| الوصف: |
Metabolism is a fundamental process of life that fuels vital biological processes including growth. The rates of metabolism and growth often correlate with other biological and ecological traits, including body size, in distinct ways. Thus, understanding variation in the body mass-scaling of growth and metabolic rate is an important area of research when studying the ecology, evolution and life histories of organisms. The overall aim of this thesis is to improve predictions of animal growth and metabolic rates, and to explain variation in these processes. Many methods for estimating individual growth rates (rate of mass increase over time) impose invalid assumptions, such as isomorphic (shape-invariant) growth. This thesis proposes a new growth curve fitting framework that relaxes the assumption of isomorphy and can capture marked diversity of growth curves, including exponential and supraexponential body mass change. Furthermore, because growth is fuelled by metabolism, the mass-scaling exponent of growth (A) and the mass-scaling exponent of metabolic rate (b_R) are predicted to positively correlate. This was explored across pelagic invertebrate species and within two oligochaete species over ontogeny. No significant relationship between A and b_R was found, suggesting organisms may differ in their proportion of metabolised energy allocated to growth and to other processes, such as locomotion, over ontogeny. In addition, I explored the relationship between A and known predictors of b_R: (i) the mass-scaling of body surface area (b_A), which may capture changes in surface area-mediated resource uptake over ontogeny for integument breathing organisms, and (ii) ambient temperature, which often correlates with body size at maturity in ectotherms and may correlate with b_R by influencing the energetic demand of locomotion over ontogeny. No significant correlations between A and b_A, or between A and temperature, were found for pelagic invertebrate species or two oligochaete species, suggesting that the rates of growth and metabolism may differ in their response to different intrinsic and extrinsic factors. To improve current understanding of the variation in metabolic rate, I explored potential predictors of b_R for pelagic invertebrate species and two oligochaete species (b_A), and mammal species (ambient temperature and reproductive parity). Ambient temperature, but not reproductive parity, was shown to be a predictor of variation in basal metabolic rate responsible for curvature patterns across mammals. A significant correlation between b_R and b_A was found for an aquatic, but not a terrestrial oligochaete species or diverse species of pelagic invertebrates. A positive relationship between b_R and b_A suggests surface area-mediated changes in resource uptake over ontogeny may be shaping metabolic scaling relationships within an aquatic oligochaete. Overall, this research highlights the importance of considering both intrinsic (e.g. body shape and size) and extrinsic factors (e.g. ambient temperature) when exploring variation in the scaling of growth and metabolic rate. Ultimately, this perspective differs from previous approaches that focus on a single-cause mechanistic explanation or universal law; rather this thesis applies a multi-mechanistic approach by considering multiple correlates, theories and mechanisms to provide a more comprehensive understanding of the diversity in metabolic and growth scaling relationships. |
