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Journal ArticleDOI

Sexual size dimorphism in hawks and owls of North America

01 Jan 1976-Ornithological Monographs (American Ornithologists' Union)-Vol. 20, Iss: 20, pp 196
About: This article is published in Ornithological Monographs.The article was published on 1976-01-01. It has received 249 citations till now. The article focuses on the topics: Sexual dimorphism.
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Journal ArticleDOI
TL;DR: Present knowledge about the species of mammals in which females are larger than males is quite rudimentary and much more information is needed before the authors will be able to speak of the selective pressures accounting for the phenomenon with any reasonable degree of certainty.
Abstract: 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.

645 citations

Journal ArticleDOI
01 Oct 1989-The Auk
TL;DR: A number of univariate and multivariate measures of body size used commonly in ornithological research are compared, including eightMultivariate measures (from principal components analyses), plus skull length, ulna length, tibiotarsus length, wing length, and weight.
Abstract: --We compared a number of univariate and multivariate measures of body size used commonly in ornithological research, including eight multivariate measures (from principal components analyses), plus skull length, ulna length, tibiotarsus length, wing length, and weight. Analyses are based on 26 measurements on three randomly selected male and three randomly selected female Savannah Sparrows (Passerculus sandwichensis) from each of 53 different geographic localities throughout the species' range. Six of the eight principal components analyses provided essentially the same information about body size. Analyses based on the variance-covariance matrix of raw or log-transformed data provided first axes that varied most from the other multivariate estimates of size. Among the univariate measures, ulna length, wing length, and body weight contributed information that diverged from the multivariate measures of overall size. Weight better represents general size (i.e. PC I) than wing length, but because of variation in reproductive condition, weight is a far better measure in males than in females. Wing length is not a representative measure of body size. Inasmuch as each principal components analysis provides information about body size on PC I, we encourage researchers to choose among the various approaches according to analytical objectives rather than methodological simplicity or general utility. Received 29 November 1988, accepted 18 May 1989. ORNITHOLOGISTS are frequently faced with the challenge of measuring body size in birds. A measure of overall size is required to test hypotheses predicting patterns of geographic variation (e.g. Bergmann's or Allen's rules; James 1970, Johnston and Selander 1971, Niles 1973, Fleischer and Johnston 1982, Handford 1983, Murphy 1985). An estimate of body size is also required to test hypotheses about the evolution of sexual dimorphism in body size (e.g. Hamilton and Johnston 1978, Johnston and Fleischer 1981, Fleischer and Johnston 1984, McGillivray and Johnston 1987, Rising 1987b). In addition, species must be ranked by body size to test models that predict size ratios among coexisting species in ecological communities (e.g. Ricklefs and Cox 1977, Ricklefs and Travis 1980, Haefner 1981, Sabo and Holmes 1983, Miles and Ricklefs 1984, PullJam 1985, Brown and Maurer 1986, Miles et al. 1987). In physiology, standard measures of metabolic activity are frequently expressed as a function of body size, and it is often useful to examine the relationship of structures or organs relative to overall body size (e.g. Fisher 1947, Kendeigh 1976, Blem 1984, Calder 1984, Paladino 1985, Rising 1987a, Packard and Boardman 1988). Body size, however, is difficult to measure. Perhaps the best measure of overall body size is total mass, but reliable information on mass is often difficult to obtain. Although recent compilations of data contribute much to our knowledge of the mass of birds (Clench and Leberman 1978, Dunning 1984), the available data on mass are all too often unsatisfactory because of seasonal and diet-related variability (e.g. Niles 1973). Consequently, ornithologists commonly use a measure of wing length as an estimate of relative body size (e.g. James 1970, Lack 1971, Snyder and Wiley 1976, Payne 1984, Jehl and Murray 1986, Zink and Remsen 1986). Wing length is measured easily on museum study skins and living birds; however, at least in some cases, it is a poor estimate of body size when compared with other, more precise measurements (Rising 1988). Even discounting measurement error, many factors that are difficult to quantify affect the wing length of a bird. First, wing feathers are subject to wear. Thus, the reliability of measurements of wing length decreases as the feathers progressively become more worn. This may be especially important in studies of sexual dimorphism, because in many species behavioral differences between the sexes lead to sexual differences in rates of feather wear. Second, the wing length of an individual varies from year to year--even though the bird's skeleton is completely ossified (and thus, in this sense, the bird is completely grown). For example, Rising (unpubL data) captured and measured (to the nearest mm) wild Savan666 The Auk 106: 666-674. October 1989 October 1989] Measurement of Body Size 667 nah Sparrows (Passerculus andwichensis) over many years. The average of the differences of measurements of wing length of males captured twice or more during the same year is -0.13 mm (range -4 to +2 mm, n = 32, SE = 0.24), but the average differences in wing length of males captured and measured during more than one year is +1.17 (range -2 to +6 mm, n = 23, SE = 0.43). A sign test shows that within seasons there are as many positive changes as negative changes in the measured wing lengths of individuals. Among years, wing lengths appear to increase more often from year to year than decrease (P = 0.05). Thus, wing length, to some extent, increases with age. Because of the difficulties of obtaining accurate information about body size from wing-length data, people have often used measures of individual bones (e.g. Johnston and Selander 1971) or of organ weight (e.g. Power 1970) as estimates of body size. Alternatively, ornithologists have computed combinations of characters (e.g. the sum of many measures; McGillivray and Johnston 1987, Rising 1987b), or multivariate measures (such as principal component or discriminant function scores; Johnston and Selander 1971, Niles 1973, Zink 1986, Rising 1988) that account for the covariation among characters and extract a \"size\" axis. There has been considerable discussion concerning which of the models of principal components analysis best extracts a \"size component\" (Jolicoeur 1963, Mosimann 1970, Mosimann and James 1979, Bookstein et al. 1985, Somers 1986, Rohlf and Bookstein 1987). Here we empirically compare size axes from eight different principal components models and five univariate measures of body size, including mass (weight) and wing length, to determine the relative similarity of these estimates of overall body size.

