Female yellowhammers (Emberiza citrinella) prefer yellower males: a laboratory experiment

The importance of male plumage coloration as a signal of male dominance and a cue for female choice in the monogamous yellowhammer, Emberiza citrinella, was tested in two sets of experiments in an indoor aviary. Dominance was tested by introducing two individuals, with no previous experience of each other, in an aviary with food and water. Aggression occurred more often between two males than between a male and a female and more frequently between pairs of males including at least one old male than between two young males. Dominance was not related to male colour in trials between males of the same age class, but it was in trials between an old and a young male, often differing in colour. Thus, age may be a more important determinant of dominance than colour. Female preference for more colourful males was tested by allowing hormone-induced females to choose between a more and a less colourful dummy male. Females spent more time in front of more colourful than drabber males and also more often perched beside colourful males than duller individuals. Although male colour cannot be ruled out as a dominance signal, the results suggest that male colour is primarily used as a signal in mate choice. Female choice may hence be responsible for maintenance of bright plumage in the male yellowhammer.     

Female preferences for multiple attributes in the acoustic signals of the Italian treefrog, <Emphasis Type="Italic">Hyla intermedia</Emphasis>

The advertisement call of frogs and toads is an example of multiple message signal because different acoustic properties encode different kinds of biologically significant information. In the Italian treefrog, Hyla intermedia, pulse rate and frequency have been found to be under stabilizing female preferences and to encode information important for mate recognition, whereas the number of calls per call group have been found to be under directional preferences and, thus, to be important for mate quality assessment. In this study, we investigate preferences for calls that differ simultaneously in frequency, pulse rate, and number of calls per call group, and we ask how these properties interact with each other in influencing female mating decisions. Results of two-choice phonotaxis experiments provide no evidence to support the hypothesis that females process multi-attribute signals in a hierarchical way. In contrast, the pattern of preferences is consistent with the ‘preference function’ hypothesis, that is, with the hypothesis that females rank signals along an ordinal scale of values and choose accordingly. Pulse rate and frequency influence mating preferences more than does the number of calls per call group. The interaction between pulse rate and frequency is not additive but multiplicative: small differences in either pulse rate or frequency that, alone, have no effects on female choice, interact synergistically so that their combination has strong influence on female preferences. A preference repeatability test shows strong among-female differences in preference for multi-attribute signals. We suggest that this result reveals not only a variation in attribute values among females, but also a variation in the way females weight and combine attribute values into a single preference score.     

Parasites and health affect multiple sexual signals in male common wall lizards, Podarcis muralis

Multiple advertising sexual traits may either advertise different characteristics of male condition or be redundant to reinforce reliability of signals. Research has focused on multiple visual traits. However, in animals that use different multiple additional sensory systems, such as chemoreception, different types of traits might have evolved to signal similar characteristics of a male quality using different sensory channels. We examined whether ventral coloration and chemicals in femoral gland secretions of male common wall lizards, Podarcis muralis, are affected by their health state (blood-parasite load and cell-mediated immune response). Our results indicated that less parasitized lizards had brighter and more yellowish ventral colorations and also femoral secretions with higher proportions of two esters of octadecenoic acid. In addition, lizards with a greater immune response had more saturated coloration and secretions with higher proportions of octadecenoic acid methyl ester. We suggest that these signals would be reliable because only healthier males seemed able to allocate more carotenoids to coloration and presumably costly chemicals to secretions. The use of multiple sensory channels may provide more opportunities to signal a male quality under different circumstances, but also may reinforce the reliability of the signal when both types of traits may be perceived simultaneously.     

Optical properties of the uropygial gland secretion: no evidence for UV cosmetics in birds

Ultraviolet (UV) reflectance of the plumage is common in birds and plays an important role in sexual signalling. Recently, it has been proposed that birds are able to modify plumage UV reflectance by the application of uropygial gland secretion. Based on a survey of the optical properties of this secretion from 51 species belonging to 12 avian orders, we show that two main types of uropygial secretions exist, one predominantly found in passerines and one in non-passerines, both reducing relative UV reflectance of a white background (Teflon™ tape). We quantified how each type of secretion (exemplified by blue tit and mallard) affected feather UV reflectance. Both secretions reduced overall brightness and relative UV reflectance of white mallard feathers but hardly affected the reflectance of UV/blue blue tit crown feathers. According to models of avian colour vision, changes in reflectance due to application of the secretion were at or below the discrimination threshold of most birds. We conclude that the uropygial secretion is unlikely to play a major role in modifying plumage UV reflectance. However, the optical properties of the uropygial secretion may have been selected to interfere as little as possible with visual signaling through plumage reflectance. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Male mating history and female fecundity in the Lepidoptera: do male virgins make better partners?

