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1.
Patterns of fluctuating asymmetry in beetle horns: an experimental examination of the honest signalling hypothesis 总被引:6,自引:0,他引:6
Recent theoretical arguments have claimed that negative relationships between the size and symmetry of secondary sexual traits
are indicative of honest signalling of male quality. The patterns of fluctuating asymmetry in beetle horns have been proposed
to support the honest signalling hypothesis. Here we examine three assumptions of the hypothesis, (1) that traits are costly
to produce; (2) the levels of fluctuating asymmetry are indicative of stress imposed during development; and (3) that males
with larger traits should have more symmetrical traits, using the horned beetle, Onthophagus taurus. Experimental manipulations of brood mass were used to manipulate horn size and asymmetry. The development of horns was found
to be environmentally determined and costly in terms of delayed development and increased risk of pre-adult mortality. Decreasing
resource availability increased relative horn asymmetry. However, horn height was positively related to absolute horn asymmetry.
While the results do support the hypothesis that sexual selection on secondary sexual traits should increase levels of fluctuating
asymmetry, they provide no support for the notion that the patterns of asymmetry honestly signal male quality. Horns are used
in disputes between males and may be indicative of male parental investment. Thus, we conclude that while horn size may be
an indication of male quality, the patterns of fluctuating asymmetry are not.
Received: 16 December 1996 / Accepted after revision: 5 May 1997 相似文献
2.
Hanna Kokko 《Behavioral ecology and sociobiology》1997,41(2):99-107
In various models of sexual selection mediated by the viability indicator (“good genes”) mechanism, a sexually selected trait
will truly reflect male quality if its expression is costly for the male. However, in long-lived species, the expression of
a trait often increases with age while the genotype of the male remains unchanged. This fact may obscure the indicator mechanism.
Hitherto, game theory models of honesty in sexual advertisement have not taken life-history effects into account, whereas
life-history models of reproductive effort have only seldom considered the dependence of mating success on the actions of
other individuals. Here, the two approaches are combined, and I examine whether honesty is maintained if males can divide
their advertisement effort over their lifetime. The model shows that an increase in the expression of the sexually selected
trait over several years is an evolutionarily stable strategy (ESS) under a wide range of situations, so that a correlated
preference for old age can emerge through a viability indicator mechanism. Honesty in the strict sense is not preserved: an
optimally behaving low-quality male will in some cases advertise more than a high-quality male of equal age, to the extent
that the strongest advertisement found in the population can be associated with a low-quality male. Due to life-history trade-offs,
however, honesty in an average sense holds true over the lifetime of individuals: “cheater” age classes will remain small
enough, that a female will obtain a higher expected mate quality if she trusts in the trait as an indicator of viability.
Received: 25 June 1996 / Accepted after revision: 15 April 1997 相似文献
3.
According to indicator models of sexual selection, mates may obtain indirect, i.e. genetic, benefits from choosing partners indicating high overall genetic quality by honest signals. In the scorpionfly Panorpa vulgaris, both sexes show mating preferences on the basis of the condition of the potential partners. Females prefer males that produce nuptial gifts (i.e. salivary secretions) during copulation, while males invest more nuptial gifts in females of high nutritional status. Both characters, males' ability to produce nuptial gifts and high nutritional status of females, are known to be reliable indicators of foraging ability. Thus, besides possible direct benefits, both sexes might also obtain indirect benefits in terms of “good foraging genes” by their choice and thereby increase the fitness of their offspring. A prerequisite for this possibility is the heritability of the respective trait. In the present study, we estimated the repeatability and the heritability of foraging ability. Our results indicate (1) a significant repeatability of individual foraging efficiencies in males and females and (2) a heritable component of this trait by a significant parent–offspring regression. These findings suggest that genetic benefits in terms of increased offspring foraging ability might contribute to selection for mating preferences in both sexes. 相似文献
4.
