Mutual Mate Choice for Colorful Traits in King Penguins

While studies of mate choice based on male color pattern are ubiquitous, studies of mate choice based on ornamental color traits in sexually monomorphic species are less common. We conducted manipulative field experiments on two color ornaments of king penguins (Aptenodytes patagonicus), the size of auricular patches of orange feathers and degree of UV reflectance from beak spots, to determine how the degree of ornamentation influenced pairing rate. In a reduction of auricular patch size, females paired significantly more quickly than males in both control and experimental samples. When this bias was taken into account statistically, pairing of individuals with reduced auricular patches was significantly delayed. We also reduced, but did not eliminate, UV reflectance from beak spots by applying a UV filter; no sex difference in pairing rate was evident in this experiment. Treated birds paired significantly more slowly than untreated control individuals, taking more than a week longer to pair on average than their unmanipulated counterparts, a result that was significant for males and approached significance for females. Our results may indicate mutual mate choice via UV reflectance of the beak spot. Given that this is a species where breeding is extremely slow and considerable investment by both males and females is required for successful reproduction, our results support the hypothesis that in such species, sexual selection might act on the same ornament in both sexes.     

Is mate fidelity related to site fidelity? A comparative analysis in Ciconiiforms

We tested for an association between divorce rate and site fidelity in 42 avian species belonging to the order Ciconiiforms, using comparative methods that account for the influences of phylogenetic relationships on the data. Our methods enabled us to detect evidence of correlated evolution and provided information on the temporal ordering of evolutionary changes in these two variables. We found a significant correlation between divorce rate and site fidelity, indicating that species with little or no site fidelity are more likely to divorce. Our data suggest that the coupled evolution of divorce and site fidelity can be summarized by three major events. The first event corresponds to a transition from species showing high divorce rate and low or no site fidelity to species that tended to reuse the same nests over consecutive breeding seasons. This was followed by a transition towards higher mate fidelity, with the preservation of pair bonds over consecutive breeding attempts. In a third stage, divorce rate and the rate of site fidelity varied, independently of each other. We discuss our results within the context of the ancestor species and the past environments in which the traits originated, and address the importance of the potential for individual recognition in shaping the observed patterns of covariation between mate fidelity and site fidelity in Ciconiiforms. Copyright 2000 The Association for the Study of Animal Behaviour.     

Ornament evolution in dragon lizards: multiple gains and widespread losses reveal a complex history of evolutionary change

The expression in females of ornaments thought to be the target of sexual selection in males is a long-standing puzzle. Two main hypotheses are proposed to account for the existence of conspicuous ornaments in both sexes (mutual ornamentation): genetic correlation between the sexes and sexual selection on females as well as males. We examined the pattern of ornament gains and losses in 240 species of dragon lizards (Agamidae) in order to elucidate the relative contribution of these two factors in the evolution of mutual ornamentation. In addition, we tested whether the type of shelter used by lizards to avoid predators predicts the evolutionary loss or constraint of ornament expression. We found evidence that the origin of female ornaments is broadly consistent with the predictions of the genetic correlation hypothesis. Ornaments appear congruently in both sexes with some lineages subsequently evolving male biased sexual dimorphism, apparently through the process of natural selection for reduced ornamentation in females. Nevertheless, ornaments have also frequently evolved in both sexes independently. This suggests that genetic correlations are potentially weak for several lineages and sexual selection on females is responsible for at least some evolutionary change in this group. Unexpectedly, we found that the evolutionary loss of some ornaments is concentrated more in males than females and this trend cannot be fully explained by our measures of natural selection.     

Mate fidelity and coloniality in waterbirds: a comparative analysis

Increased opportunities for information are one potential benefit of sociality. We apply this idea to the advantages of colonial breeding in bird species that are typically monogamous within a breeding season but often form new pair-bonds in subsequent seasons. Individuals may benefit from nesting in colonies at high density by identifying good-quality potential alternative mates among their neighbours. The opportunities for finding a better mating option are likely to increase with colony size and density. We tested this prediction with a comparative analysis of the association between mate fidelity and coloniality in waterbirds (wading birds and seabirds), where there is wide variation in both the degree of mate retention over consecutive breeding seasons and the degree of coloniality. We used two comparative statistical analyses, one based upon generalized least squares and the other based upon a continuous-time Markov model, to test whether the pattern of association between divorce rate and degree of coloniality was evidence for correlated evolutionary change in the two characters. We found a significant and positive association between divorce rate and the degree of coloniality in waterbirds. The probable ancestral state corresponds to a combination of a high degree of coloniality with no, or weak, mate fidelity. The reconstruction of the historical pattern of character origin and evolution indicates that the transition from a high to a low degree of coloniality occurred before the transition to higher mate fidelity. Received: 5 January 1998 / Accepted: 20 April 1998     

