The crest of the peafowl: a sexually dimorphic plumage ornament signals condition in both males and females

Both male and female peafowl grow crests on top of their head Á iridescent blue in males, dull iridescent green and brown in females Á but the potential signal function of this plumage ornament is unknown. In this study, peafowl crests were measured in three feral populations, and morphological variation in this ornament was studied in relation to body condition (body mass in relation to tarsus length) and health (white blood cell concentration and ectoparasite load). Prior to the start of the breeding season, male crests are wider with greater pennaceous area, and are more likely to have all feathers grown out compared with female crests. Only crest length changed with measurement date, increasing over time; in males, crest measurements were not related to the extent of train feather development. Crest morphology is a potential signal of individual health and condition in both sexes, but in different ways. In females, the amount of crest plumage grown out to its full extent was related to body condition at the start of the breeding season, whereas in males, the size and pennaceous area of the ornament were related to ectoparasite load. Observations of within-sex agonistic behaviour suggest a possible role for the crest ornament in status signaling in males, because males that engage in more aggressive interactions tend to have wider crests. There was no evidence for a relation between crest morphology and agonistic behaviour in females. Across bird species, crest plumage is often used for signaling and display. Female California quail Callipepla californica prefer males with elongated crest plumes The signal function of crest plumage can be mediated by features other than ornament size. For example, in blue tits Cyanistes caeruleus the colour of erectile crest plumage is involved in both mutual mate choice Peacocks (genus Pavo) are known for their highly elaborate train feathers, displayed during courtship and assessed by females during mate choice 405 The crest of P. cristatus is approximately 5 cm in length, so one might expect that it is fairly inexpensive to grow and maintain compared to the 150 cm long train of over 200 feathers . This raises two questions: why does the peacock have a relatively small crest in addition to such an elaborate train, and why do both sexes have crests? One possibility is that this ornament has an adaptive function in females as well as males, either with respect to mutual mate choice or status signaling (Amundsen 2000). Alternatively, the female crest could be a byproduct of genetic correlation with males (Lande 1987), with the ornament having an adaptive function in males only. In this study, I quantify sexual dimorphism in peafowl crest morphology, and consider the potential function of this ornament in both sexes. I test whether variation in the crest ornament might be an indicator of body condition (body mass controlling for tarsus length), or an indicator of health, based on hematological state and ectoparasite load. I also investigate the potential role of the crest ornament as a status signal by examining the relations between crest morphology and agonistic interactions within the sexes. Material and methods Field methods This study was conducted in three feral peafowl populations: 1) Assiniboine Park Zoo (APZ) in Winnipeg, MB, Canada, where about 60 peafowl are free-ranging over 50 ha of pens and woodland; 2) Toronto Zoo (TZ) in Toronto, ON, Canada, where about 30 peafowl are free ranging over 250 ha of pens and woodland; and 3) Los Angeles Arboretum (LAA) in Arcadia, CA, USA, where 100 peafowl live in 50 ha of parklands and surrounding residential areas. The APZ and TZ populations are housed indoors during the coldest winter months (DecÁMar), whereas birds in the LAA population are feral year-round. Birds in all three populations are lek-mating, as has been observed in the wild Birds were caught prior to the start of the breeding season for measurement (Apr 2007 at APZ and TZ; JanÁ Mar in 2008 Measuring the crest ornament To quantify crest morphology prior to the start of the breeding season, I measured the length and width of the ornament (as viewed from the side of the head) to the nearest 0.1 mm. Crest length was measured on the central crest feathers as the distance between the feather attachment and the tip, and crest width as the linear distance between the anterior and posterior extremes of the pennaceous region along the top of the crest I selected the single best photograph of each bird based on the clarity of the outline of pennaceous flags against the background, and used the 'lasso' tool in Adobe Photoshop 10.0.1 to outline the pennaceous flags and measure the area enclosed. This measurement was taken separately for: 1) the region along the top of the crest or 'top area', which includes the flags of feathers that have grown out to the maximum crest length at the time of capture, and 2) a 'new growth area' of any flags below and not contiguous with those at the top of the ornament (1 , range 052.7Á 136.6, n 0144). Body condition and health As a measure of body condition, I used body mass controlling for tarsus length, because individuals with greater mass for a 406 given skeletal size are thought to have greater energy reserves in the form of fats and other soft tissues (Green 2001). I also examined two measures of individual health describing hematological state and ectoparasite load. To assess hematological state, I stained blood smears following Campbell (1995) and examined at least 10 fields per slide at 1000) under oil immersion (mean no. fields per bird 021.1, 95% CI 020.5Á21.9, n 0142), counting the total number of leucocytes (white blood cells) and erythrocytes (red blood cells) in each field. No blood parasites were seen in any of these blood smears, consistent with a previous study of peafowl in a zoo population To quantify ectoparasite load, I examined body feathers 2Á4 months after they were collected. Parasitic chewing lice (Insecta: Phthiraptera) are