An axolotl is a larva that never stopped. It reaches adult size, it breeds, and it lives out its life still wearing the body it hatched with — gills, tail fin and all. Biologists call the condition neoteny (Rosenkilde & Ussing, 1996). For an animator it means something more specific: you are rigging an animal that never committed to land or to water, and that indecision is visible in every frame.

Two consequences settle the rig before you place a single bone.

One spine has to do two jobs. Salamanders drive terrestrial stepping with a standing wave in the trunk and swimming with a travelling wave — same axial muscles, same spinal circuit. What changes is the phase relationship along the body, not the anatomy (Delvolvé et al., 1997; Ijspeert et al., 2007). Build one spine and switch the wave. Do not build two rigs.

Six plumes sit in the middle of the silhouette and never stop. The external gills are not under moment-to-moment control: they drift, they trail, they get flicked. They are the largest single source of secondary motion on the character, and the fastest way to make it read as alive — or as a toy.

The model

Drag to turn the animal. Each point of light opens a note. The points hide themselves when they rotate to the far side, so what you see is always what faces you.

Nine landmarks, three systems

Colour groups the anatomy by what it does, not by where it sits. Every one of these has a consequence for the rig.

• Respiration

  • External gills. Three pairs of branched plumes arising from branchial arches III to V. Their fimbriae are packed with capillaries and take oxygen straight from the water, which is why they read red.
  • Rachis & branchial arch. The fleshy stalk that carries each plume. Inside it run the cartilaginous arch and the vessels that distribute blood to the fimbriae.
  • Gular fold. A fold of skin under the throat. Its buccal pumping drives water over the gills without the animal having to swim, and it also lets it gulp air at the surface: an axolotl breathes through gills, skin and lungs at once.

• Senses & feeding

  • Lidless eye. The axolotl keeps its larval eye: small, dark and without eyelids. It reads movement and contrast rather than detail.
  • Snout & suction mouth. A wide, flattened mouth. Snapping it open creates negative pressure that drags prey in whole; the tiny vomerine teeth only grip it.

• Locomotion & regeneration

  • Forelimb — 4 digits. Four clawless digits. This is the classic regeneration model: after amputation the axolotl forms a blastema and rebuilds bone, muscle, nerve and skin in the right order, leaving no scar.
  • Hindlimb — 5 digits. Five digits, one more than the hand – a normal asymmetry among salamanders. It braces against the lake bed rather than propelling: the axolotl walks slowly and swims with its tail.
  • Dorsal fin. A skin fold running from the back to the tail. A larval trait retained through neoteny, it works as a keel: it steadies the body and stops it rolling as the animal undulates.
  • Tail fin. A laterally flattened tail edged with fin. It produces all the thrust through side-to-side undulation, and its muscle stores fat as an energy reserve.

Seven rig strategies

In the order you would actually build them.

1. One spine, two wave modes

   Run a single spline-IK chain of 10–14 joints from the atlas to the tail tip. Drive lateral bend with a sine and expose phase offset per joint as the control. Near zero gives you the standing wave for walking; a constant offset per joint gives you the travelling wave for swimming, and sweeping between them is the aquatic-to-terrestrial transition itself (Ijspeert et al., 2007).

2. Sprawling limbs, outward poles, lateral sequence

   The humerus and femur project sideways, not underneath, so elbows and knees break outward — put IK pole targets out to the side or the limbs will pop. Footfall order is lateral sequence: each hind foot lands before the fore foot of the same side. Duty factor is high; this animal walks, it does not trot (Ashley-Ross, 1994).

3. Rig the gill stalks, simulate the filaments

   Three rami per side, from branchial arches III–V. Give each ramus a 3–4 bone chain and stop there. Never hand-key the fimbriae — use spring bones on the chain with the filaments parented, or a cached cloth pass. Then author one gill flick: the animal really does snap its plumes to flush the stale water trapped between the filaments, and it is the most readable idle action you have.

