Unpaired Males Research Articles

VIGILANCE BEHAVIOR IN THE HARLEQUIN DUCK (HISTRIONICUS HISTRIONICUS) DURING THE PREINCUBATION PERIOD IN LABRADOR: ARE MALES VIGILANT FOR SELF OR SOCIAL PARTNER?

Abstract Males of several avian species are more vigilant than their female partners, particularly during the preincubation period. Male vigilance may function as a deterrent of extrapair copulation attempts (EPCs) or to ensure male survival, because males of sexually dimorphic species may be more conspicuous to predators than females. However, vigilant males may act as antipredator sentinels to enhance female survivorship, which may also allow females to feed and rest more efficiently, thereby enhancing their ability to form clutches. We attempted to distinguish between the “male benefits” and “female benefits” hypotheses by studying the behavior of unpaired males and paired Harlequin Ducks (Histrionicus histrionicus) during the preincubation period. Paired males were, on average, four times more vigilant than unpaired males and their mates, which suggests that elevated levels of vigilance by paired males does not function primarily for self-protection against predators. Males were not more vigilant duri...

Vigilance Behavior in the Harlequin Duck (Histrionicus Histrionicus) During the Preincubation Period in Labrador: Are Males Vigilant for Self or Social Partner?

AbstractMales of several avian species are more vigilant than their female partners, particularly during the preincubation period. Male vigilance may function as a deterrent of extrapair copulation attempts (EPCs) or to ensure male survival, because males of sexually dimorphic species may be more conspicuous to predators than females. However, vigilant males may act as antipredator sentinels to enhance female survivorship, which may also allow females to feed and rest more efficiently, thereby enhancing their ability to form clutches. We attempted to distinguish between the “male benefits” and “female benefits” hypotheses by studying the behavior of unpaired males and paired Harlequin Ducks (Histrionicus histrionicus) during the preincubation period. Paired males were, on average, four times more vigilant than unpaired males and their mates, which suggests that elevated levels of vigilance by paired males does not function primarily for self-protection against predators. Males were not more vigilant during the female’s fertile period, contrary to the “paternity assurance” hypothesis. Male vigilance during feeding bouts was highest when pairs fed alone near shore—a condition in which feeding was likely more “risky.” Male vigilance was positively correlated with female feeding when females fed near shore, but not at other times. During resting bouts, males were vigilant 14 ± 0.01% of the time in all conditions. We suggest that male vigilance may be important to enhance female survival, especially when females feed under risky conditions.Comportement de Vigilance chez Histrionicus histrionicus durant la Période de Pré- incubation au Labrador: Les Mâles Sont-ils Vigilants pour Eux-mêmes ou pour leur Partenaire Social?

SINGING BEHAVIOR VARIES WITH BREEDING STATUS OF AMERICAN REDSTARTS (SETOPHAGA RUTICILLA)

We examined the relationship between singing behavior and breeding status in the American Redstart (Setophaga ruticilla) by analyzing song rates, singing mode (Repeat or Serial), and variability of song delivery in relation to the age and breeding status of 129 males in the Hubbard Brook Experimental Forest, New Hampshire. Unpaired males spent most of their time (>90%) after dawn singing in Repeat mode, whereas paired males sang sporadically, in Serial as well as Repeat mode (51% of their singing time). Males who lost their mates sang in Repeat mode at rates indistinguishable from males who had not yet obtained a mate. Overall, unpaired males sang in Repeat mode at significantly higher and less variable rates than did paired males. Although a larger proportion of second-year males were unpaired than after-second-year males, we found no evidence that age affected singing behavior.We also assessed the effect of pairing status on male detectability in song-based monitoring surveys (e.g., point counts), and we suggest a field protocol for identifying unpaired males. Simulations of 5-min field samples, obtained from continuous samples >3 hr in duration, suggest that human listeners would be twice as likely to detect unpaired males as paired males. This result suggests that surveys based on aural detections may be biased in favor of unpaired males. In our population, >90% of males who sang >40 Repeat songs in 5 min were unpaired. Unpaired males were >3 times as likely as paired males to sing only Repeat songs in a given 5-min period. These results suggest that it may be possible to identify unpaired male American Redstarts by their high singing rates of exclusively Repeat songs.

