John P. Swaddle

How and why environmental noise impacts animals: an integrative, mechanistic review

The scope and magnitude of anthropogenic noise pollution are often much greater than those of natural noise and are predicted to have an array of deleterious effects on wildlife. Recent work on this topic has focused mainly on behavioural responses of animals exposed to noise. Here, by outlining the effects of acoustic stimuli on animal physiology, development, neural function and genetic effects, we advocate the use of a more mechanistic approach in anthropogenic environments. Specifically, we summarise evidence and hypotheses from research on laboratory, domestic and free-living animals exposed to biotic and abiotic stimuli, studied both observationally and experimentally. We hope that this molecular- and cellular-focused literature, which examines the effects of noise on the neuroendocrine system, reproduction and development, metabolism, cardiovascular health, cognition and sleep, audition, the immune system, and DNA integrity and gene expression, will help researchers better understand results of previous work, as well as identify new avenues of future research in anthropogenic environments. Furthermore, given the interconnectedness of these physiological, cellular and genetic processes, and their effects on behaviour and fitness, we suggest that much can be learned from a more integrative framework of how and why animals are affected by environmental noise.

Avian wingtip shape reconsidered : wingtip shape indices and morphological adaptations to migration

In this paper we review the existing methods of quantifying avian wingtip shape, and compare their efficiency at detecting morphological adaptations to migration. We use multivariate methods to derive two novel measures of avian wingtip shape, pointedness C2 and convexity C3, based on measurements of primary feather lengths. Size-constrained components analysis, a modified form of principal components analysis, is used to ensure that the measures are independent of isometric size, and have a consistent interpretation in terms of the geometric shape of the wingtip. Our measures of pointedness and convexity can be calculated easily for both live birds and museum skins, and can be applied to any ecomorphological or functional analysis of avian wingtip shape. This approach circumvents many of the interpretational problems associated with previous wingtip shape indices that are often based on less accurate wing measurements. To test the suggested interpretations of previously published wingtip shape indices, we use a comparative interspecific analysis to determine the interrelations of our new shape measures with these published indices, and with aerodynamic parameters which have known functional significance. Published indices do not always measure the quantities that they are claimed to do, and are beset with awkward terminological inconsistencies. We assess the efficacy and utility of these wingtip shape measures with regard to predicted and well-known morphological adaptations in the wings for migration. Once phylogenetic bias and extraneous ecological factors are controlled, the majority of published wingtip shape indices are unable to detect morphological differences between migratory and nonmigratory species. Our measures of pointedness and convexity confirm that migrants have wingtips that are relatively more pointed and more convex; they also have wings of relatively larger aspect ratio. The biomechanical implications of these adaptations for different flight behaviours are discussed. An appendix discusses some of the statistical problems involved in the analysis of size and shape, and introduces the size-constrained components analysis (SCCA) method, which is applicable to any study of morphological variation.

Testosterone increases perceived dominance but not attractiveness in human males

Recent evidence suggests that certain features on the human face indicate hormonal levels during growth, and that women judge the attractiveness of potential partners based on the appearance of these features. One entrenched notion is male facial features that are affected by testosterone are used as direct cues in mate preference. Testosterone may be particularly revealing as it is purported to be an honest indicator of male fitness. Increased testosterone may impose an immunocompetence handicap on the bearer and only the best males can carry this handicap. To date, tests of this theory have been indirect, and have relied on digital manipulations that represent unrealistic continuums of masculine and feminine faces. We provide a much more direct test by manipulating digitally male faces to mimic known shape variation, caused by varying levels of testosterone through puberty. We produced a continuum of faces that ranged from low to high levels of testosterone in male faces and asked women to choose the points on the continuum that appeared most attractive and most physically dominant. Our data indicate that high testosterone faces reveal dominance. However, there is no evidence of directional selection for increased (or decreased) testosterone in terms of attractiveness to the opposite sex. We discuss the relevance and applicability of evolutionary interpretations of our data and, contrary to predictions, provide evidence of stabilizing selection acting on testosterone through mate preferences.

