Ambystoma Mexicanum: A brief overview of the Axolotl species

The Axolotl, Ambystoma mexicanum, is a globally recognised species, appearing regularly in popular culture and on pet shop shelves (Reiß, 2022), but it's the unique adaptations of this salamander that have enabled its significant contribution to the scientific world (Reiß et al., 2015).

The Axolotl is an Amphibian, part of the Ambystomadtidae family in the genus Ambystoma. This species is endemic to the Mexican Basin and despite its large captive bred population, it’s classified as critically endangered in the wild (IUCN 2024). Ambystoma mexicanum, is one of 17 species of Mexican Ambystomatid Salamanders, many of which are endemic to their lake or river habitats (Shaffer 1993). The presence of a multitude of related endemic species in a relatively small geographical range, suggests a previous fragmentation of an ancestral population (Ewers & Didham, 2005). Most species are found in and around the Trans-Mexican Volcanic Belt which suggests that volcanic activity during the Pleistocene caused changes in land mass, creating geographical barriers that divided the population of the common ancestor into smaller isolated groups (Parra-Olea et al., 2022). Adaptation to these new abiotic factors resulted in a burst of rapid diversification causing populations to evolve into new species, known as allopatric speciation (Bryson et al 2018), see the below table for the Ambystoma genus tree.

Figure 1. A species tree for the genus Ambystoma (Keinath et al, 2018, p.9).

Despite having many living relatives, the Axolotl is a highly unique species due to its extensive regeneration abilities (McCusker & Gardiner, 2011). Whilst regeneration is already present in the animal kingdom, Axolotl are one of the few species capable of regenerating complex biological structures, including limbs, internal organs, central nervous system, spinal cord and even the brain (Maden et al., 2013). Furthermore, this species has displayed successful resistance to tumour formation when exposed to carcinogens, presenting itself as an ideal candidate for both cancer research and anti-aging studies (Suleiman et al 2019). The process of regeneration relies on the plasticity of cells to recreate the missing structure; therefore, the Axolotl provides scientists with a textbook model of phenotypic plasticity (Galliot & Ghila 2010). Another example of phenotypic plasticity in the axolotl is its ability to camouflage. Although limited, this species can modify the size and thickness of their melanophores, resulting in a change of skin colouration (Villue-Reyndas et al 2021). This is most noticeable in juveniles or individuals in the larval stage, who use this ability to hide from potential predators. As Axolotls age, they tend to become darker and greener in colour, resembling the vegetation in their lake habitats (Björklund & Duhon 1997).

However, it’s not just the biology of the Axolotl that makes it so unique. Being endemic to just three lakes in Mexico, establishes this species as a habitat specialist. The country itself is one of 12 ‘mega diverse’ countries in the world, containing a huge variety of environments and microclimates that promote biodiversity (Mendoza-Ponce et al., 2020). The remaining habitat of the Axolotl, Lakes Xochimilco, Chalco and Chapultepe, were once part of a vast wetland, offering the species an environment rich in plant diversity, abundant food sources and favourable, stable conditions (Griffiths et al 2004). Like most Salamanders, Axolotl are carnivorous and are generalist foragers, with a diet consisting of small fish, zooplankton and a range of insect species. They are ambush predators, lying in wait for their food to swim by and use a suction technique to pull it into their mouths (Chaparro-Herrera et al 2011).

Axolotls reproduce in the same way as the rest of the Ambystoma genus. Courtship begins with ‘waltz behaviour’ and then develops into the ‘Hula Dance’ (Eisthen & Park 2005). The ‘Hula Dance’ is comparable to the mating display of male newts, Notopthalmus viridescens, reflecting the generalist courtship and reproduction of the axolotl species. The male then proceeds to lay spermatophore on the substrate, which is picked up by the female through her Cloaca to fertilise the eggs. Once she has finished picking up all the spermatophores, the female will then lay the fertilised eggs on vegetation, with an average of 300-500 eggs laid in a single spawn (Khattak et al 2014).

To reproduce, most amphibians undergo metamorphosis to attain sexual maturity. However, some Mexican Salamander populations have evolved to become paedomorphic. Paedomorphism is a process in which a species retains its juvenile traits throughout its lifespan. Due to a lack of thyroid hormone, Axolotls avoid the process of metamorphosis, remaining in their larval stage and their aquatic habitat, see below image (Crowner et al 2019). This species is also neotenic, a type of paedomorphism that slows down the development of somatic tissues but doesn’t affect reproduction, resulting in a species reaching sexual maturity whilst still in its juvenile state (Demircan et al 2016). Retaining gills and avoiding terrestrial life is a unique form of adaptive evolution. However, some possible disadvantages to paedomorphism include increased population isolation, reduced geographical range and potential for reduced genetic diversity (Bonett et al 2014). Studies have shown that stable and abundant aquatic habitats promote paedomorphism, whereas unstable, scarce habitats promote metamorphosis. Therefore, evolutionary models support the theory that species such as the Axolotl, evolved to become paedomorphic once their aquatic environment was more advantageous than the terrestrial (Percino-Daniel et al 2016).

