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Order: Caudata
Family: Ambystomatidae
The family Ambystomatidae or “the mole salamanders” is included within the four earliest or most primitive family lineages of the order “Caudata” (the salamanders), diverging from all other salamanders in the Early Cretaceous period over 140 million years ago, around five million years before the koala and dolphin lineages diverged from their common ancestor. The small number of species that represent the genus Ambystoma are highly evolutionarily distinct members of both the salamanders and the amphibians as a whole.
The axolotl exhibits some highly unusual and distinct features, indicative of its evolutionary distinctiveness, including its rare “neotenous” life history, whereby the species never develops into an adult but instead retains its juvenile characteristics throughout life, essentially achieving reproductive maturity whilst still in its undeveloped larval form. There are a couple of theories for why neoteny (also referred to a paedomorphosis) develops in some mole salamanders. One idea is that the production or effectiveness of the hormone thyroxine is compromised, either by the species living in water bodies containing insufficient iodine (which is required in the manufacture of thyroxine by the body) or in water temperatures that are too cold for the thyroxine to be effective. This impacts upon the development of the species and sexually mature adults never develop adult characteristics but remain in the larval form. A second theory suggests that species evolving in pools surrounded by hostile terrestrial environments develop aquatic lives to obviate the need to exit the relative safety of their watery home. This is a common trait in species that inhabit high-elevation ponds.
The axolotl exhibits some highly unusual and distinct features, indicative of its evolutionary distinctiveness, including its rare “neotenous” life history, whereby the species never develops into an adult but instead retains its juvenile characteristics throughout life, essentially achieving reproductive maturity whilst still in its undeveloped larval form. There are a couple of theories for why neoteny (also referred to a paedomorphosis) develops in some mole salamanders. One idea is that the production or effectiveness of the hormone thyroxine is compromised, either by the species living in water bodies containing insufficient iodine (which is required in the manufacture of thyroxine by the body) or in water temperatures that are too cold for the thyroxine to be effective. This impacts upon the development of the species and sexually mature adults never develop adult characteristics but remain in the larval form. A second theory suggests that species evolving in pools surrounded by hostile terrestrial environments develop aquatic lives to obviate the need to exit the relative safety of their watery home. This is a common trait in species that inhabit high-elevation ponds.
The axolotl is an Ambystomatid or mole salamander found only in the Laguna Alchichica in eastern Puebla, Mexico at an elevation of 2,290m above sea level. Mole salamanders are medium to large, stocky salamanders, usually measuring between 90 to 350mm from the tip of the nose to the end of the tail, which salamanders retain throughout their life. Males are often larger than females, owing to their longer tails. Ambystomatids generally exhibit both aquatic “neotenic” larval (or aquatic and permanently juvenile in form with external, feathery gills) and terrestrial “metamorphosed” (or ground-dwelling, fully developed adult in form with reduced gills) stages in their wild populations. Ambystomatids are often boldly patterned as adults, with well-developed costal grooves (successive vertical grooves along the sides of the body), especially the metamorphosing varieties. They have a rather flattened body with a wide, flattened head, a large mouth and smooth skin with many glands. The tail is roundish or laterally compressed, and, during the breeding season, males have a very swollen cloacal zone (the region around the reproductory and excretory opening in amphibians located underneath the base of the tail).
Like all neotenic Ambystoma species, the axolotl retains its larval features into adulthood. A fully grown axolotl (at age 18–24 months) ranges in length from 150–450mm, although a size close to 230mm is typical and axolotls measuring more than 300mm nose to tail are rarely observed. The axolotl grows much larger than a normal larval or neotenic salamander species, and it reaches sexual maturity in this larval stage. Axolotls have distinctive fern-like gill structures that are uncovered or external – usually three stalks on each side of the head. They have tiny teeth which are used to grip food rather than to tear and chew it. In body colour, axolotls range from albino or white (the leucistic variety) to black, through greys, tans and browns. However, leucistic axolotls are rarely found in the wild.
