Seasonal movements, migration and age determination of Burchell's Zebra (Equus burchelli antiquorum, H. Smith, 1841) in the Kruger National Park

Abstract

Since the initiation of the first management policies in the Kruger National Park in 1902 the zebra (Equus burchelli antiquorum H. Smith, 1841) population has shown a marked increase in numbers. Although this increase has largely been confined to certain regions of the Park, it has been amplified by the concomitant increase in numbers of other large herbivores, while species such as the roan antelope, sable antelope and tsessebe have declined in numbers. In addition to fluctuations in the numbers of herbivores, vegetational changes have taken place. In many instances these changes have been unacceptable to the species here considered. With the initiation of essential management practices such as dise5se control, water provision, burning programs and the fencing of the western boundary of the Park, the culling of excess zebra in certain areas has also become necessary. To calculate more reliable culling quotas and to decide on the most effective distribution of artificial watering points, it has been deemed necessary to gain a knowledge of the factors affecting the zebra's choice of habitat, distribution, home range size, and to trace their seasonal and migratory movements. Since this project is actually the forerunner of a detailed study on the reproduction and population dynamics of zebra, and since animals were regularly captured for marking purposes, a simultaneous study could also be made on a method of ageing the species. All zebra were marked with coloured "sterkolite" collars, no two animals bearing similar collars. Data on 52 zebra marked between 1963 and 1965 were augmented by over 1200 resightings collected between 1969 and 1971 on an additional 89 marked anime.ls. This data has helped to divide the zebra population of the Kruger National Park into eight ecologically self-sufficient sub-populations. These sub-populations, although not separated from each other by impassible barriers, were found to retain their identity with a minimum of mixing occurring between units. By analysing the seasonal timing of movements, the direction, the distance and the regularity, it was further possible to decide whether a particular sub-papulation is sedentary, semi-migratory or migratory. The only truly migratory zebra are found in the central district where they undertake a regular, north-south migration between winter (dry season) and summer (wet season) ranges. The extent of the migration was found to be up to 75 kilometres, while the time taken to complete the movement could vary between two and 17 days, depending on conditions. The stimuli involved in zebra migration and the question as to whether migratory behaviour is instinctive or traditional are also discussed. Semi-migratory zebra, with the exception of the zebra of the Pretoriuskop-Malelane-Skukuza area, were found to undertake west to east movements, the extent and duration of which depend on rainfall. To a large extent these zebra move from the Lebombo flats into the Lebombo hills during the wet season. Zebra in the Pretoriuskop-MalelaneSkukuza area, on the other hand, were found to undertake a very different type of movement which was largely dependent on the rotational burning programs carried out in the area. Sedentary sub-populations such as the zebra in the Batara area undertake wet season movements which take them away from the permanent waterholes to previously underutilized areas. These areas are not situated in any particular direction or distance from the winter range. Here the summer grazing areas form a mosaic pattern around the winter areas. Of the factors affecting the zebra's choice of habitat, distribution, movements and migration, water and grazing conditions were found to be the most important. In addition to these, factors such as topography, soil type, thicket density, inter- and intraspecific competition, predator pressure and sociality also affect movements. Zebra were found to avoid areas which were waterlogged and where the sticky soil conditions impeded normal locomotion, using the resighting data on marked animals it was possible to calculate the yearly home range size for 50 groups. These home range sizes were found to vary considerably from sub-population to sub-population, the largest mean size (207 sq. km) being for the Pretoriuskop-Skukuza-Malelane sub-population and the smallest (111 sq. km) being for the Crocodile Bridge sub-population. Variations in home range size could not be correlated with :inter- or intraspecific intolerance, but rather with characteristics of the particular habitat and its general acceptability to zebra. Using the skulls of over 700 culled zebra together with data collected both in the field and on captive zebra kept at Skukuza, it was possible to define 27 age classes for zebra between birth and 20 years of age. These age classes are based on the Eruption and wear of deciduous and permanent incisors, premolars and molars. The age classes of adult animals were obtained by direct measurement of the annual wear of each individual incisor. Since these animals are aged according to the shape of the table surface of each incisor and the shape of the corresponding infundibulum, the incisor teeth could be cut down by the amount that they would have worn in two or three years, and the shape could again be assessed. In addition to this, carbon prints were made of the table surface of each marked animal's incisors and these animals were recaptured after six and 12 or 18 months. By comparing prints made at these intervals it was possible to assess the changes occurring in naturally wearing incisors and to correlate this with teeth cut down by the same amount of "measured" annual wear. The accuracy of the abovemetnioned technique has aptly been substantiated by the discovery of annual growth rings in the cementum of the permanent incisors. By counting growth rings it has also been possible to decide on a maximum ecological longevity for the zebra of some 22 years.