SUERO SEPARACIÓN CONCENTRACIÓN ST

Figure 4.1 An example of a curve relating M CP to the number of relocations. The graph depicts the mean (±S E ) M CP for female JK who was resighted 37 times; it is drawn on the basis of 50 replicates generated by jackknifing for each of 12 intervals (Hooge et al., 1999).

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knifing using the above mentioned software: based on 50 replications for each interval, the mean and standard error o f MCPs were calculated for each o f around 10 increments in the number o f relocations.

The home range diameter for females in the Burrungat Study Area was estimated assuming circular ranges as was done in the home range overlap model (Bradbury et al., 1986). Duncan (1975) found that individuals generally remain with one plain system in the Western Corridor o f Serengeti National Park, thus I made a very crude estimate o f minimum female home range diameter on the Dutwa plains based on sightings o f herds during five surveys between March and July 1998. The home range diameters were compared to inter-lek distances.

4.3.3.2 Lek size in relation to density

In order to determine the relationship between mating system and density, and specifically the relationship between lek size and density, I used data from the present study as w ell as information from previous studies on topi.

In the present study mean population densities were calculated from total counts conducted in a similar way to that described in sec. 2.5.1. The areas for which densities were determined included the Burrungat Study Area (11 counts, 154 km^, sec. 3.4.1) and the Olorukoti plain (4 counts, 48 km^) in Maasai Mara National Reserve and the Dutwa plains (4 counts, 135 km^) and the Seronera wooded plains (15 counts, 60 km^) in Serengeti National Park.

4.3.4 The antipredator models

During focal watches o f 40 oestrous females, I recorded the frequency with which hyenas entered within a 100 m radius o f the focal female, and the number o f other ungulates present within the same radius at the time. From these data, I calculated the rate with which a female met hyenas on and o ff lek. I did a separate analysis taking the possibility o f safety in numbers into account by dividing hyena numbers by the mean number o f ungulates within the radius during their presence.

4.3.5 A general prediction: The ratio of females in

oestrus on/off iek

During the rut I measured the proportion o f females who were in oestrus on and o ff lek. In order to take diurnal variation into account, I sampled five different times o f

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the day, dawn (0630 hours), mid-morning (0930 hours), noon (1230 hours), afternoon (1530 hours) and dusk (1830 hours); each time o f day was sampled on 15 different days. The proportion o f females in oestrus followed a gamma distribution and was modelled as such in a generalised linear model. The independent variables included in the analysis were location on /off lek, the time o f day and the interaction term between the two; these were tested by backward regression.

4.4 Results

4.4.1 The simple hotspot model

4.4.1.1 Lek locations in relation to densities

The lek vicinity, defined as the 16 one km^ cells found at least partly within a 1 km radius o f a lek, was not characterised by uniquely high topi densities: based on 11 total counts spread out between December and June, 21% o f the study area was found to have a higher mean density (Figure 4.2).

During the drier period before the rut, i.e., between December and February, the highest concentrations o f topi were found in the w est o f the study area in the more moist areas by the Mara River; thus 35% o f the study area had higher densities than was found around the leks (Figure 4.2, Before the rut). During the rut, the density rose around the leks (Figure 4.2, The rut); however, in neither o f the two study years were the highest concentrations centred on the leks: the 'hotspot' was 3 km east o f the Ankarrani lek in 1999 and 3 km west o f the 01 Kiombo lek, not far from the Mara River, in the relatively dry rut o f 2000 (Figure 4.3). Thus during the rut, on average 14% o f the study area had higher densities than the lek vicinity. A s rutting subsided in May, the densities in the areas around the leks dropped; however, still not more than 15% o f the area had higher densities than around the lek (Figure 4.2, After the rut).

Males were generally more uniformly dispersed than females (Table 4.1, Figures 4.4 & 4.5). Looking at the distribution o f the sexes in relation to the area covered with green grass on satellite images, the high concentrations close to the Mara River during the dry period in January-February can be seen as reflecting mainly females moving to areas with green grass and water (Figure 4.4, Late January). Males were more evenly distributed, probably because o f the need for

Chapter 4 Hotspots 60

Figure 4.2 The topi distribution in the

Burrungat Study Area. The overall mean of 11 total counts as well as seasonal means are shown.

Mean density