Study population and habitat characteristics
Our study was conducted from June 2001 through April 2004 in the Masai Mara National Reserve (hereafter the Reserve) in southwestern Kenya. The 1500 km2 Reserve consists primarily of rolling grassland and scattered bushland (predominantly Croton and Euclea species), with riparian forest along the major watercourses, and supports more than fifteen species of resident ungulates totalling approximately 50,000 animals (Broten & Said, 1995; Ottichilo et al., 2000). From August to October, the Reserve also supports large migratory herds of approximately 200,000–300,000 wildebeest (Connochaetes taurinus) and an additional 10,000–50,000 zebra (Equus burchelli;Broten & Said, 1995; Serneels & Lambin, 2001). Primarily because of its diversity and abundance of predators, as well as the seasonal influx of migratory ungulates, the Reserve is Kenya’s premier wildlife tourist attraction (Norton- Griffiths, 1995), and supports 24 permanent tourist camps and lodges (Walpole et al., 2003). Although many are located just outside the Reserve boundary, a small subset of lodges (∼20%) are located within the Reserve itself, and this study focuses on one of the latter.
Spotted hyenas live in social groups called clans, and clan members cooperatively defend a stable group territory. Each clan contains one to several matrilines of adult females and their offspring, as well as a variable number of adult immigrant males. Clans are rigidly structured by hierarchical rank relationships (Kruuk, 1972; Tilson & Hamilton, 1984; Frank, 1986) that determine priority of access to food, and all adult females are socially dominant to immigrant males (Kruuk, 1972; Smale, Frank & Holekamp, 1993). Subadult individuals of both sexes maintain their maternal ranks as long as they remain in the natal clan (Smale et al., 1993). Although females are generally philopatric (Frank, 1986), most natal males disperse between the ages of 2 and 5 years (Henschel & Skinner, 1987; Smale, Nunes & Holekamp, 1997; East & Hofer, 2001). Crocuta clans are fission–fusion societies in which individuals travel, rest and forage in subgroups that typically change in composition many times each day (Kruuk, 1972; Holekamp et al., 1997). Female Crocuta bear 1–2 (rarely 3) young in isolated natal dens (Holekamp, Smale & Szykman, 1996). Cubs are typically transferred to a communal den at 2–5 weeks of age where they reside for the next 7–8 months (Kruuk, 1972). The communal den represents the social center of each clan’s territory and most clan members visit it regularly.
We monitored individuals from a single clan (the Mara River clan) that defended a territory (31 km2) near the center of the Reserve (Fig. 1). The clan included 32–43 individuals ( = 8 adult females, = 5 adult immigrant males), and each hyena was individually recognized by unique spot patterns. A single tourist lodge, which maintained an unfenced refuse pit approximately 300 m from the periphery of its grounds, was located near the northern boundary of the Mara River clan territory (Fig. 1). Garbage, mainly composed of food refuse from tourist and staff dining halls, was deposited daily into a circular earthen pit (diameter ∼8 m), usually between 9.00–10.00 hours. The pit was closed on 12 October 2002 and all garbage was removed from the site. In the following weeks the pit was filled with soil, and native shrubs and trees were planted.
Figure 1. Territory of the Mara River hyena clan and its location within the Masai Mara National Reserve. Hatched lines represent ungulate prey sampling transects. The refuse pit (filled circle) and tourist lodge are also indicated
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We anaesthetized and radiocollared six adult female hyenas from the Mara River clan of variable social rank, and made attempts to locate each individual on a daily basis. On average, we collected fifteen telemetry locations per month for each collared hyena. The majority (∼78%) of radiotracking was conducted during the morning (05.30–09.00 hours) and evening (17.30–20.00 hours), but additional locations were collected during mid-day and throughout the night. Although few locations were collected during the middle of the night, both our primary monitoring periods were largely within the active period (18.00–09.00 hours) documented for Reserve hyenas (Kolowski & Holekamp, 2007). In addition to tracking efforts, we made regular visits to the refuse pit at all times of day; however, hyenas were only observed at the site when fresh refuse was present (i.e. late morning). During these observation sessions, we recorded the number and identity of hyenas present, as well as the presence of other species at the site.
