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The identification of Seychelles giant tortoises

 

Distinguishing Aldabra-Seychelles and Galapagos giant tortoises

Although superficially similar Galapagos and Indian Ocean giant tortoises are easy to distinguish. The Galapagos species (Chelonoidis nigra) have very broad, blunt heads and the carapace lacks a nuchal scute. The Aldabra-Seychelles tortoises have smaller, narrower heads and the carapace usually (but not always has a nuchal scute).

 

Distinguishing the Aldabra-Seychelles species

The three Aldabra-Seychelles giant tortoise species can be distinguished based on carapace shape, however, many captive animals may have distorted carapaces and so may be difficult to identify. In general the Aldabra tortoise (Dipsochelys dussumieri) is a regularly domed, black coloured species. The Seychelles giant tortoise (D. hololissa) is broad, flattened on the back and with raised scute, it is usually a brownish-grey colour. Arnold's tortoise (D. arnoldi) is flattened, smooth and with a relatively high opening to the shell; it is usually black. This species usually has a depression on the suture between the first and second costal scutes, this may be a shallow depression or a distinctive pit. The plastron is less variable than the carapace and usually provides a good indication of the species - Aldabra tortoises have curved humero-pectoral sutures and a deep anal notch whilst the two Seychelles species have angled sutures and a shallow notch. More precise identification is given by the following characters:
    Aldabra tortoise Seychelles giant tortoise Arnold's tortoise
    Dipsochelys dussumieri Dipsochelys hololissa Dipsochelys arnoldi
Width posterior / anterior approximately equal approximately equal posterior 20-40% wider
height highest vertebral scute 3, sometimes 2, rarely 4
2 and 3 equal

2, 3 and 4 equal

  costal height low (73-85%) high (86-89%) high (85-89%)
Vertebral scutes 3 compared to V2 approximately equal, less than 10% difference equal or slightly shorter (0-5%) at least 4% shorter
  compared to V4 0-25% shorter or longer 0-33% shorter or longer 22-50% shorter
costal pit no pit no pit pit
costal scute proportions   costal 1/2=85-114 73-109 110-157
caudal   flat or recurved flat or recurved curved down
humero-pectoral suture   curved angled angled
abdomen-femoral suture   curved angled angled
plastron proportion of straight length 75-94% 71-91 61-82
anal notch proportion of anal scute 14-32% 0-16% 0-15%

 

Adult giant tortoise (female on left, male on right):
Aldabra tortoise
Seychelles giant tortoise
Arnold's tortoise
Dipsochelys hololissa plastron showing angled humero-pectoral and abdomino-femoral sutures (red lines) and the shallow anal notch (arrow)

Any analysis of these tortoises should include:

1. Straight length (SL)
2. Curved length (CL)
3. Height (from the bottom of the plastron to a level with the highest part of the carapace) (H)
4. Costal height (from the bottom of the plastron to a level with the top of the second costal scute) (CH)
5. Length and width of all vertebral scutes (V1-5)
6. Length of costal 1 and 2 (C1-2)
7. Plastron length (Pl)
8. Length of anal scutes at back of plastron (A)
9. Depth of notch between anal scutes (N)

 

 

 

Identification of juveniles

Hatchlings of D. dussumieri are black, D. hololissa are grey and D. arnoldi pale brown with dark scute margins but all three are similar shapes. In the Aldabra tortoise the axillary scute on the plastron has a broad contact with the abdominal whilst in the other species the scutes do not join or make a very narrow contact.

Aldabra tortoise
Seychelles giant tortoise
Arnold's tortoise
Arnold's tortoise juvenile showing the separation of axillary and abdominal  

 

Sexing giant tortoises

Females tend to be more domed than males and adult males usually have a concave plastron. However, some females can have distinctly concave plastra so this is not completely reliable. Adults are easily sexed by the length of the tail; males have long tails, some 30cm long reaching forwards to the hind legs, females have tails about 10cm long, which do not reach the legs. Juveniles all have short tails but males tend to have at least 14 scales (12-20) between the posterior margin of the cloaca and the tail tip, females tend to have no more than 10 scales (8-13).