Leslie A. Real

Search Theory and Mate Choice. I. Models of Single-Sex Discrimination

Sexual selection depends on differential patterns of mate preference and choice. Little attention has been paid, however, to the manner in which individuals acquire information about the quality of potential mates and how this information is used to form mating decisions. Different decision rules for determining mating preferences often lead to different fitness consequences for the actively searching and choosy sex. In this paper, I compare the expected fitness consequences of two alternative decision-making strategies: a best-of-n strategy (whereby searching individuals choose the best mate from a sample of size n) and a strategy based on sequential sampling (whereby the searching individual establishes a critical mate quality and continues searching until encountering a mate at or above this quality). For the same distribution of potential mate qualities, the sequential-search strategy generates higher expected fitness gains than the best-of-n strategy. This is in contrast to earlier conclusions (e.g., Janetos 1980; Halliday 1983) that the best-of-n is a dominant strategy for mate choice. In the models presented here, the cost of mate search is included; earlier models neglected this important aspect of mate choice, and this difference accounts for the different conclusions. The sequential-search model establishes a critical mate-acceptance level that equates the cost of sampling one additional potential mate and the expected fitness gain from one additional search. As the cost of search increases, the critical threshold decreases. The basic sequential-search model can be extended to include time discounting, finite time horizon, systematic search, learning, variable search costs, and mate responsiveness. In each extension, new critical thresholds for mate acceptability can be established. In some cases, such as an extension to include a finite time horizon, the critical threshold is anticipated to undergo a monotonic decline as search progresses: searchers become less choosy over time if they are unsuccessful at finding mates. The role of the arithmetic mean and variance of fitness among potential mates can be explored within these simple models. As the mean and variance of mate quality within the population increases, the critical threshold for acceptability increases. The basic model and its extensions are subject to easy empirical testing. These models will prove valuable in establishing the link between aspects of individual behavior and population-level selection processes.

The Sustainable Biosphere Initiative: An Ecological Research Agenda: A Report from the Ecological Society of America

In this document, the Ecological Society of America proposes the Sustainable Biosphere Initiative (SBI), an initiative that focuses on the necessary role of ecological science in the wise management of Earths resources and the maintenance of Earths life support systems. This document is intended as a call to arms for all ecologists, but it will also serve as a means to communicate with individuals in other disciplines with whom ecologists must join forces to address a common predicament. This document focuses primarily on the acquisition of ecological knowledge. It identifies the ecological research programs of highest priority and recommends steps required to pursue research objectives. The document also lays the groundwork for improving the communication and application of ecological knowledge. The SBI proposes three research priorities: global change; biological diversity; and sustainable ecological systems.

The Kinetics of Functional Response

Hollings Type II functional-response relationship is presented, and the formulations expressing the underlying organismal interactions which might generate such a relation arc generalized into the Type III response typical of predators showing learning behavior. An equation derived through an analogy with allosteric enzyme kinetics is given which will account for both Type II and Type III responses. The response behavior can be explained by three parameters: maximal feeding rate (F); an affinity constant (G) related to handling times, capture efficiencies, etc.; and the number of encounters (n) a predator must have with a prey item before becoming maximally efficient at utilizing the prey item as a resource. A discussion follows on the biological processes which result in shifts from Type II to Type III functional responses. Most of the learning processes involve changes in predator behavior associated with increasing encounters with prey, thus supporting parameter n as a major determinant in shifting thc functional response. Three hypotheses concerning the values parameters F, G, and n will assume under different ecological situations are posited: (l) food preferences will be reflected in an increase in resource-utilization efficiency (F) and affinity (G) and a decrease in values of n encounters; (2) a size ordering of the predators will result in shifts up in the resource-utilization efficiency (F) and affinity (G); while (3) increasing habitat complexity will generate decreases in n and F.

Fitness, Uncertainty, and the Role of Diversification in Evolution and Behavior

I have shown how a maximum principle for evolutionary processes based solely upon mean fitness of behaviors may, under certain circumstances, be misrepresentative and misleading in the analysis of biological systems. Many of the misrepresentations can be corrected by including variance of fitness associated with environmental uncertainty and variability in the structure of the maximum principle. There are many ways of incorporating this variance into a mathematical model. I have chosen to depict the modified maximum principle as a discounted function of expected fitness, where the discount is a function of the variance in fitness between behaviors. Such a function under no uncertainty reverts to the simpler mean maxim. Including uncertainty generally results in evolutionary strategies consisting of sets of diverse behaviors whose resulting fitness show negative covariance. Under certainty organisms can specialize, showing only single types of behavior. Supporting evidence has been gathered from different areas of ecology and evolutionary biology. The model appears to be consistent with what I believe to be the consequences of uncertainty in sexual processes, herbivory and pollination systems. Future research will, hopefully, demonstrate the general applicability of this approach.

Individual Variation in Nectar Production and Its Effect on Fitness in Kalmia Latifolia

During summer 1986, we assessed the effect of individual variation in per flower nectar production rate on one component of plant reproductive success (percent fruit set) mediated through differential pollinator attractiveness. Thirty-two individual mountain laurels, Kalmia latifolia, were monitored for average 24-h nectar production in individual flowers, average per flower pollinator visitation rate, average floral density, and percent fruit set. Significant positive correlations occurred between (1) average 24-h nectar production by individual flowers and average visitation rate per flower, and (2) average visitation rate per flower and percent fruit set. Pollinators appeared to be differentially attracted to individual shrubs that produce larger quantities of nectar per flower, and the increased visitation rate promoted a higher percentage fruit set because K. latifolia is pollination limited. However, there was no correlation between an individual shrubs 24-h nectar production in 1986 and that same individuals 24-h nectar production in 1985. Thus, while pollinator behavior appears to influence this component of plant reproductive success in Kalmia, any selective advantage to individuals may vary between flowering seasons.

