Biological Conservation 27 (1983) 157-170

Observations on the Sitatunga Tragelaphus spekei selousi in the Okavango Delta of Botswana

I. Games

Mammal Research Institute, University of Pretoria, Pretoria 0002, South Africa

A BSTRA CT

A study of the sitatunga Tragelaphus spekei selousi in the Okavango Delta showed the species to be restricted to the perennial swamp to which it is morphologically and behaviourally adapted. Sitatunga are essentially solitary but oJten Jorm loose groups showing little individual interaction. They are basically sedentary resting on flattened areas in the cover of reedbeds Ji'om which they move onto the surrounding areas during the morning, late evening and at night. They make extensive use of inter-connected path-systems in the large reedbeds. Seasonal movement andJeeding patterns are affected by the annual flood regime which Jorces the animals out of the reedbeds onto the flooded grassland on the fringes of the mainland during high water. At low water these Jormerly flooded areas are invaded by cattle pushing the sitatunga back to the central flood plain. The importance of these aspects Jor the conservation and management of sitatunga is examined.

I N T R O D U C T I O N

The sitatunga Tragelaphus spekei is one of Africa's least known antelopes, with published information being limited to brief accounts of its taxonomy, distribution and general biology (Walker, 1964; Sidney, 1965; Tinley, 1966; Ansell, 1968; Dorst & Dandelot, 1970; Patterson, 1976). The study of Owen (1970) on part of the population inhabiting the

157 Biol. Conserv. 0006-3207/83/$03.00 ~' Applied Science Publishers Ltd, England, 1983. Printed in Great Britain

158 1. Games

Saiwa swamps in Kenya represents, until now, the only detailed account of the life history of the species.

The relatively rapid reduction in waterflow and the extending occupation of wetland areas by cattle ranchers following modern tsetse fly control measures in the western part of the Okavango Delta (Northern Botswana) may conceivably pose a threat to a semi-aquatic species such as the sitatunga, which only occurs in suitable wetland areas throughout central Africa (see Dorst & Dandelot, 1970). The present study was therefore undertaken in an attempt to define some of the life history characteristics thought to be of importance for future management policies in the area.

STUDY AREA

The Okavango Delta is a wetland system of perennial swamp and a seasonally inundated floodplain approximately 1600 km 2 in extent (Astle & Graham, 1976). Two major geographical units can be recognised, these being the Panhandle and the Delta.

The presence of a trypanosome vector Glossina morsitans (the tsetse fly) results in present land use in the Delta being restricted~ to subsistence agriculture and hunter gathering. G. morsitans does not occur in the 470 km 2 area of the Panhandle and livestock grazing occurs along both sides of the 10 km wide floodplain.

The study area from which most information was collected is situated along the Panhandle 15km southeast of Shakawe Village (18°30'S, 2200 ' E; Fig. 1). Floodwater from Angola usually arrives in the area during November and peaks during April. This is the only area within the Okavango to receive the flood in phase with the rainfall--the water taking some five months to pass through the Del ta--and the vertical difference between high and low water can be as much as 2 m. During peak flood the Shakawe floodplain, as opposed to the channels, carries approximately 40 ~o of the flow into the Delta (Wilson & Dincer, 1976), while at low water virtually all of it is dry and subject to invasion by livestock.

The Panhandle contains vast stands of Cyperus papyrus, Phragmites mauritianus and Vossia cuspidata (Fig. 2). Other species within the flooded areas include a number of aquatic herbs, creepers, sedges and grasses. Dense riverine forest occurs on mainland fringes and islands and includes such species as Garcinia livingstonei, Acacia nigrescens, Ficus sycamorus and Phoenix reclinata.

Observations on the Sitatunga 159

NAMIBIA

Study .4

Sepopa

river

PANHANDLE

. . - -

:wane river

CAPRIVI

Linyanti

Mababe Depression

oeO°Gomare

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LEGEND

AREA SUBJECT TO FLOODING

CHANNELS

FAULT LINE

APPROXIMATE DISTRIBUTION OF SITATUNGA

ooooooo

Boteti river

Fig. I. Location of the study area and approximate distribution of sitatunga in the Okavango Delta.

160 I. Games

Fig. 2. A typical scene in the study area showing the dense stand of Cyperuspapyrus on the right with the wetland grass stand in the foreground which floods during high water.