Like all neotenic Ambystoma species, the axolotl retains its larval features into adulthood. A fully grown axolotl (at age 18–24 months) ranges in length from 150–450mm, although a size close to 230mm is typical and axolotls measuring more than 300mm nose to tail are rarely observed. The axolotl grows much larger than a normal larval or neotenic salamander species, and it reaches sexual maturity in this larval stage. Axolotls have distinctive fern-like gill structures that are uncovered or external – usually three stalks on each side of the head. They have tiny teeth which are used to grip food rather than to tear and chew it. In body colour, axolotls range from albino or white (the leucistic variety) to black, through greys, tans and browns. However, leucistic axolotls are rarely found in the wild.
The most notable feature of the life history of the axolotl is that the species exhibits an extreme form of neoteny. Axolotls remain in their aquatic larval form throughout their entire lives, which means that when they reach sexual maturity at approximately one and a half years of age, they remain in all other respects a large larva. This would be akin to a tadpole being able to breed without ever turning into a frog. Neoteny involves the retention of “paedomorphic” features (referring to those features which pertain to the juvenile form) such as external gills that persist through life, a state described as “perennibranchiate”. The paedomorphic characteristics found in adult axolotls include the maintenance of external gills, fins, non-protruding eyes, no eyelids and an associated permanently aquatic lifestyle. Young axolotls feed on algae, but as adults their diet predominantly consists of aquatic insects.
During reproduction in axolotls, the male releases sperm packets which are taken up by the female for internal fertilisation. Fertilised eggs are attached by the female to structures such as plants and hatching generally occurs after 2-3 weeks. In the wild, axolotls can live for ten to 12 years. The major predators of the axolotl are predatory birds such as herons.
Axolotls famously have a fast regeneration rate which can allow them to regrow limbs and organs. In addition to respiring via their external fearthery gills, axolotls are able to breathe through their skin and also possess lungs.
During reproduction in axolotls, the male releases sperm packets which are taken up by the female for internal fertilisation. Fertilised eggs are attached by the female to structures such as plants and hatching generally occurs after 2-3 weeks. In the wild, axolotls can live for ten to 12 years. The major predators of the axolotl are predatory birds such as herons.
Axolotls famously have a fast regeneration rate which can allow them to regrow limbs and organs. In addition to respiring via their external fearthery gills, axolotls are able to breathe through their skin and also possess lungs.
The axolotl is native to the ancient system of water channels and lakes in Mexico City. This species requires deep-water lakes and water bodies (including both natural and artificial canals) with abundant aquatic vegetation. The axolotl depends upon vegetation and other suitable structures for the attachment of their eggs, following fertilisation. Lake Xochimilco is known for its “floating gardens,” or “chinampas,” which are strips of land between drainage channels where local people grow vegetables and flowers for the market. Axolotls may be found in these channels, as well as remaining lake areas.
The axolotl is known only from central Mexico, on the southern edge of Mexico City, in canals and wetlands in the general vicinity of Xochimilco (including outside of Xochimilco proper, around the Chalco wetland). The axolotl lives only in wetlands and canals associated with Lake Xochimilco and Lake Chalco, both adjacent to Mexico City. Xochimilco and Chalco are part of a complex of five lakes, among which the Aztecs built Mexico City and around which the city has since expanded. Axolotls are not regularly distributed throughout their range but instead congregate in places where the habitat is still conducive for the survival and breeding of the axolotl.
An accurate population estimate is currently unavailable for the species, although the surviving wild population of axolotls is known to be very small despite a large captive population. Although populations are difficult to assess, recent surveys covering almost all of its known distribution range have usually captured less than 100 individuals and during 2002-2003, following more than 1,800 net casts along Xochimilco canals covering 39,173m2, the resultant catch was just 42 individuals. A recent scientific survey in 2004 revealed no axolotls in the species’ range. However, wild-caught animals are still found in local markets, indicating that the local fishermen still know where to find them. There has not been a density study of the Chalco population, but evidence suggests that this population is small and, furthermore, Chalco is a highly unstable system which runs the risk of disappearing in the near future.