BOTH MALE AND FEMALE SEXUAL ORNAMENTS REFLECT OFFSPRING PERFORMANCE IN A FISH

Secondary sexual characters are often expressed in both sexes (mutual ornamentation), but are less often studied simultaneously. We studied the adaptive signaling function of male and female ornamentation in a mutually ornamented fish, the whitefish Coregonus lavaretus. In an experimental design in which nongenetic environmental effects were minimized, we found that highly ornamented females, males, and their parental combinations had offspring with better swimming performance and predator‐avoidance ability than less ornamented individuals or combinations. Furthermore, highly ornamented females had larger offspring that also had higher yolk volume than less ornamented individuals. Offspring swimming performance was not dependent on offspring size and was only weakly affected by yolk volume, which suggest that swimming performance and measured morphological traits are independent fitness measures. In conclusion, mutual ornamentation of whitefish may signal the quality of individuals in both sexes, which may indicate ongoing directional selection for these ornamental traits. However, offspring fitness traits were also dependent on parental combination, which suggests that genetic compatibility effects may weaken the directional selection and the indicator value of the ornamentation.     

Mutual ornamentation and the parental behaviour of male and female Collared Flycatchers Ficedula albicollis during incubation

One of the benefits of mate choice based on sexually selected traits is the greater investment of more ornamented individuals in parental care. The choosy individual can also adjust its parental investment to the sexual signals of its partner. Incubation is an important stage of avian reproduction, but the relationship between behaviour during incubation and mutual ornamentation is unclear. Studying a population of Collared Flycatchers Ficedula albicollis, we monitored the behaviour of both sexes during incubation in relation to their own and their partner's plumage traits, including plumage‐level reflectance attributes and white patch sizes. There was a marginally positive relationship between male feeding rate during incubation and female incubation rate. Female but not male behavioural traits were associated with the laying date of the first egg and clutch size. The behaviour of the two sexes jointly determined the relative hatching speed of clutches and the hatching success of eggs. Females with larger white wing patches spent less time incubating eggs and left the nestbox more frequently. Males with larger white wing patches fed females less frequently, whereas males with brighter white plumage areas visited the nestbox more regularly without feeding. Females tended to leave the nest less often when mated to males with larger wing patches, and females spent less time incubating when males had more UV chromatic plumage. The behaviour of both partners during incubation therefore predicted hatching patterns and was correlated with their own and sometimes with their partner's plumage ornamentation. These results call for further studies of mutual ornamentation and reproductive effort during incubation.     

Year-round resource defence and the evolution of male and female song in suboscine birds: social armaments are mutual ornaments

The evolution of sexually monomorphic (i.e. mutual) ornamentation has attracted growing attention as a 'blind-spot' in evolutionary biology. The popular consensus is that female ornaments are subject to the same modes of sexual selection as males: intrasexual competition and mate choice. However, it remains unclear how these forces interact within and between sexes, or whether they fully capture selection on female traits. One possibility is that the 'armament-ornament' model - which proposes that traits used primarily in male-male contests are also co-opted by females as indicators of male quality - can be extended to explain signal evolution in both sexes. We examine this idea by testing the function of acoustic signals in two species of duetting antbirds. Behavioural observations and playback experiments suggest that male and female songs function primarily as armaments in competitive interactions. Removal experiments reveal that song is also a classic ornament used by unpaired males and females to advertise for mates. These results indicate that 'armament-ornament' processes may operate in reciprocal format, potentially explaining widespread mutual ornamentation in species with elevated intrasexual competition for resources. In addition, given that songs mediate competition between species outside the breeding season, our findings suggest that processes shaping monomorphic ornaments extend beyond the traditional definitions of sexual selection and are best understood in the broader framework of social selection.     

Influence of reproductive output on divorce rates in polar seabirds

The high occurrence of social monogamy in birds has led to questions about partner fidelity, or the perennial nature of monogamy from one breeding season to another. Despite the evolutionary advantages of partner fidelity, divorce occurs among 95% of bird species. We aimed to describe patterns of divorce and partner fidelity in five seabird species breeding in Arctic and Antarctic regions and investigated the influence of breeding status on pair bond maintenance. For four out of the five species considered, we observed low divorce rates (respectively 1.9%, 3.3%, 2.5%, and 0.0% for Brünnich''s guillemot, glaucous gull, Antarctic petrel, and south polar skua), while the divorce rate was much higher (19.1%) for the black‐legged kittiwake. For kittiwakes, the divorce rate was lower for pairs that managed to raise their chick to 15 days of age, while the effect of breeding success on divorce in the four other species could not be tested due to the rareness of divorce events. Our results emphasize the potentially large temporal (interannual) variations that should be taken into account in understanding divorce and partner fidelity in seabirds.     