435 citations

Journal ArticleDOI
TL;DR: Like many other such hypotheses, the fecundity-advantage model seems to have achieved the status of conventional wisdom, primarily on the bases of its simplicity and its ability to predict he directions of sexual size dimorphism on this very broad level.
Abstract: In most species of animals, females attain larger body sizes than do males. This generality was established for many groups (e.g., spiders, insects, fishes, amphibians, reptiles) by Darwin (1874), and it has been supported and extended by many subsequent workers. Even for taxa such as mammals and birds, in which males frequently are the larger sex, there are many examples of greater size in females (e.g., Ralls 1976; Snyder and Wiley 1976). The evolutionary advantages to large body size in females have attracted considerable speculation. Darwin proposed a general explanation for large female size: \"Increased size must be in some manner of more importance to the females.... and this perhaps is to allow the production of a vast number of ova\" (1874, p. 332; see also p. 275 for a similarly worded argument). Darwin invoked additional selective forces for specific groups: for example, he believed that combat between females may favor large body size in some bird species and that the smaller size of male insects is adaptive in allowing them to hatch (and be ready to mate) before the emergence of the larger females. Darwin's general hypothesis on the evolution of large body size in femalesthis character has been favored because larger females can produce more offspring-has enjoyed wide support. This \"fecundity advantage\" model has been used to explain females growing larger than males in zooplankton, insects, fishes, amphibians, reptiles, birds, and most animal species (Williams 1966; Gibbons 1972; Trivers 1972; Crump 1974; Clutton-Brock and Harvey 1978; Shine 1979; Berry and Shine 1980; Veuille 1980; Fitch 1981, 1985; Semlitsch and Gibbons 1982; Gilbert and Williamson 1983; Woolbright 1983; Mueller and Meyer 1985; Hughes and Hughes 1986), and the lack of such a fecundity advantage has been proposed as the reason for the relatively small size of females in many birds and mammals (Trivers 1972). Like many other such hypotheses, the fecundity-advantage model seems to have achieved the status of conventional wisdom, primarily on the bases of its simplicity and its ability to predict he directions of sexual size dimorphism on this very broad level. However, I am not aware of any previous attempts to examine closely the logic and assumptions of the hypothesis or to test the idea by deriving and testing predictions about sexual size dimorphism.