Asymmetries in courtship signals can result from both developmental instability during ontogeny and from temporary or permanent
damage following mating, fighting, or interactions with predators. These two types of asymmetries, which can be divided into
fluctuating asymmetry (FA) and damage asymmetry (DA), have both been suggested to play an important role in mate choice as
potential honest indicators of phenotypic and/or genetic quality, while at the same time, DA may affect ornament asymmetry
in a random manner. Interestingly, despite the massive research effort that has been devoted to the study of asymmetry during
the past decades, very little is known about how an individual’s behaviour relates to asymmetry. Here, we measure and characterise
asymmetry in morphological courtship signals in Corynopoma riisei, a fish where males carry elaborate paddle-like appendices on each side of the body that they display in front of females
during courtship. Moreover, we investigate whether male courtship display, employing this bilateral morphological trait, reflects
trait asymmetry. Finally, we assess whether males respond to phenotypic manipulations of DA with corresponding changes in
courtship behaviour. We show that male display behaviour is asymmetric in a manner that reflects asymmetry of their morphological
courtship trait and that male display behaviour responds to manipulations of asymmetry of these paddles. Our results thus
suggest that males preferentially use their best side and, hence, that males respond adaptively to temporary changes in signal
trait asymmetry. 相似文献
5.
Female preference for apparently symmetrical male sexual ornaments in the barn swallow Hirundo rustica 总被引:4,自引:0,他引:4
Anders Pape Møller 《Behavioral ecology and sociobiology》1993,32(6):371-376
Summary The outermost tail feathers of barn swallows Hirundo rustica apparently reliably signal the quality of males, because individuals with the longest tails have the lowest degree of fluctuating asymmetry (random deviations from symmetry in the otherwise symmetrical tail trait) despite the size of their secondary sexual character. I experimentally tested whether females preferred males with symmetrical tails without altering the aerodynamic properties of birds by painting the tips of the outermost tail feathers with white or black correction fluid. Unmated males were randomly assigned to one of four treatments: (i) asymmetrical tails, where the outermost 20 mm of one tail feather was painted white and the other black; (ii) symmetrical tails where the outermost 10 mm of both tail feathers was painted white (symmetric I); (iii) symmetrical tails where the outermost 20 mm of both tail feathers was painted white (symmetric II); or (iv) controls where the outermost 20 mm of both tail feathers was painted black. The experimental treatment affected the duration of the premating period since it took longer for asymmetrical males to acquire a mate than for either group of symmetrical males or control males. This gave rise to a delayed start of laying among males with apparently asymmetrical tails. The seasonal production of fledglings therefore decreased from control males through males with either symmetrical treatment to males with the asymmetrical treatment. Females therefore pay direct attention to the level of fluctuating asymmetry in secondary sexual characters even when the asymmetry does not affect the aerodynamic properties of males. 相似文献
6.
Merja Otronen 《Behavioral ecology and sociobiology》1998,42(3):185-191
The significance of male asymmetry in postcopulatory sexual selection was studied in the fly Dryomyza anilis by examining whether male asymmetry is related to fertilization success. The traits measured were wing length, tibia length
and the length of small and large claspers. The male claspers are situated at the tip of the abdomen, functional pair of claspers
consists of a large and a small clasper on the same side of the body. These claspers are used to tap the female abdomen after
sperm transfer, which has been shown to increase fertilization success for the mating male. Fertilization success was negatively
related to the fluctuating asymmetry of wing length, suggesting either female preference for more symmetrical males or a relationship
between male asymmetry and intrasexual selection which was reflected in mating performance. Fertilization success was also
related to the length of small claspers, decreasing with increasing length of the claspers. In addition, males with asymmetrical
small claspers enjoyed higher fertilization success than symmetrical ones. This study shows that fluctuating asymmetry in
wing length is an important fitness trait in postcopulatory sexual selection. Since male tapping affects sperm distribution
in the female's sperm storage organs, the higher fertilization success of males with asymmetrical small claspers could have
a functional relationship with the asymmetrical position of female sperm storage organs.
