Plumage variability and territoriality in breeding turnstone Arenaria interpres: status signalling or individual recognition?

The turnstone Arenaria interpres has marked plumage variability in comparison with many other waders (shorebirds). Two hypotheses were tested in an attempt to explain plumage variability in breeding turnstones. The first, the status signalling hypothesis (SSH), suggests that plumage type signals status, and the second, the individual recognition hypothesis (IRH), suggests that plumage differences enable territorial birds to be recognized by their neighbours. The SSH was rejected by two tests: (a) there was no correlation between plumage phenotype of males and the quality of the territories they occupied; and (b) there was no correlation between the plumage phenotype of males and females paired to each other (assuming assortative mating according to rank). The IRH was supported by a test which showed that males could discriminate between neighbours and strangers on plumage differences alone: fibre-glass models mimicking neighbours were effectively ignored whilst models mimicking strangers elicited an aggressive response. It is stressed that this result does not demonstrate that plumage variability in breeding turnstones evolved to facilitate recognition by neighbours, and the potential of the IRH as an explanation of breeding plumage variability in waders is discussed.     

Plumage variability, status signalling and individual recognition in avian flocks

A decade ago it was suggested that much of the plumage variability exhibited by flocking birds can be explained by 'status signalling', plumage variability being used to signal agonistic status(1). As a result of this suggestion, a number of studies have examined the social significance of plumage differences, but the status signalling hypothesis has not received unequivocal support. Other factors, such as the facilitation of individual recognition, also appear to be important in explaining plumage variability.     

A Young Manakin Knows His Place: Evidence for an Age‐Graded Dominance Hierarchy Among Long‐Tailed Manakins

In lek‐breeding systems where many males gather at display sites, males benefit from the establishment of dominance hierarchies to reduce intrasexual aggression and the associated risk of injuries. Long‐tailed manakins (Chiroxiphia linearis) exhibit an exploded lek‐breeding system wherein the two top‐ranking males at each display site team up to perform elaborate coordinated courtship displays for females. Young males undergo delayed plumage maturation whereby they acquire distinct pre‐definitive plumage patterns each year until they attain definitive plumage in their fifth year. This unique characteristic is thought to have evolved as a status‐signalling mechanism to aid in the establishment of an age‐graded dominance hierarchy in which older males are dominant to younger males. Previous research has shown evidence for such a dominance hierarchy among alpha and beta males; however, the presence of this hierarchy among males of other age classes has never been quantified. In this study, we investigated the presence of an age‐graded dominance hierarchy by determining whether older males direct more aggressive behaviours towards younger males. We also investigated whether status signalling is less clear within age classes than between age classes, by determining whether males within the same age class exhibit more aggression towards each other. We found that older males performed aggressive behaviours towards younger males much more frequently than younger males performed aggressive behaviours towards older males. We also found that some aggressive interactions occurred between males within the same age class more frequently than between males from different age classes. Our study provides some evidence for an age‐graded dominance hierarchy among male long‐tailed manakins of all age classes and also provides some support for the status‐signalling hypothesis. However, further research is needed to conclusively establish the presence of a linear dominance hierarchy among younger male manakins. This research may help us better understand the evolution of complex hierarchical systems in animals.     

Carotenoid-based ornamentation and status signaling in the house finch

The status signaling hypothesis (SSH) was devised primarilyto explain the adaptive significance of avian ornamental colorationduring the nonbreeding season. It proposes that conspicuousmale plumage serves as an honest signal of social status withina population of birds. However, to date this hypothesis hasbeen well tested and supported for only one type of plumage coloration, melanin-based coloration. Carotenoid-based pigmentationis known to positively reveal male health and condition duringmolt in a variety of species, but it is poorly understood whetherthis ornament type can also function as a status signal duringthe winter. We tested the SSH in male house finches (Carpodacusmexicanus) by manipulating the carotenoid-based plumage brightnessof first-year males and then pairing unfamiliar birds of differingcoloration in a series of dominance trials in captivity. Manipulated plumage color was unrelated to win/loss outcome in these trials.Similarly, the natural pigmentation of males was a poor predictorof winter dominance; as in other studies with this species,we found only a weak tendency for naturally drab males to dominatenaturally bright males. These results suggest that carotenoid-basedcoloration is not a reliable indicator of social status inmale house finches during the nonbreeding season. In fact, carotenoid-based coloration may function only in mate choice in this species,and it may be retained throughout the year either because timeconstraints preclude a second plumage molt or because it aidsin pair formation that begins in late winter.     

