Beginning with Emlen (1966) and MacArthur and Pianka (1966) and extending through the last ten years, several authors have sought to predict the foraging behavior of animals by means of mathematical models. These models are very similar,in that they all assume that the fitness of a foraging animal is a function of the efficiency of foraging measured in terms of some "currency" (Schoener, 1971) -usually energy- and that natural selection has resulted in animals that forage so as to maximize this fitness. As a result of these similarities, the models have become known as "optimal foraging models"; and the theory that embodies them, "optimal foraging theory." The situations to which optimal foraging theory has been applied, with the exception of a few recent studies, can be divided into the following four categories: (1) choice by an animal of which food types to eat (i.e., optimal diet); (2) choice of which patch type to feed in (i.e., optimal patch choice); (3) optimal allocation of time to different patches; and (4) optimal patterns and speed of movements. In this review we discuss each of these categories separately, dealing with both the theoretical developments and the data that permit tests of the predictions. The review is selective in the sense that we emphasize studies that either develop testable predictions or that attempt to test predictions in a precise quantitative manner. We also discuss what we see to be some of the future developments in the area of optimal foraging theory and how this theory can be related to other areas of biology. Our general conclusion is that the simple models so far formulated are supported are supported reasonably well by available data and that we are optimistic about the value both now and in the future of optimal foraging theory. We argue, however, that these simple models will requre much modification, espicially to deal with situations that either cannot easily be put into one or another of the above four categories or entail currencies more complicated that just energy.

Citation classic - optimal foraging - a selective review of theory and tests

Can sexual dimorphism evolve because of ecological differences between the sexes? Although several examples of this phenomenon are well known from studies on birds, the idea has often been dismissed as lacking general applicability. This dismissal does not stem from contradictory data so much as from the difficulties inherent in testing the hypothesis, and its apparent lack of parsimony, in comparison to the alternative explanation of sexual selection. The only unequivocal evidence for the evolution of sexual dimorphism through intersexual niche partitioning would be disproportionate dimorphism in trophic structures (e.g., mouthparts). This criterion offers a minimum estimate of the importance of ecological causes for dimorphism, because it may fail to identify most cases. A review of published literature reveals examples of sexually dimorphic trophic structures in most animal phyla. Many of these examples seem to be attributable to sexual selection, but others reflect adaptations for niche divergence between the sexes. For example, dwarf non-feeding males without functional mouthparts have evolved independently in many taxa. In other cases, males and females differ in trophic structures apparently because of differences in diets. Such divergence may often reflect specific nutritional requirements for reproduction in females, or extreme (sexually selected?) differences between males and females in habitats or body sizes. Ecological competition between the sexes may be responsible for intersexual niche divergence in some cases, but the independent evolution of foraging specializations by each sex may be of more general importance. If ecological causation for dimorphism can be demonstrated in so many cases, despite the inadequacies of the available criteria, the degree of sexual size dimorphism in many other animal species may well also have been influenced by ecological factors. Hence, it may be premature to dismiss this hypothesis, despite the difficulty of testing it.

Ecological Causes for the Evolution of Sexual Dimorphism: A Review of the Evidence

Three major hypotheses, based upon mechanisms of sexual selection, intersexual food competition and reproductive role division, have been advanced to explain the evolution of sexual dimorphism in body size and morphology of animals Genetic models suggest that all of the hypotheses are plausible, and empirical studies demonstrate that each of the three mechanisms operates in natural populations However, problems arise in testing hypotheses for the evolution of sexual dimorphism: more than one mechanism may be operating simultaneously, and the demonstrated occurrence of a mechanism does not indicate that it actually results in selection for dimorphism A recent statistical technique offers a solution to these problems and provides a promising new approach to the study of sexual dimorphism, in which researchers can assess the relative importance of each mechanism in present-day selection for sexual dimorphism within a species

