Autotomy and Regeneration
Hulali Alford & Rose Driscoll
Reed College Biology 342 Fall 2015
How did the behavior evolve? Did it evolve once, or multiple times? Is it shared by closely related species?
There is incredible diversity in animals that practice autotomy, from sea cucumbers and squid to spiders, snails, crabs, and worms, and a wide range of lizards. Autotomy has repeatedly evolved, been lost, and re-evolved in various different forms.
1. In lizards, intravertebral autotomy (see Mechanism) appears to be the basal condition (Evans 1981, Arnold 1984). Caudal autotomy has been lost in some derived species, but has re-evolved in a subset of these in the form of intervertebral (see Mechanism) autotomy. The evolution and loss of tail autotomy in lizards is frequently associated with differences in costs of autotomy across species, but is also strongly influenced by differences in benefits (see Adaptive Value) of autotomy across species (Arnold 1984).
2. Related species of lizards show different population-wide autotomy frequencies as a result of different conditions (see Adaptive Value.) Autotomy, the basal condition, is more common than its absence; autotomy has been lost at different timepoints in different lineages. (Bateman & Fleming 2008).
Figure 1. Frequency of autotomy in 18 lizard families indicated by the proportion of individuals showing evidence of tail breakage or regeneration. Autotomy seems to have been lost at the family level among the Chamaeleonidae and the Xenosauridae, and at the intraorder level among the Platynota (Varanoidea.) (From Bateman & Fleming, 2008.)
Figure 2. Tail (caudal) vertebrae of ancestral lizard species Gephyrosaurus bridensis showing the location of intravertebral fracture planes (f. pl). (From Evans, 1981.)
4. In six closely-related lizard species living at a range of altitudes in Mexico, lizards experience higher predation at lower elevation. Predation level is associated with the force required to induce autotomy, once phylogenetic constraints (i.e., did this animal living at high elevation recently evolve from an animal living at a low elevation?) are accounted for (Fox et al 1994).
Figure 3. Phylogenetic tree of six closely-related lizard species living at different elevations and experiencing different levels of predation. Numbers above each species name represent the height of the regression of force to elicit tail autotomy versus tail thickness, a relative measure of the force required to induce autotomy. At high altitudes, there is low predation and much more force is required to induce autotomy. At low altitudes, there is high predation and much less force is required to induce autotomy. (From Fox et al, 1994.)