Song is a reliable signal of bill morphology in Darwin's small tree finch Camarhynchus parvulus, and vocal performance predicts male pairing success

The shape and movement of the vocal tract are known to influence bird song. Current theory predicts that large bill and body size are correlated with low frequency song and slow trill rate. It is also widely accepted that song characteristics are important for mate choice by females. We investigated the relationship between bill morphology, song characteristics, and pairing success in Darwin's small tree finch Camarhynchus parvulus, on the Galapagos Islands. Contrary to predictions from a previous cross‐species study on Darwin's finches, we found that individuals with larger bill size produced songs with slow trill rate, high dominant frequency, and broad frequency bandwidth, indicating that song is a reliable signal of bill morphology. Vocal performance as indicated by the deviation from an upper performance limit was higher in paired than unpaired males. Pairing was not skewed in favour of a particular bill size, and both small and large billed males that sang high performance song had high pairing success. The reliable signalling function of song has implications for female choice and territorial defence, given that both females and conspecific competitors can assess the relative size of males’ bills through song, while females may use vocal performance as a signal of male quality.

Vocal interactions in common nightingales (Luscinia megarhynchos): males take it easy after pairing

Seasonal patterns of bird song have been studied intensively with a focus on individual males. However, little is known about seasonal patterns of singing during vocal interactions between males. Vocal interactions have been shown to be important in sexual selection as males may signal aspects of motivation or quality. Here, we investigated in nightingales (Luscinia megarhynchos) whether a male’s behaviour in vocal interactions at different stages of the breeding season is influenced by its mating status. We examined how males that differ in their subsequent mating success respond to a non-interactive, nocturnal playback presented during the period of mate attraction and subsequently during the egg-laying period. We found that mated males overlapped fewer songs and had a lower song rate during the egg-laying period compared to their responses during the mate-attraction period, whereas unpaired males did not vary in their responses between the two periods. Our results suggest that mating status is a key factor affecting singing behaviour in vocal interactions and that a time-specific singing pattern like song overlapping is used flexibly during vocal interactions. Because song overlapping is thought to be a signal of aggression in male–male vocal interactions, it seems that males vary the level of aggression in vocal interactions according to their mating status and to the stage in the breeding season.

Extra‐pair paternity in the strongly monogamous Wandering AlbatrossDiomedea exulanshas no apparent benefits for females

Although 92% of avian species are socially monogamous, extra‐pair copulation (EPC), resulting in extra‐pair paternity (EPP), is a common reproductive strategy in birds. Among seabirds, in which the rate of social monogamy reaches 100%, Procellariiformes (albatrosses and petrels) show low EPP rates, with the noticeable exception of the only albatross investigated in this regard, the Waved AlbatrossPhoebastria irrorata. This species, in which forced copulations are known to occur, showed a surprisingly high rate of EPP (25% of chicks). We investigate here EPP rates in another albatross species, the Wandering AlbatrossDiomedea exulans, subject to a demographic survey conducted for 38 years. We combined data on pair bonds with analysis of ten microsatellite loci and found that 10.7% of 75 chicks had an extra‐pair sire. Although there was some evidence for inbreeding avoidance, within‐pair and extra‐pair chicks showed similar levels of heterozygosity, and the incidence of EPP was independent of age, experience or past reproductive success. Hence, we found no evidence that females benefit from EPCs. Owing to the male‐biased sex ratio in adults, widowed and divorced males required more time to find a new mate (+28 and +72%, respectively) than did females. Combined with high sexual size dimorphism, this phenomenon might promote the forced copulations observed in this species. Our data therefore suggest that EPC is beneficial to unpaired males but occurs at random in females, consistent with the hypothesis that EPP results solely from forced EPCs. However, the importance of the latter for EPP and the part played by solitary males require further investigation.

