Tel.+49 30 838 53555 Fax.+49 30 838-53480
The study was based on hooves of 33 Przewalski horses, in total. 16 of the horses were kept in zoological gardens, and 17 in semi-reserves. Employing light, transmission electron and scanning electron microscopy besides hardness measurements, the hoof horn quality of 15 horses was determined in circumscript regions of the hoof, i.e., in the dorsal area of the hoof, particularly at hoof capsule rings (grooves), and at the waste edge of the horn chip break-outs of the hoof. Thus, a self-regulatory mechanism for adjustment of the hoof length was established. Different parameters regarding the shape of the hoof were determined by morphometry. The shape of the hooves of wild horses was characterised by a medio-lateral asymmetry and a comparatively thick sole. At an shoulder height of approximately 135 cm, the hooves of the Przewalski horses displayed an average length of 8,42 cm in the dorsal area, and a length of 4,15 cm in the quarter of the hoof. Thus, the relation of hoof and body size was well-balanced in the wild horses. The relation of toe and quarter length was smaller in horses kept in semi- reserves compared to those kept in zoological gardens. In semi-reserves, an acute-angled hoof conformation in Przewalski horses was detected merely - with one exemption - as a transitional stage coupled with a long dorsal length of the hoof. During most of the year, the hooves displayed a tendency towards an obtuse angled shape. In horses kept in semi-reserves, a distinct seasonality of the hoof shape was detectable, depending on the rate of horn production and horn abrasion, respectively. From May to September, the hoof length was self- regulatorily adjusted via break-outs of hom-chips at the weight-bearing margin of the hoof, whereas during winter the hoof length was sufficiently adjusted simply by horn abrasion at the weight bearing margin. Przewalski horses transferred from zoological gardens to semi-reserves need an adaptation period of approximately one year before this natural self-regulatory adjustment of the hoof length is established. The adaptation period is necessary because the convex and concave horn rings of the hoof capsule need to develop that are essential for the natural hoof length adjustment. Apparently, the formation of horn rings is related to the seasonal changes of food supply and intensity of horn production, therefore these horn rings are termed nutritional, hardship- or growth-rings (or grooves). The detected seasonal differences in coronary horn quality between the horn rings in przewalski horses - even in horses kept in zoological gardens - alone are not sufficient for a self-regulatory horn adjustment mechanism. Via histometry, the present study revealed that the architecture of the coronary hoof horn varied to a lesser extent in wild horses kept in zoological gardens compared to that of horses kept in semi-reserves. Additionally, under the natural environmental conditions of Przewalski horses kept in the semi-reserve, hoof capsule rings had a higher prevalence. These horn rings (or horn grooves) are predisposing for the occurrence of cross horn fissures that in turn are a prerequisite for the development of horn chip break-outs at the weight-bearing margin. The horn quality within horn ring concavities is characterised by small coronary horn tubules displaying a narrow marrow and narrow intercellular cleft between horn cells. The convex areas of horn rings, on the other hand, are characterised by wider horn tubules with wide medullar space. In addition, the intercellular cleft between horn cells is often vesicularly dilated, and thus the cellular cohesion is diminished. With regard to the break-off of hom chips at the weight-bearing margin, the concave horn rings act like a notch - comparable to the function of a saw notch in woodcutting - whereas the actual fissure extends into the convexity of the horn ring. In connection with longitudinal fissures caused by a hoof-length- related dehydration and the tissue tension in the distal part of the hoof, the break-out of horn chips at the weight-bearing margin is generated. Due to this efficient self-regulatory hoof adjustment mechanism, Przewalski horses kept in semi-reserves do not need hoof correction measures, and thus an otherwise necessary anaesthesia is avoided. Both, hoof correction and anaesthesia, are regarded as inevitable in wild horses kept in zoological gardens. Besides the physiologic nutritional or growth rings, pathological horn rings develop in Przewalski horses in the course of laminitis. These laminitic rings are more distinctive than the physiological hoof rings. Thus, the hoof can be regarded as an indicator for the nutritional and health status of the horse. Because hoof rings are distinct and ratable by adspection even from afar, it is advisable to consider them to a greater extent in the future when examining wild and also domesticated horses.