A Two-Phase Model for Smoothly Joining Disparate Growth Phases in the Macropodid Thylogale billardierii

Abstract

Generally, sigmoid curves are used to describe the growth of animals over their lifetime. However, because growth rates often differ over an animal's lifetime a single curve may not accurately capture the growth. Broken-stick models constrained to pass through a common point have been proposed to describe the different growth phases, but these are often unsatisfactory because essentially there are still two functions that describe the lifetime growth. To provide a single, converged model to age animals with disparate growth phases we developed a smoothly joining two-phase nonlinear function (SJ2P), tailored to provide a more accurate description of lifetime growth of the macropod, the Tasmanian pademelon Thylogale billardierii. The model consists of the Verhulst logistic function, which describes pouch-phase growth – joining smoothly to the Brody function, which describes post-pouch growth. Results from the model demonstrate that male pademelons grew faster and bigger than females. Our approach provides a practical means of ageing wild pademelons for life history studies but given the high variability of the data used to parametrise the second growth phase of the model, the accuracy of ageing of post-weaned animals is low: accuracy might be improved with collection of longitudinal growth data. This study provides a unique, first robust method that can be used to characterise growth over the lifespan of pademelons. The development of this method is relevant to collecting age-specific vital rates from commonly used wildlife management practices to provide crucial insights into the demographic behaviour of animal populations.

Temperature‐related variation in growth rate, size, maturation and life span in a marine herbivorous fish over a latitudinal gradient

“…The rVBGF function was fitted to observe size‐at‐age data of O. pullus and N. fucicola at each location following a smooth‐joining two‐phase (SJ2P) model approach and using the rVBGF function for both phases of growth (McMahon et al . ) (see Appendix S1, Supporting Information). To construct the SJ2P, the two growth phases are required to have the same slope at age Critage (the age at which growth switches from one phase to the other).…”

Using Spatio - Temporal Modelling as a Decision Support Tool for Management of a Native Pest Herbivore

“…An additional factor that offers to strengthen the selection of simulated density reduction targets is the estimation or quantification of damage intensity resulting from various densities of the target pest population. While the measurement of this factor was outside the original scope of our empirical data collection (see Wiggins et al, 2010;McMahon et al, 2011), upon collection this information can be inputted into STAR for the continued updating of study-specific features and the resulting improvement of model output interpretation (McMahon et al, 2010).…”