361 citations


Cites background from "Sexual size dimorphism in hawks and..."

  • ...Even for taxa such as mammals and birds, in which males frequently are the larger sex, there are many examples of greater size in females (e.g., RaIls 1976; Snyder and Wiley 1976)....

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Journal ArticleDOI
TL;DR: Rensch's rule is shown to be associated with male-biased SSD, which is consistent with the hypothesis that sexual selection acting on male size drives the evolution of this pattern of allometry.
Abstract: Rensch's rule states that sexual size dimorphism (SSD) increases with body size (hyperallometry) in taxa in which males are the larger sex and decreases with body size (hypoallometry) in those in which females are larger. We use the independent contrasts method to assess the validity and generality of Rensch's rule within 21 independent animal taxa. Allometry is estimated as the slope of the major axis regression of contrasts for log(female size) versus contrasts for log(male size). Allometry consistent with Rensch's rule is significant in 33% of the taxa examined across a diverse range of invertebrate and vertebrate taxa. Significant allometry inconsistent with Rensch's rule occurs in only one taxon. Meta-analysis of these results reveals that Rensch's rule is general and highly significant. Only owls have allometry inconsistent with this trend. Rensch's rule is also shown to be associated with male-biased SSD, which is consistent with the hypothesis that sexual selection acting on male size drives the e...

361 citations

Journal ArticleDOI
TL;DR: To explain the adaptive significance of sex role partitioning and reversed sexual size dimorphism among raptors, owls and skuas, where females are usually larger than males, this work combines several previous hypotheses with some new ideas.
Abstract: To explain the adaptive significance of sex role partitioning and reversed sexual size dimorphism among raptors, owls and skuas, where females are usually larger than males, we combine several previous hypotheses with some new ideas. Owing to their structural and behavioural adaptations for prey capture, predatory birds have better prospects than other birds of defending their offspring against nest predators. This makes sex role partitioning advantageous; one parent guards the offspring while the other forages for the family. Further, among predators hunting alert prey such as vertebrates, two mates because of interference may not procur much more food than would one mate hunting alone. By contrast, two mates feeding on less alert prey may together obtain almost twice as much food as one mate hunting alone. For these reasons, partitioning of breeding labours might be adaptive only in predatory birds. An initial imbalance favours female nest guarding and male foraging: the developing eggs might be damaged if the female attacks prey; their mass might reduce her flight performance; she must visit the nest to lay; and the male feeds her before she lays (‘courtship feeding’). Increased female body size should enhance egg production, incubation, ability to tear apart prey for the young, and, in particular, offspring protection in predatory birds. Efficient foraging during the breeding period then becomes most important for the male. This imposes great demands on aerial agility in males, particularly among predators of agile prey. Flight performance decreases with increasing size in five of six aspects explored. The male must therefore not be too large in relation to the most important prey. For these reasons, he should be smaller than the female. Among predatory birds, size dimorphism increases with the proportion of birds in the diet, which may be explained as follows. Adult birds have mainly one type of predators: other predatory birds. Because almost only these specialists exploit adult birds, they carry out most of the cropping of this prey. A predator of easier prey competes with many other kinds of predators, which considerably reduce prey abundance in its territory. This is not so for predators of adult birds. Further, because birds are extremely agile, the specialized predator can hunt efficiently only within a limited size range of birds, whose flight skill it can match. Increased size dimorphism among these predators therefore should be particularly important for enlarging the combined food base of the pair. A bird specialist may consume much of the available prey in the suitable size range during the breeding period. When the predator's young are large enough to defend themselves, the female aids better by hunting than by guarding the chicks. It is advantageous among bird specialists if she hunts prey of other sizes than does the male, who has by then reduced prey abundance in his prey size class. But among predatory birds hunting easier prey the female gains little by hunting outside the male's prey spectrum, because other kinds of predators will have reduced the prey abundance outside as well as inside the male's preferred size range. Intra-pair food separation through large sexual size dimorphism therefore should be particularly advantageous among predators of birds. This may be the main reason why the degree of size dimorphism increases with the dietary proportion of birds.

