names is unclear, as there are currently no breeding populations of any chelonian native to Britain. Any terrapins seen in the UK are invariably released pets, usually Trachemys scripta, the Red-eared Terrapin. In the past the European Pond Terrapin, Emys orbicularis, was resident in the UK but became extinct as a result of a change to a colder climate many thousands of years ago.

Among the animals I use for the talks to the public at our Amazing Animals talks is Oscar, a male Pancake Tortoise Malacochersus tornieri. People who are used to tortoises being more or less round, slow moving animals find him very surprising, as Pancake tortoises are as flat as their name suggests, and are fairly fast moving as well. Pancake tortoises originate from the more arid parts of Kenya and Tanzania, where they inhabit granite outcrops. They have their shape in order to fit more easily into the deep crevices where they spend most of their day, only emerging in the morning to feed on grasses and other plants.

As part of their adaptation for their steep habitat, they are extremely good climbers, with long, flexible legs ending in fairly long, strong nails. The soles of their feet are also covered with pointed scales, giving them additional grip as they clamber of rock surfaces. If they fall, their shape also makes it easier for them to right themselves and get back where they are going. Unlike most tortoises, their shell is flexible, which helps in their wedging themselves tightly into a crevice when threatened by a predator.

They are not very large animals, averaging around 17cm shell length as adults. The best crevices can usually accommodate several individuals, and they are often found in groups, usually with only a single adult male and a group of females. They are fairly secretive, as their flexible shells make them vulnerable when in the open. As with all animals, they have a variety of predators, with jackals, hyenas, and birds of prey being probably the most serious. Several different raptors feed on tortoises, and many have developed the same technique of getting through the armour plating – they carry the unfortunate tortoise high in the air, find a convenient boulder, and drop the tortoise onto it, killing the tortoise and breaking the shell. According to a (probably apocryphal) story, the Greek playwright Aeschylus was killed in this way by an eagle which mistook his bald head for a boulder.

Pancake tortoises are too small to lay large clutches of eggs, and instead lay a single egg at a time, but a female produces 4-6 eggs in a season. They can climb well to find suitable nest sites, and the incubation time for the egg varies considerably. This is probably an adaptation to ensure that all eggs hatch at the same time of year, when rains soften the soil and encourage plant growth, no matter when the egg was actually laid.

As dry country animals, Pancake tortoises are used to a very seasonal climate, and during the hottest and driest months of the year they may remain permanently in their crevices, avoiding the heat of the sun and conserving energy rather than trying to extract it from desiccated vegetation. The climate is never cold enough for them to go into hibernation. When the rains return, they emerge and feed on lush plants for the few months they are available, but their main diet is extremely high in fibre, low in nutrients. As a result they are even more than usually slow growing in the wild, and although the lifespan has not been definitively measured, is probably at least as long as more familiar species such as the Mediterranean Spur-Thighed tortoise, which can live to 150 years.

Unfortunately, as with many tortoises, they have been over-collected for the pet trade and are now classed as Vulnerable by the IUCN. They require a specialised vivarium in captivity, with a low humidity and a diet based around grasses and leafy plants, especially weeds and other high-fibre foods. They have been bred in captivity – in fact Bristol currently has three eggs in the incubator – but are not a prolific species.

One issue I point out when showing Oscar to the public is a shell deformity very common among captive bred tortoises. Called ‘lumpy shell’, it is a result of hatchlings and juveniles being fed a too high-protein diet when they are small. This results in the shell growing quicker than the rest of the tortoise, with a characteristic pyramiding effect. In the wild this could be a serious risk, at it means the affected individual cannot fit into a crevice as well, but in a captive environment is basically a cosmetic defect only.

While tortoise have been, and are, widely kept as pets both in the UK and elsewhere, even more than other animals they are a serious responsibility as a result of their long lifespans. Basically, if you have a pet tortoise, it needs to be in your will. If anyone is interested, please do your research first and remember that if Abraham Lincoln had been given a baby tortoise as a pet, there is a good chance it would still be grazing the White House lawn!

Next week – one of the worlds largest tortoises, the Aldabra Giant Tortoise.
(images from wikipedia)

1 comment:

  1. Correction - we now have 2 eggs waiting to htach. The first hatched this week. It is on show in the Reptile House but is pretty hard to se at present.