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

Females are larger than males in more species of mammals than is generally supposed. A provisional list of the mammalian cases is provided. The phenomenon is not correlated with an unusually large degree of male parental investment, polyandry, greater aggressiveness in females than in males, greater development of weapons in females, female dominance, or matriarchy. The phenomenon may have evolved in a variety of ways, but it is rarely, if ever, the result of sexual selection acting upon the female sex. The most common selective pressures favoring large size in female mammals are probably those associated with the fact that a big mother is often a better mother and those resulting from more intense competintion among females for some resource than among males. It appears that, in general, more than one such pressure must affect the females of a species, and that their combined effects must not be countered by even stronger selective pressures favoring large size in males, before the result is that of larger size in the female sex. Sexual selection may often be operating upon the male sux in mammals even when it is smaller. Present knowledge about the species of mammals in which females are lager than males is quite rudimentary. Much more information is needed before we will be able to speak of the selective pressures accounting for the phenomenon with any reasomable degree of certainty. Perhaps the most fruitful approach would be a series of field studies of groups of related species in which females are larger in some species and males are larger in others.

Mammals in which females are larger than males.

The different searching tactics of passerine birds foraging for arthropods among the foliage of a northern hardwoods forest result in the capture of different kinds of prey. Five major searching modes are employed by the 11 foliage-foraging bird species in the Hubbard Brook Exper- imental Forest, New Hampshire. These are distinguished primarily by the rates and distances moved by the searching birds and by the types and forms of their prey-attacking maneuvers. These in turn reflect how large an area is scanned, how thoroughly it is searched, and how the bird moves from perch to perch in its search for prey. Mean searching and prey-attacking flight distances are positively correlated, indicating that birds move just far enough on average to take them into areas they have not previously searched visually. Likewise, birds that move rapidly while searching make significantly more prey attacks per unit time and hence encounter prey more often. Slow searchers scrutinize substrates more thoroughly and seem to take more cryptic and often larger prey. The results suggest that there are limitations on the ways that birds can search for and capture arthropod prey among foliage. We hypothesize that constraints imposed by the structure of the vegetation and by the types and abundances of prey determine the available foraging opportunities. Such habitat parameters may affect, in ecological or evolutionary time, the foraging traits of birds that can successfully exploit a particular habitat, and hence influence the patterns of bird habitat selection and community structure.

https://www.jstor.org/stable/pdfplus/1940130.pdf

Foraging Behavior of Forest Birds: The Relationships Among Search Tactics, Diet, and Habitat Structure

Amer. Zool., 18:779-795(1978). Community Organization Among Neotropical Nectar-Feeding Birds Peter Feinsinger Department of Zoology, University of Florida, Gainesville, Florida 32611 AND Robert K. Colwell Department of Zoology, University of California, Berkeley, California 94720 synopsis. Assemblages of neotropical hummingbirds are organized according to parameters of available resources and morphological-behavioral attributes of particular hummingbird species. We distinguish five types of fiowers relative to hummingbird foraging, and we define six community roles for hummingbirds in exploitation of various flower types. These roles are: high-reward trapliners, which visit but do not defend nectar-rich fiowers with long corollas; territorialists, which defend dense clumps of somewhat shorter fiowers; low- reward trapliners, which forage among a variety of dispersed or nectar-poor fiowers; territory-parasites of two types (large marauders and small filchers); and generalists, which follow shifting foraging patterns among various resources. Simple communities on islands usually contain one species of low-reward trapliner or generalist and one territorial species, and sometimes support one high-reward trapliner; often these species are sexually dimorphic. More complex mainland communities support varying numbers of species in different roles, depending on the relative importance and constancy of different flower types. High-reward trapliners are particularly important in forest under-stories, while forest canopies and open habitats support large numbers of shorter-billed, mobile birds filling the other five roles. We conclude by pointing out the many parallels that exist with other consumer groups. Our geographically diverse studies have been aided by numerous persons and organizations. In addition to those acknowledged elsewhere, we are particularly grateful to: J. A. Wolfe and L. A. Swarm, for their continuing contributions to studies on Trinidad and Tobago; S. Naeem and M. Lentrell for technical assistance; M. C. King, for expert assistance in the field and in the preparation of this paper; J. Ter- borgh, for the generous loan of unpublished data; H. A. Ford, for access to an unpublished manuscript; J. A. Brown and A. Kodric-Brown, for comments on the manuscript; A. B. Bolten, for stimulating discussions on organization among neotropical bees; and M. Bon Saint Come, for hospitality and assistance on Mar- tinique. Feinsinger's field work in summer 1975 was financed by a Venture Grant and a Faculty Research Grant from the University of Denver; his current study on Trinidad and Tobago is financed by NSF grant DEB 76-20371. Colwell's work in Chile was supported by the Ford Foundation and by NSF grant GB 31195; his work in Trinidad and Berkeley was partially financed by an NIH Biomedical Sciences grant and by the Committee on Research at the University of California. INTRODUCTION Terrestrial habitats in the American tropics commonly support several species of birds that feed primarily on nectar. Repeated patterns in the morphology and behavior of coexisting species suggest that certain basic principles are involved in the ecological organization of these as? semblages. Investigation of these princi? ples is facilitated by several factors. Nectar production and nutritional value are pre? cisely measurable. Foraging patterns of birds are easily observed and quantified. The coevolution of bill morphology and flower morphology sets certain limits on the patterns of particular species. Since behavioral interactions between birds are also obvious, it is often possible to infer the effect of interference on resource use. The considerable tactability ofthe bird-flower 779