Plumage Colouration,Age, Testosterone and Dominance in Male Red Bishops (Euplectes orix): A Laboratory Experiment

Visual signals such as plumage characteristics in birds often play a key role in the establishment of dominance in contests by acting as a badge of status that can be used to assess individual fighting ability. We studied the role of plumage colouration in males of the red bishop (Euplectes orix), a sexually dimorphic and polygynous weaverbird species occurring in sub‐Saharan Africa and breeding in dense colonies around water. Males are highly territorial and often engage in competition over limited resources such as breeding sites and potential mates. By experimentally staging male–male contests over a limited food source, we wanted to determine whether the orange–red breeding plumage in this species serves as a dominance signal between individuals, with males with redder plumage being dominant over those with duller plumage. In the first set of experiments, we staged contests between unfamiliar and unmanipulated males. The setup of the second set of experiments was identical to the first, with the exception that those males with the lowest chroma and hue values had their plumage experimentally reddened within the range of the natural variation. In addition to plumage colouration, we incorporated testosterone levels, body condition and age into the analysis of factors contributing to contest outcome. Our results show a consistent and strong age effect in both sets of experiments, which seems to be independent of plumage colouration, testosterone and body condition. This suggests that in the red bishop, the outcome of male–male competitions over limited resources is determined by age‐related acquired experience rather than by status signalling through plumage colouration.     

Manipulating badges of status only fools strangers

Conflict is risky, but mechanisms that allow animals to assess dominance status without aggression can reduce such costs. Two different mechanisms of competitor assessment are expected to evolve in different contexts: badges of status are expected in larger, anonymous groups, whereas individual recognition is feasible in small, stable groups. However, both mechanisms may be important when social interactions occur both within and across stable social groups. We manipulated plumage in golden‐crowned sparrows (Zonotrichia atricapilla) and found that two known badges of status – gold and black head plumage patch sizes – independently affect dominance among strangers but manipulations had no effect on dominance among familiar flockmates. Moreover, familiar flockmates showed less aggression and increased foraging relative to strangers. Our study provides clear experimental evidence that social recognition affects badge function, and suggests that variation in social contexts maintains coexistence and context‐dependent use of these two dominance resolution mechanisms.     

Achromatic plumage reflectance predicts reproductive success in male black-capped chickadees

The size of achromatic (black, white, gray) plumage patchesserves as a male status signal in many species of birds, butvariation in the colors of these patches has received littleattention. We assessed the relation between achromatic plumagereflectance, dominance rank, body condition, and reproductivesuccess in male black-capped chickadees, Poecile atricapillus.We measured plumage reflectance for five body regions of 40male chickadees in late winter and monitored these males throughoutthe following breeding season to determine whether they survivedto breed, whether they successfully paired, whether their partnerlaid eggs, and both their apparent and realized reproductivesuccess. As expected from past studies, a male's dominance ranksignificantly predicted whether his partner laid eggs. However,only achromatic plumage reflectance significantly predictedother measures of male reproductive performance. Among maleswho fledged at least one offspring, both the brightness of whiteplumage regions and the UV-chroma of melanin-based plumage regionswere significant predictors of the proportion of within-pairyoung in their nests. When we consider all males we measured,assigning zero values to males who failed to sire any offspring,the UV-chroma of melanin-based plumage regions was a significantpredictor of realized reproductive success. Bib size was alsorelated to male realized reproductive success. Our findingssuggest that individual variation in achromatic plumage mayplay an important role in sexual signaling in chickadees.     

Longitudinal stability in adult status hierarchies among vervet monkeys (Cercopithecus aethiops)

Status hierarchies were examined in a captive group over a 12-year period (1968–1979) to establish the stability and structure of vervet monkey dominance relationships. Dyadic records of spontaneous avoid, bite, chase, and supplant behaviors are summarized into a win/lose matrix to produce a rank order of status at the end of each calendar year. The 13,717 agonistic acts tabulated are drawn from both focal and ad lib data. Relative rank is computed by dividing the rank position of each individual by the number of individuals ranked for that year. Relative rank was found to be quite stable for adults of both sexes, although males were more variable than females. There was a strong matinrline effect on all captive born animals' rank position. No gender difference in rank was demonstrable. Infants were very subordinate at the end of their first year, but rose to positions near those of their mothers by the end of the fourth year. Presence or absence of canine teeth had no demonstrable effect on status. The results emphasize the relative importance of social learning in the establishment and maintenance of dominance relationships. The apparent gender difference in rank among wild living vervets is hypothesized to be a consequence of the male intergroup mobility. If a male is restrained from emigration, he is strongly affected by his mother's rank as he establishes his own dominance position in the troop.     

