Psammobates geometricus

Geographic Range

Psammobates geometricus is found only in the southwestern portion of Cape Province, South Africa. The populations that exist there are heavily fragmented by the development of farms and towns.

• Biogeographic Regions
• ethiopian ethiopian
  • native native

Habitat

Geometric Tortoises lived in the coastal lowlands known as the renosterveld. This is a fire-prone, endangered type of shrub land habitat found in the semi-arid and sub-humid areas of southern Cape Province, South Africa. The renosterveld makes up part of the Fynbos biome, which has a chaparral, or Mediterranean climate. These tortoises were found at elevations within 80 meters above sea level. Over 90 percent of this habitat has now been converted to farmland or suburban and urban uses. Over half of remaining Geometric Tortoises live on one private nature reserve containing about 1000 hectares of useable habitat.

• Habitat Regions
• terrestrial terrestrial

• Terrestrial Biomes
• chaparral chaparral
• scrub forest scrub forest

Physical Description

Psammobates geometricus has a highly domed carapace with four to six pyramid-shaped vertebral scutes, four or five costal scutes, and 11-12 marginal scutes. The forelimbs have five digits and scattered scales of variable size, while the hindlimbs have four digits. This species lacks buttock tubercles. Yellow lines centered in yellow patches form geometric patterns that contrast the dark brown or black background of the carapace. There are 8-15 yellow lines on the vertebral scutes, 9-12 on the costal scutes, and 2-4 on the marginal scutes. There is a nuchal, an undivided supracaudal, and long, paired gulars. The neck and limbs are yellow, and the head and tail are yellow with black splotches.

This species is sexually dimorphic; males have longer tails and concave plastrons. Males are also known to be smaller than females, with average adult carapace lengths of 100 mm and 125 mm and masses of 200 g and 430 g, respectively. The largest specimens found had carapace lengths of 120 mm for a male and 200 mm for a female. The heaviest specimens had masses of 270 g for a male and 600 g for a female.

• Other Physical Features
• ectothermic ectothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• female larger
• sexes shaped differently

Development

This is an oviparous species. Juveniles and sub-adults grow at an average of 10.45 mm per year, and adult males and females grow at 1.70 mm and 1.56 mm per year, respectively. This rapid growth results in two yearly annuli added to the carapace scutes. It is unclear if this species has environmental sex determination or genetic sex determination. Males mature at a carapace length of about 90 mm; size at maturity for females in unreported but they undoubtedly mature at a larger size than males.

• Development - Life Cycle
• indeterminate growth indeterminate growth

Reproduction

The mating behavior of Psammobates geometricus in the wild has not been extensively observed or reported. In captivity, mating has occurred from January through March and in May and September.

Psammobates geometricus nests from spring to early summer. This species is iteroparous, so females will have several reproductive events throughout their lives. In captivity, egg-laying generally occurred from August through December, but nesting also took place in May and June. Females lay one or two clutches (per year) of two to eight eggs that are about 24 x 32 mm in size. The eggs hatch in 120 to 210 days. The hatchlings emerge in March, April, or May, once the winter rains help to soften the ground, and can range in size from 30-40 mm and weigh six to eight grams. There is no parental investment once the offspring hatch, so the hatchlings are considered independent from the start. Male age at maturity has been found to be 5 to 6 years; female age at maturity is unknown, but it has been suggested that they probably mature in 7 to 8 years.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
• oviparous oviparous

Like most other turtles, P. geometricus does not exhibit parental care once eggs are laid. Post-fertilization, the female will dig a nest, lay her eggs, and cover the nest. This marks the extent of parental care in the species.

• Parental Investment
• no parental involvement
• pre-hatching/birth
  • provisioning
    • female

Lifespan/Longevity

After reaching maturity at five to eight years, individual Geometric Tortoises may live for over 30 years. Other than this, there has not been any significant research on the lifespan of these tortoises. Limits on lifespan include habitat destruction and fires and, to a lesser extent, predation and illegal collection for the pet trade.

Behavior

Psammobates geometricus can be active throughout the year, but generally moves and feeds in the morning and late afternoon when temperatures are lower. Otherwise, the tortoises may go into a daily torpor while waiting out the hot weather. When food and water are scarce and temperatures are high, this tortoise may aestivate, remaining dormant without actively searching out food, for weeks at a time. The body temperatures of active tortoises were 3 to 6 degrees C above the ambient temperature.

• Key Behaviors
• terricolous
• diurnal diurnal
• motile motile
• aestivation
• daily torpor
• solitary solitary

Home Range

Habitat fragmentation has resulted in spotty populations of P. geometricus throughout the species' range. Individual geometric tortoises may have annual home ranges of from 1.02 ha to 44.85 ha (10200 m^2 and 448500 m^2). Females tend to have larger home ranges and move more frequently than males. During the non-nesting season however, males move over more distance, possibly to increase future mating opportunities.