Received: 6 March 1997 / Accepted after revision: 8 November 1997 相似文献
7.
Oddmund Kleven Frode Jacobsen Rasa Izadnegahdar Raleigh J. Robertson Jan T. Lifjeld 《Behavioral ecology and sociobiology》2006,59(3):412-418
In socially monogamous species, extra-pair paternity has the potential to increase the variance in male reproductive success,
thereby affecting the opportunity for sexual selection on male extravagant ornamentation. In the European barn swallow (Hirundo rustica rustica), the tail streamer length is a sexually selected male ornament and an honest indicator of viability. The North American
barn swallow (Hirundo rustica erythrogaster) also shows sexual dimorphism in tail streamer length, but whether this trait holds the same signalling function in this
subspecies is a controversial issue, and the available literature is presently scarce. Here, we present data on paternity
in the North American barn swallow, including a complete sampling of extra-pair sires in four colonies. We analysed how extra-pair
paternity affected the variance in male fertilization success and examined whether male tail streamer (i.e. the outermost
tail feather) length correlated with fertilization success (n=86 males). Extra-pair paternity constituted 31% of all offspring and significantly increased the variance in male fertilization
success. The number of offspring sired by extra-pair males accounted for almost half of the total variance in male fertilization
success. Males with naturally long tail streamers had a higher fertilization success than males with shorter tail streamers,
and this pattern was mainly caused by a higher extra-pair success for males with long tail streamers. Males with long tail
streamers also paired with early breeding females in prime body condition. These results are consistent with the idea that
there is directional sexual selection on male tail streamer length, possibly mediated through male extra-pair mating success
or the timing of breeding onset. 相似文献
8.
The hypothesis that sexual ornaments are honest signals of quality because their expression is dependent on hormones with
immune-depressive effects has received ambiguous support. The hypothesis might be correct for those signals that are carotenoid-dependent
because the required carotenoid deposition in the signal, stimulated by testosterone, might lower the carotenoid-dependent
immune defence of the organism. Two pathways underlying this androgen-dependent honest signaling have been suggested. Firstly,
androgens that are needed for ornament expression may suppress immune defence, a cost that only high-quality animals can afford.
Alternatively, immune activation may downregulate the production of androgens in low-quality individuals. Which of these alternatives
is correct, and to what extent these effects are mediated by the different metabolites of androgens, remain open questions.
To provide answers to these questions, we manipulated the levels of testosterone (T), 5α-dihydrotestosterone (DHT), and 17-β-estradiol
(E2) in diamond doves Geopelia cuneata, a species in which both sexes exhibit a carotenoid-dependent, androgen-regulated red–orange periorbital ring of bare skin.
On the first day of the experiment (day 0), we inserted steroid-releasing implants into groups of birds and on day 14, we
subjected half of the birds to an immunological challenge by immunizing them with sheep red blood cells (SRBC). In females,
but not in males, androgen but not estradiol treatments reduced antibody production to SRBC. In addition, the immunological
challenge reduced redness and size of the trait as well as androgens levels in both sexes and in all treatments. This indicates
that an immunological challenge can lower circulating T at the cost of the trait expression. These findings are in accordance
with both pathways postulated in the immunocompetence-handicap hypothesis, but do not entirely support the idea that the immunosuppressive
effect of androgens yields honest signaling since both T and DHT were not immunosuppressive in males, for which sexual signaling
is supposed to be especially important. 相似文献
9.