As one moves further into the perennial swamps of the Delta the area of floodplain decreases while that of islands increases where Miscanthidium teretifolium (tambookie) and Typha latifolia become dominant. C. papyrus beds diminish in height and extent and the growth of competitor plants becomes more vigorous. Termitaria islands (up to 40 m diameter) also increase in number and are often associated with C. papyrus or M. teretifolium beds.

METHODS

Field work was undertaken from a camp established on the edge of the Shakawe floodplains. Routine observations were made from tall trees overlooking parts of the swamp, and a survey was conducted along the Moanachira river system in the north of the Delta and along the Boro

Observations on the Sitatunga 161

River to Maun (Fig. 1). In an attempt to determine density an aerial survey was conducted over the Shakawe floodplain using a Centurian 210 aircraft equipped with a VLF navigational unit. Flying height was 100 m with a strip width of 100m on either side of the aeroplane. Systematic transects were 0"93 km apart and air speed was 150 km h- 1. Counts were done in the early morning and late evening while most sitatunga were still active.

Faeces were collected from various areas within the Panhandle and Delta and analysed for percentage C. papyrus content using the method described by Storr (1961). These were then related to a visual estimate of C. papyrus abundance in the vicinity. Percentage utilisation of C. papyrus by sitatunga was estimated by counting all the culms (stems) with and without umbels (flowering heads) within fifteen 16 m 2 a r e a s flattened by sitatunga.

Nocturnal use of a peninsula was measured by counting sitatunga tracks along a wet sand area, 500 m in length, between C. papyrus and dry land in the early morning. Tracks were obliterated in the late evening to ensure a fresh count every morning.

RESULTS AND DISCUSSION

Distribution and density

The present approximate distribution of sitatunga within the Okavango Delta is shown in Fig.. 1. Their range appears to have shrunk over the past 100-150 years as Andersson (1856) and Selous (188 l) reported sitatunga sightings at Lake Ngami and along the Boteti, Thamalakane and Mababe Rivers (Mababe depression) (Fig. 1). The changes in their pattern of distribution may be ascribed to the reedbeds along these areas being less extensive today than in the past. There are recent unconfirmed reports of sightings near Maun on the Santantadibe River and on the lower Boro system but these are unusual. Elsewhere in Botswana they occur along the Kwando River and in the Linyanti swamp system (Smithers, 1971) and there are records of animals 40km above the Victoria Falls on the Zambezi River (D. Games, pers. comm.).

An aerial survey of the Shakawe floodplain during the peak flood suggested the presence of 234 animals (95 ~ confidence limits + 138) over an area of approximately 300 km 2. Sitatunga show a preference for beds

162 1. Games

of C. papyrus and P. mauritianus rather than the shallow flooded grassland, which provides limited cover and is easily traversed by local inhabitants using dugout canoes. The survey also suggested a clumped distribution pattern, probably resulting from social aggregation or habitat heterogeneity within the Shakawe floodplain.

The annual flood regime has a marked effect on the seasonal movement of the sitatunga within the Panhandle. During periods of high water (January to June) floodplains up to the mainland fringes are utilised and during periods of low water only areas near the channels are utilised, which are away from the mainland when these are then dry and invaded by livestock. A possible reason for the mats fringing the channels being favoured is that they capture considerable sediment, thereby contributing to a more solid substrate than occurs in the interior. At peak flood the C. papyrus fringe (62~ of the total) around a reed island 37km in circumference showed 13.5~ utilisation while at low water this had increased to 37.1 ~o (a factor of 2.5 in four months). During the same period livestock invasion resulted in a 56 ~ reduction in the area available for use by sitatunga and the area is often further reduced by burning, which is prevalent at low water. Conversely, once the new growth commenced, feeding occurred on these mats but the animals always returned to the cover of dense C. papyrus, Several sightings of three known subadult males suggested that they do not move very far during high water. They were never recorded more than 100m from their original sightings.

Social structure activity

For the observed Shakawe floodplain population, 60 ~o of individuals occurred solitarily and 34 ~ in groups of two, with most of these groups comprising two adult females (15 ~o, Table 1). Owen (1970) recorded 26 ~o of individuals as occurring singly and 39 ~ in pairs and an analysis of the distribution of group sizes showed that the two populations differed significantly (X2=35.97; df3; P<0.001) , possibly due to density differences resulting from availability of swamp area. It should be noted that most sightings within the Shakawe floodplain were recorded during the five-month period of high water. Increased density during low water may, however, have an influence on the frequency distribution of group sizes.