Increased Avian Diversity Is Associated with Lower Incidence of Human West Nile Infection: Observation of the Dilution Effect

Recent infectious disease models illustrate a suite of mechanisms that can result in lower incidence of disease in areas of higher disease host diversity–the ‘dilution effect’. These models are particularly applicable to human zoonoses, which are infectious diseases of wildlife that spill over into human populations. As many recent emerging infectious diseases are zoonoses, the mechanisms that underlie the ‘dilution effect’ are potentially widely applicable and could contribute greatly to our understanding of a suite of diseases. The dilution effect has largely been observed in the context of Lyme disease and the predictions of the underlying models have rarely been examined for other infectious diseases on a broad geographic scale. Here, we explored whether the dilution effect can be observed in the relationship between the incidence of human West Nile virus (WNV) infection and bird (host) diversity in the eastern US. We constructed a novel geospatial contrasts analysis that compares the small differences in avian diversity of neighboring US counties (where one county reported human cases of WNV and the other reported no cases) with associated between-county differences in human disease. We also controlled for confounding factors of climate, regional variation in mosquito vector type, urbanization, and human socioeconomic factors that are all likely to affect human disease incidence. We found there is lower incidence of human WNV in eastern US counties that have greater avian (viral host) diversity. This pattern exists when examining diversity-disease relationships both before WNV reached the US (in 1998) and once the epidemic was underway (in 2002). The robust disease-diversity relationships confirm that the dilution effect can be observed in another emerging infectious disease and illustrate an important ecosystem service provided by biodiversity, further supporting the growing view that protecting biodiversity should be considered in public health and safety plans.

Plumage condition affects flight performance in common starlings: implications for developmental homeostasis, abrasion and moult

Variation in length and asymmetry of wing primary feathers can arise from a breakdown of developmental homeostasis, feather abrasion and incomplete growth during moult. Indirect predictions have been made concerning the impact of primary length and asymmetry on the flight ability of birds, but they have not been explicitly tested. Here we provide evidence from both natural variation in primary feather condition and experimental manipulations of primary feather length and asymmetry to indicate that these factors influence aspects of flight performance in the Common Starling Sturnus vulgaris. Damaged and incompletely grown primary feathers reduce escape flight performance. Experimentally reduced primary lengths reduce take-off speed; increased primary asymmetry decreases aerial manoeuvrability. A comparison of the experimental and natural plumage data indicates that birds may be able to adapt to a change in wing morphology, perhaps reducing the effects of feather loss or damage on flight. The results from this study indicate that primary feathers are under strong stabilising selection to maximise developmental homeostasis and reduce feather asymmetry. These findings are also of ecological importance to the damageavoidance and moult strategies of these birds. This is the first experimental evidence to indicate a quantitative reduction in flight performance with feather lengths and asymmetries typical of those observed during flight feather moult and feather damage in any species.

Asymmetry and Human Facial Attractiveness: Symmetry May not Always be Beautiful

It has been postulated that levels of fluctuating asymmetry in human faces may be negatively related to components of fitness such as parasite-resistance; hence potential mates with low levels of asymmetry may appear more attractive. However, previous investigations of the relationship between asymmetry and facial attractiveness have confounded manipulations of asymmetry with facial ‘averageness’ and mean trait size. In this experiment we performed a manipulation that altered asymmetry within a face without altering the mean size of facial features. These faces were then rated on attractiveness. Contrary to what was predicted, faces that were made more symmetrical were perceived as being less attractive. These results do not support the hypothesis that attractiveness is related to low levels of fluctuating asymmetry. The observed positive relationship between asymmetry and facial attractiveness may be because certain facial features (including those contributing to attractiveness) in fact show directional asymmetry or antisymmetry. Our manipulations thus render naturally asymmetric features symmetrical. This may make symmetric faces less attractive because of the reduction of natural directional asymmetries, perhaps making the faces appear unemotional. The role of fluctuating asymmetries alone in assessments of facial beauty is still unknown, although this experiment suggests fluctuating asymmetry is relatively unimportant compared with directional asymmetry.