Individual Recognition Affects Aggression and Dominance Relations in Rainbow Trout,Oncorhynchus mykiss

In this study, the hypothesis that individual recognition is used as a cue to reduce the cost of contesting resources in rainbow trout Oncorhynchus mykiss, was addressed. The predictions were that the second contest between familiar individuals should be settled with less aggression, and lower probability of status reversal, than a contest between strangers. To test these predictions, similar-sized juvenile rainbow trout were subjected to two dyadic dominance contests in one of four treatment groups: in half of the pairs, the initially subordinate and dominant individuals were staged against an unfamiliar opponent (of opposite rank to their own) in a second contest, while in the other the initial pair was reunited for the second contest. Further, in half of the pairs, contestants were separated for 3 d between contests, while in the other the second contest was staged immediately after the first. Levels of aggression in contests between familiar individuals were lower than in contests between strangers, which supports the hypothesis that individual recognition reduces aggression. Dominance status was reversed in 31% of the 62 tested pairs. Although the probability of status reversal was not significantly affected by individual recognition, the degree of change in competitive success appeared to be lower in familiar pairs. Separation interacted with familiarity so that the effect of individual recognition generally was less pronounced when pairs were separated between contests, suggesting that the ability to remember opponents is time/limited. In natural streams, salmonids may use individual recognition to reduce the cost of contesting resources within groups, to reduce aggression between territory neighbours and to distinguish ‘cheaters’ from honest signallers.     

Behavioural dominance and reproductive success in female Japanese monkeys (Macaca fuscata)

Eight years of reproductive data (including 248 births) from a translocated troop of Japanese monkeys (Macaca fuscata) living in a 42-ha enclosure provided three measures of female reproductive success: fecundity, survival of infants to 1 year of age, and age at first parturition. No significant relationship was found between social dominance and these measures. Social dominance was considered with respect to both matrilineal and individual female rank. Additional data on female dominance ranks over four generations of adult females revealed no significant concordance over time. The finding that ranks may not be stable over the lifetime of a female is a significant one because the variation in reproductive success among the females of a group is likely to be further diminished by any instability. For 34 females that were adults for the 8-year period considered, there was no significant correlation between the average rank of a female and either fecundity or survivorship of infants to 1 year of age. These data considered along with the results of other studies of female dominance and reproduction suggest that any effect of female social dominance on reproductive success is probably dependent upon resource availability, with significant benefits accruing to high-ranking individuals only during subsistence periods. It is suggested that dominance competition among female macaques may be a behavioural strategy with a variable payoff.     

Signals of status in wintering white-crowned sparrows, Zonotrichia leucophrys gambelii

The possibility of plumage status signalling within the social systems of wintering birds has been a controversial issue. Our results are the first to demonstrate conclusively the reality of such signalling. Data from eight groups of captive white-crowned sparrows (Zonotrichia leucophrys gambelii), each with 8 to 11 different individuals, show that immature and adult females with crowns painted to resemble more brightly coloured, dominant adult males consistently win encounters with control birds of their own age and sex. These experiments demonstrate that signals that correlate with age (adult versus immature) and sex (adult male versus adult female) are used by the birds as reliable indicators of relative dominance position. Our demonstration of status signalling draws attention to the need to explain how such a system can be evolutionarily stable and we discuss some suitable models.     

The function of the variation in the breast stripe of the great tit (Parus major)

Many winter flocking bird species possess a plumage colour polymorphism. Two hypotheses have been proposed to explain the function of this variability in plumage structure: the signalling of (1) individual identity (IRH), or (2) social status (SSH). These two hypotheses were tested on winter flocking great tits, a species showing intraspecific variation in its breast stripe plumage. The SSH was considered the most probable, because a close correlation between social status and breast stripe width was found. The SSH was further tested by using dummy great tits bearing breast stripes of different widths. Great tits that possessed a breast stripe wider than that of the dummy approached it aggressively, and those with a narrower behaved submissively. We therefore conclude that the function of the variation in width of the breast stripe plumage is to proclaim the social status of the individual.     

Seasonal Changes in Colour: A Comparison of Structural,Melanin- and Carotenoid-Based Plumage Colours

Background

Plumage coloration is important for bird communication, most notably in sexual signalling. Colour is often considered a good quality indicator, and the expression of exaggerated colours may depend on individual condition during moult. After moult, plumage coloration has been deemed fixed due to the fact that feathers are dead structures. Still, many plumage colours change after moult, although whether this affects signalling has not been sufficiently assessed.