Communication and Perception

Communication and perception has not been studied in P. geometricus , but general trends in tortoises include visual, tactile, and chemical signals used for social and sexual communication. Tortoises primarily rely on these signals, but can also detect audio cues and vibrations for input about their surrounding environment.

• Communication Channels
• visual visual
• tactile tactile
• chemical chemical

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• vibrations vibrations
• chemical chemical

Food Habits

Geometric Tortoises have a relatively specialized diet; they have been documented to eat various grasses including Briza maxima , Cynodon dactylon , Themeda triandra , Ehrharta calycina , and Pentaschistis curvifolia , as well as members of the geophyte genera Oxalis and Pelargonium . It has been noted that geometric tortoises use a "grab and pull" method of eating, so plants with high tensile strength are often avoided. When food and non-food plants are compared, P. geometricus has been found to eat food plants with high levels of phosphorus, iron, soluble carbohydrates, and low levels of acid detergent fiber and lipids. In captivity these tortoises eat various fruits, garden weeds, succulents, the shells of crushed snails and even softened chicken bones. This suggests possible scavenging in the wild.

• Primary Diet
• carnivore carnivore
  • molluscivore molluscivore
  • scavenger scavenger
• herbivore herbivore
  • folivore folivore
  • frugivore frugivore

• Animal Foods
• mollusks

• Plant Foods
• leaves
• fruit
• flowers

Predation

The starred pattern on the back of P. geometricus may serve to camouflage these tortoises in grasses and brush to hide them from potential predators. It has been noted that when wildfires and other forces push P. geometricus into the open, the incidence of avian predation increases. Noted mammalian predators include baboons, jackals, and genets. Ostriches, secretary birds, various hawks, crows, and storks are among the potential natural avian predators. Introduced mammals, such as dogs, cats, rats, mongooses, and swine, are also known or suspected predators on the tortoises.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

Psammobates geometricus does not burrow, so its effects on its ecosystem are the result of its place in the food web. The food plants are kept in check by P. geometricus , which in turn serves as a food source for birds and other predators. However, with the large amount of habitat destruction and deadly fires, P. geometricus is rare, just as the renosterveld habitat itself is rare. In a study of South African reptile parasites, the ticks Hyalomma truncatum , Rhipicephalus gertrudae , Amblyomma marmoreum , Amblyomma sylvaticum , and Ornithodoros compactus were found to parasitize P. geometricus , but the potential for disease transmission has not been studied.

• Ecosystem Impact
• disperses seeds

Economic Importance for Humans: Positive

Despite being endangered and protected by law, Geometric Tortoises were (and may still be) sold in the pet trade and used for medicinal purposes. There are, however, benefits that P. geometricus provides to humans that are not harmful to the species. A wide range of research has been conducted on P. geometricus in relation to species conservation and habitat conservation because of the rarity of the tortoises and of the renosterveld in which they live. The reserves that were established for Geometric Tortoises help to preserve other species, boost ecotourism and to increase interest in research and education.

• Positive Impacts
• pet trade pet trade
• ecotourism ecotourism
• source of medicine or drug source of medicine or drug
• research and education

Economic Importance for Humans: Negative

There are no known adverse effects of Psammobates geometricus on humans.

Conservation Status

Psammobates geometricus is the rarest chelonian in Africa. Up to 97% of its renosterveld habitat has been lost due to human development and fires, making P. geometricus gravely endangered. The species is listed on the IUCN Red List as Endangered, and is a CITES Appendix I species, meaning it is threatened with extinction and may only be traded in exceptional circumstances. Because it is not native to the United States, the species is absent from the US Federal List and the State of Michigan List. Another cause for the decline of P. geometricus is invasive plant species such as members of the genus Acacia , which replace the native plants that serve as food sources for the tortoises. It has also been suggested that the yellow mongoose has been introduced to the area and serves as a predator to P. geometricus , causing further population decline.

There are several reserves dedicated to the conservation of P. geometricus spread across the natural range of the species. These include Eenzaamheid, Romans River, Hartebeest River, and Elandsberg Reserve.

Other Comments

The small geographic range of P. geometricus is due to the small range in which the renosterveld habitat exists. This habitat range is still being reduced and converted into land for agriculture. It may also be noted that no subspecies of P. geometricus are recognized.

There has been some confusion over Carl Linnaeus's description of Testudo geometrica , which was eventually proven to be Geochelone elegans (Indian Star Tortoise), not P. geometricus as previously thought.

Additional Links

Encyclopedia of Life

Contributors

Hunter Craig (author), Michigan State University, James Harding (editor), Michigan State University, Tanya Dewey (editor), University of Michigan-Ann Arbor.

References

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