Implications of mate search, mate choice and divorce rate for population dynamics of sexually reproducing species

In this paper we examine how the process of mate search, degree of mate choice and degree of mate fidelity may interact to affect long‐term population dynamics of sexually reproducing species. In particular, we address the following questions: are degree of mate choice and degree of mate fidelity correlated? How does mate search shape this relationship? How does longevity affect mating behaviour? To resolve these questions, we develop a spatially explicit, individual‐based model of a sexually reproducing population with single (i.e. unpaired) males, single females, and pairs as focal individuals. Both this model and its non‐spatial approximation give rise to the Allee effect due to lack of mating possibilities, and sufficiently small/sparse populations always go extinct. We quantify combinations of mate choice and divorce rate under which populations persist or go extinct even from high sizes. We thus show that there exist ecological constraints for possible (co)evolution of mate choice and pair maintenance behaviour. Our models also suggest that colonial species with active mate search strategy survive at higher divorce rates than less colonial animals that search for their mates randomly, and that long‐lived species sustain at higher degrees of mate choice and lower degrees of mate fidelity compared to the short‐lived ones. Connection of these findings to other theoretical results and some empirical observations is discussed.     

Burrow and mate fidelity in the Little Penguin Eudyptula minor at Lion Island, New South Wales, Australia

Although both mate and nest fidelity are thought to exist in Little Penguins Eudyptula minor , few studies have statistically assessed the relationship between both types of fidelity and breeding success. Observations of the breeding biology of the Little Penguin were carried out between 1990 and 1998 at Lion Island, New South Wales, Australia. The existence of fidelity and the effect of breeding success on subsequent fidelity were assessed for this population. Strong evidence for the existence of nest and mate fidelity was found with high rates of return to the same burrow and mate being recorded for both sexes. For all years of the study, the average number of female and male birds faithful to their nest was 76 and 79%, respectively, with only two noted cases of intraseasonal burrow swapping. The rate of divorce varied widely between years, with a range of 0–40%. A marginal association of a log linear model showed a highly significant relationship ( P  < 0.001) between success of a clutch in one year and fidelity to a mate in the following year. Consequently, parents successfully raising a fledgling in one year were more likely to reunite in the following year than were pairs that attempted unsuccessfully to breed. Pairs failing to fledge chicks were much more likely to separate owing to the absence of one parent in the following year. We were unable to detect any association between breeding success in one year and subsequent change of nest in the next.     

Mate fidelity in Cory's Shearwater Calonectris diomedea on Selvagem Grande

In the Cory's Shearwater Calonectris diomedis boreadis breeding on Selvagem Grande (30̀ 09'N, 15̀52'W), mate fidelity averaged 71.4%. Two-thirds of the changes were the result of the absence of the former mate and one-third only, carried out in its presence, were true divorces. The rate of divorce and the rate of replacement of an absent former mate had increased since the beginning of the 1980s. Among young and inexperienced birds, the rate of divorce decreased with the increase of age and experience before becoming stable. Breeding failure during the previous year increased the probability of divorce. Temporary non-breeding increased both the rate of divorce and the rate of replacement of an absent former mate.     

Female finery is not for males

There has recently been an increase in interest in the notion that female ornamentation is selected through male choice, rather than being an artefact of selection on male ornamentation. There are, however, key differences between the sexes in the type of selection pressures that are likely to generate ornamentation and important differences in investment tradeoffs. Here, I discuss that female ornamentation might be selected more often through female competition over resources than through competition over mates, as exemplified in a recent study by Heinsohn and colleagues.     

Mate fidelity in a polygamous shorebird,the snowy plover (Charadrius nivosus)

Social monogamy has evolved multiple times and is particularly common in birds. However, it is not well understood why some species live in long‐lasting monogamous partnerships while others change mates between breeding attempts. Here, we investigate mate fidelity in a sequential polygamous shorebird, the snowy plover (Charadrius nivosus), a species in which both males and females may have several breeding attempts within a breeding season with the same or different mates. Using 6 years of data from a well‐monitored population in Bahía de Ceuta, Mexico, we investigated predictors and fitness implications of mate fidelity both within and between years. We show that in order to maximize reproductive success within a season, individuals divorce after successful nesting and re‐mate with the same partner after nest failure. Therefore, divorced plovers, counterintuitively, achieve higher reproductive success than individuals that retain their mate. We also show that different mating decisions between sexes predict different breeding dispersal patterns. Taken together, our findings imply that divorce is an adaptive strategy to improve reproductive success in a stochastic environment. Understanding mate fidelity is important for the evolution of monogamy and polygamy, and these mating behaviors have implications for reproductive success and population productivity.     

