Animal Diversity Web

Ge­o­graphic Range

Am­bystoma mex­i­canum is his­tor­i­cally found in Lakes Chalco and Xochim­ilco of the Val­ley of Mex­ico near Mex­ico City, Mex­ico. (Smith, et al., 1989; Bran­don, et al., 1989; Smith, 1969)

• Biogeographic Regions
• neotropical

Habi­tat

The na­tive habi­tats of A. mex­i­canum are large, rel­a­tively per­ma­nent (until re­cently), high-al­ti­tude lakes lo­cated near Mex­ico City. Of the two lakes - Chalco and Xochim­ilco - where these an­i­mals are his­tor­i­cally na­tive, only Xochim­ilco (el­e­va­tion: ~ 2,274 m) re­mains. Ax­olotls are al­most ex­tinct in their na­tive habi­tat, largely due to the in­tro­duc­tion of preda­tory fishes and habi­tat loss. ("Xochim­ilco", 2003; Shaf­fer, 1989)

• Habitat Regions
• freshwater

• Aquatic Biomes
• lakes and ponds

• Average elevation

  2290 m

  7513.12 ft

Phys­i­cal De­scrip­tion

Ax­olotls are pae­do­mor­phic or neotenic aquatic sala­man­ders, mean­ing they re­tain cer­tain lar­val char­ac­ter­is­tics in the adult, re­pro­duc­tive state. They pos­sess feath­ery ex­ter­nal gills and finned tails for swim­ming. Lab­o­ra­tory an­i­mals exist in sev­eral color morphs, rang­ing from wild type (dark, mot­tled brown­ish-green) to al­bino. Ax­olotls reach lengths on av­er­age of 20 cm (9 inches), but can grow to more than 30 cm (12 inches) in length. (Brunst, 1955a)

The sexes can be eas­ily dis­tin­guished in adult ax­olotls. Males can be iden­ti­fied by their en­larged cloaca (sim­i­lar to other urode­les), while fe­males have a smaller cloaca and round, plump bod­ies. (Brunst, 1955a)

• Other Physical Features
• ectothermic
• bilateral symmetry

• Sexual Dimorphism
• sexes shaped differently

• Range mass

  60 to 110 g

  2.11 to 3.88 oz

• Range length

  30 (high) cm

  11.81 (high) in

• Average length

  23 cm

  9.06 in

De­vel­op­ment

A. mex­i­canum is pae­do­mor­phic, which means that it re­tains lar­val char­ac­ter­is­tics in the re­pro­duc­tively ma­ture adult form. Ju­ve­nile and adult ax­olotls pos­sess feath­ery, ex­ter­nal gills and tail fins suited to an aquatic lifestyle. Meta­mor­pho­sis can be in­duced in ax­olotls via thy­roid hor­mone in­jec­tions. In the wild, ax­olotls rarely, if ever, meta­mor­phose.

• Development - Life Cycle
• neotenic/paedomorphic
• metamorphosis

Re­pro­duc­tion

The courtship be­hav­ior of A. mex­i­canum fol­lows the gen­eral Am­bystoma pat­tern; it first in­v­oles each an­i­mal nudg­ing the other's cloa­cal re­gion, even­tu­ally lead­ing to a "waltz," with both an­i­mals mov­ing in a cir­cle. Next, the male moves away while un­du­lat­ing the pos­te­rior part of his body and tail (re­sem­bling a "hula dance"), and the fe­male fol­lows. The male will de­posit a sper­matophore (a cone-shaped jelly mass with a sperm cap) by vig­or­ously shak­ing his tail for about half a minute, and will then move for­ward one body length. The fe­male then moves over the sper­matophore, also shak­ing her tail, and picks up the sper­matophore with her cloaca. (Eis­then, 1989)

• Mating System
• polygynandrous (promiscuous)

Ax­olotls breed in the wild gen­er­ally from March to June. From 100 to 300 eggs are de­posited in the water and at­tached to sub­strates. Eggs hatch at 10 to 14 days and the young are im­me­di­ately in­de­pen­dent. Sex­ual ma­tu­rity is reached in the next breed­ing sea­son. (Eis­then, 1989)

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

• Breeding interval

  Axolotls in the wild breed once yearly.

• Breeding season

  Breeding laboratory axolotls can be accomplished at almost any time; in the wild, it is thought that the best time for spawning is March to June.