Sex steroids modulate changes in social and sexual preference during juvenile development in zebra finches

Zebra finches, like many other animals, have close social relationships mainly with the family at young ages but begin to express interest in opposite-sex extra-family animals as they enter the late juvenile period and sexual maturity. This experiment tested a set of hypotheses that sex steroids are involved in this developmental transition. At 25–30 days, subjects were implanted subcutaneously with Silastic tubes that were empty (controls), filled with testosterone propionate, filled with estradiol benzoate, or filled with a combination of ATD (an aromatization inhibitor) and flutamide (an anti-androgen). Once a week between ages 30 and 90 days, they were given three-choice tests where the three stimulus types were the family members, unpaired males, or unpaired females. The preferred category was defined as the one adjacent to the proximity zone in which the subject spent the most time. Control males were more likely to prefer females and less likely to prefer the family as they got older, and control females were increasingly likely to prefer males. Males treated with testosterone or estradiol showed a premature increase in preferences for females. Females treated with ATD plus flutamide failed to show the normal increase in preferences for males shown by controls. These results indicate an involvement of sex steroids in the maturation of sexual preferences in a socially monogamous species that relies on visual and auditory, rather than olfactory, cues for sexual or other social behavior.

Breeding-Season Survival of Male and Female Mallards in Canada's Prairie-Parklands

Abstract Despite greater harvest rates of males, mallard (Anas platyrhynchos) populations exhibit male-biased sex ratios, presumably because females experience greater mortality during breeding seasons than do males. Nest success and adult female survival during the breeding season greatly impact mallard population growth; however, no study has evaluated breeding-season survival of males and females simultaneously using radiotelemetry. We captured, radiomarked, and monitored 90 male and 272 female mallards during 2 breeding seasons in the Prairie-Parkland region of Canada (Manitoba 1998, Saskatchewan 1999). Model-averaged estimates of female breeding season (i.e., 15 Apr–14 Jul) survival probabilities were 0.84 (SE = 0.031) in 1998 and 0.71 (SE = 0.040) in 1999. Estimated survival probabilities of paired males were 0.99 (SE = 0.016) in 1998 and 0.98 (SE = 0.025) in 1999; survival estimates for unpaired males were 0.92 (SE = 0.052) in 1998 and 0.85 (SE = 0.083) in 1999. Female mortality was greatest during...

Vocal interactions in nightingales, Luscinia megarhynchos: more aggressive males have higher pairing success

Song overlapping in birds is used and perceived as a signal of aggression, and evidence suggests that eavesdropping females base their extrapair mating decisions on the performance of males in vocal contests. In our study population of nightingales a large proportion of territorial males remain unpaired throughout the breeding season. A comparison between subsequently mated males and unpaired males may reveal whether females could use singing performance during vocal interactions in their choice of a social mate. We investigated how males that differed in their subsequent pairing status overlapped a noninteractive playback during the period of mate attraction, and how males used specific structural song components in response to playback. Subsequently mated males overlapped more playback songs than did males that remained unpaired throughout the breeding season. Males also adjusted the use of specific song components and decreased song rate during playback, suggesting that the flexible use of structural song components is more important in vocal contests than increasing song output. Because song overlapping is thought to be a signal of aggression, more aggressive males seem to have greater pairing success.

Is male song quality important in maintaining pair bonds?

AbstractThis study investigated a possible mechanism for maintaining long-term pair bonds in a socially monogamous songbird, the zebra finch (Taeniopygia guttata). Only males sing in this species. Song is thought to be important in female choice, and our earlier research showed that distortion of song by tracheosyringeal nerve transection (TS) and temporary muting by puncturing the interclavicular air sac (AS) both had profound effects on female's choice of male mates and pair formation. Males continue to sing when paired, though function and importance of this song is not well understood. The current study investigated whether these same vocal manipulations affected the maintenance of pair bonds. A total of 27 males and females formed pairs in aviaries. After 3 weeks of pairing and one week after the start of egg laying, males were experimentally manipulated. Eggs were then removed, so that females were forced to decide whether or not to engage in another breeding attempt with their mate. Novel unpaired males and females were added to the aviaries for potential extra pair copulation partners or new mates and pairs were then observed for four weeks. Only two pairs separated after song-altering surgery (both in the TS group), and one of these TS males quickly paired with another female. Of the pairs that remained together, there were no significant differences in courtship or pairing behaviors compared with control pairs. These results suggest that song quality has surprisingly little effect on female pairing decisions once the pair has formed, and that the song quality mechanisms of pair bond formation are not required in the maintenance of the pair bond.