Methodology/Principal Findings

We studied changes in coloration after moult in four passerine birds (robin, Erithacus rubecula; blackbird, Turdus merula; blue tit, Cyanistes caeruleus; and great tit, Parus major) displaying various coloration types (melanin-, carotenoid-based and structural). Birds were caught regularly during three years to measure plumage reflectance. We used models of avian colour vision to derive two variables, one describing chromatic and the other achromatic variation over the year that can be compared in magnitude among different colour types. All studied plumage patches but one (yellow breast of the blue tit) showed significant chromatic changes over the year, although these were smaller than for a typical dynamic trait (bill colour). Overall, structural colours showed a reduction in relative reflectance at shorter wavelengths, carotenoid-based colours the opposite pattern, while no general pattern was found for melanin-based colours. Achromatic changes were also common, but there were no consistent patterns of change for the different types of colours.

Conclusions/Significance

Changes of plumage coloration independent of moult are probably widespread; they should be perceivable by birds and have the potential to affect colour signalling.     

Allogrooming, partner choice, and dominance in male anubis baboons

This study was designed to test the hypothesis that among unrelated male baboons (Papio cynocephalus anubis) in single-gender social groups there is no significant association between dominance status and allogrooming performance. The hypothesis was tested using behavioral measures obtained by focal animal sampling techniques. The results indicate that unrelated male baboons established well-defined linear dominance hierarchies, formed allogrooming relationships with one another, and exhibited a nonrandom distribution of allogrooming; however, there were no significant relationships between dominance rank and the frequency of allogrooming. We further tested our results by grouping individuals into three dominance status classes (high, middle, and low) and comparing the classes. Analysis of variance demonstrated no significant differences in rates of allogrooming by dominance class. These results suggest that dominance did not account for the variation in observed allogrooming behavior: Dominance status did not appear to determine the frequency with which animals groomed others, the number of grooming partners, or frequency of grooming that any individual received in comparison to that performed. High-ranking animals did not have significantly more grooming partners than low-ranking animals, and there appeared to be little competition within the groups for subordinates to groom high-ranking animals. When age, kinship, and group tenure are controlled, performance and reception of allogrooming are not strongly associated with dominance in single-gender social groups of male anubis baboons.     

Aerobic performance does not affect social rank in female Red Jungle Fowl

1. Exercise capacity ultimately constrains behaviour, and therefore may influence social interactions. The hypothesis was tested that individual differences in maximal rates of oxygen consumption ( V· _O2max) – a primary determinant of sustainable exercise capacity – affect dominance hierarchies in experimental all-female flocks of the highly social Red Jungle Fowl ( Gallus gallus ). It was also examined whether social rank could in turn influence V· _O2max (which is quite plastic in most vertebrates), and whether these relationships were influenced by a common and relatively benign parasite, the intestinal nematode Ascaridia galli .
2. Substantial between-individual variation was found in V· _O2max that was significantly repeatable over time, but there was no indication that V· _O2max was affected by A. galli infection. Stable social hierarchies were quickly established in 26 of 28 experimental flocks (each contained three females previously isolated from each other). Infection status affected social rank, but there was no consistent pattern between rank and infection.
3. No indication was found that individual differences in V· _O2max either predicted the social rank in newly formed flocks, or were affected by social status in established flocks.     

Dominance status of adult male Japanese macaques: Relationship to female dominance status,male mating behaviour,seasonal changes,and developmental changes

Aggressive dominance orders of all adults in a confined troop of Japanese macaques (Macaca fuscata) were determined each mating and birth season during a 4-year interval. Males outranked more females in the mating than in the birth season, and some males shifted back and forth from ranks lower than female ranks in the birth season to ranks higher than female ranks in the mating season. Mating behaviour (number of female partners in mount and ejaculation series and ejaculation frequency) did not differ among males with ranks higher than, as high as, or lower than those of most females, nor did individual males mate more in years, when they were high-ranking than in years when they were not. There was a correlation, however, between ejaculation frequency and the number of females defeated by males. A pattern of increasing male rank with age was found.     

Carotenoid-based plumage coloration of male greenfinches reflects health and immunocompetence

Hypotheses of parasite-mediated sexual selection (PMSS) propose that elaborate male ornaments have evolved due to female preferences. Females would benefit from mating with more ornamental males if males' ornamentation signals their health status and ability to provide parasite resistance genes for the offspring. Carotenoid-based plumage coloration of birds has been hypothesised to honestly reflect an individual's health status due to trade-off in allocation of carotenoids between maintenance and signalling functions. The prediction of this hypothesis, namely that individuals with brighter plumage are able to mount stronger immune responses against novel antigens and reveal generally better health state, was tested in captive male greenfinches (Carduelis chloris). Greenfinches with brighter yellow breast feathers showed stronger humoral immune response against novel antigen (SRBC) while no relationship between plumage coloration and an estimate of cell-mediated immune responsiveness (PHA response) was detected. Elaborately ornamental individuals had better general health state as indicated by the negative correlations between plumage brightness and heterophil haemoconcentration. Consistent with the concept of PMSS, these results suggest that carotenoid-based plumage coloration in greenfinches honestly signals immunocompetence and health status.     

