Evolutionary aspects of tail shedding in lizards and their relatives

Summary

The ability to shed (autotomize) all or part of the tail, usually in response to predator attack, and often to subsequently regenerate it is widespread in lizards and amphisbaenians and also occurs in a few snakes and in the tuatara. Most species possess a sophisticated intravertebral autotomy mechanism which seems to be primitive in the Squamata. This appears to have been independently lost in members of many groups, but some agamids and snakes have regained the ability to shed their tails by a simpler intervertebral means and a number of agamids have also redeveloped tail regeneration as well.

Breakable tails are used to evade capture in two main ways: by enabling reptiles to break away from predators that have grasped them by the tail and by providing a distraction which deflects the attention of the attacker away from the vulnerable head and body. It is argued that loss of caudal autotomy has occurred when the costs of tail shedding outweigh its benefits. Likely costs include the expense of regrowing the tail and the loss of a variety of possible tail functions that may cause partial incapacitation, at least until the tail regenerates. Benefits of autotomy are liable to be low if predation is rare, if the animal is able to protect itself effectively in other ways, if it is too slow to evade further pursuit after the tail is shed, or if the tail is small or unpalatable and consequently not likely to distract a predator. Benefit variation may well be greater than cost variation and therefore more important in initiating the loss of autotomy mechanisms. Many taxa that do not shed the tail appear to conform to the above interpretation, but in some cases, such as the Platynota, Agamidae and Chamaeleonidae, lack of intravertebral autotomy may reflect the history of these groups rather than being a direct result of present ecological pressures. The distribution of intervertebral autotomy in the Agamidae suggests that it may have evolved only in rather special circumstances where tail fragility is advantageous even in the absence of the ability to regenerate.

Restriction of autotomy planes to the tail-base, so that the whole organ is lost, a condition found in a number of relatively slow-moving geckoes, is interpreted as a means of ensuring that enough of the tail is shed to distract a predator from further pursuit. The stimulus necessary to induce autotomy can vary rapidly in individual lizards and at least some of these changes probably maximize the effectiveness of the tail-shedding mechanism. Differences in the readiness with which all or part of the tail is shed exist between species and are likely to reflect the balance of costs and benefits in particular cases. Variations in incidence of broken tails between species and populations may be due to such differences in fragility but many other factors may play a part, including the age structure of samples, incidence of unsuccessful attacks by predators and ability to evade predators after autotomy. There is a clear tendency for climbing lizards, especially those living on rock surfaces, to have higher incidences of broken tails than ground-dwelling species, perhaps because the tail is usually less important in locomotion in the first group. Many lizards possess conspicuously coloured tails and tail movements that seem likely to help distract attention from the head and body. Conspicuous tail colouring is more frequent and often better developed in young animals, which tend to be more vulnerable than adults, and in active species from open habitats where crypsis may not always be very effective. Conspicuous tails usually have contrasting light and dark areas in nocturnal forms but are often a single bright colour in diurnal ones, probably reflecting the visual capacities of their respective predators. The predominance of blue tails in day-active species may be because this colour is striking close to but not very arresting at a distance, so it may not attract predators from far away while still drawing their attention at close quarters.