More Information

Additional Information

Psammobates geometricusGeometric Tortoise

By Hunter Craig

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. (Baard, 1995a; Hofmeyr, et al., 2012)

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. (Boycott and Bourquin, 2000; Cowling, et al., 1986; Cunningham, et al., 2002; Hildyard, 2001; Kemper, et al., 1999)

  • Range elevation
    0 to 80 m
    0.00 to 262.47 ft

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. (Baard, 1989; Boycott and Bourquin, 2000)

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. (Baard, 1989; Boycott and Bourquin, 2000)

  • Sexual Dimorphism
  • female larger
  • sexes shaped differently
  • Range mass
    270 (Male) and 600 (Female) (high) g
    oz
  • Average mass
    200 (Male) and 430 (Female) g
    oz
  • Range length
    120 (Male) and 200 (Female) (high) mm
    in
  • Average length
    100 (Male) and 125 (Female) mm
    in

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. (Baard, 1989; Baard, 1995b; Baard, 1996; Boycott and Bourquin, 2000)

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. (Boycott and Bourquin, 2000)

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. (Baard, 1989; Baard, 1995b; Boycott and Bourquin, 2000; Iverson, et al., 1993)

  • Breeding interval
    Geometric tortoises lay one or occasionally two clutches of eggs yearly (Baard, 1989).
  • Breeding season
    Breeding occurs from September to November, which is spring to early summer (Baard, 1989).
  • Range number of offspring
    2 to 8
  • Range gestation period
    120 to 210 days
  • Range age at sexual or reproductive maturity (female)
    7 to 8 years
  • Range age at sexual or reproductive maturity (male)
    5 to 6 years
  • Average age at sexual or reproductive maturity (male)
    5.9 years

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. (Boycott and Bourquin, 2000; Iverson, et al., 1993)

  • 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. (Baard, 1989; Baard, 1996; Boycott and Bourquin, 2000)

  • Range lifespan
    Status: wild
    30+ (high) years
  • Range lifespan
    Status: captivity
    30+ (high) years
  • Typical lifespan
    Status: captivity
    19 to 30 years

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. (Balsamo, et al., 2004; Boycott and Bourquin, 2000)

  • Range territory size
    10200 to 448500 m^2

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. (Boycott and Bourquin, 2000; Hofmeyr, et al., 2012)

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. (Auffenberg, 1977; Baard, 1989)

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. (Balsamo, et al., 2004; Boycott and Bourquin, 2000; Henen, et al., 2005)

  • 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. (Baard, 1996; Boycott and Bourquin, 2000)

  • Anti-predator Adaptations
  • 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. (Baard, 1996; Horak, et al., 2006; Kemper, et al., 1999)

  • Ecosystem Impact
  • disperses seeds
Commensal/Parasitic Species

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. (Baard, 1989; Boycott and Bourquin, 2000; Kemper, et al., 1999; Petersen, et al., 2012)

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. (Baard, 1989; Balsamo, et al., 2004; Boycott and Bourquin, 2000; Leuteritz and Ekbia, 2008)

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. (Baard, 1989; Boycott and Bourquin, 2000)

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. (Baard, 1989; Baard, 1993; Boycott and Bourquin, 2000)

Contributors

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

Glossary

Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chaparral

Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.

chemical

uses smells or other chemicals to communicate

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

diurnal
  1. active during the day, 2. lasting for one day.
drug

a substance used for the diagnosis, cure, mitigation, treatment, or prevention of disease

ecotourism

humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

fertilization

union of egg and spermatozoan

folivore

an animal that mainly eats leaves.

frugivore

an animal that mainly eats fruit

herbivore

An animal that eats mainly plants or parts of plants.

indeterminate growth

Animals with indeterminate growth continue to grow throughout their lives.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

molluscivore

eats mollusks, members of Phylum Mollusca

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

pet trade

the business of buying and selling animals for people to keep in their homes as pets.

scavenger

an animal that mainly eats dead animals

scrub forest

scrub forests develop in areas that experience dry seasons.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

tactile

uses touch to communicate

terrestrial

Living on the ground.

vibrations

movements of a hard surface that are produced by animals as signals to others

visual

uses sight to communicate

References

Auffenberg, W. 1977. Display Behavior in Tortoises. Integrative and Comparative Biology, 17/1: 241-250. Accessed November 29, 2013 at http://icb.oxfordjournals.org/content/17/1/241.short.

Baard, E. 1995. A preliminary analysis of the habitat of the geometric tortoise,Psammobates geometricus. South African Journal of Wildlife Research, 25/1: 8-13. Accessed November 03, 2013 at http://www.sabinet.co.za/abstracts/wild/wild_v25_n1_a2.html.

Baard, E. 1995. Growth, age at maturity and sexual dimorphism in the geometric tortoise, Psammobates geometricus. The Journal of the Herpetological Association of Africa, 44/1: 10-15. Accessed November 03, 2013 at http://www.tandfonline.com/doi/abs/10.1080/04416651.1995.9650383#.UpkmksRDtFt.