Using tracking locations from five of the six monitored adult female hyenas (one female was not monitored after pit closure), we compared space use patterns before and after pit closure using the following variables: 95% fixed kernel home range size, 50% fixed kernel home range (core area) size, presence/absence of the refuse pit within the core area, and average hyena distance to the refuse pit. Home range size during the full 16-month preclosure period was strongly influenced by the consecutive use of communal dens separated from each other by as much as 6 km. We therefore limited the preclosure locations for this analysis to those collected in the 10 months prior to pit closure, when consecutive den locations were consistently only short distances apart. Throughout this 10-month period, and the entire postclosure period, the communal den was always located in the northeast portion of the territory, and therefore, the influence of den shifts on space use comparisons was effectively removed. Because space use of female hyenas is dependent on whether they have cubs residing at the communal den (Boydston et al., 2003), we categorized all preclosure locations for each female with respect to whether or not she had den-dwelling cubs. We then randomly subsampled locations collected in the 19-month postclosure period for each female to equalize the number of locations collected before and after closure with and without den-dwelling cubs (minimum of 20 locations per female pre- and postclosure). Because female space use is even further restricted during use of natal dens, we excluded locations collected at these dens from all analyses. Using this restricted dataset, we calculated a single 95% and 50% home range (with smoothing factors determined by least squares cross-validation –Seaman & Powell, 1996), and an average distance at which each female was tracked from the refuse pit, for the pre- and postclosure periods.
We compared average distances to the refuse pit before and after closure using a Wilcoxon matched-pairs test. The pooling of locations from multiple individuals to calculate home range and core area size, while necessary for reasons of sample size requirements, prevented statistical comparisons of these two variables before and after pit closure. All distances and fixed kernel home ranges were calculated using Animal Movement Analyst (Hooge & Eichenlaub, 2000) and ArcView GIS 3.2 (Environmental Systems Research Institute, Redlands, CA, U.S.A.).
Because variability in prey abundance over time may influence space use patterns, we monitored prey abundance within the clan territory throughout the study period. As a monthly index of local prey abundance, we counted the total number of wild ungulates within 100 m of 2 4-km road transects twice each month (Fig. 1), and calculated the average number of prey animals counted per sampling event. To investigate differences in prey abundance between pre- and postclosure periods while controlling for the influence of the seasonal ungulate migration, we compared the average prey count for the same 10 months before and after pit closure using a t-test.
To investigate the influence of natural prey abundance on use of the refuse pit, we compared pit utilization during months of high prey abundance, when migratory herds were present in the clan territory, with that during months when only resident herds were present. Migration months were defined as those months when wildebeest were seen within the territory because no resident herds of wildebeest utilize this portion of the Reserve. We focused only on hyenas that were closely monitored in the 16-month preclosure period (≥12 months of tracking), and that were known to use the site with some frequency (>5% of locations within 500 m of the refuse pit). We compared the relative frequency of locations of these hyenas (pooled) within 500 m of the refuse pit during migration months and nonmigration months using a chi-square test.
To investigate the influence of social rank on individual variation in space use relative to the refuse pit, we conducted two separate analyses with females assigned to low or high rank categories relative to the median adult female rank. Individual ranks were assigned based on the outcomes of dyadic agonistic interactions as in Holekamp & Smale (1990). Immigrant males were treated separately as the lowest ranking group. First, to compare qualitatively use of the pit between individuals of different ranks, we calculated the proportion of tracking locations collected within 500 m of the refuse pit for those adult females tracked for ≥12 months while the pit was in use. Second, independent of radiotracking, we documented the composition of hyena groups observed feeding at the refuse pit based on eighteen observation sessions. We compared the proportion of these sessions recording the presence of low-ranking females, high-ranking females and immigrant males. We then compared the number of hyenas present from each rank group at these visits using a Friedman ANOVA, with the number of hyenas from each rank group representing three repeated samples for each observation session. Each rank group contained a total of five individuals in this preclosure period.
All tests were considered statistically significant at α = 0.05 (two-sided), and all analyses were conducted using the STATISTICA software package (Statsoft, 2002). Descriptive statistics are presented as mean ± SE throughout.