No sightings of groups of adult males were recorded from the Saiwa

Observations on the Sitatunga 163

TABLE 1 Frequency Distribution of Group Sizes of Sitatunga on the Shakawe Floodplain

Group size Total

I 2 3 4

Numbers of groups 205 58 4 2 269 Number of individuals 205 116 12 8 341 Percentage groups 76-2 21 '6 1'5 0.7 Percentage individuals 60.1 34.0 3" 5 2.3

swamp which possibly indicates mutual intolerance and perhaps an exclusive home range, while in the Okavango population this association represented nearly 5 ~o of individual groupings (Table 2). Once again this was mainly recorded during the high water period with a suggested decrease in density. Increased density could result in greater contact and more opportunity for aggressive encounters, which could lead to intolerance.

Immature animals represented 12 ~ of the population (Table 2). This figure is probably negatively biased as size differentiation at a distance

TABLE 2 Sex andAge Structure of the Shakawe Sitatunga Population

Number of Number of Percentage Percentage groups individuals groups individuals

Single male 47 47 17.4 13.8 Single female 137 137 50.9 40.2 Female immature 20 40 7.4 11.7 Male immature 21 21 7.8 6.2 Female & female 25 50 9.3 14.7 Male & male 8 16 3.0 4.7 Male & female 5 10 1.9 2.9 2 females & male 1 3 0.4 0.9 3 females 2 6 0.7 1.8 4 females 2 8 0.7 2.3 Female & 2 immatures 1 3 0.4 0.9

Total 269 341

164 I. Games

was difficult and many immature females may have been ascribed to the adult female class. Calves were not recorded but Walther (1964, in Lert, 1974) recorded the young sitatunga as 'lying out' in a concealed place for much of their early life.

The sex ratio of the Okavango sitatunga as observed from ground level sightings was 1 male:2.5 females (n = 319) compared with 1:2.1 for the Saiwa population (Owen, 1970). An analysis of the difference of activity between males and females in the Shakawe population showed that males spent 34 ~ of the time feeding and 54 ~ standing, while the females spent only 17 ~ standing and 77 ~ feeding. This suggests a greater degree of altertness in males, which is possibly stress-linked and could be a partial cause of the disparate sex ratio. Concentration of the population during low water could further intensify the situation.

The activities recorded in Table 3 are for a single adult male which was sufficiently close to the edge of the C. papyrus to be observed for two consecutive days. Other activities which were also recorded but are not represented, as they constitute only 9 ~ of activity, were walking, lying and grooming.

Feeding activity during the day shows a crepuscular pattern with greater activity during the morning from 0600 h to 0900 h/1000 h. Owen (1970) states that sitatunga are most active between 0600 h and 1100 h and then again after 1700 h. Smithers (1971) observed a crepuscular feeding pattern in the Kwando-Linyanti swamps (Fig. 1). The central part of the day was generally spent secluded in beds of C. papyrus or P. mauritianus, although individuals have been sighted in isolated islands. Occasional feeding, by taking C. papyrus umbels from a resting position, was often noted during the period from 1600 h to 1700 h. On occasions individuals were noted actively feeding during midday, usually on higher ground adjacent to the channels.

Recording activity at night was attempted but distances proved too great to achieve results. Sounds of feeding and breaking C. papyrus stems were often heard within the reedbeds at night. Figure 3 shows an index of activity of sitatunga on the mainland, as suggested by the number of fresh tracks recorded, and its relationship to the phases of the moon. A decrease in activity is apparent during the period of full moon. Owen (1970) also noted a marked tendency for the Saiwa swamp animals to remain in the reedbeds on moonlit nights. Smithers (1971) observed a group of five individuals on a moonless night several hundred metres from the swamp.

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166 1. Games

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Fig. 3.

No Moon

I

oFull Moon ~ 0

is 2b 2's ao DAYS

The relationship between sitatunga activity, as suggested by the number of fresh tracks recorded, and the phases of the moon.

Feeding

Seasonal changes in the flood levels have a marked effect on their diet. During periods of high water sitatunga have access to V. cuspidata beds which are then extensively utilised (65.2 % of all sightings). Once the water level drops they return to the central floodplain where C. papyrus and P. mauritianus predominate. This obviously reduces feed density and sitatunga then consume up to 80 % of the papyrus umbels available.