Testis size variation in the greenfinch Carduelis chloris : relevance for some recent models of sexual selection 总被引:3,自引:0,他引:3
Interspecific evidence that testis size responds to selection caused by sperm competition has been obtained from many taxa. However, little is known about the sources of intraspecific variation in testis size, although such variation may have functional significance. Variation in testis size and asymmetry was studied within and between eight geographically separated (and genetically differentiated) populations of greenfinches Carduelis chloris. The relationships between testis size and plumage brightness (degree of yellowness) and the prevalence of haematozoan infections were also investigated in three of these populations, as they related to the predictions of the immunocompetence handicap hypothesis, and Møller's hypothesis relating directional testis asymmetry to phenotypic quality. There were large differences between populations in testis size, with males from northern populations having larger testes than those from southern populations. Within populations, large testes were associated with larger body size and greater age. When the influence of these factors was removed statistically, males with large testes were more likely to be infected with haematozoan parasites, and had brighter yellow plumage. No evidence was found that directional asymmetry in testis size was related to either of these measures of phenotypic quality. These results are consistent with the hypothesis that males with large testes, while signalling higher phenotypic quality as revealed by increased plumage brightness, also pay a cost in terms of reduced immunocompetence, revealed by the increased probability of infection in these males. That these patterns were similar in three different populations adds further strength to these conclusions. Our results suggest that studying the sources of variation in testis size among individuals can reveal interesting processes in sexual selection. 相似文献
10.
Sjouke A. Kingma István Szentirmai Tamás Székely Veronika Bókony Maarten Bleeker András Liker Jan Komdeur 《Behavioral ecology and sociobiology》2008,62(8):1277-1288
Melanin-based ornaments are often involved in signaling aggression and dominance, and their role in sexual selection is increasingly
recognized. We investigated the functions of a melanin-based plumage ornament (facial ‘mask’) in male Eurasian penduline tits
Remiz pendulinus in the contexts of male–male aggression, mating success, and parental care. The penduline tit is a passerine bird with a
unique mating system in which both sexes may mate with several mates in a breeding season, and one (or both) parent deserts
the clutch. Our study revealed that mask size of males is more likely an honest signal used by females in their mate choice
decisions than a trait involved in male–male competition. First, mask size increased with both age and body condition, indicating
that the mask may signal male quality. Second, males with larger masks paired more quickly and had more mates over the breeding
season than males with smaller masks. Third, we found no evidence that male mask size signals male–male aggression or dominance
during competitive encounters. The increased mating success of large-masked males, however, did not translate into higher
reproductive success, as nestling survival decreased with mask size. Therefore, we conclude that there is either no directional
selection on male mask size or males with larger masks receive indirect, long-term benefits. 相似文献
11.
Individuals under stress and those of intrinsically low quality may have insufficient resources to invest in developing equally on both sides. Such individuals may thus display higher levels of fluctuating asymmetry (FA)—small, random departures from perfect bilateral symmetry. These small deviations from symmetry are often less than 1 % of trait size, similar to variation in symmetry generated by measurement error. Studies regarding FA must thoroughly assess measurement error if reliable conclusions regarding FA and fitness parameters are to be drawn. Although many studies regarding facial symmetry in humans have been conducted, few have been carried out on non-human primates. The primary aim of this study was to assess whether facial FA in a non-human primate, the olive baboon (Papio anubis) can be reliably measured from digital photographs. Facial FA was measured in three sets of bilaterally symmetrical landmarks from digital images of 35 olive baboons at Gashaka-Gumti National Park, Nigeria. Measurement error was found to be low indicating a high level of reliability based on comparisons of values from two photos of the same individual (r?=?0.85). Measurement error was found to be related to the size of the face. This suggests that orientation error increases in large faces and could potentially influence FA scores. These findings highlight the need for FA studies to measure and control error caused by orientation and position of subjects if FA is to be of utility to behavioural ecology and conservation (e.g. if FA reflects whether individuals in a population are suffering from environmental or genetic stress). 相似文献
12.