The evolution of sexual dimorphism in animals: Hypotheses and tests

The objectives of this study were to determine responses by red mangrove (Rhizophora mangle) and its primary consumers to nutrient enrichment, to determine if nutrient limitation is responsible for the dwarf plant form of red mangrove, and to evaluate four competing hypotheses for the relation of nutrient status and invertebrate herbivory. In a factorial-designed experiment, I fertilized 48 dwarf trees along gradients of tidal elevation and water depth at Twin Cays, an intertidal mangrove island in Belize, Central America, and measured plant growth responses and herbivory for 2 yr. At the end of year- 2, I compared biomass accumulation and analyzed plant tissue for chemical and structural composition. Dwarf red mangrove trees on this tidal island responded quickly and grew vigorously when treated with P and NPK fertilizers. Leaf number, leaf area, branching, shoot length, and aerial root production increased dramatically over 2 yr. N-fertilized trees grew very slowly and their responses were not different from Control trees. N-fertilized and Control trees changed little over 2 yr. Some responses to phosphorus-containing fer- tilizers vary by water depth and tidal elevation, but physicochemical factors do not explain the differences in growth responses. I conclude that phosphorus availability is a major factor limiting red mangrove growth at my study site in the interior of Twin Cays. Herbivory by two specialized, endophytic insect species (Ecdytolopha sp., which feeds in apical buds, and Marmara sp., which mines stem periderm) increased in P- and NPK-fertilized trees compared to N-fertilized and Control trees. Twice as many apical buds were damaged or destroyed and the frequency of mines increased by 6-8 fold. However, fertilization had no effect on feeding rates and standing damage by a leaf-feeding guild of generalist herbivores or on the frequency of shoots killed by stem borers. Herbivory by the two specialists was not related to C:N ratios, but it was inversely related to concentrations of phenolic com- pounds. Neither nutrient ratios nor concentrations of phenolics affected rates of herbivory by the generalist folivores. Although sclerophylly of red mangrove leaves decreased in P- and NPK-fertilized trees but not in N-fertilized and Control trees, there was no relationship between leaf toughness and herbivory by generalist folivores. These data suggest that sclerophylly in oligotrophic ecosystems may be an adaptive mechanism related to nutrient conservation, and that it is associated with red mangrove survival in phosphorus-deficient soil rather than an adaptation to herbivory.

Effects of Nutrient Enrichment on Growth and Herbivory of Dwarf Red Mangrove (Rhizophora Mangle)

Effects of nutrient enrichment on growth and herbivory of dwarf red mangrove

Both tannin and fat concentration in acorns have been postulated to affect squirrel preferences and foraging strategy. Our study directly tests the effects of tannin and fat on the food preferences of free—ranging squirrels (Sciurus carolinensis). Using food items processed from white oak (Quercus alba) acorns with various amounts of tannin and fat added, we find that added tannin significantly reduces the probability that a food item will be eaten and the length of time spent eating it. The addition of fat is shown to attenuate the effects of tannin, but the overall effect is that squirrels do not forage in a manner that maximizes daily energy intake. These results are contrary to the findings of previous studies. A hypothesis to presented to account for the apparent contradiction: The squirrels may maximize energy intake over the entire fall/winter season rather than over a shorter period, such as each foraging bout or day.

Grey Squirrel Food Preferences: The Effects of Tannin and Fat Concentration

We tested the alternative hypotheses that the different foraging niches of male and female Downy Woodpeckers (Picoides pubescens) (1) were determined primarily by genetic prefer- ence, or (2) were due to displacement of foraging behavior of one sex in the presence of the other. In one experimental woodlot where males had been removed, the remaining females became male- like in their foraging behavior. Specifically, females selected branch diameters, foraging heights, and substrate angles normally used only by males. In a second experimental woodlot where females were removed, males did not change their foraging behavior. Controls supported the conclusion that the change in foraging by females could only have been caused by the removal of males. These results support the hypothesis that sex-specific foraging niches in Downy Woodpeckers are caused by female avoidance of the foraging microhabitat of socially dominant males. Our results also refute the hypothesis that intersexual niche partitioning in the Downy Woodpecker occurs because each sex has a different foraging microhabitat that is genetically determined.

Wm. David Peters

Papers

Grey Squirrel Food Preferences: The Effects of Tannin and Fat Concentration

An Experimental Analysis of Sex-Specific Foraging in the Downy Woodpecker, Picoides Pubescens