Acta Veterinaria Scandinavica 2008, 50:41 http://www.actavetscand.com/content/50/1/41 [16], boar (from 2.93 for rugosity or shape factor 1 to 9.38 for elongation) [20], but lower than those of the Cynomolgus monkey (from 2.90 for shape factor 1 to 16.39 for ellipticity) [42], or alpaca (from 4.7 for shape factor 1 to 17.8 for ellipticity) [18]. Between-animal group percentage CVs were higher in the sperm of T group animals (from 11.2 for elongation to 32.0 for shape factor 3) than in F group stallions (from 1.6 for perimeter to 26.1 for shape factor 1). Identification of individual animals might be possible if one focuses on those parameters that have low within-animal and high between-animal CVs. The literature suggest that suitable parameters for other species might include perimeter (5.42 versus 35.45) and shape factor 1 (7.33 versus 36.98) for rams [16], and perimeter (2.69 versus 14.43), shape factor 1 (rugosity; 2.93 versus 26.26) and shape factor 3 (regularity; 2.45 versus 16.31) for boars [20]. Meanwhile, our study suggested that perimeter (2.9 versus 19.2), shape factor 1 (2.6 versus 25.8) and shape factor 3 (3.8 versus 32.0) for T and shape factor 1 (2.9 versus 26.1) for F sperm were suitable parameters. The crossbred genetic background may result in increased between-animal sperm dimensional variability as compared with purebred groups. Conclusion The results presented here indicate that the variability in percentages of normal and abnormal morphological char- acteristics of sperm in individual Thai native crossbred stallions was similar to that of purebred stallions. Further- more, the morphometric characteristics of normal sperm heads also varied substantially between stallions, with the sperm heads of Thai native crossbred stallions being larger and rounder than those of purebred stallions. Perimeter, shape factor 1 and shape factor 3 were identified as parameters that could potentially be used as a means of identifying individual T stallions. Competing interests The authors declare that they have no competing interests. Acknowledgements We would like to thank Assistant Professor Dr. Pariwat Poolperm for his critical comments and edits to this manuscript. We would like to thank the Department of Veterinary and Remount, Royal Thai Army, Kanchanaburi province and the Thai Horse Club, Saraburi province, for their management of the experimental animals. We also would like to thank Ms Sudarat Amornsak and Ms Piyawan Suthanmapinanh for their assistance with equip- ment and logistics. 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