Sitatunga browse on trees when available and A. nigrescens and P. mespiliformis saplings are often denuded of leaves. In areas where various tree species are browsed, the occurrence of the sedge C. papyrus in faecal samples dropped to 45 %. The seasonal fall in water level, however, results in their return to the depths of the swamp where trees are unavailable, and in some areas the entire diet may then comprise C. papyrus. The sitatunga

Obsert, ations on the Sitatunga 167

furthermore do not appear to be selective feeders as in only 58 ~ of the area specific variation in the occurrence of C. papyrus remnants in faecal samples can be accounted for by availability alone.

Burning is of major importance in the Delta with as much as 75 ~ of the total area being burnt annually (Patterson, 1976). This gives sitatunga access to new shoots of certain plants. In the Moanachira system thick M. teretifolium growth often accompanies C. papyrus and feeding by the sitatunga is limited to umbels close to the paths. Burning removes unpalatable M. teretifolium and gives the animals access to shoots of Cyclosorus interrutus (a fern) and Eragrostis inamoena (a grass).

General observations

Sitatunga make use of paths through stands of C. papyrus and Phragmites spp. The majority of these are formed by the animals themselves although tracks indicate some use of paths in the shallower sections created by hippopotamus Hippopotamus amphibius. In C. papyrus these paths are often well defined for some distance before becoming indistinct. The distinct sections usually have a semi-solid substrate.

In some Phragmites spp. stands the path system is often well-defined with a myriad of interconnected passageways and small clearings. These were very narrow and in many cases difficult for a human to traverse. Path formation in Phragmites spp. and M. teretifolium is probably easier than in C. papyrus because of shallow water and a solid substrate.

On the larger islands predation by lions Panthera leo (Carp, 1940 in Smithers, 1971), leopards Panthera pardus and wild dogs Lycaon pictus (R. Wilmot, pers. comm.) occurs. Hunting probably accounts for a large part of the annual mortality. However, although up to 200 licences can be sold annually in Botswana (visiting hunters) only 20-30 animals are actually shot by trophy hunters (Von Richter & Butynski, 1974).

CONCLUSIONS

Sitatunga are medium-sized bovids confined in their distribution to areas of swamp. They are predominantly a solitary species but in the Okavango Swamp, Botswana, have been observed in groups of up to eight. Estes (1974) stated that increased group size is regarded as an anti-predatory

168 1. Games

strategy. Sitatunga, as opposed to bovids living on dry land, do not require to employ this strategy as they are afforded a high measure of protection against predation by the nature of their habitat, with its thick cover of C. papyrus and watery substrate. They have a tendency to avoid the more open areas of water and therefore, predation by crocodiles Crocodylus niloticus is lessened.

From 1957 to 1969 approximately 10000 crocodiles were shot for commercial purposes in the Okavango (Blomberg, 1976), the majority being taken from the Panhandle. This has reduced both the numbers and average size of crocodiles (Tinley, 1966). As the incidence of vertebrates in crocodile stomachs increased with increased size (Blomberg, 1976) it is reasonable to assume that the numbers of sitatunga taken have decreased in the Panhandle, resulting in an increase in the population (substantiated by R. Wilmot, pers. comm.).

Leuthold (1974) advances the view that group structure is similar in the genus Tragelaphus but that group size decreases from the greater kudu Tragelaphus strepsiceros through mountain nyala T. buxtoni/lesser kudu T. imberbis/nyala T. augasi/sitatunga and finally bushbuck T. scriptus. Data from the present study tend to support this hypothesis but the Shakawe population shows a closer affinity to the bushbuck than the Saiwa swamp sitatunga, which according to Leuthold (1974) is similar to the lesser kudu.

Sitatunga utilise a large proportion of the plants available to them and will generally feed on the more abundant species within an area. Owen (1970) found that the Saiwa swamp sitatunga were partially selective in their feeding habits but that they were mainly feeding on the most abundant species. By being non-selective and capable of modifying their diet to include aquatic grasses during low water they are able to survive despite fluctuating swamp levels and annual reductions in feeding area. Availability of water is not a problem as the swamp is unlikely to dry up entirely during any given year.

Considering the vast area of the Okavango Delta and the different habitat types within it, it is possible that other sitatunga populations, especially those on the permanent swamp/seasonal swamp ecotone, may differ greatly from those of the Shakawe floodplain. Future studies could help to clarify the situation.

