“Axolotls” are problematic: Dicamptodon as a short case study of neoteny in salamanders
A Tumblr-specific pet peeve set me off and inspired me to spread some propaganda for how cool Dicamptodon giant salamanders are, so yea. Sorry.
I understand that it is a bit classist and snobbish to be pedantic about technical jargon; it can constitute un-cool gatekeeping to criticize the vernacular language of beginning naturalists. So, I’m just being half-serious for the sake of a fun salamander discussion.
Tumblr pet peeve: Sharing pictures of any species of larval or neotenic salamander and calling them all an “axolotl” whether or not they’re actually an axolotl (Ambystoma mexicanum).
So “axolotls” are really popular on Tumblr.
Mole salamanders - the genus Ambystoma, North American salamanders with adaptations to live in desert and prairie habitats - all have pretty physically-similar larvae, but the axolotl is a singular, unique species. The term people are often looking for to describe the gilled salamanders in these popular images on Tumblr is either larval (if the animal is young and sexually immature) or neotenic/paedomorphic (meaning it’s a sexually-mature adult that has retained a larval body-type rather than transforming into a terrestrial form). It is less problematic to confuse other Ambystoma species with axolotls, since they’re in the same genus and closely related. But when pictures of mudpuppies, American cave salamanders, or olms are shared … no excuse for calling those girls “axolotl.”
Most salamander species have a young larval stage with gills and a paddle-tail that look superficially similar to axolotls. But they’re not axolotls.
Similarly, “mudpuppy” and “waterdog” are not catch-all terms, but also refer to a a couple of species in the genus Necturus. No:
Not an axolotl, but closely related.This is an eastern tiger salamander (Ambystoma tigrinum). No:
Not an axolotl, but closely related. This is a blotched tiger salamander (Ambystoma mavoritum melanosticum).
No:
Not an axolotl, but closely related. This is a common mole salamander (Ambystoma talpoideum). EXTRA-BIG NO:
Not even close! This is a northern red salamander (Pseudotriton ruber ruber). EXTRA-BIG NO:
Nope! She’s a Texas blind salamander (Eurycea rathbuni).
EXTRA-BIG NO:
No way! It’s the beloved olm (Proteus anguinus).
EXTRA-BIG NO:
Totally unrelated! This is a common mudpuppy (Necturus maculosus).
How about a case study in neoteny? The Dicamptodon species:
The terms “neotenic” and “paedomorphic” as descriptors of adult animals with larval bodies can basically be used interchangeably.
Neoteny can be common in salamander populations existing at: (1) … cold, high elevation areas (where living underwater allows the salamander to avoid the extremely cold temperature) (2) … extra-wet areas; environments with lots and lots of reliable year-round water (3) … or extra-dry areas with no moist habitat on land but with deep, totally permanent water Salamanders living in dry/arid environments can’t risk retaining a water-dependent larval body form because water sources are at risk of drying-up. However, sometimes arid environments DO incentivize retaining the gilled aquatic form, but only if the water body is permanent; as in, there might be no moist habitats on land in this arid place, so the salamanders can only live within the permanent water body, and thus will retain their larval form.
Paedomorphosis is common in many populations of tiger salamanders - quite closely related to axolotls.
This neoteny can also be common in other mostly-terrestrial salamander genera, like the Pacific giant salamanders - Dicamptodon, endemic to the rainforest of the Pacific Northwest.
These salamanders are extremely unique, relict species of a bygone era.
Here’s a couple of paedomorphic coastal giant salamander (Dicamptodon tenebrosus):
(Photo above by the talented David Herasimtschuck)
The Dicamptodon species are the largest terrestrial or land-dwelling salamanders in the world. They are also the only salamanders that can vocalize; they bark when alarmed! Adults - both terrestrial and paedomorphic - can reach total lengths of 28-35 centimeters or 12-14 inches.
The actual larvae of Dicamptodon don’t get that long or robust, but the two salamanders pictured above aren’t larvae; instead, they’re sexually-mature adults that have retained their gills. The temperate rainforests of the Pacific Northwest aren’t really at risk of suffering the drying of streams, so the salamanders can more often afford to genetically select for neoteny.