Figure 2. An example of how the presence of a Thyroid Hormone affects Salamander Paedomorphosis (Johnson & Voss, 2013)

The Axolotl is the only species to have developed compulsory ontogenetic paedomorphism through evolution (De Groef et al 2018). For the Eastern Tiger Salamander, paedomorphism is present in some populations yet not completely obligatory. It seems that for this species, metamorphosis and paedomorphism are dependent on the quality and stability of the aquatic habitats the populations occupy (McCarthy et al 2017). This data suggests that the obligatory paedomorphic evolution of the axolotl may be significantly damaging as their endemic habitat becomes more vulnerable.

Comparing members of the Ambystoma family, allows scientists to locate the main drivers for species vulnerability (Kloetstra, 2023). One species of Ambystoma that are currently labelled as ‘least concern’ is the Eastern Tiger Salamander, Ambystoma tigrinum (IUCN 2024). This species has an expansive geographical distribution with populations throughout Mexico, the USA and Canada, inhabiting many ponds, and lakes (Everson et al 2021). However, the Axolotl is much more restricted in its habitat selection. Being a microendemic salamander, Axolotl are confined to their ecological niche in the Xochimilco region, making extinction more likely through habitat degradation and stochastic events (Contreras et al 2009).

However, it’s not just species-specific characteristics of the axolotl that make them vulnerable to extinction. Most of the threats faced by this species are due to human activity. Whilst Axolotl did hold a prominent position in Aztec mythology, the native Mexican civilisation were known to hunt, trap, and consume this species due to their abundance in the basin lake system (Tate 2010). With the arrival of modern industrial processes, the water quality of Lake Xochimilco is becoming increasingly toxic, with chemical runoff from farmland and industrial effluent resulting in increasing mortality rates and morphological abnormalities in resident aquatic wildlife (Robles-Mendoza et al 2009). To further exacerbate the situation, the introduction of invasive non-native species, such as the Nile Tilapia Oreochromis niloticus, pose further threats, as they prey on Axolotl eggs and small juveniles (Zambrano et al., 2010). Furthermore, the addition of these invasive species causes an increase in competition for resources and pose a huge threat for the Axolotl, who’s paedomorphic evolution was established in a previously predator free habitat (Alcaraz et al 2015). Perhaps the biggest threat Axolotl face overall, is urbanisation and the rapid expansion of Mexico City, causing habitat degradation and fragmentation (Zambrano et al 2020). The result of these combined pressures is a significant reduction in Axolotl numbers, with less than 35 individuals per square kilometre remaining, see below for a map of Mexico City expansion and development (Vance 2017).

Figure 3. A map showing the southern lakes of Mexico City, Xochimilco and Chalco, with selected natural and cultural features against the modern landscape (Cordova, 2022, p.165).

There are many drivers pushing the Axolot species closer to extinction. Investigations into the drivers of extinction risk reveal that species geographical range size was the best predictor of extinction, with specialists facing a greater threat of extinction than generalists (Chichorro et al 2019). Comparing Axolotl against these findings reveal many areas for extinction vulnerability. However, most of these drivers are human based, promoting the theory that were human pressures not involved, this species would not be at threat of extinction. Although, their biological endemism does make them more vulnerable to natural events, such as volcanic activity, that would occur regardless of human presence. New data generates bleak predictions for the future of our global flora and fauna, but current global horizon scans are addressing some threats faced by the Axolotl, such as chemical toxicity, which may help to relieve some of the pressure on this species (Sutherland et al 2024).

To conclude, the Axolotl is an excellent example of a highly specialised species facing extinction as a direct result of human pressures. Whilst their captive population continues to thrive, it’s vital to protect their wild counterparts to preserve biodiversity in what remains of the Mexican river basin. As the impact of climate change and man-made pressures increases globally, the Axolotl serves as an important reminder that not all species may be able to evolve in time to survive an increasingly human dominated world.

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• Figures

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  3. Cordova, C.E. (2022) ‘A geographic sketch of the Historic Lakes’, The Lakes of the Basin of Mexico, 1(1), p. 165. doi:10.1007/978-3-031-12733-5_7. 

     Cover Image: Röhl, R. (2015) ‘Axolotl Overkill’, Flickr, https://www.flickr.com/photos/127957725@N08/16820055642