common in feral peafowl Behavioural observations To determine the potential role of the crest ornament in competitive dominance interactions, I quantified agonistic behaviour that occurred on leks during the breeding season. In the APZ and TZ populations (May and Jun 2007), as well as in the LAA population (MarÁApr 2008), leks were observed during peak lekking periods of 07:00Á12:00 and 16:00Á18:00 local times (Petrie et al. 1991, pers. obs.). A total of 80 h of observations were conducted on 8 focal leks in the APZ population, 115 h on 5 leks in TZ, and 160 h on 7 leks in LAA in 2008. I also quantified agonistic interactions at LAA in 2010, using continuous observations on 4 leks from 8:00 to 18:00 local times, over a 13-day period (15Á27 Mar, total observation time 506 h). During these observations, all 'displacements' between two individuals of the same sex were recorded The distribution of displacement events among individuals was highly right-skewed, with 48/71 females and 12/38 males never seen engaging in these interactions (males: mean no. interactions05.7, 95% CI 02.8Á8.6, n 038; females: mean01.0, 95% CI 00.5Á1.6, n 071), in spite of the fact that all of the studied males were adults with regular display court territories on the leks (i.e. none were floaters). The birds that did not engage in these agonistic interactions on the leks were not a random sample, however. Among birds observed in at least one interaction, the total number of interactions was highly correlated with the number of interactions where that bird was the aggressor (Pearson correlations, male: r00.97, pB0.0001, n 025; female: 0.93, p B0.0001, n 022), but not with the number of interactions where it was the target (male: r00.29, p 00.16, n 025; female: r00.07, p 0 0.76, n 022), excluding a single outlier for each sex that was targeted exceptionally frequently (38/40 interactions for the male as target; 9/10 for the female). This pattern indicates that the birds not seen engaging in these interactions were mainly subordinate individuals that avoided direct competition. To summarize these data, I assigned birds to the following three categories that correspond to increasing dominance status: 0 0birds that did not engage in any agonistic interactions (n 012 males, 48 females), 10birds that were the target at least half of the time (n 09 males, 11 females) and 2 0birds that were the aggressor in agonistic interactions more than half of the time (n 017 males, 12 females). Data analysis Analyses were performed using JMP 9.0.0 and R 2.13.0 (R Development Core Team 2011). In the LAA population where some birds were measured and observed in multiple years, I analyzed males in the first year they were captured. Only 2 LAA females were captured in more than one year (2009 and 2010 for both), and because focal observations on leks were not conducted in 2009, I analyzed data from these females in 2010. I also excluded one adult male from APZ who was missing all crest feathers. Total sample sizes for measured birds were: APZ, n 013 males, 34 females; TZ, n 06 males, 15 females; LAA, n 030 males, 46 females. 407 I used general linear models to test for sex differences in crest morphology controlling for the effects of study population and measurement date. Most males (31/49) had no new growth though some had substantial new growth; in contrast, only a few (15/95) females had no new growth. Thus, I analyzed new growth area as a categorical variable (presence/absence of new pennaceous growth) using a generalized linear model with a quasibinomial distribution of errors (to correct for overdispersion) and logit link function, because the raw data were not normally distributed and no transformation could be found to correct this. To test whether crest morphology is related to feather development in males, I examined the relation between train length and male crest measurements in general linear models controlling for population and measurement date, as train length increases with date in these populations (Dakin and Montgomerie 2011). As above, I used a generalized linear model with a quasibinomial distribution to model the presence or absence of new growth area in relation to train development. The exact age of birds was not known. However, because some males were captured in more than one year at LAA, I used linear mixed models with male identity as a random variable to test for an effect of increasing age on male crest measurements (n 044 measurements of 16 males). This was not examined in females because only 2 were captured in more than one year. To investigate the potential signal content of the crest ornament, I examined general linear models of body condition, WBC concentration and ectoparasite load using the four crest measurements and study population as potential predictors. As an index of body condition, I analyzed body mass controlling for average tarsus length. I applied a 4th root transformation to WBC concentration and a square root transformation to both ectoparasite load and new growth area of female crests to normalize model residuals involving these zero-inflated variables (Quinn and Keough 2002); new growth area in males was analyzed as a categorical variable as described above. To control for collinearity and the problem of multiple comparisons, I used AICc (Akaike's Information Criterion corrected for small sample sizes) to select the best model from the set of all possible models. I also examined models with DAICc 52 in each set, as these 'top models' are also considered to be plausible, given the data To investigate the role of the crest ornament in within-sex agonistic behaviour, dominance category was treated as an ordinal response and analyzed using logistic regression models with cumulative response probabilities fit by maximum likelihood (lrm function in the R package Design ver.2.3-0). I followed the same model selection procedure described above, using the four crest measurements and study population as predictors. A previous study of peafowl reported that a male's ability to maintain a lek territory is related to his tarsus lengt

