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Rubenstein, D. I., & Hohmann, M. E. (1989). Parasites and Social Behavior of Island Feral Horses. Oikos, 3, 312–320.
Abstract: The horses of Shackelford Banks, North Carolina, USA, are heavily parasitized by intestinal worms and harassed by dipterans, and although both types influence behavior only internal parasites affect bodily condition and the structuring of horse society. Thirteen species of internal parasites were identified, but only 4 of 13 groups contain them all and even within groups differences among individuals are large. Among individuals ova emissions vary ranging from 50 to 76,875 eggs per gram. The most important environmental factors influencing egg production are season and a group's location on the island, presumably because of salinity and soil differences and their effects on ova survival. Of the social and life history factors, age, and group size, but neither reproductive state nor dominance status are important. The fitness consequences of internal parasitism may be large since the number emitted is negatively correlated with next year's bodily condition. Biting fly burdens are also affected by a variety of environmental factors. In general, horses are covered with more flies on sunny days, when winds are moderately brisk, when occupying dunes, and around mid-day. In contrast to endoparasites, fly burden is affected by reproductive condition and dominance status and tends to decrease as groups increase in size. Since groups do not grow very large, nor do females attempt to bring groups together, the negative effects of endoparasites appear to overide those associated with ectoparasites. Consequently, endoparasites appear to exert a stronger influence on social structure, even though ectoparasites seem to play a stronger role in shaping details of behavior.
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Gordon, I. J., & Lindsay, W. K. (1990). Could Mammalian Herbivores “Manage” Their Resources? Oikos, 59(2), 270–280.
Abstract: The concept of resource management has gone hand in hand with group selection arguments. For this reason, it has been cast aside in the era of evolutionary theory which assumes that foraging strategies must have evolved under selection operating to maximise an individual's inclusive fitness. However, results from empirical studies show that under favourable environmental and social circumstances, resource management could be selectively advantageous. Much of the recent literature on plant-herbivore interactions suggest that herbivory can result in changes in the resource base which are assumed to increase the intake and fitness of the herbivore. As a result, a number of authors suggest that herbivores manage their resource utilisation to maximise the flow of nutrients from these resources. Long term territoriality or the exclusive use of a home range are the social systems most likely to favour selection for prudent resource exploitation. This review argues that, in many habitats, resource management strategies are not feasible, as individuals have little control over the way resources are depleted and renewed. Thus far, very little evidence is available showing that herbivorous mammals actively manage the resources which they utilise.
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