• Range number of offspring

  100 to 300

• Range time to hatching

  10 to 14 days

• Range time to independence

  10 to 14 days

• Average age at sexual or reproductive maturity (female)

  1 years

• Average age at sexual or reproductive maturity (male)

  1 years

Eggs are sur­rounded by a pro­tec­tive jelly coat and are laid singly, un­like frog eggs (which are laid in clumped masses), be­cause they pos­sess higher oxy­gen re­quire­ments. They are often at­tached to sub­strates such as rocks or float­ing veg­e­ta­tion.

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

Lifes­pan/Longevity

Ex­pected lab­o­ra­tory longevity is 5 to 6 years; how­ever, some an­i­mals have been known to live as long as 10 to 15 years. Most lab­o­ra­tory an­i­mals die shortly after meta­mor­pho­sis. (Brunst, 1955a)

• Range lifespan
  Status: captivity

  15 (high) years

• Average lifespan
  Status: captivity

  5 - 6 years

• Typical lifespan
  Status: captivity

  6 (high) years

Be­hav­ior

Ax­olotls are soli­tary and may be ac­tive at any time of the day.

• Key Behaviors
• natatorial
• diurnal
• nocturnal
• motile
• sedentary
• solitary

Com­mu­ni­ca­tion and Per­cep­tion

Ax­olotls com­mu­ni­cate mainly via vi­sual cues and chem­i­cal cues dur­ing mat­ing. At other times of the year there is lit­tle to no in­traspe­cific com­mu­ni­ca­tion.

Ax­olotls can de­tect elec­tri­cal fields and also use their vi­sion and chem­i­cal cues to per­ceive their en­vi­ron­ment and dis­cover prey.

• Communication Channels
• visual
• chemical

• Other Communication Modes
• pheromones

• Perception Channels
• visual
• chemical
• electric

Food Habits

Gen­er­ally the top preda­tor in their nat­ural en­vi­ron­ment, ax­olotls will eat any­thing that they can catch, in­clud­ing mol­luscs, fishes, and arthro­pods, as well as con­specifics. (Shaf­fer, 1989)

• Primary Diet
• carnivore
  • piscivore
  • insectivore
  • eats non-insect arthropods
  • molluscivore

• Animal Foods
• amphibians
• fish
• insects
• mollusks
• terrestrial worms
• zooplankton

Pre­da­tion

Ax­olotls may be preyed on by large fish and con­specifics. Large fish have only re­cently been in­tro­duced into the lakes where ax­olotls are found, con­tribut­ing to the demise of their pop­u­la­tions.

Ecosys­tem Roles

Ax­olotls were the top preda­tor in their na­tive en­vi­ron­ment, mak­ing them im­por­tant in struc­tur­ing com­mu­nity dy­nam­ics.

Eco­nomic Im­por­tance for Hu­mans: Pos­i­tive

Ax­olotls are an im­por­tant re­search an­i­mal and have been used in stud­ies of the reg­u­la­tion of gene ex­pres­sion, em­bry­ol­ogy, neu­ro­bi­ol­ogy, and re­gen­er­a­tion. Oc­ca­sion­ally taken as a food item (sub­sti­tuted for fish), ax­olotls are pre­pared by ei­ther roast­ing or boil­ing and the tail is eaten with vine­gar or cayenne pep­per. They have also been used for med­i­c­i­nal pur­poses. (Smith, et al., 1989; Brunst, 1955a)

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

Eco­nomic Im­por­tance for Hu­mans: Neg­a­tive

There are no neg­a­tive ef­fects of ax­olotls on hu­mans.

Con­ser­va­tion Sta­tus

The nat­ural habi­tat of A. mex­i­canum is nearly gone. His­tor­i­cally, they have been known to live in high al­ti­tude lakes near Mex­ico City. Lake Chalco is gone com­pletely, drained for drink­ing water, and Lake Xochim­ilco is now noth­ing more than a scat­ter­ing of canals and swamps. Be­cause known pop­u­la­tions are few and far be­tween, very lit­tle is known about the ecol­ogy and nat­ural his­tory of A. mex­i­canum; there have been few eco­log­i­cal stud­ies on wild pop­u­la­tions. (Bran­don, et al., 1989)

• IUCN Red List

  Critically Endangered
  More information

• IUCN Red List

  Critically Endangered
  More information

• CITES

  Appendix II

Other Com­ments

The word "ax­olotl" comes from the na­tive Aztec lan­guage, or nahu­atl. It roughly trans­lates to: water slave, water ser­vant, water sprite, water player, water mon­stros­ity, water twin, or water dog. All of these names refer to the Aztec god Xolotl, brother to Quet­za­coatl and pa­tron of the dead and ressur­rected (where he took the form of a dog), games, grotesque (read: ugly) be­ings, and twins. Aztec lore states that Xolotl trans­formed him­self into, among other things, an ax­olotl to es­cape ban­ish­ment. He was cap­tured, killed, and used to feed the sun and moon. (Shaf­fer, 1989; Smith, 1969)