Male and Female Prolactin Receptor mRNA Expression in the Brain of a Biparental and a Uniparental Hamster, Phodopus, Before and After the Birth of a Litter

Prolactin receptor (PRL-R) mRNA transcript level was quantified in the choroid plexus (ChP) of a naturally biparental hamster, Phodopus campbelli, and its otherwise similar, yet nonpaternal, sibling species, Phodopus sungorus. Pair-housed males and females on the day before the birth of their first litter (G17), the day after birth (L1), lactation day 5 (L5), and unpaired animals that were sexually naïve, were tested. PRL-R mRNA transcript level relative to total RNA, was evaluated by reverse transcriptase-polymerase chain reaction using primers common to the long- and short-form of the PRL-R in Phodopus. In the ChP, a region implicated in prolactin transport into the central nervous system, females had the expected increase in PRL-R mRNA transcript from dioestrus to L5, consistent with known actions of prolactin. As predicted, males and females of the biparental species were similar, although PRL-R mRNA in naive males was higher than in dioestrus females. Males of the two species also differed as predicted. PRL-R mRNA transcript levels were higher in the biparental males. In addition, P. campbelli males had low PRL-R mRNA at G17 compared to L5. By contrast, non-paternal P. sungorus males had elevated PRL-R mRNA transcript levels on G17 relative to unpaired males. We conclude that PRL-R mRNA in the ChP is differentially regulated before and after birth in a paternal and a nonpaternal male.

Revirstruktur och häckningsframgång hos ett unikt bestånd av skärpiplärka Anthus petrosus littoralis i hamnmiljö

A unique Rock Pipit Anthus petrosus littoralis population was established in an industrial environment in the harbour of Varberg in the late 1980s. It rose to 17 territories in 1994, then declined to extinction in 2004. In a detailed study (1993—1996), 57 territories were found (41 breeding pairs, 8 non-breeding pairs, and 8 unpaired males). Most nests lay on the ground but several up to 5.5 m above ground. 52 clutches were found: 38 first clutches, 19 failed; 9 replacement clutches, 6 failed; 5 true second clutches, 4 failed. 65% of the failures were caused by predators. Fledging success was 1.6 young per breeding attempt and 2.2 per breeding pair. 61% of males, 40% of females, but only 1% of fledglings returned to the study area in the following spring, adults always to the same territory. The establishment of this unique harbour population may be explained by habitat deterioration (higher vegetation, bushes), possibly enhanced by increasing predation, in adjacent natural coastal habitats. Later, harbour development and pavement of former weedy feeding grounds, and the high depredation of nests, explains the decline and extinction of the harbour population.

Effect of Extra-Pair Paternity on Effective Population Size in a Reintroduced Population of the Endangered Hihi, and Potential for Behavioural Management

We collected genetic and behavioural data on hihi (Notiomysts cincta, an endangered New Zealand bird) after reintroduction to Mokoia Island to assess the effect of extrapair copulation on effective population size (N e), and investigate the potential for increasing N e through behavioural management. DNA fingerprinting revealed that 46% of chicks (n = 188) resulted from extra-pair paternity, and 82% of broods (n = 56) had at least one extra-pair chick. Of the extra-pair young, 34%(n = 89) were from unpaired males, and the remainder were from paired males. Variance in reproductive success (VRS) among individuals changed between years, and the relative variance among males and females depended on the sex ratio. VRS increased when measured over longer time scales, the variance in recruits being three times higher than the variance in the number of hatchlings. Extra-pair copulation increased VRS by 150% in 1 year and decreased it by 30% in another year, but this only caused a 4% decrease and 8% increase, respectively, to N e/N. Although there is potential to manage VRS in this species through behavioural management, a more important factor is adult lifespan, which is the main correlate of lifetime reproductive success as well as the determinant of generation time. The high annual mortality rate in Mokoia hihi (females = 64%, males = 52%) has prevented the population from growing, so the key factors limiting N and N e/N are the same.

