Abstract: The formation of collaborating pairs by individuals belonging to two different classes occurs in the contexts of reproduction and intea-specific cooperation as well as of inter-specific mutualism. There is potential for partner choice and for competition for access to preferred partners in all three contexts. These selective forces have long been recognised as important in sexual selection, but their impact is not yet appreciated in cooperative and mutualistic systems. The formation of partnerships between members of different classes has much in common with the conclusion of trade agreements in human markets with two classes of traders, like producers and consumers, or employers and employees. Similar game-theoretical models can be used to predict the behaviour of rational traders in human markets and the evolutionarily stable strategies used in biological markets. We present a formal model in which the influence of the market mechanism on selection is made explicit. We restrict ourselves to biological markets in which: (1) Individuals do not compete over access to partners in an agonistic manner, but rather by outcompeting each other in those aspects that are preferred by the choosing party. (2) The commodity the partner has to offer cannot be obtained by the use of force, but requires the consent of the partner. These two restrictions ensure a dominant role for partner choice in the formation of partnerships. In a biological market model the decision to cooperate is based on the comparison between the offers of several potential partners, rather than on the behaviour of a single potential partner, as is implicitly assumed in currently accepted models of cooperation. In our example the members of one class A offer a commodity of fixed value in exchange for a commodity of variable value supplied by the other class, B. We show that when the B-class outnumbers the A-class sufficiently and the cost for the A-class to sample the offers of the B-class are low, the choosiness of the A-class will lead to selection for the supply of high value commodities by the B-class (Fig. 3a). Under the same market conditions, but with a high sampling cost this may still be the evolutionariy stable outcome, but another pair of strategies proves to be stable too: relaxed choosiness of class A coupled with low value commodities supplied by class B (Fig. 3b). We give a number of examples of mating, cooperative and mutualistic markets that resemble the low sampling cost situation depicted in Fig. 3a.

Abstract: Habitat types were classified in a semi-arid nature reserve in South Africa in order to assess the spatial requirements of resident ungulates, namely zebra, wildebeest and impala. Multivariate analyses showed patterns of soil factors and plant species associations that corresponded with variations of local geological formations and the abundance of plants. The response of ungulates to habitats of different degrees of complexity in terms of soils and plant species associations was examined on the basis of annual occurrence. New habitat types were described through merging or subdividing the existing classification. New habitat categories which corresponded with high occurrences of ungulates provided better indications of the resource requirements for large herbivores. Wildebeest were restricted in their habitat requirements and were characterized by high seasonal densities in bottom lands, particularly during the late wet period. There was a considerable overlap in the preference of habitat types between wildebeest and zebra although zebra were aggregated during longer periods within the dolerite formation. Impala had a consistent annual preference for the granite formation where seepage lines and bottom lands were inhabited seasonally by large herd concentrations.
Résumé
On a classifié les types d'habitat dans une réserve naturelle semiaride d'Afrique du Sud, dans le but d'évaluer les exigences spatiales des ongulés qui y vivent, c'est à dire les zèbres, les gnous et les impalas. Des analyses multivariées ont révélé des schémas pour les facteurs du sol et pour les associations d'espèces végétales qui correspondent aux variations des formations géologiques locales et à l'abon-dance des plantes. On a examiné la réponse des ongulés à des habitats de complexité différente en termes de sols et d'associations d'espéces végétales, d'après leur présence annuelle. On a décrit de nouveaux types d'habitats en fusionnant ou en subdivisant la classification existante. Les nouvelles catégories d'habitats qui correspondaient à des présences abondantes d'ongulés ont fourni de meilleures indications sur les ressources exigées par les grands herbivores. Les gnous se limitaient aux endroits qui répondaient a leurs exigences et se caractérisaient par de hautes densités saisonnières dans les régions basses, spécialement pendant la dernière saison des pluies. Il y avait un recouvrement considérable des types d'habitats préferés par les gnous et les zébres, encore que les zébres se rassemblent plus longtemps dans la formation doléritique. Les impalas marquent une préférence annuelle constante pour la formation granitique où les sources et les terres basses sont occupées de façon saisonnière par des hardes très concentrées.

Abstract: Triver's model of reciprocal altruism, and its descendants based on the Prisoner's Dilemma model, have dominated thinking about cooperation and altruism between non-relatives. However, there are three alternative models of altruism directed to non-relatives. These models, which are not based on the Prisoner's Dilemma, may explain a variety of phenomena, from allogrooming among impala to helping by non-relatives in cooperatively breeding birds and mammals.

Abstract: Group foragers may be able to assess patch quality more efficiently by paying attention to the sampling activities of conspecifics foraging in the same patch. In a previous field experiment, we showed that starlings foraging on patches of hidden food could use the successful foraging activities of others to help them assess patch quality. In order to determine whether a starling could also use another individual's lack of foraging success to assess and depart from empty patches more quickly, we carried out two experimental studies which compared the behaviour of captive starlings sampling artificial patches both when alone and when in pairs. Solitary starlings were first trained to assess patch quality in our experimental two-patch system, and were then tested on an empty patch both alone and with two types of partner bird. One partner sampled very few holes and thus provided a low amount of public information; the other sampled numerous holes and thus provided a high amount of public information. In experiment 1, we found no evidence of vicarious sampling. Subjects sampled a similar number of empty holes when alone as when with the low and high information partners; thus they continued to rely on their own personal information to make their patch departure decisions. In experiment 2, we modified the experimental patches, increasing the ease with which a bird could watch another's sampling activities, and increasing the difficulty of acquiring accurate personal sampling information. This time, subjects apparently did use public information, sampling fewer empty holes before departure when with the high-information partner than when with the low-information partner, and sampling fewer holes when with the low-information partner than when alone. We suggest that the degree to which personal and public information are used is likely to depend both on a forager's ability to remember where it has already sampled and on the type of environment in which foraging takes place.

Abstract: In the literature on dominance hierarchies, “winner” and “loser” effects usually are denned as an increased probability of winning at time T, bated on victories at time T-l, T-2, etc, and an increased probability of losing at time T, based on losing at T-1, T-2, etc., respectively. Despite some early theoretical work on winner and loser effects, these factors and how they affect the structure of dominance hierarchies have not been examined in detail. I developed a computer simulation to examine winner and loser effects when such effects are independent of one another (as well as when they interact) and when combatants assess each other's resource-holding power. When winner effects alone were important, a hierarchy in which all individuals held an unambiguous rank was found. When only loser effects were important, a dear alpha individual always emerged, but the rank of others in the group was often unclear because of the scarcity of aggressive interactions. Increasing winner effects for a given value of the loser effect increase the number of individuals with unambiguous positions in a hierarchy and the converse is true for increasing the value of the loser effect for a given winner effect Although winner and loser effects have been documented in a number of species, no study has documented both winner and loser effects (using some controlled, pairwise testing system) and the detailed nature of behavioral interactions when individuals are in groups. I hope the results of this model will spur such studies in the future.