335 citations

References
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Journal ArticleDOI
03 Apr 2008-Ibis
TL;DR: There is a strong tendency for those young which are hatched earliest in the season to have the greatest chance of surviving to breed, and not all species are likely to be prevented, by food shortage, from breeding at the best time for raising young.
Abstract: Summary Examination of survival rdtes of nestlings and fledglings of some species show that there is a strong tendency for those young which are hatched earliest in the season to have the greatest chance of surviving to breed. Since natural selection so strongly favours parents who leave many surviving young, the question arises as to why other birds breed later than the date at which they could most successfully raise their young. It is suggested that the food supply for the breeding females immediately prior to the breeding season may limit their ability to form eggs and the females may thus not be able to lay at the time which would result in young being in the nest at the best time for raising them, but as soon after this time as the female is able to produce her eggs. Not all species are likely to be prevented, by food shortage, from breeding at the best time for raising young and the groups of birds most likely to be affected are discussed.

1,364 citations

Journal ArticleDOI
TL;DR: Most cases of polygyny in birds, a group in which monogamy is the most common mating pattern, can be explained on the basis of the model, and those cases not apparently fitting into the predictions are clearly indicated.
Abstract: Predictions from a theory assuming mate selection on the part of females, which maximizes reproductive success of individuals, are found to accord closely, though not completely, with known mating patterns. These predictions are that (1) polyandry should be rare, (2) polygyny should be more common among mammals than among birds, (3) polygyny should be more prevalent among precocial than among altricial birds, (4) conditions for polygyny should be met in marshes more regularly than among terrestrial environments, (5) polygyny should be more prevalent among species of early successional habitats, (6) polygyny should be more prevalent among species in which feeding areas are widespread but nesting sites are restricted, and (7) polygyny should evolve more readily among species in which clutch size is strongly influenced by factors other than the ability of the adults to provide food for the young. Most cases of polygyny in birds, a group in which monogamy is the most common mating pattern, can be explained on...

1,363 citations

Journal ArticleDOI
03 Apr 2008-Ibis
TL;DR: Evidence is presented to support the hypothesis that communal roosts, breeding colonies and certain other bird assemblages have been evolved primarily for the efficient exploitation of unevenly-distributed food sources by serving as “information-centres”.
Abstract: Summary Evidence is presented to support the hypothesis that communal roosts, breeding colonies and certain other bird assemblages have been evolved primarily for the efficient exploitation of unevenly-distributed food sources by serving as “information-centres”. Predation-pressure is regarded as being the most important factor “shaping” the assemblages. The shaping involves the choice of inaccessible or otherwise safe sites, optimum dispersal, mutual awareness of attack and joint defensive tactics, and serves to minimise the vulnerability to predation which would otherwise result when birds mass together in conspicuous, and often predictable centres.