The function of female and male ornaments in the Inca Tern: evidence for links between ornament expression and both adult condition and reproductive performance

Inca Terns Larosterna inca are medium-size seabirds that breed along the Peruvian and Chilean coast. They are monogamous and both sexes incubate and contribute to chick provisioning. The sexes are similar in appearance and have elaborate ornaments, including a long white moustache of feathers and fleshy yellow wattles. In this paper we report the differences in ornamentation between sexes and examine whether the trait predicts body condition, reproductive performance or chick quality in either sex. The ornaments were similar in size and coloration between the sexes, except for the wattle length, the difference in which can be due to greater head length in males. Moustache length was the most reliable signal of body condition in both sexes. Moreover, there was a significant relationship between the moustache length and reproductive category of adults (non-breeder, unsuccessful breeder, or one or two chicks fledged). Both asymptotic chick body mass and the T-cell mediated response of chicks (a measure of immunocompetence) were related to the moustache length of male and female adults. These results provide support for the role of ornaments in mutual signalling of condition in this species. Female and male ornaments predict body condition, reproductive performance and chick quality, as predicted by sexual selection models.     

Assortative Pairings in Diamond Firetails (Stagonopleura guttata) are not the Result of Mutual Mate Choice for an Ornament

Positive assortative mating occurs when individuals with similar phenotypes mate more frequently with each other than is expected by chance. In species in which both the males and females are ornamented, assortative pairings could arise from mutual mate choice on the same trait. We test this mechanism of mate choice and assortative pairing in the Diamond Firetail (Stagonopleura guttata), an Australian estrildid finch in which both sexes are ornamented with red bills, red rumps and white flank spots. We have previously shown sex differences in the degree of ornamentation as females have more flank spots than males. These white flank spots are used during sexual display, being fully displayed by courting males and by females when approaching a displaying male. Here, we experimentally test whether mutual mate preference is based on the number of flank spots. There was no evidence for a direct mutual preference for spot number. Given a choice of potential mates with a natural or experimentally manipulated number of flank spots, males preferred females with more spots, while female preference was not solely based on flank spots. Intriguingly, in both wild and captive Diamond Firetails, we found the number of flank spots in pairs was correlated suggesting a basis for positive assortative pairing. Nevertheless, we conclude that assortative pairing in Diamond Firetails is not due to mutual choice of mates based on the number of flank spots. We discuss different selection pathways for this trait in each sex.     

Bill redness is positively associated with reproduction and survival in male and female zebra finches

Sexual traits can serve as honest indicators of phenotypic quality when they are costly. Brightly coloured yellow to red traits, which are pigmented by carotenoids, are relatively common in birds, and feature in sexual selection. Carotenoids have been linked to immune and antioxidant function, and the trade-off between ornamentation and these physiological functions provides a potential mechanism rendering carotenoid based signals costly. Mutual ornamentation is also common in birds and can be maintained by mutual mate choice for this ornament or by a correlated response in one sex to selection on the other sex. When selection pressures differ between the sexes this can cause intralocus sexual conflict. Sexually antagonistic selection pressures have been demonstrated for few sexual traits, and for carotenoid-dependent traits there is a single example: bill redness was found to be positively associated with survival and reproductive output in male zebra finches, but negatively so in females. We retested these associations in our captive zebra finch population without two possible limitations of this earlier study. Contrary to the earlier findings, we found no evidence for sexually antagonistic selection. In both sexes, individuals with redder bills showed higher survival. This association disappeared among the females with the reddest bills. Furthermore, females with redder bills achieved higher reproductive output. We conclude that bill redness of male and female zebra finches honestly signals phenotypic quality, and discuss the possible causes of the differences between our results and earlier findings.     