In insects, large ejaculate and associated materials, including spermatophores, appear to have evolved via sexual selection acting on males to either delay female remating or to increase the rate of egg-laying. It is also possible, however, that females use nutrients transferred during mating to increase their lifetime fecundity. If so, male ejaculate size may also have evolved under natural selection as a form of paternal investment. In Lepidoptera, males with a greater number of prior matings tend to produce smaller spermatophores. However, the reported effects of male mating history on female fecundity vary widely among species. We therefore performed a meta-analysis using data from 29 studies of 25 species. Overall, the reproductive output of females mated to virgin males was significantly higher than that of females mated to sexually experienced males (Hedges d=0.33, P<0.01). A sample size of around 145 females per male mating type is required to detect an effect of this size with 80% statistical power at =0.05 (two-tailed). There was no difference in mean effect size between butterflies/skippers and moths. After controlling for any effect of taxonomic group, however, the mean effect size for polyandrous species was significantly greater than that for monandrous species (Hedges d=0.45 vs 0.25, P=0.01). We then discuss possible reasons why male mating history, presumably acting through its effect on spermatophore size, might have a stronger effect in polyandrous than monandrous species.Communicated by A. Cockburn     

Cloacal erection promotes vent apposition during forced copulation in the New Zealand stitchbird (hihi): implications for copulation efficiency in other species

Cloacal protuberances (CP) in male birds result from spermatic engorgement of storage tubules around the cloaca during the breeding season. We examined seasonal changes in the volume and orientation of the CP in the New Zealand stitchbird Notiomystis cincta. The male stitchbird has one of the largest recorded CPs for any species (max = 1,570 mm³), with CP volume increasing by almost 400% between the non-breeding and breeding seasons. While sperm competition has been positively correlated with the magnitude of CP storage in other species, no evidence previously existed for the CP improving copulation efficiency. By measuring the relative orientation of the CP throughout the year, we show that not only does the CP increase in size as males become sexually active, it also changes its orientation by approximately 60°. This results in it shifting from facing posteriorly to becoming almost perpendicular to the abdomen. This cloacal erection improves the apposition of the male and female cloacal openings during face-to-face forced copulation in this species. This provides the first reported evidence supporting the copulation efficiency hypothesis of the avian CP. While the magnitude of seasonal changes in female cloacal volume was similar to males, female cloacal orientation remained virtually unchanged across seasons. This difference between the sexes is likely to reflect differing selection pressures for optimizing sperm transfer. In females, a posterior-facing cloaca is ideal for both waste evacuation and sperm reception, whereas, for the male, a posterior-facing cloaca is well suited for waste evacuation, but possibly hinders sperm delivery. Changes in male cloacal orientation from the non-breeding to the breeding season are a likely reflection of conflict in this dual function. Evidence of changes in CP orientation in another passerine species suggests this phenomenon is widespread and also important for understanding related fields such as sperm competition, forced copulation and constraints on the evolution of the avian intromittent organ.     

Daring females,devoted males,and reversed sexual size dimorphism in the sand-dwelling spider <Emphasis Type="Italic">Allocosa brasiliensis</Emphasis> (Araneae,Lycosidae)

Sexual selection theory predicts that a higher investment in offspring will turn females into the selective sex, while males will compete for accessing and courting them. However, there are exceptions to the rule. When males present a high reproductive investment, sex roles can reverse from typical patterns, turning males into the choosy sex, while females locate males and initiate courtship. In many spiders, males are smaller than females, wandering in search of sedentary females and maximizing the number of copulations. In the present study, we present findings on the sand-dwelling wolf spider, Allocosa brasiliensis, evidencing a reversal in typical courtship roles reported for the first time in spiders. Males were bigger than females. Females located males and initiated courtship. Copulation always occurred in male burrows and took place mainly in long burrows. Males donated their burrows to the females after copulation, closing the entrance before leaving with female cooperation from inside. Males would provide females with a secure place for ovipositing, being exposed to predation and diminishing their future mating possibilities until constructing a new burrow. The cost of vacating the burrow and losing the refuge in an unpredictable habitat, such as sand dunes, would explain the courtship roles reversal in this spider species. Results turn A. brasiliensis as a promising model for discussing the determinants of sex roles and the pressures that drive their evolution and maintenance. Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users.     