Food, Feathers and Fluctuating Asymmetries

Nutritional, or energetic, stress has been implicated as a causal factor in the inter-individual differences in levels of fluctuating asymmetry in the elongated tails of male swallows (Hirundo rustica). However, there has been no direct experimental test of this hypothesis. We report results from an experiment that has investigated the effects of sequential food deprivations on levels of fluctuating asymmetry in primary feathers and chest plumage of the moulting female European starling (Sturnus vulgaris). Birds were housed individually and taken through their full moult by means of photoperiodic manipulation. During this period, some of the birds experienced food deprivations. At regular intervals, we recorded the length of every primary feather, quantified the chest plumage via video-image analysis, and recorded body mass and subcutaneous fat score measurements. We found that increasing levels of nutritional or energetic stress caused larger development asymmetries, which would appear to have increased fitness costs. Primary feather asymmetry showed a negative relation with amount of subcutaneous fat stored during moult and a positive relation with social dominance. This is the first study to show, directly, the importance of energetic stress in the production of feather and plumage asymmetries. Low levels of feather asymmetries were also associated with an indicator of reproductive status (chest ‘spottiness’). These findings support the recent literature, suggesting that degree of fluctuating asymmetry, even in traits under stabilizing selection, may reveal aspects of individual fitness value.

Ultraviolet vision and band-colour preferences in female zebra finches,Taeniopygia guttata

Zebra finches have previously been found to have preferences for particular colours of both natural and artificial traits among opposite sex conspecifics. For example, in some studies female zebra finches preferred males wearing red leg bands to orange-banded and unbanded birds and rejected light green-banded males. In other studies, females also preferred males with red beaks to orange-beaked males. However, several authors have failed to replicate these results. We show that females may fail to show a colour preference because of the absence or removal of ultraviolet light under experimental conditions. In mate-choice trials, females observing males through filters that transmitted ultraviolet preferred red-banded males but where females viewed males through ultraviolet-blocking filters, no such preference was observed. Further investigation revealed that the lack of a colour preference when ultraviolet was absent was probably due to the change in overall appearance of the bird, rather than the change in appearance of the rings themselves. This work highlights the importance of proper consider-ation of the sensory capabilities of animals in experimental design, particularly with regard to the role of ultraviolet light in avian colour perception.Copyright 1997 The Association for the Study of Animal BehaviourCopyright 1997The Association for the Study of Animal Behaviour.

Female zebra finches prefer males with symmetric chest plumage

The level of fluctuating asymmetry in a secondary sexual character is believed to reveal aspects of male quality. Previous investigations have demonstrated that females may pay attention to this information when making mate choice decisions; females prefer symmetric over asymmetric males. However, these studies have involved either manipulation of functionally important flight feathers, or of artificial ornaments. Here, we manipulate an existing secondary sexual plumage trait, one that does not influence flight performance, within the boundaries of natural asymmetry. Through manipulations of chest plumage, we demonstrate that female zebra finches choose to display more and for longer in front of males with symmetric, as opposed to asymmetric, chest plumage.

Reproductive success and symmetry in zebra finches

Abstract Fluctuating asymmetries are small, random developmental accidents in morphological traits that would otherwise be described as being bilaterally symmetric. Experimental evidence indicates that in mate choice situations individuals possessing symmetric arbitrary traits or symmetric secondary sexual characters are preferred over those with larger asymmetries. This experiment investigated the effect of symmetric and asymmetric arbitrary traits (arrangements of coloured leg-bands) on reproductive success in captive zebra finches,Taeniopygia guttata. Six groups of six males and six females were separated into six free-flight breeding aviaries. Nestboxes and nesting material were freely available throughout the experiment. Males wore symmetric and asymmetric arrangements of the same coloured leg-bands. Each male wore two orange and two light green bands, two on each leg. There were six possible arrangements of these leg-bands. One male from each of these six leg-band treatments was present in each aviary, along with six females wearing numbered orange bands. Symmetrically banded males produced more offspring that survived past the period of parental care than males in either of the asymmetric treatments; this appeared to be the effect of female choice processes and female-based parental investment and not male intra-sexual dominance. This is the first experiment to indicate that symmetrically manipulated males gain reproductive advantages in controlled laboratory conditions and further supports recent theories indicating the evolutionary importance of symmetry in signalling-trait design.