The crest of the peafowl: A sexually dimorphic plumage ornament signals condition in both males and females

Both male and female peafowl grow crests on top of their head - iridescent blue in males, dull iridescent green and brown in females - but the potential signal function of this plumage ornament is unknown. In this study, peafowl crests were measured in three feral populations, and morphological variation in this ornament was studied in relation to body condition (body mass in relation to tarsus length) and health (white blood cell concentration and ectoparasite load). Prior to the start of the breeding season, male crests are wider with greater pennaceous area, and are more likely to have all feathers grown out compared with female crests. Only crest length changed with measurement date, increasing over time; in males, crest measurements were not related to the extent of train feather development. Crest morphology is a potential signal of individual health and condition in both sexes, but in different ways. In females, the amount of crest plumage grown out to its full extent was related to body condition at the start of the breeding season, whereas in males, the size and pennaceous area of the ornament were related to ectoparasite load. Observations of within-sex agonistic behaviour suggest a possible role for the crest ornament in status signaling in males, because males that engage in more aggressive interactions tend to have wider crests. There was no evidence for a relation between crest morphology and agonistic behaviour in females

Peacocks orient their courtship displays towards the sun

We studied two courtship displays of male peafowl (Pavo cristatus), focusing particularly on male orientation relative to the position of the sun. During the "wing-shaking" display, females were generally behind the displaying male, and male orientation with respect to the position of the sun was not significantly different from random. However, during the pre-copulatory "train-rattling" display, males were on average directed at about 45° to the right of the sun azimuth with the female positioned directly in front, suggesting that this behaviour is involved in the communication of a visual signal. A model presentation experiment confirmed that courting peacocks were more likely to perform the train-rattling display when the female was on the sunny side of their erect train, but more likely to perform wing-shaking behaviour when the female was on the shaded side of the male. This study underscores the importance of visual signalling in peafowl courtship, and we suggest that an angle of about 45° relative to the sun may allow males to enhance the appearance of their iridescent eyespot feathers

Deceptive copulation calls attract female visitors to peacock leks

Theory holds that dishonest signaling can be stable if it is rare. We report here that some peacocks perform specialized copulation calls (hoots) when females are not present and the peacocks are clearly not attempting to copulate. Because these solo hoots are almost always given out of view of females, they may be dishonest signals of male mating attempts. These dishonest calls are surprisingly common, making up about a third of all hoot calls in our study populations. Females are more likely to visit males after they give a solo hoot call, and we confirm using a playback experiment that females are attracted to the sound of the hoot. Our findings suggest that both sexes use the hoot call tactically: females to locate potential mates and males to attract female visitors. We suggest that the solo hoot may be a deceptive signal that is acquired and maintained through reward-based learning

