Geometric tortoise  (Psammobates geometricus)

The geometric tortoise, a medium-sized terrestrial tortoise, is the rarest of the three Psammobates (tent tortoise) species restricted to southern Africa (4) (5). It is named for the pattern adorning its high, domed shell, which occasionally may be perfectly geometric (6). The shell is dark brown or black and each scute has yellow stripes radiating out from a yellow centre (7). The rear edges of the shell are slightly upturned. Female geometric tortoises can be distinguished by their larger size and smaller tail (6), while the males typically have a concave plastron, or lower shell (2).

This species is restricted to the extreme south-western part of the Western Cape Province, South Africa (8), from near Cape Town, 160 kilometres north to Eendekuil, and east to the Upper Breede River Valley and the Ceres Valley (4) (5) (8).

The geometric tortoise primarily inhabits a relatively narrow strip of coastal lowland, between the Cape Fold Mountains and the sea, with two isolated populations east of the mountains (4) (5) (8). The vegetation of this region forms part of the Cape Floral Kingdom or fynbos biome, and the geometric tortoise is restricted to a specific vegetation type within the fynbos biome, called renosterveld (8).

The geometric tortoise lays one to two clutches of two to five eggs in spring and early summer, which are incubated for a period of five to eight months before they hatch (5) (9). Typically, eggs hatch at the onset of the winter rains in the Western Cape (late April to May). This long-lived tortoise reaches sexual maturity at seven to eight years and may live for over 30 years (4) (5) (10). The geometric tortoise feeds on a wide range of grasses, reeds, sedges, herbs and shrubs (9).

The destruction of more than 90 percent of the renosterveld habitat in which the geometric tortoise resides has resulted in the depletion of tortoise numbers, and the range of this species continues to be reduced (2) (8) (11). This habitat loss has been the result of indiscriminate urban and agricultural development and unfortunately the geometric tortoise has proved to be intolerant of habitat modification (4) (12). The fynbos biome and renosterveld vegetation is adapted to fire, but too frequent unplanned and uncontrolled fires can result in local extinction of the geometric tortoise (4) (8). In addition, the spread of invasive alien vegetation in the region competes with, and excludes, important food items (8). The geometric tortoise is occasionally eaten by farm workers, and in the past it has been much sought after for the pet trade. However the impact of these activities is negligible compared to the major threat of habitat degradation and destruction (4).

The geometric tortoise is protected by Western Cape Provincial legislation, which is rigidly and effectively enforced (4), and it is listed on Appendix I of the Convention on International Trade in Endangered Species (CITES), meaning that trade in this species is only permitted in exceptional circumstances (3). In addition, around 75 percent of the total population exists in formal protected areas (4), such as the Elandsberg Private Nature Reserve and a number of smaller provincial reserves (8). However, the majority of remaining geometric tortoise habitat is in the hands of private landowners and efforts are being made to secure the future of some of these sites through a conservation stewardship programme (13). Captive breeding of the geometric tortoise has had limited success and is currently not considered a priority, thus the primary focus of conservation efforts is to ensure its continued survival in the wild (4). The continued enforcement of conservation legislation and vigilance towards the unscrupulous collection of animals for the pet trade is called for (2), while the maintenance of protected areas is essential. In addition, it is imperative that private landowners take custodianship of the last remaining renosterveld remnants that contain geometric tortoise populations, in partnership with conservation agencies, since it is practically not possible to buy enough land to protect formally (2). Without such measures, the renosterveld habitat and its endangered inhabitants may disappear completely.

For further information on conservation of renosterveld habitat see:

• Renosterveld Trust:
  www.renosterveld.org

Authenticated (17/04/08) by Dr Ernst Baard, Senior Manager: Scientific Services, CapeNature.
http://www.capenature.co.za