Exact population data are currently unavailable for the axolotl, although the population trend is assumed to be in decline in the IUCN Red List of Threatened Species based on notable declines in survey catches performed since 2002 along a 39,173m2 area of the Xochimilco canals.
Listed as Critically Endangered in the IUCN Red List of Threatened Species because its area of occupancy is less than 10 km sq., its distribution is severely fragmented, there is a continuing decline in the extent and quality of its habitat and a sustained decline in the number of mature individuals.
Axolotl populations are suffering as a result of land drainage and the growth of Mexico City. Various efforts at flood control and sewage disposal starting in the seventeenth century have led to serious damage to the Xochimilco and Chalco lake complex. The digging of wells for the burgeoning population of Mexico City has also caused drying of the valley in which the lakes are located. The largest of these lakes, Texcoco, has been greatly diminished in size, while Lake Chalco has all but disappeared. Xochimilco has likewise suffered a decline in size and water quality. The major threat to the survival of the axolotl is therefore the desiccation, pollution and general degradation of the canal system and lakes in Xochimilco and Chalco, as a result of urbanization. The species is under pressure from traditional harvesting for consumption by local people and axolotls are also captured for medicinal purposes. The harvesting is targeted at animals that are less than one year old, and therefore generally before the individuals have had the opportunity to breed since axolotls reach sexual maturity at approximately one and a half years of age. Formerly, axolotls have also been captured for the international pet trade, although it is thought that no axolotls commercially available today are wild caught since doing so is strictly forbidden. The majority of axolotls currently available on the international market probably originated from captive or laboratory populations.
Introduced predatory fish (such as tilapia and carp) have increased to high abundances – a recent study collected 600kg of tilapia in one small channel using a 100m net. These introduced fish species have had a negative impact upon axolotl populations through competition and predation. The species is also being adversely affected by poor water quality, arising from factors such as pollution and disease, probably spread via invasive species. Although the water regime has changed in the last ten years, and it is reported that pollution levels are decreasing, factors such as very high levels of bacterial contamination could still pose a serious threat to axolotls in the wild.
Introduced predatory fish (such as tilapia and carp) have increased to high abundances – a recent study collected 600kg of tilapia in one small channel using a 100m net. These introduced fish species have had a negative impact upon axolotl populations through competition and predation. The species is also being adversely affected by poor water quality, arising from factors such as pollution and disease, probably spread via invasive species. Although the water regime has changed in the last ten years, and it is reported that pollution levels are decreasing, factors such as very high levels of bacterial contamination could still pose a serious threat to axolotls in the wild.
Conservation action to protect axolotl populations in the wild is focusing on raising the profile of Lake Xochimilco through conservation education and a nature tourism initiative, coupled with work on habitat restoration and bioremediation. A species action plan is in draft. This species is protected under the category Pr (Special Protection) by the Government of Mexico and is in the process of being amended to a higher risk category. The axolotl is currently on Appendix II of CITES (the Convention on International Trade in Endangered Species of Wild Fauna and Flora), restricting its international trade to protect this species from over-harvesting in the wild, where it has been listed since 1975. However, it is currently under the process of "Periodic Review of Species included in CITES Appendices".
A Darwin project was recently completed focusing on the conservation of the axolotl, led by the Durrell Institute of Conservation and Ecology. This was designed to assist Mexico in the development of a sustainable development programme to conserve the axolotl and other endemic fauna and flora of Xochimilco through the promotion of nature tourism.
A Darwin project was recently completed focusing on the conservation of the axolotl, led by the Durrell Institute of Conservation and Ecology. This was designed to assist Mexico in the development of a sustainable development programme to conserve the axolotl and other endemic fauna and flora of Xochimilco through the promotion of nature tourism.
Protection of the axolotl’s habitat in the Xochimilco and Chalco canals and wetlands is an urgent priority in order to prevent this species from becoming extinct in the wild.
