Abstract
Compensatory growth is faster than optimal growth, and commonly occurs following dietary restriction in early development. This alternative growth strategy allows an animal to reach a "normal" adult size, influencing reproductive fitness. However, the rapid growth required to reach a large size after growth restriction comes at a cost of accumulated cellular damage. Compensatory growth has evolved as an alternative growth strategy because these costs are often incurred late in life, after the reproductive period. The aim of this thesis was to address the issue of compensatory growth on two levels: first, in an empirical study of one species, and second, in a quantitative review of numerous taxa.
I investigated the effects of early dietary restriction on the growth, development and morphology of Litoria ewingii tadpoles, as well as on three fitness-related behavioural traits. This is the first known study to follow the effects of compensatory growth in an amphibian beyond metamorphosis, and to simultaneously explore the effects of immune activation. Restricted tadpoles were fed half-rations for two weeks in early development, and tadpoles in half of each feeding treatment received an injection of phytohemagglutinin, PHA, a known immune-activating lectin. Dietary restriction prolonged the larval period of the tadpoles but resulted in larger, heavier frogs which were faster to capture prey and had increased survival. In contrast, immune activation caused high initial mortality but showed weak long-term effects. Whole-body corticosterone levels, as analysed by radioimmunoassay, were not affected by the dietary treatment. These results are unique for showing the rare effect of "over-compensation" and suggest dietary restriction is a stronger developmental influence than immune activation. The impact of compensatory growth on the post-metamorphic fitness of L. ewingii was contrary to theoretical expectations and may possess some value as an alternative conservation strategy for amphibians.
The quantitative review, the first in this field, clarified the terminology of compensatory growth and catch-up growth (achieving the same final size as controls) and was able to confirm both growth patterns as reliable, wide-spread responses to dietary restriction. Meta-analysis and meta-regression analysis techniques were used to conduct eight analyses of the size, growth slopes and fitness outcomes related to compensatory growth, based on data collected from 88 studies, spanning 11 taxonomic classes. Overall, animals experienced higher mortality and reduced reproductive output as a result of the dietary treatment. Taken together, the results of the quantitative review verified the basic assumptions of compensatory growth but also highlighted a number of aspects which could guide future research, such as the significance of diet protocols, appropriate fitness correlates and possible effect of age-dependent growth.
Compensatory growth is a broad field of research, ranging from the small-scale physiological mechanisms to the vast evolutionary perspective. This research has far-reaching implications, from human health to agriculture to evolutionary theory. In addressing two levels of this field, this thesis provides answers for previous gaps in knowledge. In addition, these results open up further avenues of research, which not only extend the field of compensatory growth, but also have real-world medical and economic applications.