977 citations

Journal ArticleDOI
TL;DR: Evidence of an adaptive function of sexual dimorphism in size in woodpeckers is presented by relating degrees of morphologicalDimorphism and sexual divergence in foraging behavior in two melanerpine species, the stronglyDimorphic Hispaniolan Woodpecker of Haiti and the Dominican Republic and the moderately dimorphic Golden-fronted Woodpeker of continental North and Central America.
Abstract: Adaptive radiation has been defined as the evolutionary divergence of members of a phyletic line into different niches or adaptive zones (Mayr, 1963:633). Although it has been customary to think of adaptive radiation solely in terms of species or races, a growing body of evidence indicates that some degree of radiation occurs also within populations, as individuals come to occupy different subniches or adaptive subzones, subdividing and, perhaps, expanding the total niche or zone utilized by the population. Probably all species show some degree of ecological variation, either polymorphic or continuous. But this phenomenon is being studied in only a few groups of organisms, notably in Drosophila, in which chromosomal polymorphism has been interpreted as a. means of adaptation of populations to heterogeneous environments (Dobzhansky, * 1961, 1963, 1965). Theoretical bases for research on ecological variation in animal populations have been provided by Ludwig (1950), Levene (1953)) da Cunha and Dobzhansky (1954), Dempster (1955), Li (1955), Carson (1959), and Levins (1962, 1963). In birds, as in other vertebrates, the sexes usually differ in size if not also in proportions of body parts, including those used in feeding (Amadon, 1959) ; and, especially where the degree of sexual dimorphism, which is a form of polymorphism (Ford, 1961: 12), is marked, it seems probable that the morphological divergence has ecological significance in adapting the sexes to different subniches. However, there is only an occasional reference in the literature to sexual dimorphism in relation to niche utilization (e.g., Pitelka, 1950; Rand, 19.52), and, in general, the whole problem of ecological variation in populations has been neglected by vertebrate ecologists. The primary purpose of this report is to present evidence of an adaptive function of sexual dimorphism in size in woodpeckers by relating degrees of morphological dimorphism and sexual divergence in foraging behavior in two melanerpine species, the strongly dimorphic Hispaniolan Woodpecker (Centurus striatus) of Haiti and the Dominican Republic and the moderately dimorphic Golden-fronted Woodpecker (Ce&zmus awifrons) of continental North and Central America. In addition, the paper surveys other evidence that sexual dimorphism in birds is related to differential niche utilization. Finally, some evolutionary aspects of sexual dimorphism and ecological variation are considered.

789 citations


"Sexual size dimorphism in hawks and..." refers background in this paper

  • ...Many recent authors (e.g. Rand 1952; Storer 1966; Selander 1966, 1972; Earhart and Johnson 1970) have argued that size dimorphism in raptors is primarily a device to reduce intersexual competition for food, but reduction of competition does not in itself explain the reversed nature of the…...

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Journal ArticleDOI
01 Jan 1968-Ecology
TL;DR: The smaller spatial needs for omnivorous and herbivorous birds of a given biomass and perhaps the greater patchiness of their food when compared to predators are used to explain the higher occurrence of gregarious nesting in the former group.
Abstract: This analysis deals with size variations in the breeding territories of land birds which obtain most or all of their food on the territory. For the species studied as a whole, territory size shows a strong positive relationship to body weight. Predators tend to have larger territories than omnivores or herbivores of the same weight, presumably due to the relatively denser food of the latter species. The home ranges of raptors inhabiting two areas were found to be significantly correlated with an index of the numerical density of their prey and in one area with raptor weight. Higher clutch size is not significantly associated with larger territories in any category of birds tested. The number of individuals defending the territory and the number feeding the young are probably not correlated with territory size. The exponential relationships between body weight of the consumer and three dependent variables–food biomass consumed per unit time, average prey weight and territory or home range size–are used to derive three predictions: a) Heavier predators take fewer individuals per unit time than lighter species; b) If certain restrictions are satisfied, the collective biomasses in a given large area increase as individual biomasses become larger for omnivorous species and decrease as individual biomasses increase for predators; c) For predators, the density of acceptable and accessible food in biomass per unit area decreases as the weight of the consumer increases. Territory or home range size increases more rapidly with body weight for predators than for omnivores or herbivores. This relationship holds true for both birds and mammals and presumably reflects a rapidly decreasing food density for predators of increasing weight. Since smaller predators do not feed over a wider range of food size than larger species, and since there are less species or individuals feeding on large food than on small, the predators of the areas studied probably consume food whose distribution of biomass with food size is declining. The habit of feeding on exclusive areas is considerably more widespread among predators than among omnivores and herbivores. The smaller spatial needs for omnivorous and herbivorous birds of a given biomass and perhaps the greater patchiness of their food when compared to predators are used to explain the higher occurrence of gregarious nesting in the former group. Implications of this study for the functions of feeding territories maintained during the breeding season are discussed.

618 citations