In addition to conserving native habitat for this species, the IUCN Technical Guidelines for the Management of Ex situ Populations, part of the IUCN Red List of Threatened Species, recommend that all Critically Endangered species should have an ex situ population managed to guard against the extinction of the species. An ex situ population is ideally a breeding colony of a species maintained outside its natural habitat, giving rise to individuals from that species that are sheltered from problems associated with their situation in the wild. This can be located within the species’ range or in a foreign country that has the facilities to support a captive breeding programme for the species. There are already several captive colonies around the world for the axolotl, since this species is used in physiological and biomedical research, as well as in the pet trade, but the re-introduction of captive-bred axolotls is not recommended until threats can be neutralised and disease and genetic risks to wild populations assessed. Furthermore, captive bred populations lose affinity for their wild habitat with each successive generation away from their wild descendents. Most captive bred populations of axolotls bear little resemblance to their wild ancestors, especially the leucistic (or albino) varieties. It is hopeful that axolotls are known to readily breed in captivity, rendering a captive breeding programme for conservation purposes a viable option, but any such programme should ideally utilise wild caught individuals to give resulting progeny the best chance of survival in the wild.
However, any ex situ conservation measure is rendered ineffective if there remains insufficient natural habitat in which to release captive bred populations in the future. Clearly protected areas, sensitive management of the axolotl’s Xochimilco and Chalco canal and wetland habitat, control of predatory fish populations in important axolotl water bodies, axolotl disease research and control and general habitat restoration are of paramount importance in rescuing the Critically Endangered axolotl from extinction in the wild.
In addition to conserving native habitat for this species, the IUCN Technical Guidelines for the Management of Ex situ Populations, part of the IUCN Red List of Threatened Species, recommend that all Critically Endangered species should have an ex situ population managed to guard against the extinction of the species. An ex situ population is ideally a breeding colony of a species maintained outside its natural habitat, giving rise to individuals from that species that are sheltered from problems associated with their situation in the wild. This can be located within the species’ range or in a foreign country that has the facilities to support a captive breeding programme for the species. There are already several captive colonies around the world for the axolotl, since this species is used in physiological and biomedical research, as well as in the pet trade, but the re-introduction of captive-bred axolotls is not recommended until threats can be neutralised and disease and genetic risks to wild populations assessed. Furthermore, captive bred populations lose affinity for their wild habitat with each successive generation away from their wild descendents. Most captive bred populations of axolotls bear little resemblance to their wild ancestors, especially the leucistic (or albino) varieties. It is hopeful that axolotls are known to readily breed in captivity, rendering a captive breeding programme for conservation purposes a viable option, but any such programme should ideally utilise wild caught individuals to give resulting progeny the best chance of survival in the wild.
However, any ex situ conservation measure is rendered ineffective if there remains insufficient natural habitat in which to release captive bred populations in the future. Clearly protected areas, sensitive management of the axolotl’s Xochimilco and Chalco canal and wetland habitat, control of predatory fish populations in important axolotl water bodies, axolotl disease research and control and general habitat restoration are of paramount importance in rescuing the Critically Endangered axolotl from extinction in the wild.
There are axolotls resident in both ZSL London and ZSL Whipsnade Zoos
AmphibiaWeb: Information on amphibian biology and conservation [web application]. 2006. Berkeley, California: AmphibiaWeb. Available: amphibiaweb. Accessed: 08 December 2006.
Armstrong, J.B. and Malacinski, G.M. (eds) 1989. Developmental Biology of the Axolotl. Oxford University Press, New York.
Axolotls.org
Brandon, R.A. 1972. Hybridization between the Mexican salamanders Ambystoma dumerilii and Ambystoma mexicanum under laboratory conditions. Herpetologica 28: 199-207.
Brandon, R.A. 1989. Natural history of the axolotl and its relationship to other ambystomatic salamanders.. In: J.B. Armstrong and G.M. Malacinski (eds), Development Biology of the Axolotl, pp. 12-21. Oxford University Press, New York.
Frost, Darrel R. 2006. Amphibian Species of the World: an Online Reference. Version 4 (17 August 2006). Electronic Database accessible at: . American Museum of Natural History, New York, USA.