Some Distinguishing Features of Models of Search Behavior and Mate Choice

We develop analytical predictions about how females should behave when either a (infinite time horizon) sequential search tactic or best-of-n rule is employed to search for mates. The likelihood that a male is rejected on first encounter with a female should decline with male quality if females use a sequential search tactic. If females employ a best-of-n rule, however, the probability that a first encountered male is rejected should not depend on male quality; the proportion of males of any quality rejected on first encounter with a female equals the proportion of females that perceive the optimal number of males to be sampled prior to mating to be more than one. The behavior of females using either rule, however, depends on the distribution of quality among potential mates. We show that a variance-preserving increase of mean male quality results in an increase of the critical value of the threshold of acceptance under a sequential search rule; that is, females become more choosy. However, such a shift is not expected to change female search behavior if a best-of-n tactic is employed. A mean-preserving increase of the variance of male quality results in an increase of n among females that employ a best-of-n rule, whereas the number of males sampled prior to mating may increase, decrease, or remain unchanged following such a shift if females use a sequential search rule. The threshold acceptance criterion under sequential search, however, increases (decreases) with a mean-preserving increase (reduction) of the variance of male quality. Patterns of resampling of males by females and the position of accepted males in a search sequence have often been used to distinguish between the best-of-n and sequential search tactics. Predictions of each pattern under a sequential search model depend on both whether the time horizon over which search takes place is assumed to be finite or infinite and whether or not recall (resampling) of previously encountered males is permitted and sometimes resemble predictions of a best-of-n model of search behavior. Because finite time horizon models of sequential search are currently poorly developed, such information is not likely to provide good evidence that females are using either rule The predictions that we derive, however, should be relatively robust to assumptions about recall and the time horizon over which sampling takes place. An experimental protocol in which mate quality is altered may consequently provide a valuable approach to the study of search behavior and mate choice.

Effects of recent experience on foraging decisions by bumble bees

The temporal and spatial scales employed by foraging bees in sampling their environment and making foraging decisions should depend both on the limits of bumble bee memory and on the spatial and temporal pattern of rewards in the habitat. We analyzed data from previous experiments to determine how recent foraging experience by bumble bees affects their flight distances to subsequent flowers. A single visit to a flower as sufficient to affect the flight distance to the next flower. However, longer sequences of two or three visits had an additional effect on the subsequent flight distance of individual foragers. This suggests that bumble bees can integrate information from at least three flowers for making a subsequent foraging decision. The existence of memory for floral characteristics at least at this scale may have significance for floral selection in natural environments.

Patterns of Individual Variability in Floral Resources

The distribution and presentation of floral rewards to insects has been dem- onstrated to influence pollinator visitation rates, pollinator movements, and consequent plant fitness. However, very little is known about the extant variation in nectar rewards among individuals in a single population. Patterns of nectar production within individual shrubs of Kalmia latifolia L. (Ericaceae) were examined during summer 1985, in a Southern Appalachian heath bald in Giles County, Virginia. Five inflorescences from each of 20 shrubs at two sites in the study area were bagged to exclude insect visitors. Twenty-four hour nectar production was measured using microcapillary extractions in each open flower within each of the bagged inflorescences. For each individual we monitored arithmetic mean reward per flower and variance in reward per flower over the flowering season. There is significant variation among individuals in the mean reward/flower at both sites, but the variance in reward/flower is significant at only one site. The mean reward and the variance in reward are positively correlated among shrubs. If the coefficient of variation is used to uncouple the intrinsic correlation between these two plant variables, then no significant differences emerge among shrubs in levels of relative variability at either site.

Information Processing and the Evolutionary Ecology of Cognitive Architecture

The individuals behavioral response to the environment is a consequence of three processes. Information from the environment must be encoded into a form that can be manipulated and stored in the nervous system. Encoded information is then subject to computational operations that lead to a representation of the environment that forms the basis for decisions and actions. The design of information-processing schemes acts as both a constraint on behavioral response and an aspect of the organisms phenotype that may be subject to natural selection. My experiments with bumblebees suggest that individual workers process information about floral rewards according to expected utility theory. Utility is here defined by a biomechanical function relating floral reward sizes to rates of net energetic gain. This model also suggests that bees process information from successive visits so as to maximize expected short-term (rather than long-term) energetic gain. Short-term computational algorithms may prove advantageous when environments are spatially or temporally autocorrelated or when individuals are subject to short-term memory constraints. Short-term computational algorithms may lead to perceptual biases, for example, underestimation of low-probability events. Observations on the computational rules employed by individuals may reveal aspects of nervous system organization and operation. At the same time, the floral preferences that result from individual decision making may have profound consequences for the evolution of pollinator-plant interactions and ecological community organization.

Toward a cognitive ecology

The emergence of cognitive psychology as the dominant approach to understanding human behaviors and actions acknowledges the importance of internal mental operations in generating specific behavioral responses to sets of external stimuli. Traditional behaviorist interpretations that rely primarily on external inputs as the precursors of action have been largely replaced by cognitive approaches. The main intent of this article is to outline the major areas that require exploration if we wish to apply fully the principles and insight of cognitive science to behavioral ecology.