Researcher: Michael HensmanRegion: Botswana
BackgroundThe sable antelope (Hippotragus niger), has shown some dramatic declines in parts of its range in southern Africa. Despite fairly intensive study in the Kruger National Park and other areas, the reasons for these declines are imperfectly understood.
ObjectivesThe sable antelope (Hippotragus niger), has shown some dramatic declines in parts of its range in southern Africa. Despite fairly intensive study in the Kruger National Park and other areas, the reasons for these declines are imperfectly understood.
It is for this reason that an investigation into the home range and habitat use of the species has been initiated in the Vumbura area in the Okavango Delta. Here a relative stronghold of sable exists, allowing an investigation into what factors allow the species to thrive here. Michael Hensman, an MSc student from the University of Witwatersrand and HOORC (being jointly supervised by Prof Norman Owen-Smith and Dr Casper Bonyongo), has just begun his research project using the relatively new technology of Geographical Positioning Systems (GPS) collars to produce fine scale movement data from three different herds.
MethodologyWe have so far managed to fit two collars onto two sable cows and are indebted to the assistance of veterinarian Dr Dane Hawk and gyrocopter pilot Mark Muller without whom the exercise would not have been possible. Dr Casper Bonyongo also lent valuable field experience during the operation.
The first cow darted was part of a herd of 22 animals that use the area in the north-west of the Vumbura Concession around the airstrip. This herd is well known and is comfortable with vehicles, allowing a close enough approach for darting from a vehicle. After some initial challenges this cow was successfully darted, immobilised and collared by Dr. Hawk. All relevant measurements and samples were taken from the cow before she was ‘reversed’ and allowed to rejoin the rest of the herd. Further observations on that day, the following morning and subsequently, suggest she has recovered completely and has continued to hold her position as the dominant female within the herd.
The second cow darted was located from the air by Mark Muller who guided us into an area to the north-east of Vumbura Plains Camp. We eventually located the 15-strong herd in an area of open grassland within mopane woodland. We were again able to successfully dart a selected cow from the vehicle and following all checks and processing of the animal, Dr Hawk successfully revived her, allowing her to rejoin the herd without incident. She was again located the following day and found to be behaving normally.
The third herd intended for inclusion in the study had in the meantime moved into a temporarily inundated area we were unable to access. We will attempt a second operation later in September and are excited to see the results from this study and what role these might play in the conserva
Having finished the fieldwork on sable in the Kwedi Concession I would like to take this opportunity to thank everyone for their help, support, enthusiasm and friendship during the fieldwork process.
Unfortunately, in August, one of the collared female sables was killed on an island, which we were unable to access during the high floods. As soon as the floods had receded enough we located the carcass (which suggested that lion had been the culprits) and also managed to recover the collar. The other two collars were removed at the beginning of December 2010 after more than 20 000 hourly GPS positions and associated data were collected. In the late dry season, the herds avoiding the teak woodlands until after the first rains when there was a fresh flush of green grass in the understorey.
The sable also avoided floodplain grasslands where other herbivores concentrated. All three herds supplemented their grazing again this year by spending a lot of time eating leaves and stems of Philenoptera nelsii as well as the fallen flowers of Kigelia africana in mid- to late- September. Observations of geophaegea (particularly on the edges of termite mounds) and osteophagea were also common during the late dry season which may be an effort by the sable to supplement the diet with calcium and phosphorus.
In May, a well known sable bull named “Stompie” (easily recognisable by his missing tail) was killed by lion on the edge of the Vumbura airstrip and in July, two adult female sable were killed by lion. A big concern is that only four of 15 calves born at the beginning of the year are currently surviving. Two of these are males, leaving little breeding stock for future years. Sable hide their calves for up to three weeks while the adults forage and it is during this period that I suspect they are picked up and killed most likely by hyaena and other smaller predators.
In August 2009, female sable antelope from two separate herds consisting of twelve and nineteen individuals on Vumbura Plains, were fitted with GPS collars and a female representing a third herd, numbering twelve individuals, was collared in October.
Over 12 000 hourly GPS positions for the three females and their respective herds have been recorded since the collars were fitted. The new technology allows remote downloads via UHF to a portable receiver and has worked extremely well. Apart from revealing areas of high utilisation, the GPS telemetry has interestingly revealed the areas sable avoid.
Thus far, the downloaded GPS positions have shown an avoidance of dry floodplain grasslands where wildebeest, zebra and tsessebe concentrate. Whether this is due to direct competition from the above-mentioned species or due to predators that follow these species is still to be established. However, predation is clearly a threat to the sable in the area as herd sizes reflect a gradient in the presence of lion.
Sable also avoided the teak woodlands that presented very little grass during the late dry season. The rains in late November and early December resulted in a flush of new grass within the teak woodlands and as a result the sable have begun using it extensively. The characteristics of this and other habitat types will continue to be measured for the duration of the study, in order to determine how they change seasonally and if the changes influence use by sable.
Although literature already recognises that sable browse to some extent during the dry season, very little has been documented about which species are browsed and what proportion of the diet consists of browse. An opportunity for detailed feeding observations has been made possible at Vumbura because the sable are used to vehicles and so allow us to approach to within a few meters of them. During the observations made during the dry season, all three herds spent a large portion of their time browsing the leaves and soft stems of Combretum mossambicense; the leaves and flowers of Lonchocarpus nelsii; and the flowers of Kigelia africana. To support these observations, tail hairs were collected from the sable when they were darted and more will be collected when the collars are removed to determine the proportion of C3 and C4 plants in their diet through isotope analysis. Over 50 faecal samples have also been collected and will be analysed to determine crude protein, phosphorus and nitrogen content. It will indicate the nutritional status of the sable during different seasons as well as the quality of the graze and browse being utilized.