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Resource limitation and population ecology of white-eared kob Fryxell, John M.


In this study I examine the effect of seasonal resource limitation on the behaviour and population dynamics of white-eared kob, Kobus kob leucotis, in the Boma region of the southern Sudan. This population, numbering over 800,000, migrates seasonally between savannah grasslands in areas with low rainfall and ephemerally swamped grasslands in areas with high rainfall. The aims of the study were: (1) to examine whether kob migration tracks ephemeral distributions of food or water resources, (2) to test the hypothesis that the Boma kob population is limited by food availability, (3) to determine if calf production is cued to seasonal peaks in food abundance, and (4) to evaluate the effect of breeding synchrony on lekking behavior and male competition. Seasonal climatic changes produced pronounced changes in the distribution and abundance of both green forage and water supplies. Dry season migration primarily tracked limited supplies of water. Within the dry season range, kob aggregated at high densities (over 1,000 per km²) in low-lying meadows that supported grass re-growth when little green grass was available elsewhere in the ecosystem. However, southerly movements in the wet season were not explainable by the resource hypothesis, since both food and water were widely distributed during the wet season. I suggest that kob may move southward in order to avoid surface flooding during the wet season. Kob mortality during the dry seasons of 1982 and 1983 was considerably higher than estimated mortality during the wet season. Unusual rainfall during the dry season of 1982 provided a "natural experiment" to test the food limitation hypothesis. Adult mortality was significantly lower during the dry season of 1982 than during the more typical dry season of 1983. Calf mortality did not vary significantly between years. Adult mortality rates were related to dry season duration. Dry season mortality was related to sub-maintenance food intake and declining fat reserves. The age structure of the kob population in 1983 suggests that large-scale mortality (ca. 40%) occurred in the 1980 drought that immediately preceded this study. These findings support the food limitation hypothesis. Kob exhibited a 4 month period of calf production during the late wet season, when food availability was highest. As a consequence, females continued lactation through the dry season period of food scarcity. I suggest that kob reproductive phenology may result from an obligatory delay during which females restore their fat reserves prior to calving or selection pressures imposed by predation during the vulnerable post-partum period. Synchronous breeding in the Boma kob was related to increased rates of aggression between males and increased color dimorphism, in comparison to the asynchronous breeding Uganda kob, Kobus kob thomasi. Male aggression served not only to establish dominance relations between males on leks, but also disrupted the mating activities of neighboring males. Young adult males suffered higher age-specific mortality than females, possibly resulting from injuries incurred during strenuous fighting on leks. In order to analyze the age structure of the kob population, I devised a new method for estimating age-specific mortality rates that is free of the restrictive assumptions that underlie most conventional techniques. The proposed method has somewhat greater sampling variation, but is considerably more robust, than two current methods. Moreover, the proposed method permits calculation of age-specific mortality at frequent intervals during periods of population fluctuation and, under some circumstances, population numerical trends may be directly determined from age structure.

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