Baard, E. 1989. The Conservation Biology of Tortoises. Glaad, Switzerland: IUCN. Accessed November 29, 2013 at http://books.google.com/books?id=pL2vyHjC4akC&pg=PA87&dq=geometric+tortoise&hl=en&sa=X&ei=jhmZUt6LOpfhoASu3YHIBw&ved=0CDoQ6AEwAA#v=onepage&q=geometric%20tortoise&f=false.

Baard, E. 1993. Distribution and status of the geometric tortoise Psammobates geometricus in South Africa. Biological Conservation, 63/3: 235-239. Accessed November 30, 2013 at http://www.sciencedirect.com/science/article/pii/000632079390718G.

Baard, E. 1996. THE CONSERVATION BIOLOGY OF SOUTH AFRICAN TORTOISES - PERSPECTIVES AND OPPORTUNITIES. The Desert Tortoise Council 1995 Symposium, 15: 4-9. Accessed November 29, 2013 at http://www.deserttortoise.org/ocr_DTCdocs/1995DTCProceedings-OCR.pdf.

Balsamo, R., M. Hofmeyr, B. Henen, A. Bauer. 2004. Leaf biomechanics as a potential tool to predict feeding preferences of the geometric tortoise Psammobates geometricus. African Zoology, 39/2: 175-181. Accessed November 29, 2013 at http://196.21.45.52/usrfiles/importcms/gen11Srv7Nme54_2531_1210050507/Balsamo_Zool.pdf.

Boycott, R., O. Bourquin. 2000. The Southern African Tortoise Book. KwaZulu-Natal, South Africa: O. Bourquin.

Cowling, R., S. Pierce, E. Moll. 1986. Conservation and utilisation of South Coast renosterveld, an endangered South African vegetation type. Biological Conservation, 37/4: 363-377. Accessed November 29, 2013 at http://www.sciencedirect.com/science/article/pii/0006320786900789.

Cunningham, J., E. Baard, E. Harley, C. O'Ryan. 2002. Investigation of genetic diversity in fragmented geometric tortoise (Psammobates geometricus) populations. Conservation Genetics, 3/3: 215-223. Accessed November 29, 2013 at http://link.springer.com/article/10.1023/A:1019909515202#page-1.

Henen, B., M. Hofmeyr, R. Balsamo, F. Weitz. 2005. Lessons from the food choices of the endangered geometric tortoise Psammobates geometricus. South African Journal of Science, 101/9-10: 435-438. Accessed November 29, 2013 at http://apps.webofknowledge.com.proxy1.cl.msu.edu/full_record.do?product=WOS&search_mode=Refine&qid=2&SID=2F5xhccsBrUEXm7SksB&page=1&doc=2.

Hildyard, A. 2001. Endangered Wildlife and Plants of the World. United States: Marshall Cavendish Corporation. Accessed November 29, 2013 at http://books.google.com/books?id=wFdWlrnz_uoC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false.

Hofmeyr, M., U. van Bloemestein, B. Henen, C. Weatherby. 2012. Sexual and environmental variation in the space requirements of the Critically Endangered geometric tortoise, Psammobates geometricus. Amphibia-Reptilia, 33: 185-197. Accessed November 29, 2013 at http://www.ingentaconnect.com/content/brill/amre/2012/00000033/00000002/art00003.

Horak, I., I. McKay, B. Henen, H. Heyne, M. Hofmeyr, A. De Villiers. 2006. Parasites of domestic and wild animals in South Africa. XLVII. Ticks of tortoises and other reptiles. Onderstepoort Journal of Veterinary Research, 73: 215-227. Accessed November 30, 2013 at http://www.ojvr.org/index.php/ojvr/article/viewFile/148/143.

Iverson, J., C. Balgooyen, K. Byrd, K. Lyddan. 1993. Latitudinal variation in egg and clutch size in turtles. Canadian Journal of Zoology, 71/12: 2448-2461. Accessed November 29, 2013 at http://www.nrcresearchpress.com/doi/abs/10.1139/z93-341#.Uplg58RDtFs.

Kemper, J., R. Cowling, D. Richardson. 1999. Fragmentation of South African renosterveld shrublands: effects on plant community structure and conservation implications. Biological Conservation, 90/2: 103-111. Accessed November 29, 2013 at http://www.sciencedirect.com/science/article/pii/S000632079900021X.

Leuteritz, T., H. Ekbia. 2008. Not All Roads Lead to Resilience: a Complex Systems Approach to the Comparative Analysis of Tortoises in Arid Ecosystems. Ecology and Society, 13/1: 1. Accessed November 29, 2013 at http://www.ibcperu.org/doc/isis/8549.pdf.

Petersen, L., E. Moll, R. Collins, M. Hockings. 2012. Development of a Compendium of Local, Wild-Harvested Species Used in the Informal Economy Trade, Cape Town, South Africa. Ecology and Society, 17/2: 26. Accessed November 30, 2013 at http://www.ecologyandsociety.org/articles/4537.html.

Search

Navigation Links

Information
Classification

Classification