Males in seemingly female‐like plumage do not mimic females: UV reflectance reveals temporal cryptic dimorphism in a manakin species exhibiting delayed plumage maturation

Manakins (Pipridae) are neotropical birds that usually exhibit delayed plumage maturation (DPM). Thus, while plumage of most adult male manakins is brightly conspicuous, subadult males and females are basically dull‐olive green. Although sexual dichromatism in some bird species may be evident only through UV reflectance, this phenomenon, known as hidden sexual dichromatism, has not been previously studied in manakins to compare subadult males and females. Within this framework, we carried out spectrophotometric analyses in searching for hidden sexual dichromatism in the white‐bearded manakin Manacus manacus, through comparison of UV spectra in females and subadult males in green plumage. Our results revealed UV reflectance in both sexes in green plumage. Moreover, we found UV spectral differences in homologous color patches between sexes, particularly at belly. Since the observed differences may allow intraspecific sex recognition of individuals in green plumage, our results do not support the female‐mimicry hypothesis to explain delayed plumage maturation in the white‐bearded manakin. Although our findings dismiss the female mimicry hypothesis, we cannot state whether these results support the non‐mutually exclusive cryptic and status signaling hypotheses. We propose then, that dull coloration of subadult males may serve both as a cryptic trait and to limit the energetic costs of acquiring the adult plumage before sexual maturity. Meanwhile, differential UV color traits between sexes in green plumage may allow adult males to avoid unnecessary energy expenditures in courtship displays in the presence of males near leks, and to selectively focus their the courtship displays on females. In accordance with the status signaling hypothesis, subadult males can be recognized both as males and subordinates and consequently may practice courtship displays without suffering aggressions by adult males. Our results highlight the importance to include a wider range of spectrophotometric information analyses for testing hypotheses regarding delayed plumage maturation.     

Social relations in a free-ranging troop of stumptail macaques (Macaca arctoides): Male-care behaviour I

The occurrence of male-care behaviour directed from juvenile and adult males to infants was studied in a free-ranging troop of Stumptail macaques. The study period lasted two months comprising about 140 hours of recorded observations. Infants were a focal subgroup and their interactions with older males were recorded. The following variables were examined in relationship to the sending and receiving of male-care: the infant (its age, sex, and dominance rank), older males (their age and dominance rank), and genetic ties. Infants I received more male-care than infants II and differences in the type of male-care received by infants I and II were found. Male infants received more male-care than female infants and sex differences in the type of care received were evident. No relationship was found between the infant’s dominance rank and the amount of male-care received. A substantial amount of male-care behaviour was sent to genetic kin. Two-three year olds displayed more male care than yearlings. Juveniles as a class displayed more male-care than adults. A positive association was found between the juveniles’ dominance rank and the sending of male-care. However, among the adults, the subordinate male displayed more care behaviour than the alpha male. The presence or absence of the mother was found to influence the older males’ interest in the infant. The results are discussed and compared with data available on other primate species. Supported by grants from the Behavioral Science Foundation (U.S.) and by the Mexican Institute of Anthropology.     

Does Male Badge Size Signal Status in Small Island Populations of House Sparrows,Passer domesticus?

The relationship between dominance and throat badge size, body size, and body condition was examined in three small island populations of house sparrows, Passer domesticus, on the coast of northern Norway. Individual dominance rank in males was determined by observation of agonistic interactions in an artificial observation cage (one replicate in Dec. and one in Mar. in two populations and one replicate in Mar. in the third population). Previous studies had indicated that badge size signals status in house sparrows. In this study, because of the small population sizes and stability in their membership, we expected that prior information about fighting ability would reduce the importance of badge size for the outcome of conflicts in favour of traits most directly associated with fighting ability. In two of five replicates, significant relationships were found between components of body size and dominance rank, but in opposite directions. The relationship between badge size and dominance rank was consistently positive, but non-significant. In a pooled sample of the three independent replicates, badge size was the best predictor of dominance, and no other variable explained any variance in dominance. However, the coefficient of determination was low and linearity of dominance hierarchies was poor. We therefore suggest that individual recognition may influence the dominance relationships.