Do the determinants of dominance index and social rank in adult rams vary by breed?

Abstract

The aim of the present study was to determine factors influencing the establishment of dominance hierarchy in Hemsin, Chios and Karakul rams. Rams aged 6–7 years old from Hemsin (n = 13), Chios (n = 12) and Karakul (n = 14) sheep breeds were used. The feed competition test was applied to determine the dominance status of rams within their own breeds. After testing all possible pairs, dominance index and social ranks for each ram was calculated. Body weight, numerous body measurements, horn length and testosterone concentration were evaluated as predictor variables in stepwise regression analysis. Improved linearity index value was 0.522, 0.6224 and 0.7033 for Hemsin, Chios and Karakul rams (Pr<0.001), respectively. In Hemsin rams, the regression model including body weight and tail width explained 75.3% of the variation in dominance index (P = 0.001). The model, which included body weight and testosterone concentration, explained 68% of the variance and was the best model for prediction of the dominance index in Chios rams (P < 0.01). The model including rump width, tail width and linear horn length explained 79.5% of the variation in dominance index in Karakul rams (P = 0.001). When the principal component scores determined by principal component analysis were utilised as predictors in stepwise regression analysis, no variables were incorporated into the model for Karakul rams. Moreover, worse predictions were obtained with principal component scores compared to the analysis conducted with raw data in Hemşin and Chios rams. When the results of the current study are evaluated observationally, factors determining the dominance hierarchy vary by breed of rams.