Frost, D. R., Grant, T., Faivovich, J., Bain, R.H., Haas, A., Haddad, C. F. B., De Sá, R.O., Channing, A., Wilkinson, M., Donnellan, S.C., Raxworthy, C.J., Campbell, J.A., Blotto, B.L., Moler, P., Drewes, R.C., Nussbaum, R.A., Lynch, J.D., Green, D.M., and Wheeler, W.C. 2006. The Amphibian Tree of Life. Bulletin of the American Museum of Natural History 297: 1-370.
Graue, V. 1998. Estudio genético y demográfico de le población del anfibio Ambystoma mexicanum (Caudata: Ambystomatidae) del lago Xochimilco. Unpublished thesis, Instituto de Ciencias del Mar y Limnologica, Universidad Nacional Autónoma de México, Mexico City.
Griffiths, H.I. and Thomas, D.H. 1988. What is the status of the Mexican axolotl?. British Herpetological Society Bulletin 26: 3-5.
Griffiths, R.A., Graue, V. and Bride, I.G. 2003. The axolotls of Lake Xochimilco: the evolution of a conservation programme. Axolotl News 30: 12-18.
Griffiths, R.A., Graue, V., Bride, I.G. and McKay, J.E. 2004. Conservation of the axolotl (Ambystoma mexicanum) at Lake Xochimilco, Mexico. Herpetological Bulletin 89: 4-11.
Groombridge, B. (ed.) 1994. 1994 IUCN Red List of Threatened Animals. IUCN, Gland, Switzerland.
Highton, R. 2000. Detecting cryptic species using allozyme data. In: R.C. Bruce, R.G. Jaeger and L.D. Houck (eds), The Biology of Plethodontid Salamanders, pp. 215-241. Kluwer Academic/Plenum Publishers, New York.
IUCN, Conservation International and NatureServe. 2006. Global Amphibian Assessment. Global Amphibian Assessment. Accessed on 08 December 2006.
IUCN Conservation Monitoring Centre. 1986. 1986 IUCN Red List of Threatened Animals. IUCN, Gland, Switzerland and Cambridge, UK.
IUCN Conservation Monitoring Centre. 1988. 1988 IUCN Red List of Threatened Animals. IUCN, Gland, Switzerland and Cambridge, UK.
IUCN. 1990. 1990 IUCN Red List of Threatened Animals. IUCN, Gland, Switzerland and Cambridge, UK.
IUCN. 2004. 2004 IUCN Red List of Threatened Species. IUCN Red List of Threatened Species. Downloaded on 23 November 2004.
Jones, C. 2002. Water quality model for the reintroduction of the axolotl (Ambystoma mexicanum) into the canals of Xochimilco, Mexico City. Undergraduate Honours theses, Trent University Peterborough, Canada.
McKay, J.E. 2003. An evaluation of captive breeding and sustainable use of the Mexican axolotl (Ambystoma mexicanum). M.Sc. dissertation, University of Kent Canterbury, U.K.
Mattison, C. 2005. Encyclopedia of reptiles and amphibians: An essential guide to reptiles and amphibians of the world. Grange Books Plc. Kent.
Obst, F.J., Richter, K. and Jacob, U. 1984. The Completely Illustrated Atlas of Reptiles and Amphibians for the Terrarium. T.F.H. Publication Inc., N.J., U.S.A.
Roelants, K., Gower, D. J., Wilkinson, M., Loader, S. P., Biju, S. D., Guillaume, K., Moiau, L. and Bossuyt, F. 2007. Global patterns of diversification in the history of modern amphibians. Proceedings of the National Academy of Sciences 104: 887-892.
Shaffer, H.B. 1984. Evolution in a paedomorphic lineage. I. An electrophoretic analysis of the Mexican ambystomatid salamanders. Evolution 38: 1194-1206.
Shaffer, H.B. 1984. Evolution in a paedomorphic lineage. II. Allometry and form in the Mexican ambystomatid salamanders. Evolution 38: 1207-1218.
Shaffer, H.B. 1989. Natural history and ecology of the Mexican axolotls. Axolotl News 18: 5-11.
