Modeling the Growth of Laboratory and Farm Animals: Part 2. Polytocus Animals and the Relationship of Birth Weight, Gestation Time, and Mature Size

Abstract

<abstract> <bold><sc>Abstract.</sc></bold> This article presents an analysis of the growth for three polytocus species (mice, cats, and dogs) and the development of a procedure to determine the mature size of individual animals based on the parameters of birth weight and gestation time. Previously (part 1 of this article), it was determined that the mature size of three monotocus species (sheep, cattle, and horses) could be reasonably estimated by using their birth parameters. The growth curves were normalized, and a term designated as the birth ratio (gestation time divided by the fraction of maturity at birth) was formed for each animal. The birth ratio was found to be similar for the three monotocus species. However, polytocus species, which produce offspring in litters, first require establishing the fraction of mature size for the litter (Fb_L) before determining that same value for each individual animal. The litter birth ratio was found to be very similar within a given species, indicating that the fraction of maturity at birth for the litter is an important factor in determining the length of gestation for polytocus animals. Two relationships were developed: one for predicting the litter fraction of maturity (Fb_L) as a function of litter weight, and one for predicting the fraction of maturity for the individual animal (Fb_i) as a function of Fb_L and the length of gestation. Once Fb_i was determined, an estimate of the animal’s mature weight can be obtained using birth weight and gestation time. Examples are shown indicating the efficacy of the predictions compared to the values of mature size found from the growth curve regressions.