Behavioural and genetic assessment of reproductive success in a spawning aggregation of the Australian giant cuttlefish, Sepia apama

The only known spawning aggregation of cuttlefish occurs in winter in southern Australia. The operational sex ratio in this aggregation is highly skewed towards males (range 11:1 to 4:1). Using SCUBA, we videotaped females as they mated with up to six males/h. Females rejected and accepted mates of any status or size. We obtained individual eggs and tissue samples from females and their mates and determined paternity by microsatellite DNA analysis. Thirty-nine eggs were sampled during 17 observational sequences (trials); 22 eggs were sired by a sampled male. One-third of the females mated with multiple males and 67% of females' eggs had multiple sires (N=9). Given our restricted sampling, these results probably underestimate actual levels of multiple paternity. Males competed to guard and copulate with females, and spent most of the copulation time flushing water into the female's buccal area. Thus, we predicted a high probability of fertilization of the next-laid egg(s) by the last male to mate, but this prediction was not statistically supported by our data. Fertilization success did not differ significantly between paired consorts (36%) and unpaired males (31%), or between large (33%) and small males (29%). Females used sperm from paired, unpaired and sneaker males as well as from previous matings to fertilize eggs. Our results indicate that this mating system has a high level of multiple mating and multiple paternity and that males of any size or status can obtain successful fertilizations.

Territorial aggression, circulating levels of testosterone, and brain aromatase activity in free-living pied flycatchers

Testosterone (T) is a critical endocrine factor for the activation of many aspects of reproductive behavior in vertebrates. Castration completely eliminates the display of aggressive and sexual behaviors that are restored to intact level by a treatment with exogenous T. There is usually a tight correlation between the temporal changes in plasma T and the frequency of reproductive behaviors during the annual cycle. In contrast, individual levels of behavioral activity are often not related to plasma T concentration at the peak of the reproductive season suggesting that T is available in quantities larger than necessary to activate behavior and that other factors limit the expression of behavior. There is some indication from work in rodents that individual levels of brain aromatase activity (AA) may be a key factor that limits the expression of aggressive behavior, and in agreement with this idea, many studies indicate that estrogens produced in the brain by the aromatization of T may contribute to the activation of reproductive behavior, including aggression. We investigated here in pied flycatcher ( Ficedula hypoleuca) the relationships among territorial aggression, plasma T, and brain AA at the peak of the reproductive season. In a first experiment, blood samples were collected from unpaired males holding a primary territory and, 1 or 2 days later, their aggressive behavior was quantified during standardized simulated territorial intrusions. No relationship was found between individual differences in aggressive behavior and plasma T or dihydrotestosterone levels but a significant negative correlation was observed between number of attacks and plasma corticosterone. In a second experiment, aggressive behavior was measured during a simulated territorial intrusion in 22 unpaired males holding primary territories. They were then immediately captured and AA was measured in their anterior and posterior diencephalon and in the entire telencephalon. Five males that had attracted a female (who had started egg-laying) were also studied. The paired males were less aggressive and correlatively had a lower AA in the anterior diencephalon but not in the posterior diencephalon and telencephalon than the 22 birds holding a territory before arrival of a female. In these 22 birds, a significant correlation was observed between number of attacks/min displayed during the simulated territorial intrusion and AA in the anterior diencephalon but no correlation was found between these variables in the two other brain areas. Taken together, these data indicate that the level of aggression displayed by males defending their primary territory may be limited by the activity of the preoptic aromatase, but plasma T is not playing an important role in establishing individual differences in aggression. Alternatively, it is also possible that brain AA is rapidly affected by agonistic interactions and additional work should be carried out to determine whether the correlation observed between brain AA and aggressive behavior is the result of an effect of the enzyme on behavior or vice versa. In any case, the present data show that preoptic AA can change quite rapidly during the reproductive cycle (within a few days after arrival of the female) indicating that this enzymatic activity is able to regulate rapid behavioral transitions during the reproductive cycle in this species.

State-dependent pairing behaviour in male Gammarus pulex (L.) (Crustacea, Amphipoda): effects of time left to moult and prior pairing status

Because mating can be costly in terms of time and energy, an individual's propensity to engage in courtship and mating activities might be modulated by its physiological state. However, so far, state-dependent mate choice has received little attention The present study examined the effect of both prior pairing status and time left to the moult on the ability of male Gammarus pulex (Crustacea, Amphipoda) to enter in precopula with receptive females. In the lab, males that were freshly collected in precopula pairs in the field had a higher probability of re-pairing and were quicker to enter in precopula with receptive females compared to males of similar size that were freshly collected unpaired. In addition, unpaired males found in the field were closer to their moult than paired males. Considered together, our results strongly suggest that time left to the moult and prior mating status directly influence male propensity to pair in G. pulex.

