Summary1. Over the last three decades the main prey species (wildebeest Connochaetes taurinus , zebra Equus burchelli , Thomson's gazelle Gazella thomsoni , and Grant's gazelle Gazella granti ) of spotted hyaenas Crocuta crocuta in the Ngorongoro Crater, Tanzania, substantially declined in numbers, whereas buffalo Syncerus caffer numbers increased strongly. This provided a 'natural experiment' to investigate how a generalist predator such as the spotted hyaena responds to long-term changes in prey populations. Here we compare data on the feeding ecology of Crater hyaenas from the late 1990s ( period II) with equivalent data from the late 1960s (period I). 2. Hyaenas showed strong hunting preferences for wildebeest calves and gazelle fawns during both periods I and II. Adult buffaloes, prey difficult to hunt for hyaenas, were rarely hunted in either period despite a large increase in buffalo numbers from period I to II. 3. Hyaenas exhibited a functional response to the changes in prey populations by killing more buffalo calves and adult wildebeest during period II, relatively easy prey categories for hyaenas to hunt. 4. The proportion of carcasses scavenged and acquired by kleptoparasitizing other predators increased from period I to II. This was facilitated by an increase in the relative number of lion kills available to hyaenas during the same period. 5. Hyaenas defended a similar proportion of their kills against kleptoparasitic attempts by lions during both periods. Since the ratio of kills hyaenas kleptoparasitized from lions in relation to kills lost to lions increased from period I to period II, hyaenas obtained more food resources from lions during period II than I. 6. The success of hyaenas kleptoparasitizing lions was influenced by the presence of male lions and the ability of hyaenas to recruit sufficient clan members to appropriate carcasses from lions. 7. This study is the first to demonstrate both a functional response of a predator to changes in the size of prey populations and an associated change in foraging behaviour of the predator.

Dispersal has a significant impact on lifetime reproductive success, and is often more prevalent in one sex than the other. In group-living mammals, dispersal is normally male-biased and in theory this sexual bias could be a response by males to female mate preferences, competition for access to females or resources, or the result of males avoiding inbreeding. There is a lack of studies on social mammals that simultaneously assess these factors and measure the fitness consequences of male dispersal decisions. Here we show that male-biased dispersal in the spotted hyaena (Crocuta crocuta) most probably results from an adaptive response by males to simple female mate-choice rules that have evolved to avoid inbreeding. Microsatellite profiling revealed that females preferred sires that were born into or immigrated into the female's group after the female was born. Furthermore, young females preferred short-tenured sires and older females preferred longer-tenured sires. Males responded to these female mate preferences by initiating their reproductive careers in groups containing the highest number of young females. As a consequence, 11% of males started their reproductive career in their natal group and 89% of males dispersed. Males that started reproduction in groups containing the highest number of young females had a higher long-term reproductive success than males that did not. The female mate-choice rules ensured that females effectively avoided inbreeding without the need to discriminate directly against close kin or males born in their own group, or to favour immigrant males. The extent of male dispersal as a response to such female mate preferences depends on the demographic structure of breeding groups, rather than the genetic relatedness between females and males.