Sex ratios,mating behavior and sexual size dimorphism of the northern water snake,Nerodia sipedon

Competition among males to mate is generally associated with male-biased size dimorphism. In this study we examine mating behavior in the northern water snake (Nerodia sipedon), a species in which males are much smaller than females despite substantial competition among males to mate. Competition among males was a consequence of a male-biased operational sex ratio due to slightly higher female mortality from a birth sex ratio of 1 : 1, and, in 1 year, more synchronous and longer mating activity by males. Approximately one-third of both males and females appeared not to mate in a given year. Larger males were generally more likely to attempt mating, but size did not explain the variance in the number of aggregations in which individual males participated. Within aggregations, males that were successful at achieving intromission were larger than unsuccessful males in 1 of 2 years. Variation in condition (mass relative to length) and relative tail length were not generally useful predictors of either mating effort or success in males. Because large size was often advantageous to males, sexual size dimorphism appeared not to be a consequence of sexual selection favoring smaller males. Because sexual dimorphism was evident at birth, and both males and females matured sexually at about 4 years, sexual dimorphism was not simply a consequence of one sex growing at the maximum rate for longer. Female fecundity increased with size, and sex differences in size-fecundity relations may underly the pattern of sexual size dimorphism. However, because multiple mating by females is common, sperm competition is likely to be important in determining male reproductive success. Therefore, allocation of energy to sperm rather than growth may also prove to be an important influence on male growth rates and sexual size dimorphism.     

Assessing the aerodynamic effects of tail elongations in the house martin (Delichon urbica): implications for the initial selection pressures in hirundines

Of the three species of hirundine that breed sympatrically across the U.K., one, the barn swallow, has outer tail feathers elongated into streamers, whereas the other two species, the house martin and the sand martin, do not. The tail streamer of the barn swallow is regarded as a classic example of a sexually selected trait. Recent evidence, however, has suggested that streamers may have evolved largely through natural selection for enhanced flight performance and increased maneuverability. We tested the hypotheses that small streamers (1) increase performance in turning flight, but (2) decrease performance in flight variables related to velocity. We manipulated the lengths of house martin outer tail feathers and measured changes in their free-flight performance, using stereo-video to reconstruct the birds" three-dimensional flight paths. Five flight variables were found to best describe individual variation in flight performance. Of these five, the three variables determining maneuverability predicted that flight performance would be optimized by a 6- to 10-mm increase in the length of the outer tail feathers. In contrast, for mean velocity and mean acceleration, extension of the outer tail feathers appears to have a detrimental effect on flight performance. We suggest that the initial selection pressure for streamers in ancestral short-tailed "barn swallows" was via natural selection for increased maneuverability. In addition, we propose that the benefits of increased maneuverability have differed between hirundines in the past, such that the cost of increasing the length of the outer tail feather has, to date, outweighed the benefits of doing so in streamerless hirundines. Received: 14 February 2000 / Revised: 2 July 2000 / Accepted 18 July 2000     

Can male-male competition stabilize speciation? A test in Lake Victoria haplochromine cichlid fish

It has been suggested that sympatric speciation can be driven by sexual selection on male mating traits alone. However, a fundamental problem for this process is the lack of ecological differentiation that would stabilize the coexistence of incipient species through frequency-dependent selection. Such selection can also occur if male aggression is primarily directed towards similar rather than towards dissimilar phenotypes, so that rare male phenotypes would enjoy a negatively frequency-dependent fitness advantage. We experimentally tested such an aggression bias in two recently diverged, ecologically and anatomically similar sympatric cichlid species pairs of the genus Pundamilia from Lake Victoria. Territorial males of a pair of partially reproductively isolated species with red and blue nuptial coloration, respectively, studied in the laboratory were confronted simultaneously with both colour types enclosed in transparent tubes. Red males were more aggressive to red stimuli under white light but not when colour differences were masked under green light. Blue males were equally aggressive to both stimuli in both light conditions. Males of two apparently fully reproductively isolated species, again one with red and one with blue nuptial coloration, studied in the field, both directed more aggressive behaviour towards conspecific than towards heterospecific stimulus males. The differential allocation of aggression would create an advantage for males of the less abundant phenotype or species, thereby potentially supporting stable coexistence of the phenotypes. The finding that this effect was less clear in the partially reproductively isolated species pair than in the fully isolated species pair is discussed.