Does size matter? Examining the possible mechanisms of multi-stallion groups in horse societies

In some feral horse populations, adult females are either associated with a single male or multiple males (stallions). However, little is known on why such groups with more than one male exist, considering that stallions fight to monopolize females. Body size is often an important determinant of male fighting ability and/or dominance rank and, consequently, reproductive success. Stallions may, therefore, vary in "quality" which could be a differentiating factor between single-stallion and multi-stallion males. We observed feral Garrano horses in Northern Portugal and examined the relationship between body size, sex, group type and number of females. Using a non-invasive laser distance meter, we determined individual body length and height by overlapping photos of the horses and a measuring tape. We tested the accuracy of remote measurements by comparison with manually measuring domestic horses and found no statistical differences between the two methods. Thus, we assume the values obtained were reliable and the methodology used was a useful tool for assessing morphological characteristics. Results showed no significant differences in body length and height for all parameters assessed, suggesting that size is not a determinant factor for feral horse society. Thus, other factors may be more important to explain the existence of single and multi-stallion groups.