Condition-dependent mate assessment and choice by peahens: Implications for sexual selection

Condition-dependent mate choice is thought to affect the strength and direction of sexual selection. Although there is ample evidence of this from studies that experimentally manipulate female condition, few studies to date have examined condition dependence of natural mate choice wherein females interact with a pool of available males. We examined mate assessment by free-living Pavo cristatus peafowl, focusing on 3 measures of the choosing female's condition: mass condition (scaled mass index), ectoparasite load, and white blood cell (WBC) count. Females with greater mass condition were more active on peacock leks, approaching and visiting males more often, and spending more time near the males they visited. Condition did not affect the total number of males visited or the probability that a female would visit a male that she had already approached. Peacock mating success is strongly correlated with the iridescent colors of the eyespots on their train, suggesting that color signals are a major focus of female choice. Here, we show that females in better condition (higher mass condition and lower WBC count) allocate a greater proportion of their visit time to the most iridescent males displaying on leks. Our results provide evidence of strong condition-dependent effects on female mate assessment and choice in the absence of experimentally induced stress. We suggest that condition-dependent mate choice may help maintain variation in sexually selected male color traits

Peahens prefer peacocks displaying more eyespots, but rarely

Recent studies have reported conflicting evidence for the influence of train-feather eyespots on the mating success of peacocks, Pavo cristatus. In this study we address this controversy, using observation and experiment to evaluate the effect of train morphology and the number of eyespots displayed during courtship on female choice. We show first that the maximum number of eyespots in the train is consistent among adult peacocks in feral populations at about 165-170 eyespots, and that most of the observed variation in eyespot number appears to be due to feather breakage or loss. Although we confirm a previous report that removing a large number (≥20) of the outermost eyespots from a male's train decreases his mating success compared to unmanipulated males, this experimental modification produces an ornament that is outside the range of eyespot number typically displayed during courtship. Thus, the considerable variation in the mating success of feral peacocks cannot be explained by natural variation in the number of eyespots visible in the train. Peafowl mate choice is clearly more complex than previously thought: females may reject a few males with substantially reduced eyespot number, while using some other cue to choose among males with typical trains

Reciprocity and behavioral heterogeneity govern the stability of social networks

The dynamics of social networks can determine the transmission of information, the spread of diseases, and the evolution of behavior. Despite this broad importance, a general framework for predicting social network stability has not been proposed. Here we present longitudinal data on the social dynamics of a cooperative bird species, the wire-tailed manakin, to evaluate the potential causes of temporal network stability. We find that when partners interact less frequently and when social connectedness increases, the network is subsequently less stable. Social connectivity was also negatively associated with the temporal persistence of coalition partnerships on an annual timescale. This negative association between connectivity and stability was surprising, especially given that individual manakins who were more connected also had more stable partnerships. This apparent paradox arises from a within-individual behavioral trade-off between partnership quantity and quality. Crucially, this trade-off is easily masked by behavioral variation among individuals. Using a simulation, we show that these results are explained by a simple model that combines among-individual behavioral heterogeneity and reciprocity within the network. As social networks become more connected, individuals face a trade-off between partnership quantity and maintenance. This model also demonstrates how among-individual behavioral heterogeneity, a ubiquitous feature of natural societies, can improve social stability. Together, these findings provide unifying principles that are expected to govern diverse social systems

How conspicuous are peacock eyespots and other colorful feathers in the eyes of mammalian predators?

Colorful feathers have long been assumed to be conspicuous to predators, and hence likely to incur costs due to enhanced predation risk. However, many mammals that prey on birds have dichromatic visual systems with only two types of color-sensitive visual receptors, rather than the three and four photoreceptor

Morphology, muscle capacity, skill, and maneuvering ability in hummingbirds

How does agility evolve? This question is challenging because natural movement has many degrees of freedom and can be influenced by multiple traits. We used computer vision to record thousands of translations, rotations, and turns from more than 200 hummingbirds from 25 species, revealing that distinct performance metrics are correlated and that species diverge in their maneuvering style. Our analysis demonstrates that the enhanced maneuverability of larger species is explained by their proportionately greate