Nonlinear Models to Describe the Testicular Size Growth Curve of Mangalarga Marchador Stallions.

This research aimed to chart the testicular growth curve in Mangalarga Marchador stallions by applying nonlinear models to data relating to the total width of the scrotum and the volume of the testicles. The study found that different growth stages followed separate mathematical models and also established a moderate correlation between the size of the animal and these testicular traits.

Study Execution

• The researchers carried out a cross-sectional study involving 120 Mangalarga Marchador stallions aged between 1.75 years and 21.93 years.
• They gathered data on the total scrotal width (TSW) and testicular volume (TV) of the stallions.
• The parameters of ten different nonlinear models were assessed using an iterative mathematical process.

Assessing Model Fit

• Each model’s “goodness of fit” to the gathered data was evaluated using several statistical criteria, including both Bayesian and Corrected Akaike’s Information Criterion, error sum of squares, and various predictive error measurements.
• The Logistic I model was found to best represent TSW growth, whilst the Logistic II model was the best fit for TV growth.

Key Findings

• The researchers found that the ‘inflection point’ of TSW growth (when the rate of growth changes direction), was estimated at 5.23 cm after 0.76 years. In contrast, the inflection point for TV growth curve was found at a much later stage (139.36 cm after 2.57 years).
• An intriguing discovery was that the TV growth curve plateaued later than the TSW growth curve. The reason suggested was that the height of the testicles continues growing for an extended period.
• Moderate and positive correlations were found between the height of the horse and both the TSW and TV measurements.

Implications of the Research

• The researchers inferred that TV is a better overall indicator of sexual maturity in these horses than indicators of early sexual development.
• The findings suggest that traits related to testicular size could be important factors to include in selection programs for this breed of horse. These traits appear to have similar relevance to more conventional traits related to physical size.