A New Mechanistic Model for Individual Growth Applied to Insects under
Ad Libitum Conditions
Abstract
Metabolic theories in ecology interpret ecological patterns at different
levels through the lens of metabolism, typically applying allometric
power scaling laws to describe rates of energy use. This requires a
sound theory for metabolism at the individual level. Commonly used
mechanistic growth models, such as von Bertalanffy, DEB and the
ontogenetic growth model lack a number of potentially important aspects
and fail to accurately capture a growth pattern often observed in
insects. Recently, a new model (MGM – the Maintenance-Growth Model) was
developed for ontogenetic and post-mature growth, based on an energy
balance that expresses growth as the net result of assimilation and
metabolic costs for maintenance and feeding. The most important
contributions of MGM are: 1) the division of maintenance costs into a
non-negotiable and a negotiable part, potentially resulting in
non-linear allometric scaling of maintenance and lowered maintenance
under food restriction; 2) differentiated energy allocation strategies
between sexes and 3) inclusion of costs for finding and processing food.
MGM may also account for effects of body composition and type of growth
at the cellular level. The model was here calibrated and evaluated using
empirical data from an experiment on house crickets growing under ad
libitum conditions. The procedure involved parameter estimations from
the literature and collected data, using statistical models to account
for individual variation in parameter values. It was found that
ingestion rates cannot be generally described by simple allometries,
here requiring more complex descriptions after maturation. By the
unusual assumption of super-linear scaling of maintenance with body
mass, MGM could well capture the differentiated growth patterns of male
and female crickets. Other mechanistic growth models have also been able
to provide good predictions of insect growth during early ontogeny, but
MGM seems to be unique in its ability to accurately describe the
trajectory until terminated growth.