4. The head is a pump, not a mouth

   Ambystomatid feeding is suction. The gape and the floor of the mouth drop together, pressure falls, and the prey arrives whole; the vomerine teeth only grip (Lauder & Shaffer, 1985; Deban & Wake, 2000). So rig a jaw and a gular bone and drive both from one strike control. The event is measured in tens of milliseconds — on twos that is two to four frames. Budget your face effort here; nothing else in it moves.

5. No eyelids, so no blink system

   The larval eye is lidless: submerged, the cornea needs no wiper. That deletes an entire facial subsystem — no blink, no lid follow, no lashes. What is left is a small, near-black, highly specular bead. Spend the saved time on the catchlight, because it is the only thing in that face that will read as alive.

6. Buoyancy over weight

   In water, gravity is nearly cancelled. Curves want slow-in, slow-out and drift, not the snap-and-settle of a terrestrial rig. On the substrate the same animal is a walker: limbs brace, belly rides low, tail drags. Animating the handover between the two states is a shot in its own right (Ashley-Ross & Bechtel, 2004).

7. Order your follow-through

   Delay, fastest and largest first: gill rami, then tail fin, then toes. Offset each by two to four frames from the spine that drives them. If everything moves on the same frame the animal reads as moulded plastic, no matter how good the model is.

The clip list

The set worth building, in the order that de-risks the rig. Frame counts assume 24 fps. None of these are animated yet — this page will carry them as they are finished.

A note on the evidence

References

1. Ashley-Ross, M. A. (1994). Hindlimb kinematics during terrestrial locomotion in a salamander (Dicamptodon tenebrosus). Journal of Experimental Biology, 193(1), 255–283.
2. Ashley-Ross, M. A., & Bechtel, B. F. (2004). Kinematics of the transition between aquatic and terrestrial locomotion in the newt Taricha torosa. Journal of Experimental Biology, 207(3), 461–474.
3. Contreras, V., Martínez-Meyer, E., Valiente, E., & Zambrano, L. (2009). Recent decline and potential distribution in the last remnant area of the microendemic Mexican axolotl (Ambystoma mexicanum). Biological Conservation, 142(12), 2881–2885.
4. Deban, S. M., & Wake, D. B. (2000). Aquatic feeding in salamanders. In K. Schwenk (Ed.), Feeding: Form, function and evolution in tetrapod vertebrates (pp. 65–94). Academic Press.
5. Delvolvé, I., Bem, T., & Cabelguen, J.-M. (1997). Epaxial and limb muscle activity during swimming and terrestrial stepping in the adult newt, Pleurodeles waltl. Journal of Neurophysiology, 78(2), 638–650.
6. Ijspeert, A. J., Crespi, A., Ryczko, D., & Cabelguen, J.-M. (2007). From swimming to walking with a salamander robot driven by a spinal cord model. Science, 315(5817), 1416–1420.
7. Lauder, G. V., & Shaffer, H. B. (1985). Functional morphology of the feeding mechanism in aquatic ambystomatid salamanders. Journal of Morphology, 185(3), 297–326.
8. Northcutt, R. G., Catania, K. C., & Criley, B. B. (1994). Development of lateral line organs in the axolotl. Journal of Comparative Neurology, 340(4), 480–514.
9. Rosenkilde, P., & Ussing, A. P. (1996). What mechanisms control neoteny and regulate induced metamorphosis in urodeles? International Journal of Developmental Biology, 40(4), 665–673.
10. Tanaka, E. M. (2016). The molecular and cellular choreography of appendage regeneration. Cell, 165(7), 1598–1608.
11. Voss, S. R., Epperlein, H. H., & Tanaka, E. M. (2009). Ambystoma mexicanum, the axolotl: A versatile amphibian model for regeneration, development, and evolution studies. Cold Spring Harbor Protocols, 2009(8), pdb.emo128.
12. Zambrano, L., Mosig Reidl, P., McKay, J., Griffiths, R., Shaffer, B., Flores-Villela, O., Parra-Olea, G., & Wake, D. (2010). Ambystoma mexicanum. The IUCN Red List of Threatened Species 2010, e.T1095A3229615.

Model, renders and interactive viewer built in Blender for animalanimator.com. Anatomy checked against the sources above.