The plasticity of life histories during larval development and metamorphosis, using amphibians as study organisms

The ability of animals to vary growth, development rate and behaviour in response to environmental conditions has been well documented, particularly during the larval phase in animals with complex life cycles. The evolution and maintenance of plasticity in response to environmental conditions is likely to be adaptive in animals that face unpredictable environments. However, there are two aspects of life histories in animals with complex life cycles, which would be expected to favour plasticity, that have received limited attention: traits during metamorphic climax and variation in the life history phase at which temperate species spend the winter. Therefore the aims of this thesis were to consider the environmental factors that are likely to result in plasticity in the timing and duration of metamorphic climax and contribute to variation in the over-wintering life stage, using amphibians as study animals. To assess the ability of animals to respond to environmental conditions during metamorphic climax conditions were manipulated during metamorphosis independent of larval treatment. Accordingly all larvae entered metamorphic climax having experienced the same conditions. The African clawed toad, Xenopus laevis, was used. I examined the influence of environmental temperature, predation risk and starting body size on several traits during the transitional stage (e.g. mass, snout-vent length (SVL), head width, tail morphology, duration and locomotor performance). Morphological measures and the duration of the life stage were shown to vary with temperature and predation risk. As predicted, higher temperatures and the risk of predation resulted in faster development through metamorphosis and smaller sizes on completion. The acceleration of metamorphosis was demonstrated to have potential costs, not in the form of reduced locomotor performance as predicted, but in a reduction in juvenile size as a result of faster metamorphic development. This suggests that, during this potentially vulnerable stage, it would be advantageous to take more time to complete in the absence of predators. Greater body size at the onset of metamorphosis requires a longer time to complete metamorphic climax suggesting that having a greater quantity of tissue to reconfigure during metamorphosis takes more time. Therefore, the conditions experienced during metamorphosis may have important implications for juvenile fitness and should be considered in studies of life history plasticity. In many temperate species with complex life cycles, the life history stage at which a species can survive the winter is generally fixed, imposing time limits on the timing of development. Most of these species must therefore often modify developmental rate to reach the appropriate stage or size at the onset of winter, usually at a cost to other traits. However, variation in the stage or developmental group that some amphibian, fish and insect species spend the winter has been observed, such as in the common frog Rana temporaria in the UK, which can spend the first winter as either a tadpole or as a juvenile frog. To investigate the factors that contribute to this variation in life history, I examined the influence of environmental temperature, food availability and water depth on the rate of larval development and growth. Data on development, growth and environmental temperature of a field population of R. temporaria, which have been observed to over-winter as larvae, were collected to determine how and when the two divergent early life history patterns of development were established. Development rate was slowed by reduced temperatures and food availability and greater water depth during rearing. Temperature and food availability also had a significant impact on the proportion of larvae that over-wintered, but in the field other factors are likely to contribute to the within-population variation in wintering strategy. While a greater water depth did prolong larval development, as predicted, this does not appear to be due to the cost of surfacing to respire acting as a constraint on development, since a similar slowing in development was observed in the lung-less Bufo bufo tadpoles. The results of these studies did not allow a definitive assessment of whether over-wintering as larvae represents an adaptive strategy or occurs as the result of developmental constraints. There is some evidence that over-wintering as larvae might be adaptive, since on completion of metamorphosis individuals that wintered as larvae were larger than those that completed metamorphosis late in the summer. Further work is necessary to identify other factors contributing to the over-wintering of larvae in Rana temporaria and to determine the adaptive significance, if any, of the alternative life history patterns.