Community Organization Among Neotropical Nectar-Feeding Birds

Evolving relationships between animals and plants are often patchy in their occurrence in both space and time. If the relationship is mutualistic, selection should act to maximize the probability of interaction between the participants. In the mutualistic interactions between temperate fleshy fruits and birds, the timing of fruit ripening affects the probability of seed dispersal by birds. Rapid removal of ripe fruits is important for some summer and fall fruiting species because of a high probability of fruit destruction by invertebrates (Thompson and Willson, 1978). Production of fruits at a time when birds do not remove them may be selectively disadvantageous in that destruction by invertebrates precludes successful dispersal. Therefore, bird-dispersed plants in temperate regions are commonly supposed to ripen their fruits in fall, after the avian breeding season during which birds often concentrate on insects (Morton, 1973) and during the fall migration (Snow, 1971; Stebbins, 1971). This paper examines the selective pressures acting on the seasonal timing of mutualistic interactions between temperate fleshy fruits and birds. Specifically, it (1) compares the removal rates of mid-latitude fruits before, during and after fall bird migrations, (2) analyzes seasonal variation in the ripening patterns of plants in the context of frugivore availability, and (3) provides a theoretical framework for future work on the phenology of temperate fruit/bird interactions. A fleshy

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1558-5646.1979.tb04751.x

Evolution of temperate fruit/bird interactions: phenological strategies

The sexes of the Red—eyed Vireo, an abundant insectivorous bird in eastern deciduous woodlands, differed in their foraging heights, with an overlap of only about 35%. The males foraged higher than the females, and non—random sequences of movements were employed to maintain this separation. The foraging level and the nest height of female Red—eyes was similar, whereas the males foraged closer to their singing perches. Thus the foraging efficiency of both sexes was likely greater in accordance with their differing behavior roles. The "small" territory (1.3—1.7 acres) of this species actually consists of a cylinder extending from the forest canopy to the low understory. The associated White—eyed Vireo (Vireo griseus) was generally separated from Red— eyed Vireo by habitat, and the Yellow—throated Vireo (Vireo flavifrons) overlaps in habitat but possesses structural and behavioral differences which indicate different prey preferences. Foraging behavior of migrating Red—eyed Vireos differs markedly from that of breeding birds. Differences were also noted between summer and winter foraging behavior patterns in the White—eyed Vireo. All three vireo species exhibit species—specific patterns of foraging movements. Two other foliage—gleaning insectivorous birds occupying the same forests, the American Redstart (Setophaga ruticilla) and the Acadian Flycatcher (Empidonax virescens), also possess species—specific foraging behaviors (niche—exploitation patterns).

Penelope Williamson

Papers

Feeding ecology of the Red-eyed Vireo (Vireo olivaceus) and associated foliage-gleaning birds.