Constance Dubuc William L. Allen Dario Maestripieri James P. Higham 《Behavioral ecology and sociobiology》2014,68(7):1215-1224
Male sexually selected traits can evolve through different mechanisms: conspicuous and colorful ornaments usually evolve through intersexual selection, while weapons usually evolve through intra-sexual selection. Male ornaments are rare among mammals in comparison to birds, leading to the notion that female mate choice generally plays little role in trait evolution in this taxon. Supporting this view, when ornaments are present in mammals, they typically indicate social status and are products of male-male competition. This general mammalian pattern, however, may not apply to rhesus macaques (Macaca mulatta). Males of this species display conspicuous skin coloration, but this expression is not correlated to dominance rank and is therefore unlikely to have evolved due to male-male competition. Here, we investigate whether male color expression influences female proceptivity toward males in the Cayo Santiago free-ranging rhesus macaque population. We collected face images of 24 adult males varying in dominance rank and age at the peak of the mating season and modeled these to rhesus macaque visual perception. We also recorded female sociosexual behaviors toward these males. Results show that dark red males received more sexual solicitations, by more females, than pale pink ones. Together with previous results, our study suggests that male color ornaments are more likely to be a product of inter- rather than intra-sexual selection. This may especially be the case in rhesus macaques due to the particular characteristics of male-male competition in this species. 相似文献
13.
14.
Joanna M. Setchell Marie J. E. Charpentier Issa-Ben Bedjabaga Patricia Reed E. Jean Wickings Leslie A. Knapp 《Behavioral ecology and sociobiology》2006,61(2):305-315
Honest advertisement models of sexual selection propose that exaggerated secondary sexual ornaments are condition-dependent, and that only individuals with superior disease resistance will be able to express costly ornamentation. Studies of secondary sexual ornamentation and their maintenance by sexual selection tend to focus on males. However, females may also possess showy ornaments. We investigated whether female ornaments, in the form of sexual swellings, reliably signal female fitness in a semifree-ranging colony of mandrills (Mandrillus sphinx) at the Centre International de Recherches Médicales, Franceville (CIRMF), Gabon. We measured swelling height and width using photographs of periovulatory females over three mating seasons and compared swelling size with parasitism (using fecal analysis over one annual cycle), immune status (ratio of lymphocytes to neutrophils in blood smears made during captures), and genetic diversity (microsatellite heterozygosity). Swelling size varied by up to 10% between cycles in individual females, giving some support to the hypothesis that size differences may indicate the quality of individual swelling cycles. However, there was no significant difference in swelling size between conceptive and nonconceptive cycles. Measures of swelling size varied more between females than within females across swelling cycles, implying that swelling size was a relatively consistent characteristic of individual females. Swelling size was not significantly related to either general measures of parasitism and immune status, or to the closest available measures to each swelling cycle. Nor was swelling size significantly related to genetic diversity. The healthy, provisioned nature of the colony and problems associated with observational, correlational studies restrict interpretation of our data. However, in combination with previous findings that females of higher reproductive success do not show larger swellings, and that males do not allocate mating effort as a function of swelling size, these results imply that sexual swelling size does not indicate female quality in these semifree-ranging mandrills. 相似文献
15.
Wendt Müller Dieter Heylen Marcel Eens Hector Fabio Rivera-Gutierrez Ton G. G. Groothuis 《Behavioral ecology and sociobiology》2013,67(11):1791-1798
Bird song is considered to have evolved via sexual selection and should as such honestly signal aspects of the quality of its bearer. To ensure honesty, the immunocompetence handicap hypothesis proposed a dual role of testosterone, having positive effects on sexual signalling but suppressive effects on immune function. However, recent studies showed that it is rather an immune activation that suppresses the androgen production. This reversed chain of causation may significantly alter the pathways, which translate the effects of parasites and pathogens into changes in the expression of male sexual traits. We infested male canaries with Ixodes ricinus tick nymphs to investigate the causal relationships between (ecto-)parasites, testosterone and sexual signalling, here singing behaviour. We focused on flexible song traits, which may quickly reflect changes in the infestation status, and tested whether these effects relate to changes in the plasma testosterone levels or health state. The experimental tick infestation altered the males’ song performance by reducing song consistency, a trait that had previously been identified to reflect male quality. The tick infestation lowered the plasma testosterone levels and had a negative effect on the health status in terms of a reduced hematocrit. Our pathway analysis then revealed that it is the parasite-induced reduction of the plasma testosterone levels but not of the health state that caused the changes in song consistency. Thus, our study supports the view that it is the effect of parasites and immune activation on plasma testosterone levels that generates the trade-off between immunocompetence and sexual signalling. 相似文献
16.