Several factors might in the future become vital to the continued viable population of sitatunga in the Okavango Swamp. Major interference with the hydrological conditions of the swamp could seriously reduce the

Observations on the Sitatunga 169

habitat available to the species. With the apparent success of the tsetse-fly control measures further pressures may be brought to bear to increase the numbers of cattle in the fringes of sitatunga habitat. Utilisation of fringes of the southeastern parts of their habitat by cattle may well be the reason for their disappearance in this sector for, at low water, cattle graze deep into the swamp fringe and are therefore using grazing areas which are important to the species which are forced out of the swamp at times of high flood. Moreover, the numbers taken by hunters should be carefully controlled and checked. This is not difficult for trophy hunting although the effect of removal of prime males with the longest horns may eventually affect the population. In addition, sitatunga are utilised by the local population. During June in the Linyanti, and presumably January/ February at Sepopa, the calf crop is subject to culling by them and calves are apparently easily caught while lying up. Nor has the extent to which firearms are used by the local inhabitants for hunting been accurately assessed.

Protection provided by the nature of the Okavango Delta should ensure the future existence of this species. However, as it is adapted to survive in swamp conditions only, an artificial reduction in waterflow and the increasing occupation of wetland areas by cattle ranchers may well result in a reduction in sitatunga numbers.

A C K N O W L E D G E M E N T S

The study was conducted under the auspices of the Director, Professor J. D. Skinner of the Mammal Research Institute, University of Pretoria. The Department of Wildlife, National Parks and Tourism in Botswana kindly gave permission for the project to be carried out. Financial and logistic support was provided by Messrs V. Duina and D. Hartley and Mr and Mrs B. Pryce. P. Smith identified many of the plant specimens. Mr R. J. van Aarde gave me invaluable assistance in the preparation of the manuscript.

REFERENCES

Andersson, C. J. (1856). Lake Ngami. London, Hurst and Blacket. Ansell, W. H. F. 0968). Preliminary identification manual for African mammals.

Washington, Smithsonian Institution.

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Astle, W. L. & Graham, A. (1976). Ecological investigations of the UNDP in the Okavango Delta. Proc. Symp. Okavango Delta, held at Gabarone, Botswana, 30 August-2 September 1976, 81-93. Gabarone, Botswana Society.

Blomberg, G. E. D. (1976). Feeding and nesting ecology and habitat preference of Okavango crocodiles. Proc. Symp. Okavango Delta, held at Gabarone, Botswana, 30 August-2 September 1976, 131-140. Gabarone, Botswana Society.

Dorst, J. & Dandelot, P. (1970). A field guide to the larger mammals of Africa. London, Collins.

Estes, R. D. (1974). Social organization of the African Bovidae. 1UCN Pubis, New Series, 24, 166-205.

Lert, P. C. (1974). Mother-infant relationships in ungulates. IUCN Pubis, New Series, 24, 14-55.

Leuthold, W. (1974). Observations on the home range and social organization of lesser kudu (Tragelaphus imberbis). 1UCN Publs, New Series, 24, 206-34.

Owen, R. E. A. (1970). Some observations on the sitatunga in Kenya. E. Air. Wildl. J., 8, 181-95.

Patterson, L. (1976). An introduction to the ecology and zoogeography of the Okavango Delta. Proc. Symp. Okavango Delta, held at Gabarone, Botswana, 30 August-2 September 1976, 55-60. Gabarone, Botswana Society.

Selous, F. C. (1881). A hunter's wanderings in AJrica. London, Richard Bently and Son.

Sidney, J. (1965). The past and present distribution of some African ungulates. Trans. Zool. Soc. Lond., 30, 1-397.

Smithers, R. H. N. (1971). The mammals of Botswana. Salisbury, Mardon. Storr, G. M. (1961). Microscopic analysis of faeces, a technique for ascertaining

the diet of herbivorous mammals. Aust. J. Biol. Sci., 14, 157-64. Tinley, K. L. (1966). An ecological reconnaissance of the Moremi Wildlife

Reserve, Botswana. Johannesburg, Okavango Wildlife Society. Von Richter, W. & Butynski, T. M. (1974). Wildlife utilization in Botswana: A

review and evaluation of hunters' returns as a source of administrative and biological data. S. Afr. Wildl. Mgmt Ass., 4, 167-76.

Walker, E. P. (1964). Mammals of the world. Baltimore, Johns Hopkins Press. Wilson, B. H. & Dincer, T. (1976). An introduction to the hydrology and

hydrography of the Okavango Delta. Proc. of the Symp. Okavango Delta, held at Gabarone, Botswana, 30 August-2 September 1976, 33-47. Gabarone, Botswana Society.