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: Investigation of tree growth in Isle Royale National Park in Michigan revealed the influence of herbivores and carnivores on plants in an intimately linked food chain. Plant growth rates were regulated by cycles in animal density and responded to annual changes in primary productivity only when released from herbivory by wolf predation. Isle Royale's dendrochronology complements a rich literature on food chain control in aquatic systems, which often supports a trophic cascade model. This study provides evidence of top-down control in a forested ecosystem.

Abstract: Two mazes were used to test the learning ability and memory of horses, and changes in these abilities. Testing was done on four occasions. On Occasion 1, the horses were run through Maze A until they had reached the criterion of three consecutive correct runs. A week later (Occasion 2), they were retested in Maze A to the same criterion as a measure of memory. On Occasion 3,2 months later, the horses were run through Mazes A and B until they reached the criterion. Occasion 4 took place 1 week later when they were run through Mazes A and B. An estimation of changes in ability to learn came from a comparison of results from Occasions 1 and 3. Similarly, changes in ability to remember came from a comparison of results from Occasions 2 and 4. Nine horses with a variable amount of riding training were the subjects. All horses were able to learn the maze, but the ability varied among horses. There was no obvious correlation between quality of handling of the horses and learning ability. Once the horses had learned the maze, they remembered it perfectly on subsequent occasions. There were changes in the memory and learning ability of the horses, but no clear explanation for this could be found.

Abstract: Twenty-three foals were used to determine if different amounts of handling between birth and weaning affected their later learning ability and manageability. Foals were assigned to one of three treatments: non-handled (NH) foals were not handled except for necessary maintenance procedures; intermediately handled (IH) foals were handled daily in two 10-min sessions for 7 days after birth and then not handled except for necessary maintenance procedures; extensively handled (EH) foals were handled daily for 7 days as were IH foals and then handled for 10 min once weekly until weaning. Foals were weaned at 120 +/- 10 days of age. On days 1, 3, and 15 after weaning, foals were subjected to a one-trial learning test. The learning test consisted of placing the foal in a familiar pen with an 1.5 X .6-m apparatus containing 40 15 X 15-cm compartments. Number of visits to the apparatus and compartment visited were recorded for 5 min. A small amount of concentrate feed then was placed in a target compartment, and visits were recorded for an additional 5 min. On day 16 after weaning, foals were subjected to a manageability test in which flight distance from an unfamiliar handler and reaction to a novel stimulus were recorded. Split-plot analysis of variance revealed no treatment differences in performance on the learning test (P > .05). Foal performance on the test was greater on day 15 than on day 1 or day 3 (P < .01). Analysis of variance indicated handling treatment had no effect (P > .05) on foal performance during the manageability test. Results indicate that this preweaning handling regimen has no effect on foal learning ability or manageability as measured by these procedures.