'Costs' of caudal autotomy in the metallic skink, Niveoscincus metallicus

Caudal autotomy is a defensive mechanism utilised by squamate reptiles to survive predatory attacks. Although tail loss is an effective escape mechanism in lizards, autotomy may result in severe 'costs' being inflicted upon the animal. However, since the tail is capable of regeneration, these 'costs' may only be transient. An animal's ability to balance the costs and benefits of autotomy determines the adaptive advantage of tail loss. The locomotory, thermoregulatory, energetic, reproductive and behavioural 'costs' of caudal autotomy were investigated in the metallic skink, Niveoscincus metallicus. Niveoscincus metallicus is a small ground-dwelling skink that inhabits a wide range of microhabitats. It is a viviparous species that possesses a sinusoidal locomotory mode, and both caudal and abdominal fat reserves. High levels of tail loss are evident in most populations. Caudal autotomy was found to have two main impacts on N. metallicus: 1) restriction of mobility; and 2) depletion of energetic reserves. However, the species was found to possess several behavioural and anatomical modifications to limit the 'costs' incurred. Tail autotomy was found to severely restrict locomotory performance m N. metallicus. Terrestrial locomotion, sprinting and stamina, was found to be preferentially inhibited by caudal autotomy. Climbing ability was not affected by tail loss. In females, locomotory inhibition caused by caudal autotomy was relatively short-lived. However, in males there was no evidence of restoration of locomotory performance during this study. This was the first investigation of the temporal impact of caudal autotomy on performance in lizards. Behavioural modification was evident following caudal autotomy. Tailless N. metallicus were found to compensate for diminished locomotory abilities by selecting more cryptically located basking sites and remaining closer to refuge. This modification of basking preference was assumed to be the result of the animal adopting an alternative defensive strategy.Thermoregulatory behaviour was not modified following tail loss. However, females were found to lower their thermal preferences during gestation, presumably to enhance embryonic development. It was concluded that reproductive success represented higher priority than tail regeneration. Niveoscincus metallicus was demonstrated to store the majority ( -50-75%) of its energetic reserves in its tail. Most ( -90%) of these reserves were located within the proximal third of the tail. Depletion of abdominal fat stores was found to be related to periods of reproductive investment. There was some evidence that abdominal fat reserves were preferentially allocated to reproductive effort. Caudal autotomy during vitellogenesis was associated with a reduction in the clutch size of tailless females. This reduced reproductive investment was related to the diversion of energy towards tail regeneration rather than the direct depletion of caudal fat stores. These tailless females were found to modify their reproductive strategy by producing a smaller number of larger and 'better' quality offspring. However, tail autotomy during gestation did not influence the weight or size of offspring. Maternal tail size and the presumed environment during gestation was not related to the phenotype of a female's offspring. The frequency and position of tail loss were found to vary between altitudes and populations. Predation pressure or basking behaviour failed to explain these differences. Predation efficiency, frequency of repeated tail breaks and the age structure of the population all appeared to be related to the frequency and position of tail loss. Niveoscincus metallicus was found to be capable of 'economy of autotomy', preferentially losing the more distal portions of its tail that do not contain significant fat stores. It is concluded that caudal autotomy inflicts several costs on N. metallicus. However, the existence of behavioural and anatomical modifications limits the impacts of these 'costs'. This study suggests that the costs and benefits of caudal autotomy have coevolved in N. metallicus, allowing it retain a successful defensive mechanism while limiting the associated 'costs'. iii