Fluctuating asymmetries are subtle random deviations from perfect symmetry in bilateral traits caused by the inability of individuals to buffer their development against the disruptive effects of genetic and environmental stresses. The degree of asymmetry of secondary sexual characters is supposed to convey information about a male's phenotypic and/or genetic quality, and females may thus be expected to use bilateral symmetry as a cue in mate choice. We independently and simultaneously manipulated the length and the relative symmetry of the pelvic spines of computer-animated three-spined sticklebacks, Gasterosteus aculeatus . In all mate choice tests involving two male models differing in spine symmetry, females preferred the symmetric model, irrespective of spine length, though only significantly so when both male models had short spines. Females did not significantly discriminate between male models differing in spine length alone. The preference for symmetry was stronger the higher the females' physical condition. Our study demonstrates that female sticklebacks master the perceptual abilities required to discriminate between potential mates basing exclusively on small length differences between paired structures, and provides experimental evidence that spine symmetry is subject to sexual selection through female choice in this species. 相似文献
17.
Whether sexual selection and species recognition involve distinct preferences and signals is still debated. Earlier work showed that traits under sexual selection can reduce the efficiency of species recognition but remains uncertain on how frequently such a conflict occurs. We can, however, hypothesise that overlapping distributions of sexual signals may enhance the hybridization risk in many species. We tested this hypothesis in a newt, Lissotriton vulgaris, which hybridises with Lissotriton helveticus. The two species also share an ultraviolet (UV) colour trait, which influences male attractiveness in L. vulgaris, though this trait is probably not functional for sexual communication in L. helveticus. We predicted that the shared trait would affect species recognition when UV radiation is present in the environment. We staged binary choice preference tests under UV+ and UV? conditions. In the UV+ treatment, female preference depended on the values of the shared UV trait and total brightness, regardless of male species identity. Thus, species recognition was enhanced or reversed depending on the difference in the male trait. Females preferred no male type in the UV? treatment, likely explained by our design, which alternated different sensory environments, and the limited prior exposure of subjects to the other species’ morph. We conclude that the presence of this shared trait used in sexual communication contributes to the production of hybrids in syntopic ponds. To our knowledge, this is the first experimental evidence of the influence of an UV sexual signal modulating species recognition. 相似文献
18.
Testosterone stress does not increase asymmetry of a hormonally mediated sexual ornament in a lizard
José P. Veiga Alfredo Salvador Pilar López José Martin 《Behavioral ecology and sociobiology》1997,41(3):171-176
The theory that fluctuating asymmetry is sensitive to both environmental and genetic stress is gaining acceptance among evolutionary
biologists. Most empirical work has focused on ornamental traits on the assumption that they are more susceptible to stress.
In tegumentary coloration is a common ornament in nature, and frequently has a hormonal basis. Earlier studies in the lizard
Psammodromus algirus indicate that testosterone induces the development of head nuptial coloration in large males and, at the same time, produces
an increase of the ectoparasite load and higher mortality. Hence, the manipulation of testosterone levels may be a way to
increase ornament expression and simultaneously create conditions that may make symmetrical development difficult. This positive
covariation between character size and symmetry is opposite to that expected in theory for sexually selected traits, so the
predicted elevation of asymmetry due to the treatment cannot be confounded by any intrinsic association between symmetry and
character size. We firstly consider the effect that testosterone supplementation has on two variables that reflect the symmetry
of bilateral throat nuptial coloration in large P. algirus males. Also, we examine whether ornament symmetry is positively associated with reproductive success, a prediction of theory
of sexually selected symmetry. Testosterone treatment did not increase the fluctuating asymmetry of throat coloration. Size
asymmetry increased with character size in individuals with a fragmented colour pattern, but changed suddenly to a highly
symmetric pattern in individuals with non-fragmented coloration. Mirror asymmetry decreased steadily with character size.