Shaffer, H.B. 1993. Phylogenetics of model organisms: the laboratory axolotl, Ambystoma mexicanum. Systematic Biology 42: 508-522.
Shaffer, H.B. and Lauder, G.V. 1985. Patterns of variation in aquatic ambystomatid salamanders: Kinematics of the feeding mechanism. Evolution 39(1): 83-92.
Smith, H.B. 1989. Discovery of the axolotl and its early history in biological research. In: J.B. Armstrong and G.M. Malacinski, G.M. (eds), Developmental Biology of the Axolotl, pp. 3-12. Oxford University Press, New York.
Smith, H.B. 1989. The axolotl in its native habitat. Axolotl News 18: 12-17.
Vergara, G. 1990. Contribución al estudio histológico del aparato reproductor masculino del anfibio urodelo Ambystoma mexicanum. Unpublished thesis, Facultad de Ciencias, Universidad Nacional Autónoma de México Mexico City.
Zambrano, L., Mosig Reidl, P., McKay, J., Griffiths, R., Shaffer, B., Flores-Villela, O., Parra Olea, G. & Wake, D. 2006. Ambystoma mexicanum. In: IUCN 2006. 2006 IUCN Red List of Threatened Species.. Downloaded on 08 December 2006.
Zambrano, L., Reynoso, V.H. and Herrera, G. 2004. Abundancia y estructura poblacional del axolotl (Ambystoma mexicanum) en los sistemas dulceacuícolas de Xochimilco y Chalco. Unpublished report, Instituto de Biología, Universidad Nacional Autónoma de México. Base de datos SNIB-Conabio proyecto AS004. Mexico City.
Armstrong, J.B. and Malacinski, G.M. (eds) 1989. Developmental Biology of the Axolotl. Oxford University Press, New York.
Axolotls.org
Brandon, R.A. 1972. Hybridization between the Mexican salamanders Ambystoma dumerilii and Ambystoma mexicanum under laboratory conditions. Herpetologica 28: 199-207.
Brandon, R.A. 1989. Natural history of the axolotl and its relationship to other ambystomatic salamanders.. In: J.B. Armstrong and G.M. Malacinski (eds), Development Biology of the Axolotl, pp. 12-21. Oxford University Press, New York.
Frost, Darrel R. 2006. Amphibian Species of the World: an Online Reference. Version 4 (17 August 2006). Electronic Database accessible at: . American Museum of Natural History, New York, USA.
Frost, D. R., Grant, T., Faivovich, J., Bain, R.H., Haas, A., Haddad, C. F. B., De Sá, R.O., Channing, A., Wilkinson, M., Donnellan, S.C., Raxworthy, C.J., Campbell, J.A., Blotto, B.L., Moler, P., Drewes, R.C., Nussbaum, R.A., Lynch, J.D., Green, D.M., and Wheeler, W.C. 2006. The Amphibian Tree of Life. Bulletin of the American Museum of Natural History 297: 1-370.
Graue, V. 1998. Estudio genético y demográfico de le población del anfibio Ambystoma mexicanum (Caudata: Ambystomatidae) del lago Xochimilco. Unpublished thesis, Instituto de Ciencias del Mar y Limnologica, Universidad Nacional Autónoma de México, Mexico City.
Griffiths, H.I. and Thomas, D.H. 1988. What is the status of the Mexican axolotl?. British Herpetological Society Bulletin 26: 3-5.
Griffiths, R.A., Graue, V. and Bride, I.G. 2003. The axolotls of Lake Xochimilco: the evolution of a conservation programme. Axolotl News 30: 12-18.
Griffiths, R.A., Graue, V., Bride, I.G. and McKay, J.E. 2004. Conservation of the axolotl (Ambystoma mexicanum) at Lake Xochimilco, Mexico. Herpetological Bulletin 89: 4-11.
Groombridge, B. (ed.) 1994. 1994 IUCN Red List of Threatened Animals. IUCN, Gland, Switzerland.
Highton, R. 2000. Detecting cryptic species using allozyme data. In: R.C. Bruce, R.G. Jaeger and L.D. Houck (eds), The Biology of Plethodontid Salamanders, pp. 215-241. Kluwer Academic/Plenum Publishers, New York.
