Common features of the Chelonians
The most prominent feature is that they all have an outer shell, consisting of the upper carapace and the lower plastron.
They have an internal skeleton contained within the shell.
The order Chelonia consists of two sub-orders, which can be distinguished by the way the head and neck are withdrawn into the shell. In both sub-orders the eight neck bones move in an ‘S’ shape.
Tortoises (Sub-order Cryptodoria): The neck is withdrawn in a vertical plane i.e. straight back
Terrapins (Sub-order Pleurodira): Bend their necks in a horizontal plane i.e. sideways
Both forms of head withdrawal have been used for over 130 million years. The tortoise is the oldest form of living reptile in the world, having undergone very little change in the past 200 million years. The first fossil record of chelonians dates back 210 million years.
Being reptiles, Chelonians need warmth to become active. They are most active between 25 and 30 degrees Celsius, the lower the heat, the less the activity. When too hot, activity is also reduced and tortoises will actually fully extend all their limbs including head and neck in an effort to cool down.
Common Terrapins species found in Botswana are:
Okavango hinged terrapin – found in the Okavango swamps, Linyanti and Chobe Rivers.
Marsh or Helmeted terrapin – found throughout Botswana except for the southwest Kgalagkadi area.
Serrated hinged terrapin – found in south Tuli area.
Pan hinged terrapin – found in Central eastern area along border of Zimbabwe and Okavango swamps, Linyanti and Chobe Rivers.
Mashona hinged terrapin – found only on the north-western side of the Okavango swamps in the panhandle and towards Namibia’s border.
Okavango Hinged Terrapin (Pelusios bechuanicus)
This terrapin h as a carapace of approximately 330mm large. The carapace is generally dark olive to black in colour. As with most terrapins, the plastron is hinged (refer to diagram). It has a very large head, which is black with distinctive yellow patterns. Terrapins have 5 claws on hind feet.
The Okavango terrapin lays between 21 – 38 eggs in a brood. The eggs are laid in October (late spring/early summer). The eggs hatch in late summer and upon hatching, the hatchlings make their way to the nearest water.
The female will lay her eggs in a hole approx. 125mm deep. She will use her hind legs to dig the hole and will constantly urinate to soften the soil while digging. Once the eggs are laid, the hole is filled with mud and gently smoothed over.
Terrapins are carnivorous. Prey items include carrion, fish, amphibians, tadpoles and limited amounts of plant matter. They have also been known to hunt birds, which they drag into the water to be drowned.
The Okavango hinged terrapin and the serrated hinged terrapin are the only two species, which prefer deep, open water. Terrapins have been known to cover great distances over land when water dries up but they will also scratch and burrow underground, retreat in the mud where they will remain in a state of aestivation until the return of the rains.
Terrapins give off a very strong, vile odour as a means of defence. When protecting themselves, they will also retract their limbs and their head. The head is retracted in a sideways motion and the hinged plastron is closed over the head to protect it.