These results suggest that the development of coloration on both sides of the throat midline follows a random pattern. Asymmetry
did not correlate with variables that estimated reproductive success, suggesting that asymmetry is not affected by physiological
stress and that this trait is not a sexually selected signal in P. algirus.
Received: 1 July 1996 / Accepted after revision: 4 May 1997 相似文献
19.
Guillermo Gonzalez Gabriele Sorci Linda C. Smith Florentino Lope 《Behavioral ecology and sociobiology》2001,50(6):557-562
Female preference for mates with elaborated ornaments has often been explained on the basis that exaggerated secondary sexual traits might reflect individual quality and females might gather direct and indirect benefits in mating with such males. Sexual signals must however also entail costs to be reliable indicators of male quality. Androgens have been suggested as a physiological link between sexual signals and individual quality for several reasons, including their immunosuppressive effect. In this study, we tested two hypotheses linked to the hormonal basis of sexual signal expression. First, we investigated whether testosterone is correlated with the size of the black feather bib on the throat of male house sparrows (Passer domesticus) - a trait involved in intra- and inter-sexual selection. Second, we tested whether testosterone affects the seasonal exposure of the trait. Observational work conducted in 1998 showed that the testosterone level was positively correlated with badge size both in spring and in the subsequent fall, after molt. In 1999, we experimentally reduced spring testosterone levels using silastic implants filled with cyproterone acetate, an antiandrogen. Male house sparrows implanted with cyproterone acetate showed reduced exposure of the badge, because the white tips of the black feathers of the badge wore off later than in control males implanted with empty silastic tubes. This result suggests that testosterone can be causally involved in the expression of a secondary sexual trait in house sparrows, at least in terms of its seasonal exposure. 相似文献
20.
Matthew R. Evans Thais L. F. Martins Michael P. Haley 《Behavioral ecology and sociobiology》1995,37(1):15-23
Recent work on fluctuating asymmetry has suggested that ornaments should have higher levels of fluctuating asymmetry than (1) non-ornaments and (2) homologous structures in the non-ornamented sex. In addition, as both ornament size and symmetry should increase with individual quality there should be a tendency for ornament symmetry to increase with ornament size. In non-ornaments, a U-shaped relationship between symmetry and size is expected, with the individuals at the extremes being more asymmetrical than individuals around the optimum. We tested these predictions in the red-billed streamertail (Trochilus polytmus), a sexually dimorphic endemic Jamaican hummingbird. The lengths of four bilaterally symmetrical traits (first and second outermost tail feathers, tarsi and wings) in 43 adult males and 42 females were measured. The second outermost tail feathers of adult males (which are elongated into streamers) were absolutely but not relatively more asymmetrical than non-ornaments (including the homologous feathers in females). When character size was controlled for, wings were shown to be relatively more symmetrical than other traits. Symmetry did not increase with increasing trait size in any of the morphological traits measured. There was a U-shaped relationship between asymmetry and trait size for four traits (adult male streamers, adult male wings and female outer tail feathers). These results do not support any of the predictions made by fluctuating asymmetry hypotheses and suggest that stabilising selection may act on ornaments as well as non-ornaments. These predictions have been supported in swallows and peafowl but not in sunbirds; this may be due to differences in female perception of tail ornaments. Perhaps male tails do not convey information about quality in some species, or there may be inter-specific differences in the relative costs of tail ornaments and the benefit of marginal increases in tail length and symmetry. 相似文献