IUCN, Conservation International and NatureServe. 2006. Global Amphibian Assessment. Global Amphibian Assessment. Accessed on 08 December 2006.
IUCN Conservation Monitoring Centre. 1986. 1986 IUCN Red List of Threatened Animals. IUCN, Gland, Switzerland and Cambridge, UK.
IUCN Conservation Monitoring Centre. 1988. 1988 IUCN Red List of Threatened Animals. IUCN, Gland, Switzerland and Cambridge, UK.
IUCN. 1990. 1990 IUCN Red List of Threatened Animals. IUCN, Gland, Switzerland and Cambridge, UK.
IUCN. 2004. 2004 IUCN Red List of Threatened Species. IUCN Red List of Threatened Species. Downloaded on 23 November 2004.
Jones, C. 2002. Water quality model for the reintroduction of the axolotl (Ambystoma mexicanum) into the canals of Xochimilco, Mexico City. Undergraduate Honours theses, Trent University Peterborough, Canada.
McKay, J.E. 2003. An evaluation of captive breeding and sustainable use of the Mexican axolotl (Ambystoma mexicanum). M.Sc. dissertation, University of Kent Canterbury, U.K.
Mattison, C. 2005. Encyclopedia of reptiles and amphibians: An essential guide to reptiles and amphibians of the world. Grange Books Plc. Kent.
Obst, F.J., Richter, K. and Jacob, U. 1984. The Completely Illustrated Atlas of Reptiles and Amphibians for the Terrarium. T.F.H. Publication Inc., N.J., U.S.A.
Roelants, K., Gower, D. J., Wilkinson, M., Loader, S. P., Biju, S. D., Guillaume, K., Moiau, L. and Bossuyt, F. 2007. Global patterns of diversification in the history of modern amphibians. Proceedings of the National Academy of Sciences 104: 887-892.
Shaffer, H.B. 1984. Evolution in a paedomorphic lineage. I. An electrophoretic analysis of the Mexican ambystomatid salamanders. Evolution 38: 1194-1206.
Shaffer, H.B. 1984. Evolution in a paedomorphic lineage. II. Allometry and form in the Mexican ambystomatid salamanders. Evolution 38: 1207-1218.
Shaffer, H.B. 1989. Natural history and ecology of the Mexican axolotls. Axolotl News 18: 5-11.
Shaffer, H.B. 1993. Phylogenetics of model organisms: the laboratory axolotl, Ambystoma mexicanum. Systematic Biology 42: 508-522.
Shaffer, H.B. and Lauder, G.V. 1985. Patterns of variation in aquatic ambystomatid salamanders: Kinematics of the feeding mechanism. Evolution 39(1): 83-92.
Smith, H.B. 1989. Discovery of the axolotl and its early history in biological research. In: J.B. Armstrong and G.M. Malacinski, G.M. (eds), Developmental Biology of the Axolotl, pp. 3-12. Oxford University Press, New York.
Smith, H.B. 1989. The axolotl in its native habitat. Axolotl News 18: 12-17.
Vergara, G. 1990. Contribución al estudio histológico del aparato reproductor masculino del anfibio urodelo Ambystoma mexicanum. Unpublished thesis, Facultad de Ciencias, Universidad Nacional Autónoma de México Mexico City.
Zambrano, L., Mosig Reidl, P., McKay, J., Griffiths, R., Shaffer, B., Flores-Villela, O., Parra Olea, G. & Wake, D. 2006. Ambystoma mexicanum. In: IUCN 2006. 2006 IUCN Red List of Threatened Species.
Zambrano, L., Reynoso, V.H. and Herrera, G. 2004. Abundancia y estructura poblacional del axolotl (Ambystoma mexicanum) en los sistemas dulceacuícolas de Xochimilco y Chalco. Unpublished report, Instituto de Biología, Universidad Nacional Autónoma de México. Base de datos SNIB-Conabio proyecto AS004. Mexico City.
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