The terrestrial adults are also very pretty, featuring diverse arrays of gold, brown, and black mottling:
There are only 4 species of the remarkable rainforest-dwelling Dicamptodon salamanders. Though they often transform into terrestrial adults, one of the species - the Olympic or Cope’s giant salamander, Dicamptodon copei - almost never, ever transforms into a terrestrial adult. This is likely because the other 3 giant salamander species live in relatively drier forests, and the Cope’s giant lives in the much wetter forests of the Olympic Peninsula and St. Helens/Mt. Rainier rainforests. Thus, since there is relatively more water and moisture available year-round, it would rather stick to the streams full-time since there is less incentive for the Cope’s giant to transform into a land-dwelling salamander.
Other large terrestrial salamander species usually use still water to breed in, thus their larvae hang-out in ponds, lakes, or seasonal rainwater pools. But the Dicamptodon giant salamanders, quite uniquely, live in little mountain STREAMS. (A fact that I just love.)
How can 30-centimeter salamanders with gills find enough underwater space to remain submerged if the streams they inhabit are so small and shallow? Step-pool morphology, that’s how!
Dicamptodon prefers smaller, fast-flowing, cold, clear, steep streams in heavily-forest mountain areas with ample moss and decaying logs. These are the kind of streams that have little 1-meter waterfalls every few meters along the stream’s course. This is what we call “step-pool” morphology, when there’s (1) some fast-flowing running water, (2) then a waterfall, (3) then a nice still pool under the waterfall before the sequence starts over again. In this space, larger salamanders might dominate a two-meter wide pool under one waterfall; and then 5 meters downstream, there will be another waterfall with its own pool. These pools can allow the larger gilled salamanders to have ample space to move about, while smaller two-inch-long baby salamanders can have relatively still water so they’re not swept-away by the fast current. In the Pacific Northwest, these streams usually occur in cedar-hemlock forests with ferns, moss, and fungus covering the forest floor, providing space for terrestrial adults to crawl around unexposed to the sun or open air.
Since the climate in these rainforests is mild and temperate, moderated by moist Pacific air, the streams rarely freeze over. Meaning that there is incentive for the salamanders to often retain larval form. I mean, look how wet their habitat is:
So, consistently wet environments and reliable water can cause salamander populations, sometimes regardless of species, to select for neoteny.
In the case of the world-famous olm (pictured earlier in this post) and the several extremely rare species of Texas blind salamanders, the animals’ subterranean cave habitats are permanently inundated with water. Therefore, salamander species inhabiting caves are most often neotenic.
In other species that usually are expected to transform into terrestrial adults, neoteny often occurs as a response to either a dry/arid landscape or a high-elevation location.
In other words, if a tiger salamander population is living at 3,250 meters - which is pretty common - it is often advantageous for the sexually-mature adults to keep their larval physique and maintain a fully-aquatic lifestyle in the depths of a montane lake which avoids freezing-over.
This allows the tiger salamander to inhabit glacial lakes and cirques in alpine areas above the treeline throughout the Rocky Mountains. Despite the frigidly cold winters and excessive snowfall, tiger salamanders can continue to be active throughout the year because their gilled, larval bodies allow them to swim around in the depths of these high-altitude lakes.
Anyway.
Re-cap: – Axolotl is a single species of mole salamander, the “Mexican salamander” or Ambystoma mexicanum. – Other species in the Ambystoma genus have larvae that are similar to the axolotl’s adult form, but they most often will transform into terrestrial adults and lose the gills and paddle-tail. – When a salamander retains its gills and paddle-tail, but becomes sexually-mature and ceases to be a technical larva, this is called neoteny or paedomorphosis. – Other larval or neotenic salamanders outside of the genus Ambystoma are not closely related to axolotls. – Mudpuppies, sometimes known as waterdogs, are also distinct species unrelated to axolotls. – Neoteny occurs in salamanders living in (1) extremely wet areas; (2) high-elevation areas; or (3) permanent water bodies within arid areas.
Bonus: – Dicamptodon giant salamanders are great.
Not all neotenic salamanders are axolotls.