Mating and remating of least auklets (Aethia pusilla) relative to ornamental traits

We investigated mating patterns of the least auklet, a smallmonogamous seabird, at St. Paul Island, Alaska, during threebreeding seasons. Least auklets mated assortatively with respectto both plumage color, a trait important in status signaling,and tarsus length, an index of body size. Least auklets mateddisassortatively with respect to the extent of facial plumes,but neither assortatively nor disassortatively for any otherornamental trait (bill color, bill ornament size). Mate fidelitywas lower in least auklets than in some long-lived seabird species;when both members survived to a following year, only about two-thirdsof pairs reunited. Nearly half of the auklets paired in 1 yearobtained a new mate in the following year, either because ofmate disappearance or divorce. Interyear fidelity to mates wasrelated only to male ornamentation; males with larger facialplumes were more likely to reunite with their mates the nextyear than males with smaller plumes. There were no significantdifferences in the ornaments of females in reunited and divorcedpairs. Pairs that reunited also had significantly lighter plumagethan pairs that divorced, and the plumage of males reunitingwith their mates was significantly paler than that of divorcedmales. We conclude that the probability of both divorce andremating in this species is influenced by ornamental traits.Our finding that remating was related to male plumage colorand ornaments is consistent with the idea that remating is influencedby female choice. Pairs that reunited also bred earlier in theseason and had higher reproductive success than pairs with experiencedindividuals breeding together for the first time. We also foundevidence that failure to breed in a given year increased theprobability of subsequent divorce.     

Carotenoid-based ornaments of female and male American goldfinches (Spinus tristis) show sex-specific correlations with immune function and metabolic rate

Conspicuous ornamentation has been linked to immunological and physiological condition in males of many species. In species where both sexes are ornamented, it is unclear whether the signal content of ornaments differs between males and females. We examined the immunological and physiological correlates of carotenoid-based bill and plumage ornamentation in American goldfinches Spinus tristis, a species in which bright orange bills are sexually monomorphic but yellow plumage is sexually dimorphic during the breeding season. Because bill color is dynamic over short periods while plumage color is static over longer time frames, we tested whether these signals have the potential to provide temporal information about immunity and condition. In both sexes, bill color (but not plumage color) was negatively related to leukocyte differential, a measure of recent stress, while plumage color (but not bill color) was positively related to resting metabolic rate. In females, bill color also positively correlated with immunoglobulin Y, a component of acquired immunity, while plumage color positively predicted natural antibody levels, a component of innate immunity. In males, neither bill color nor plumage color predicted immune function, suggesting that the mechanisms underlying these signals vary with sex. Our results demonstrate that dynamic signals such as bill coloration do not merely reflect the same information provided by static signals but that these two classes of signal provide information about different temporal aspects of phenotypic quality. Furthermore, our results indicate that a signal expressed in both sexes has the potential to provide different information depending on the sex of the bearer.     

Male and Female Mating Behavior is Dependent on Social Context in the Butterfly Bicyclus anynana

Reproduction is often more costly to females than it is to males, leading to the evolution of ornamented or competitive males and choosy females. Reproduction costs to females, however, can be reduced through nuptial gifts provided by males. These gifts, by increasing female survival or fecundity, can promote the evolution of mutual mate choice, ornamentation, or competition in both sexes, as well as plasticity in mating behavior dependent on social context. We tested for plasticity in male and female mating behavior in a species of butterfly, Bicyclus anynana, where male spermatophore gifts contribute to female survival and fecundity, and where mutual mate choice and ornamentation were previously established. We examined the effect of a sexual competitor on male–female interactions by observing and comparing the behavior of male–female pairs with that of triads containing either an extra male or an extra female. In the presence of a sexual competitor both males and females copulated less than when in male–female pairs, regardless of the direction of sex-ratio skew. Active males increased their own likelihood to copulate, while active females increased their likelihood of being courted. In addition, there was an effect of social context on relative rates of male and female courting and flying. These results suggest that both males and females change their mating behavior in response to social context in the butterfly Bicyclus anynana.     

Assortative mating by multiple ornaments in northern cardinals (Cardinalis cardinalis)

In positive assortative mating, individuals of similar phenotypemate together more frequently than expected by chance. Assortativemating by a variety of qualities, including ornamentation, iswell documented in birds. Studies of assortative mating by ornamentshave focused on single, highly conspicuous ornaments, but manyspecies of birds possess multiple ornaments in both sexes. Wecompared ornament expressions between mates of northern cardinals(Cardinalis cardinalis) to determine if assortative mating occurredby one or more of the four ornaments displayed by both sexes.All cardinals possess tall head crests and red-orange bills.In addition, males have black face masks and entirely red bodyplumage, whereas females have blackish face masks and red underwingcoverts. We predicted that cardinals mate assortatively by plumagecolor because red plumage expression has been shown to indicatequality in both sexes. We found that cardinals mate assortativelyby plumage and bill color, the two ornaments colored by carotenoidpigments, but not by mask expression or crest length. Whetherthis mating pattern arises by mutual mate choice or intrasexualselection is not known.