To understand which morphological/anatomical parts may be responsible in artiodactyl ungulates for the clicking sound made when moving, this research focuses on the forelimb tendon apparatus where an undescribed opening in the fibrous cuff (manica flexoria), called hereafter for its shape as an "oval window" in the manica flexoria (OWMF), was detected. This oval window was found in 24 of the 25 species of four families (Camelidae, Giraffidae, Cervidae, and Bovidae) evaluated; the exception being in Bos taurus taurus (Domestic cattle). The length and width of the OWMF enabled correct species discrimination between the majority of species, but remained conservative intraspecifically, as it did not differ between the left and right side of the forelimb, third and fourth digits, or between sexes. When evaluating the shape of OWMF in individual species, and measuring its length and width, 18 out of the 24 species investigated had this window as an oval shape, the remaining 25% of species exhibited more oval-oblong shapes with either proximal or distal asymmetry. The function of the OWMF in the thoracic autopodium of most ruminant even-toed ungulates is not yet fully understood. Its most likely function is to help balance the pressure inside the ligament cuff and reduce the friction of the touching surfaces of the muscle tendons-thus facilitating the movement of the digits when walking. None of the absolute or relative OWMF parameters fit exclusively with the occurrence and distribution of knee-clicks produced by some bovids and cervids during movement, so the mechanism responsible for this sound remains cryptic from the present anatomical perspective.

Centre for Nutrition and Food Sciences Queensland Alliance for Agriculture and Food Innovation The University of Queensland Digital Agricultural Building 8115 Office 110 Gatton 4343 Australia

Department of Anatomy Histology and Embryology Faculty of Veterinary Medicine University of Veterinary Sciences Brno Palackého třída 1946 1 612 42 Brno Czech Republic

Department of Animal Science and Food Processing Faculty of Tropical AgriSciences Czech University of Life Sciences Prague Kamýcká 129 165 00 Praha 6 Suchdol Czech Republic

Department of Animal Sciences University of Stellenbosch Matieland Private Bag X1 Stellenbosch 7602 South Africa

Department of Ethology Institute of Animal Science 104 00 Prague 10 Uhříněves Czech Republic

Department of Zoology Faculty of Science University of South Bohemia Branišovská 1760 370 05 České Budějovice Czech Republic

Jiří Orten Grammar School Jaselská 932 284 80 Kutná Hora Czech Republic

Liberec Zoo Lidové sady 425 1 460 01 Liberec Czech Republic

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