Effects of Environmental and Social Conditions on Homosexual Pairing in the Japanese Beetle (Popillia japonica Newman)

Homosexual pairing between males occurs under natural conditions in a wide variety of taxa, including many insect species, but few studies have investigated how environmental and social conditions affect same-sex pairing in insects. We investigated factors affecting homosexual pairing in male Japanese beetles (Popillia japonica Newman) in the field and in the laboratory. Specifically, we investigated how time of day, sex ratio, beetle density, and temperature affected the likelihood of homosexual pairing. In the field, male–male pairs constituted 1–6% of the pairs we collected. Homosexual pairs were more common in the afternoon than in the morning and the evening. Sex ratio, density, and temperature were all related to the likelihood of finding a homosexual pair, but the relationships were not linear. In the laboratory, higher male densities and relatively male-biased sex ratios were associated with an increase in the frequency of homosexual pairs. Homosexual pairs were more frequent at relatively low and relatively high temperatures. Males that mounted other males tended to be smaller than the males that they mounted. In addition, compared to males that were not homosexually paired, there was some indication that the mounting males were smaller, and the mounted males larger, than the unpaired males. Our data suggest that homosexual pairs are a result of males mistaking other males for females, and we hypothesize that the environmental and social factors cause changes in homosexual pairing through their effects on the frequency of pair formation and pair duration.

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.

Breeding ecology and social behaviour of an antiphonal duetter, the eastern whipbird (Psophodes olivaceus)

This 3-year study provides the first detailed information on the ecology and breeding behaviour of a colour-banded population of the duetting eastern whipbird (Psophodes olivaceus). Molecular sexing of banded individuals unambiguously demonstrated that the eastern whipbird duet is male-led. Stable, socially monogamous pair bonds were formed, with a low rate of divorce. Pairs jointly defended a territory year-round, with those inhabiting open tussock grass areas defending significantly larger territories than pairs with a predominantly bracken and Macrozamia spiralis understorey. Territory turnover was low, adult survival was high, and habitat was apparently saturated, suggesting that competition for territories at this site was high. The ratio of unpaired females to unpaired males was 7 : 1, indicating that female competition might be particularly intense. Eastern whipbirds displayed characteristic breeding features of old endemic Australian passerines, with a small clutch size (two eggs), multiple broods in a long breeding-season (5–6 months), a delayed renesting period and an extended period of post-fledging care (~6 weeks). Only the female incubated and brooded young, but both males and females fed offspring. Males provisioned nestlings at a rate almost 1.5 times greater than females and played an equal role in post-fledging care, with each parent caring exclusively for one fledgling. We discuss the value of such ecological information for understanding the function of duets.

Mate Guarding Conflict, Extra-Pair Courtship And Signalling in the Harlequin Duck, Histrionicus Histrionicus

AbstractSocially monogamous birds pursuing extra-pair reproductive strategies may be in conflict, both sexes seeking new mates or copulations outside the pair, while simultaneously attempting to prevent infidelity by the partner. Intra-pair conflicts are augmented by inter-pair conflicts when pairs meet, when all four individuals may be sexually attracted to one member of the other pair while seeking to prevent their mate from copulating, or deserting, with the other. We studied the dynamics, signalling and resolution of these conflicts in a field experiment with the harlequin duck, recording responses to single model birds of both sexes, and to model pairs. Both sexes mate guarded by placing themselves between the mate and a model competitor, and by signalling with the head nod display. Females were closer to model pairs than their mates. Female mate guarding responded adaptively to infidelity risk, increasing to models of paired females, single females and single females inviting copulation, respectively. Males head nodded to signal the goal of (extra-pair) copulation, pairing or mate switching. Courtship by unpaired males increased to models of paired females, single females and single females inviting copulation, respectively. Paired males courted these models equally, perhaps inhibited by the parallel increase in mate guarding of their mates. Males signalled mate guarding using the head nod more than females, but this predicted difference in mate guarding was not shown for the measure of relative proximity to the models. In spite of male extra-pair courtship the harlequin is strictly monogamous, due to female fidelity and close mate guarding. The use of models revealed mate guarding by females, and failed extra-pair reproductive attempts by males, neither of which were apparent from observational studies alone.