Lar­vae of other am­bysto­mids, such as the lar­val stage of the tiger sala­man­der, A. tigrinum, are often er­ro­neously re­ferred to as ax­olotls. The name ax­olotl should be used only when re­fer­ring to A. mex­i­canum and not to any other am­bysto­mid sala­man­der. His­tor­i­cally, the Mex­i­can ax­olotl has been listed under more than 40 dif­fer­ent names and spellings; all, ex­cept A. mex­i­canum, have been re­jected by the In­ter­na­tional Com­mis­sion on Zo­o­log­i­cal Nomen­cla­ture (ICZN). (Brunst, 1955a; Brunst, 1955b; Smith, 1969; Smith, et al., 1989)

The clos­est rel­a­tive of A. mex­i­canum is thought to be A. tigrinum, the tiger sala­man­der. In­deed, the lar­vae of these species are vi­su­ally very sim­i­lar. Some even con­sider the ax­olotl to be a sub­species of the tiger sala­man­der; vi­able off­spring can be pro­duced be­tween the two species in the lab­o­ra­tory, though no hy­brids have as of yet been dis­cov­ered in the wild. (Bran­don, et al., 1989; Smith, 1969; Smith, et al., 1989)

Ax­olotls are ex­cel­lent lab spec­i­mens as they are easy to raise and in­ex­pen­sive to feed. They are renowned for their amaz­ing re­gen­er­a­tive ca­pa­bil­i­ties, have been used widely in de­vel­op­men­tal stud­ies, and, be­cause of their large cells (they are poly­ploid), are often used in his­to­log­i­cal stud­ies. (Brunst, 1955a; Smith, et al., 1989)

Nearly all mod­ern lab­o­ra­tory ax­olotls can be traced back to 33 an­i­mals shipped from Xochim­ilco to Paris in 1864. They are one of the most widely used and stud­ied lab­o­ra­tory an­i­mals. (Smith, 1969; Smith, et al., 1989)

Con­trib­u­tors

Tanya Dewey (ed­i­tor), An­i­mal Di­ver­sity Web.

Amy Ma­jchrzak (au­thor), Michi­gan State Uni­ver­sity.

Ref­er­ences

En­cy­clopædia Bri­tan­nica Pre­mium Ser­vice. 2003. Xochim­ilco. En­cy­clopae­dia Bri­tan­nica. Ac­cessed 06/13/03 at http://​www.​britannica.​com/​eb/​article?.​eu=79786.

Bran­don, R., J. Arm­strong, G. Malacin­ski. 1989. Nat­ural his­tory of the ax­olotl and its re­la­tion­ship to other am­bysto­mid sala­man­ders. Pp. 13-21 in De­vel­op­men­tal bi­ol­ogy of the ax­olotl. New York, NY: Ox­ford Uni­ver­sity Press, Inc..

Brunst, V. 1955. The ax­olotl (Sire­don mex­i­canum) I. As ma­te­r­ial for sci­en­tific re­search. Lab­o­ra­tory In­ves­ti­ga­tion, 4: 45-64.

Brunst, V. 1955. The ax­olotl (Sire­don mex­i­canum) II. Mor­phol­ogy and pathol­ogy. Lab­o­ra­tory In­ves­ti­ga­tion, 4: 429-449.

Eis­then, H. 1989. Courtship and mat­ing be­hav­ior in the ax­olotl. Ax­olotl Newslet­ter, 18: 18-19.

Shaf­fer, H. 1989. Nat­ural his­tory, ecol­ogy, and evo­lu­tion of the Mex­i­can "ax­olotls". Ax­olotl Newslet­ter, 18: 5-11.

Smith, H. 1969. The Mex­i­can ax­olotl: some mis­con­cep­tions and prob­lems. Bio­Science, 19: 593-597.

Smith, H., J. Arm­strong, G. Malacin­ski. 1989. Dis­cov­ery of the ax­olotl and its early his­tory in bi­o­log­i­cal re­search. Pp. 3-12 in De­vel­op­men­tal bi­ol­ogy of the ax­olotl. New York, NY: Ox­ford Uni­ver­sity Press, Inc..